Otto Piechowski's: Owner's Review of the MK67 / Argonaut 150 MCT
Telescope, Mount, Accessories and Observatory Optical and Mechanical Characteristics of the MK67-standard Manufacturer, Marketers and Accessories Negative Characteristics Positive Characteristics Observations Otto's Parting Reflection Addendum: MCT Scope Type Comparison Addendum: MK-67 Eyepiece Selection
A telescope is a time machine. Through it we look into the past. We gaze at objects, not as they now are, but as they existed back then. We look through our telescope at the moons of Jupiter and see them as they existed an hour ago. Shifting the telescope further east we seek out a quasar and behold it as it existed three billion years ago. Earlier in the evening, we may have looked at the Andromeda galaxy, seeing its nearer edge as it was, fifty thousand years younger than the edge farthest away.
The telescope is a multi-dimensional time machine in that its primary element is a piece of glass. To the eye it appears to be a three-dimensional solid, but to a sentient being who could observe four-dimensionally, it would become apparent that this piece of glass acts as and is a liquid.
And finally, a telescope is an anachronistic multi-dimensional time machine in that this liquid gathers, distills and brings to our attention photonized information about these objects, near and far. In the modern era of frantic digitized information, we stargazers leisurely consume photonized information just as it was consumed a century ago and a millennium ago. The telescope is a time machine. And we stargazers are living anachronisms.
Telescope, Mount, Accessories and Observatory
The telescope under review is an MK67-standard. It is used with a Celestron g4 mount and hand controlled clock drive made for the cg4 that allows adjustments in both declination and right ascenion. Eyepieces are 38, 16 and 11.4 millimeter semi-Plossls, and 7, 5 and 4 millimeter orthoscopics. On occasion eyepieces are used with a 2X two element Barlow. These are supported by a Celestron Star diagonal using an elliptical beveled thick secondary type mirror. Two finders are mounted on the OTA; Daisy and 7X35 millimeter achromatic refractor.
MK67 riding on a Vixen Great Polaris mount. Photo courtesy of Tahir Saban.
The MK67 model, when purchased as an optical tube assembly (OTA), is received in its own canvass carrying case. The case includes a side pocket for accessories - but none are supplied. Received with the OTA, is the 7x35mm achromatic finderscope, metal L-bracket carrying handle, and four dust caps (for meniscus, visual back, finderscope eyepiece, and finderscope dewshield). To use the telescope, one must supply a two inch and / or inch and a quarter diagonal plus adapter and eyepieces. (NOTE: Due to limited focuser travel, the adapter can not be used with 2 inch diagonal for inch and a quarter eyepieces.) Finally one must supply one's own mount. (A CG-4 or equivalent Orion Skyview Deluxe are recommended minimum equatorial mounts for visual use.)
The reviewer's observing site is a backyard abutting a thirteen acre vacant lot with a good view of the southern sky. This site is within the urban area of a city of nearly a third of a million people situated within the Ohio River Valley. There is substantial light and aerosol pollution. On a clear night with unaided vision, the Milky Way is barely visible and only four or five of the stars in the Little Dipper are to be seen. However, there are times of good transparency and stability that improve the view by perhaps half a magnitude. As with most urban areas, the stability and transparency of the sky improves as night moves toward morning; all other things being equal. The climate of the region creates useable skies more than half the nights of the year.
From that observing site, the reviewer is principly concerned with lunar, planetary, and double star observation. Forays into deepsky observation (planetary nebulae and clusters in particular) are undertaken on better nights of transparency.
Optical and Mechanical Characteristics of the MK67-standard
The particular telescope under review has an aperture of 150 millimeters. The more recent version has an aperture of 152 millimeters. However, I've been told that the manufacturer continues to use only one size of meniscus, but that the meniscus holder varies in opening size between 150 and 152 millimeters. In fact, even before this scope was offered with a standard 152 millimeter opening, some individual scopes had a size greater than 150 millimeters.
The secondary mirror holder is 52 millimeters across; just under 35% of the diameter of the aperture opening. This is much larger than the holder on more expensive and less portable Maksutov-Newtonian telescopes and roughly equivalent with secondary to correcting lens ratios on most Schmidt-Cassegrain scopes.
The secondary on the MK67 is an attached mirror. The Rumak (Sigler) design is superior to the original Maksutov architecture which contained a secondary separated from the meniscus, and from the Maksutov-Gregorian which used an aluminized spot on the inside center of the meniscus as a secondary. The Rumak design provides a flatter field of view and allows for finer collimation adjustment. The secondary is actually smaller than the 52mm's cited above. It is contained within a secondary baffle that is said to improve background sky depth by minimizing stray light. (Certainly this must be so, it is estimated that had the baffle not been included in the design the linear central obstruction percentage would drop to less than 30%.) The distance between secondary and primary may be controlled during collimation to assist in achieving focus across a range of eyepiece types and focal lengths.
The mirrors are made of Pyrex. All elements have excellent multi-coatings. (The Deluxe model of this scope has Sital mirrors with somewhat different coatings. This allows a light grasp of perhaps another half magnitude (giving image illumination closer to that of a 7 inch version.) The meniscus is made of the Russian equivalent of the optically excellent BAK-7 glass. The primary mirror is securely fitted to the mirror cell and includes an interior concentric baffle tube. Like the secondary, the primary's position can be adjusted mechanically for optimum spacing between primary and secondary. Once set, the mirror remains fixed in position (unlike SCT's or MCT models that use mirror shift focusing). It is believed by the author that proper spacing and orientation between the two mirror surfaces and the meniscus results in a superior star test and noticeable improvements in image contrast. (Please see Collimating the INTES MK67 / ORION ARGONAUT 150.
Unlike the classical cassegrain, newtonian or SCT, the Maksutov-Cassegrain design uses all spherical optical surfaces. Since it is easier to produce such surfaces with great accuracy and repeatability than paraboloids etc., high optical quality can be consistently achieved by manufacturers such as Intes and Intes Micro of Russia. This particular scope (a Standard model) has a wavefront error of 1/6 wave (the Deluxe version approaches 1/8th wave), an RMS of around 1/30, a Strehl ratio of .96, and provides image spot sizes between 13 and 15 microns. (Slightly greater than a comparable apochromatic refractor, and well less than mass-produced SCT's.) Meniscus coatings appear to be uniform and of proper thickness. Optical surfaces, very smooth, with no sleeking. The focal length of this scope varies between 1650 millimeters (F11) and 1950 millimeters (F13) - depending primarily on the (adjustable) distance between primary and secondary mirrors.
| Focused Star | Disparate Double | Extrafocal Startest | Planet Saturn | Planet Jupiter |
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Aberrator Simulation of MK-67 Standard Performance Under Perfect Seeing (1/6th wave SA)
| Focused Star | Disparate Double | Extrafocal Startest | Planet Saturn | Planet Jupiter |
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Aberrator Simulation of MK-67 Deluxe Performance Under Perfect Seeing (1/8 wave SA)
As mentioned, the primary mirror is fixed in place. Focusing is accomplished by means of Crayford-style focuser. On the focuser are two screws. Loosening the screw nearest the optical tube allows focal adjustments. Loosening the other screw varies the tension on adjusting the focus; this by means of a focusing bi-wheel. The entire Crayford focuser assembly is fixed to the optical tube by means of three Allen screws. By loosening these, one can rotate the focuser knobs 360 degrees. This is of some benefit in placing the focusing wheels in a convenient spot, or in helping a defective diagonal remain square-on to the optical axis of the scope. The Crayford focuser is designed to accept both 1/1/4 and two inch accessories and eyepieces. As mentioned, limited focuser travel means that separate diagonals are needed to support each type.
The finder is a non-illuminated straight through 7X35 achromatic refractor. (7X50 and 10X50, straight through or right angled are available but not optional). The finder is focused by means of rotating eyepiece. Images are very clear and flat (over the middle two-thirds of the 6 degree field). The finder is held in place by two holders, each with three screws enabling alignment with the main tube. Adjustment is rarely required.
Secondary mirror collimating adjustment is supported by means of three Phillips head screws attached to the back of the secondary holder. Like finder scope alignment, secondary collimation is stable and secure requiring infrequent attention. And this is fortunate given the fact that wear can accumulate to both screwheads and interior adjustment plate under heavy use.
NOTE: Fellow MK67 user Jeff Barbour has modified the secondary adjustment mechanism on his scope by fixing the adjustment screws into the adjustment plate and adding wingnuts to make "tool-less" collimation possible. (See "Collimation Mechanism Re-design".)
Similarly, adjustments can be made to primary mirror alignment by means of three Allen screws on the rear of the optical tube. There one find's three sets of two screws. In each set, is a larger and a smaller Allen screw. The larger screws loosen the mirror holder and allow for adjustment, or tighten an adjustment in place. The smaller screws adjust the position of the main mirror once it has been loosened.
NOTE: Because the MK67 has a relatively large central obstruction, planetary performance may suffer whenever precise collimation is not achieved. Readers considering acquisition of an MK67 may wish to become familiar with the collimation procedure (goals, methods, and expected results) embodied in the document Collimating the INTES MK67 / ORION ARGONAUT 150.
Manufacturer, Marketers and Accessories
This telescope is made by the optical and mechanical craftsmen of the INTES company in Russia. The mount and drive on my scope are made by Celestron but a similar mount is available through Orion Binocular and Telescope (Skyview Deluxe). Plossl oculars are made by Paul Rini and the orthoscopics by University Optics, Celestron and Meade. The Barlow is an Orion Shorty. The diagonal is a Celestron Star Diagonal (the one which looks like two tubes slammed together).
The optical tube assembly is marketed through dozens of retailers worldwide. Examples include Hands On Optics, Earth and Sky Adventures, Internet Telescope Exchange and APM Telescopes. Orion telescope company of California no longer sells the identical version of the MK67 (called the Argonaut 150). Two of the Rini eyepieces were purchased through Surplus Shed of Pennsylvania. The Barlow was purchased through Orion in California.
As mentioned a CG4 (or equivalent mount) may be used to support the MK67 OTA for visual use. Those who purchase the telescope without mount should be appraised that the scope may be used with extremely high magnifications. Thus it is not recommended that it be mounted in any way that it can not be supported by clock drive or one-axis manual tracking. So mounts of the German Equatorial type are strongly recommended. In addition, though the CG4 mount is a satisfactory ride for the OTA, the frequent user will probably ultimately wish to support the scope with something on the order of Orion Binocular and Telescope's Super Polaris or Losmandy's G8. As such the scope will give superior results at high magnifications and forays into astrophotography and imaging become a possibility.
Finally, the OTA needs incorporation of a diagonal and eyepiece set to complete its usefulness. A full discussion of eyepieces is added as an addendum to this review - but a word about diagonals is in order. Because the scope has a relatively large central obstruction, it is imperative that any diagonal not introduce coma into the image plane. Inexpensive diagonals invariably are improperly aligned and tend to send the light cone careening off to one side or another. Collimating to such a diagonal results in mis-alignment whenever the diagonal is rotated to any degree needed to improve eyepiece access. Finally, due to the number of light-effecting surfaces associated with the optical path, it is helpful to purchase a diagonal which supports maximum image brightness. Certainly, like inexpensive eyepieces, an inexpensive diagonal may be used initially, but plans should be made to upgrade and no attempt should be made to collimate the scope with such a diagonal installed...
Negative Characteristics
All telescope architectures have inherent strengths and weaknesses. This follows from the fact that, over the centuries, the simple two element "optik tube" Galileo used to discover the satellite moons of Jupiter, has undergone a process of refinement to become the multi-elemental apochromatic refractor of the day. Meanwhile, the simple single lens and mirror combination Gregorian telescope was refined by Newton and has evolved into the sophisticated Maksutov-Newtonian design - again of the current era. Beyond this, even contemporary and competing architectures have inherent limitations. Were this not the case, there would in fact be only "one telescope" design that meets all current needs - in all places.
But of course, individual scopes of a particular type or manufacture show variations in quality, aesthetic or utility. These variations may be indivdual to a specific scope or collective based on the manufacturers implementation of the design type. The following "negatives" are based on the peculiarities of the MK67 scope as a class...
- Cooldown:
Depending on the temperature differential between in-house and outside, it may take between 15 minutes and three hours for the scope to reach thermal equalibrium. To accommodate this characteristic of maksutov systems, we set the scope up before sunset (in the shade) and come out to observe a couple hours later. On those occasions when the temperature fluctuates throughout the night, this scope may have difficultly reaching and keeping equilibrium.
- Back Focus:
The Crayford focuser allows only 35 millimeters of focal movement. This causes some difficulty with bringing certain eyepieces, especially in two inch mode, to focus. It also presents some difficulty when attempting to observe straight through for collimation purposes. Finally this may be a serious issue for primary focus photography - especially if the scope is used visually and through eyepiece projection as well.
- Focusing Distance:
Many stargazers are also scopists. Scopists enjoy scoping on terrestrial objects, sometimes as near as a few feet away. This might includes a hornets' nest across the street, ants on the Peonies in the backyard, birds at the feeder. Unfortunately, this scope does not accommodate this aesthetic pleasure of nearby terrestrial focus. The nearest objects one can bring to focus are between 100 and 200 feet away.
- Slipping Collimation Adjustments:
Though the collimating screws on the secondary are firm, and the adjustment on the primary can be fixed in place, it seems that slight collimating adjustments are needed every-so-often (i.e once a month or so).
- Dewing:
Because of its upfront exposed nature, the meniscus will often dew up on a warm night of high humidity or cold night with high dew point. Even on nights when there is a great deal of dewing, many hours of observing can be done before dewing occurs by using a dew-cap-tube out of construction paper or some such material extending up to a foot beyond the meniscus holder. Once dewing occurs, a hair blower can take care of the dewing while creating only a few minutes of thermal dis-equilibrium.
NOTE: Although the MK67 is well baffled, use of a dew shield as an additional block against extraneous light is encouraged. Another reason to add a dew shield is to minimize contamination from sources of dust in the environment.
- Finder Scope:
The finder is sometimes found in an uncomfortable viewing position. The addition of a daisy finder does make some tasks easier. Further, the rotating focuser on the finder is rather loose and can get out of perfect focus.
NOTE: A retaining ring (such as a rubber band) can be installed in the finderscope to eyepiece gap to offset this issue.
- Secondary Collimating Screws:
The Phillips head screws on the back of the secondary are close to the meniscus and somewhat small, allowing slippage of a screw driver if one is not careful, hazarding damage to the meniscus or its coatings. Also, these screws can be loosened so far that the secondary mirror detaches.
NOTE: As mentioned earlier, enterprising owner's can rework the collimation mechanism to introduce wingnuts as replacements for the phillips screw BUT this requires dis-assembly of the secondary and fixing the leadscrews at their base in the secondary backing plate.
- Long Focal Length:
Maksutov-Cassegrain scopes as a type, are usually limited to F10 focal ratios or greater. As such, the lowest power 2 inch eyepieces (50mm) give a magnification of 36x (at the MK67's 1800mm focal length). This magnification is barely acceptable as a low power sweeper magnification. Since many users may not prefer to add the necessary 2 inch diagonal needed to support 2 degree fields, the scope is limited to about 50x (one degree fields) when using longest focal length 1 1/4 inch eyepieces.
Positive Characteristics:
And of course, every scope type (and individual model) has its virtues...
- Optical Performance:
When properly collimated, through this telescope one can see 14th magnitude stars and split double stars of equal brightness separated by less than .8 arc seconds. The field of view is just under one degree when using a 38 millimeter eyepiece. The field is flat. At low magnifications (.3X to 1X per millimeter) stars appear as pinpoints clear across the field. At high magnifications (2X to 4X per millimeter) the sharp in focus structure of objects remains unchanged across the entire field of view. At high magnifications there is a faint light-haze around an extended object such as the planet Saturn stretching out in a circle four to five times the width of the object itself. With the 2X Barlow, when used with the orthoscopic eyepieces at high magnifications (515X, 720X, 900X), observations at the edges of an extended bright object such as the moon tends to be hampered by glare. However, if the edges of the negative lenses in the Barlow are removed and darkened, this significantly removes the glare. Finally, this optical system does reveal increasing detail on some objects up to 360X (2.4X per millimeter, 60X per inch). The image does not seriously deteriorate even when pushed up to twice these amounts (720X; 4.8X per millimeter, 120X per inch). Some objects become visible only when these very high magnifications are used. Examples include very tight double stars and especially, the very faint ring hugging moons of Saturn.
- Portability and Storage:
The optical tube assembly is less than a foot and a half in length and weighs less than ten pounds. This allows the use of a substantially less bulky equatorial mounting than would be needed for a similar aperture Newtonian or Refractor. Because of the reduced weight, it can be inferred that battery packs used to power the drive drain more slowly. This lack of bulk makes it easier to transfer and set up the telescope. The entire telescope, mount, tripod and accessories can easily be stowed in the trunk of a sub-compact car. The entire set-up can be packed up into the bag used for the scope and drive, a small eyepiece case and a regular size tool box for tools and accessories. Storage room takes up one shelf of about six cubic feet, and three square feet of space on a floor for the contracted tripod/mount. Further, it has been said that the OTA in its bag fits in the overhead compartment of a jet.
Scope assembly is furthered by the very useful "L"-handle mounted on the OTA. This allows the scope to be held securely while fastening the scope to the mount. The L-handle may also be used to slew the scope rapidly between studies and makes a practiceable "1x finder" when locating guide stars.
Once assembled, the scope need not be broken down. Given a solid equatorial mount - of not too excessive a weight, scope and mount combination can be carried as a unit to more advantageous viewing locations or returned to garage, study, or first floor apartment for stowage.
- Robustness and User/Owner-Friendliness:
This scope is designed to take a great deal of use under normal field observing conditions. It can even take some misuse with little or no damage. Jars to the optical tube will probably not damage the optics and might not dislodge the collimation. Because of the curve of the meniscus, this element is further from the front edge of the scope than a comparable Schmidt-Cassegrain collector plate. Though an impact against the meniscus would scratch the optical element and its coatings, it is unlikely that a comparable impact which would crack a Schmidt corrector element would crack the meniscus due to the thickness of the meniscus. Similarly, the Maksutov meniscus is further from the dust cap then the corrector plate of a Schmidt-Cassegrain, thus giving a Maksutov a further degree of robustness. Another aspect of the usability of this telescope is that it is designed to have maintenance performed on it by the amateur scopist. Both the secondary and the primary have regular sized and shaped screws which allow the owner to handle all collimation needs. Advanced amateurs also completely disassemble the entire scope and all of its components so as to study the assembly, to clean optical elements and to experiment with adjustments. Though this should be done under the tutelage of one who is experienced in doing so, this nonetheless highlights the user/owner-friendliness of this scope.
- Price and Value:
The value of a telescope must be measured by a qualitative algorithm that includes the three elements of optical excellence, optical performance and usability.
The optical excellence of a system is a function of the efficiency of the optical design and of the quality of the optical elements (i.e. curve accuracy, smoothness, coatings, etc.).
Optical performance, then, is a function of aperture in addition to this optical excellence. An optically excellent scope with no secondary obstruction (or an obstruction of less than 20% of the diameter) is said to perform better than a somewhat larger apertured obstructed scope. The algorithm often cited is that the diameter of the aperture of the unobstructed (D-unobstructed) or of the slightly obstructed system is equal in image contrast to an obstructed system whose aperture diameter (D-obstructed) minus the diameter of the obstruction (d-obstruction) equals D-unobstructed. (E.g. a refractor of 4 inch aperture is equal to a 6 inch maksutov with 2 inch obstruction in terms of low contrast planetary detail; i.e. 4=6-2).
Usability, the third factor in the algorithm of value, refers to the famous MacRobert Principle: a smaller lighter easier to handle telescope will show more than a larger bulkier harder to handle telescope because the smaller scope will be used more and more often.
In terms of optical excellence, optical performance and usability, the MK67 might be the best telescope available today per inch aperture cost. The optical tube assembly can be obtained new for less than $850. The optical tube assembly with a suitable mount, tripod and drive can be obtained new for between $1300 and $1400. In good second hand condition, the same can be obtained for under $1000.
It has been said, correctly, that this optical tube assembly gives low contrast planetary optical performance equivalent to apochromatic refractors of 4 inch aperture costing 2 to 3 times as much; and in the case of a very tight double star of equally bright components, equals or surpasses the performance of an equal aperture apo-chromatic refractor costing 4 to 5 times as much. This favorable comparison is somewhat deceptive. It is deceptive in that one can also obtain the same optical performance in a 6 inch Newtonian reflector with a focal ratio of F8 to F10 costing half as much as the MK67.
There is a beautiful justice in the fact that a well made 6 inch F10 Newtonian performs as well as a 152 millimeter apo-chromatic refractor or a maksutov-cassegrain of similar optical quality. The stargazer of limited means or the scopist possessing mechanical aptitude is able to obtain an instrument that optically performs as well as any telescope of equivalent aperture for a quarter or an eighth of the cost. In this regard, the equality of amateur stargazing is not unlike the fairness of fishing: the adolescent with only a cane-pole often catches more fish and enjoys herself more than the serious and sophisticated recreational fisherman who seeks relaxation in owning boats, expensive rigs and the most current accoutrements of the sport. The only difference between the fisherwoman and the stargazing scopist is that the latter catches and consumes planets, galaxies, clusters and nebulae.
Further, the apochromatic refractor is superior in its ability to reach thermal equilibrium more quickly than the maksutov cassegrain and in its greater field of view presented at the eyepiece. On the other hand, though the Newtonian costs less, it is bulkier, much longer and weightier. The apochromat, despite the benefits of its thermal characteristics and field of view, costs much more, is somewhat longer than the MK67, and will always have a touch more chromatic aberration than the maksutov design.
Value, of course, is dependent on many subjective factors. For example, $1000 for one person might be no more significant than an expenditure of $4000 for another person. Further, one might be willing to spend more for what she feels is an aesthetically superior instrument. Or he might be willing to spend more for an instrument that allows the possessor entrance to a particular fellowship. In this aesthetic-relational regard the MK67 might be said to appeal to a certain group of persons. Specifically, the MK67 has been described as the perfect example of champagne taste on beer budget; one can obtain the equivalent of an apochromatic refractor (champagne) at half or a third the price (beer). This metaphor of the beer budget also, interestingly dove-tails into the MK67's robustness (brute-force-design) and its Russian origins. This is a scope made by and for persons who tend to be laborers by circumstance or choice, perhaps hard-working and intelligent, desiring a modicum of learning and having a taste for aesthetic refinement. Though hoi polloi themselves, they are often appreciative of another's good fortune, grateful for the blessings and nice things they possess and passionate in their affections. This might be a good definition of the amateur (devoted lover) stargazer and scopist.
Observations:
Oh my! What fun! What things one can see, even from within an urban area.
- The Sun:
Using a Baader filter, the most pleasing views of the sun occur at 50X. Additional detail can be obtained up to 158X and, on occasion, 257X can be serviceably used. At 50X one can see the rice grain across the entire surface. Faculae can easily be seen at the edges and noticed even near the center of the sun. Great detail can be seen in the delicate and well delineated penumbra surrounding the umbra of sun spots.
- The Moon:
The moon presents an overwhelming wealth of detail. When the scope is well collimated the area around Copernicus appears inundated with an infinite iteration of craterlets of decreasing size. When near the terminator one can see a half dozen depressions within Ptolemaeus, and the caldera in the domes of Hortensius in the Copernicus region. Under a higher sun, one can see at least four craterlets inside the crater Plato. Under finest seeing conditions Plato-A can be seen as a true craterlet - and not a simple brightening. Due to high contrast of lunar features, the MK67's full 150mm of aperture is brought to bear in observing our nearest neighbor.
- Saturn:
Saturn does not present the wealth of detail that Jupiter reveals, but what it presents it lavishes on the MK67. One can easily see a well-defined light band across the planet and the shadow of the ball of the planet against the rings. Six moons (perhaps seven) can be viewed. The Cassini division looks like a deep black ribbon. The crepe ring is easily noticed against the disk of the planet and in the ansae. Darkened areas, spoke like features, brightenings and the illusory Encke Minima can be glimpsed with frequency. Satisfying views are obtained up to 360X. Very little image degradation is noticed up to and including magnifications of 720X. Like the Moon, Saturn's high contrast ring structure allows the MK67 to fully exploit its 150mm of aperture - irresepective of central obstruction size.
- Jupiter:
Though heavily dependent on sky transparency and stability, Jupiter presents the proverbial "wealth of detail". All four moons appear as disks. One can distinguish the relative albedo of each - as opposed to merely comparative brightnesses. Eclipse shadows against the globe of the planet are jet black disks. The Galileans themselves can easily be seen as they transit in front of the planet against the background of a dark belt. Even when immersed in one of the creamy white areas of the planet near an edge, one can see a moons disk in transit. Detail can be seen within and around the Great Red Spot. The two main belts are always visible. Bright circles, dark bars, thin jets of dark material and festoons can be seen in and around the main belts. Under 8/10 seeing (Pickering) the thin equatorial belt can be seen bisecting the Equatorial Zone.. And, though usually the polar areas are noticeably shaded, occasionally these too resolve into one or more discernable belts. Due to large central obstruction, detail and color tends to wash out in the MK67 - compared to a comparably sized apochromat of similar optical quality. In poor seeing, 158X is the practical limit of useable magnification. Under excellent skies, this can be increased to 360X which better displays detail associated with the north and south equatorial belts.
- Outer Planets:
Uranus and Neptune are beautiful, well delineated, aquamarine disks. Pluto, at magnitude 13.5, is within the scopes grasp - under truly dark skies.
- Double and Multiple Stars:
Though it would be difficult to describe the appearance of double and multiple star systems, a listing of what has been seen might be of value. In general, the MK67 surpasses its theoretical limits. This is due in part to the nature of a Rumak variation of a maksutov design with a large secondary obstruction. The following are beautiful views in a steady sky: the double-double in Lyrae, Castor in Gemini, and the triple system zeta cancri. Doubles such as Rigel, 16 orionis, 32 orionis and eta orionis are easy splits. Much closer or disparate doubles due to limited separation or a great magnitudinal difference such as Antares, gamma ceti, and epsilon equulei, have also been resolved. No amount of magnification seems to allow this scope to split 68 tauri or 37 pegasi. Although fellow observers Jeff Barbour and Cor Boerrevoets (using a similaly constructed Intes Micro 150mm MCT) report elongations of matched pair doubles separated down to .55 seconds of arc.
- Globular clusters:
Globular clusters of Messier designation and to magnitude 8.4 often resolve to at least some individual stars. The Great Clusters (Hercules, Sagittarius etc.) though dimmer than in larger apertures, are resolved to utter perfection. Galactic (open) clusters such as the Praesepe, the double cluster in Pegasus, the Pleiades, the messiers in Auriga and Wild Duck in Scutum give extremely satisfying appearances. As such they take on the appearance of a large handful of small white diamonds cast onto a black felt background.
- Stars and Nebulae:
Though color rendition is not a great virtue in this telescope, carbon stars reveal their deep red hue. Similarly, and probably due more to light pollution than to the secondary obstruction of the MK67, the greenish tone of the Great Nebula in Orion is quite muted. However, this nebula extends well beyond the edges of the field of view of the 38 millimeter eyepiece. At high magnifications, the texture of the Great Nebula becomes very interesting; it appears as a bubbling cauldron of black flames a la Harry Potter. The four stars of the Trapezium are always visible. On steady nights the E component is easily seen and F component is glimpsed with slightly averted vision.
Hundreds of galaxies can be seen, and detail in some. M82 shows its notch and M51 shows its companion at the end of the arm. M110 next to the Andromeda galaxy is visible but only as a faint wisp.
NOTE: Under 5.5 ZULM skies, fellow observer Jeff Barbour has succeeded in tracking down and observing all five main member's of the M31 galaxy family (M31, 32, 110, NGC147 & 185.)
Some emission nebulae and planetary nebulae can be seen and some cannot. Those which can be seen under light polluted skies include the Ring Nebula in Lyrae, the Dumbell in Vulpecula, the Eskimo in Gemini, the Owl in Ursa Maior, Hubble's variable nebula, and many smaller or fainter nebulae. The Flame nebula in Orion and the Horsehead cannot be seen, as neither can the Veil Nebula in Cygnus.
NOTE: Under more transparent skies (ZULM=5.5), Flame and Veil Complex Nebulae have been seen with and without nebula filter. Under 6.0 skies efforts have been made to locate the Horsehead without success.
There is every reason to believe that esoteric objects such as Pluto, the quasar 3C-273, the moons of Mars and Triton are within the reach of this scope.
NOTE: Jeff has also located 3 billion light year distant quasar 3C-273 but due to variability in its luminosity at the time, the Quasar required averted vision. Those interested in more detail's regarding specific deepsky results seen under decent - but not exceptional - skies need look no further than Astro.Geekjoy.Com .
Parting Reflection
Consider the stargazer: an ephemeral creature, fully alive only in the present, consuming aeons old electro-magnetic energy transformed into chemical energy by his optic nerves and brain; who, in this act of feeding that transfers cosmic information into the confines of the mind, reveals himself to be an anachronism which seeks future enlightenment.
To a universe of electro-magnetic radiations, he represents nothing more significant than any cloud of organic molecules obstructing its paths, only somewhat more condensed. But to these radiations understood to be information, he is their goal. They are his temporal purpose. To him, they are food. To them, he is rest.
Otto Piechowski,
Piechowski2@aol.com
Addendum: MCT Scope Type Comparison
There is no one optimal optical performance standard that all amateur astronomers and scopists would agree on. However, that doesn't mean that astro.geekjoy can't weigh in on this matter!
- Encircled Energy: 80% of all photons from a white light point source of magnitude zero should fall within an aperture-constrained and idealized airy disk. The arc-second diameter of such a disk may be calculated based on the formula 260/D(mm).
Those who often refer to Dawes limit for scope resolution know that the airy disk size value is more tightly constrained. (This by more than half.) Dawes based his research on a pair of 6.5 magnitude stars. Due to lower magnitude, much of the true airy disk is invisible. The result has been an improper characterization by subsequent observers that all stars show airy disks .8 arc seconds in diameter through a 6 inch scope. An additional benefit of standardizing to a zero magnitude point source is that chromatically tinged photons will smear outside the airy disk. This effectively reduces the encircled energy percentage provided by otherwise fine optics suffering from chromatic and sphero-chromatic aberrations.
Where does the MK67 settle on this criteria? Tough question! The scope's strehl (encompassed energy) rating is 92% - significantly better than the criteria cited above. However, the presence of a 35% central obstruction means that considerable light is thrown into the first diffraction ring. Thus some kind of CO and optical quality compensation is needed. The general rule for "unobstructed aperture equivalence" on low contrast studies (such as Jupiter) is D'mm = D(mm) - CO(mm). By this standard an MK67 is the equivalent of a 98mm unobstructed scope. But that rule applies to 98mm's aperture meeting a 80% encompassed energy requirement. The MK67 is superior to this.
Another way of tackling this question is to assume that half a stars "unencompassed energy" falls into the first diffraction ring. That ring is located within a region 360/Dmms in diameter. For the MK67, this region is 2.4 arc seconds. The equivalent unobstructed scope aperture which produces a zero magnitude airy disk of 2.4 arc secs has an aperture of 108mm's. Generally then, we can assume that the equivalent low contrast feature aperture of an obstructed scope falls somewhere between these two values - based on the quality of the optics. Given the scopes 92% strehl ratio, a good approximation is 104mms... - Field Flatness: An optimal telescope should meet the above encircled energy requirement anywhere within the 60 degree apparent field of view of an eyepiece whose focal length renders 1mm exit pupil magnification. Thus a 150mm scope operating at 150x should display a truly "flat field" anywhere within 12 arc minutes of field center.
As aperture increases, the radius posed by this "flat field" requirement shrinks. However, the size of the airy disk also becomes smaller. Most 300mm scopes will have great difficulty meeting the 80% encircled energy requirement on a zero magnitude star anywhere in the field of view. However, an idealized scope of great aperture should be able to achieve this desideratum somewhere. (Of course, atmosphere seeing imposes the ultimate constraint...)
MK67 experience has shown that star images anywhere within a 52 degree apparent field / 120x eyepiece meet this requirement - but that 72x does not. The effective flat field is probably greater than 15 but less than 20 arc minutes in radius.
- Range of Magnification: An ideal scope should allow magnification to range across the full domain of the eye's entrance pupil capability. (For most of us, this is .25 to 7mm.) Thus, a 150mm scope should support a range of magnifications from super-wide fields (21.5x) to absolute tunnelvision (600x).
NOTE: This upper limit (of 600x) is only useful in elongating super close doubles. The effect of such magnification is to pare back the airy disk to the smallest possible point of greatest brilliance and highest contrast. Otherwise, .4mm exit pupils (360x in a 150mm scope) is probably the upper limit of magnification useful in drawing out intermediate contrast detail. To achieve 7mm exit pupils using 50mm 2 inch eyepieces, the optimal scope has a focal ratio of roughly F7.2. Therefore, the ideal 150mm scope should have a focal length of roughly 1080mms. To achieve 600x such a scope would require the use of a high quality barlow lense.
This is the main failing of the MK67. The need to keep central obstruction sizes as small as possible, and physical limits on meniscus thickness, demand that longer focal ratios be implemented. The most carefully designed MCT's (such as the more expensive Intes-Micro M603) are only able to accomplish this at F10. The MK67 is definitely NOT a rich field scope. - Transmissivity/Reflectivity: A scope should be able to pass 80% of the useful light incident to the optic tube to the final image.
Glass elements in the optic path absorb or scatter about 2% of the light passing through them. Mirror surfaces scatter roughly 6%. Random or incidental light entering the field of view contributes to background glare and reduced image definition. Contrast is compromised and magnitudinal reach, diminished. Thus scopes lacking sufficient baffling or possessing a multitude of glass and reflecting surfaces reduce image luminosity.
Based on the 2/6 rule, all refractors and Newtonian scopes can achieve 80% transmission of effective light using garden variety coatings and surfaces. Refractors lend themselves to ease of baffling while Newtonians do not. SCTs and MCTs can easily fail to meet the 80% standard. As such they are candidates for refined coatings and super-reflective surfaces. A quality diagonal can make a significant difference as well. SCTs and MCTs can be readily baffled.
The MK67 Standard model requires the use of a quality diagonal to achieve 80% illumination. The use of a dewshield, along with manufactured-in secondary and primary baffles does a good job of blocking incidental light just outside the field of view. However, there are certain angles of incidence from strong light sources (Moon, planets, and bright stars), that contaminate the field when placed in selected moderately approximating positions.
There are of course, other features that may be of parochial interest to the amateur astronomer or scopist. Cost, portability, maintainability, storage, ergonomics, and visual appeal all have there place when discussing a particular scope (and accessories) merits. But the above factors are those that establish a baseline for optical quality and performance...
The following table attempts to give the reader an idea of how well the MK67 stacks up against the performance criteria established earlier on various types of asronomical studies. It can be seen from the earlier discussion that the scope is equivalent in low contrast feature resolution to a diffraction-limited, apochromat of about 104mms aperture. Due to the presence of a large number of mirror and lens surfaces, and the half inch cumulative aperture loss caused by its two inch central obstruction, the scope's effective aperture is equivalent to about 130mms for many types of studies. Of course, that same central obstruction improve's the MK67's ability to resolve close matched-magnitude double star pairs to the equivalent of a diffraction-limited 175mm apochromat. Where doubles are disparate in magnitude, this unique feature may be lost - especially if the companion falls on the first diffraction ring of the primary...
| Study Class | Effective Aperture | Illumination | Size | Contrast Level | Comment |
| Solar | 75mm | Super-bright | Extended | Mid-Contrast | Poor Seeing Limits Aperture |
| Lunar | 150mm | Bright | Extended | High Contrast | Superb! |
| Mars | 130mm | Bright | Intermediate | Mid-Contrast | Approaching Opposition |
| Saturn | 130mm | Bright | Intermediate | Mid-Contrast | Cloudtops not an issue |
| Jupiter | 104mm | Bright | Intermediate | Low | Falls down here |
| Faint Stars | 130mm | Faint | Point source | Intermediate Contrast | Good Baffling & Image Coherence! |
| Planetary Nebulae | 130mm | Faint | Intermediate | Mixed | Good Baffling and Image Coherence |
| Matched Double Stars | 175mm | Bright | Point Sources | Super High Contrast | Overachiever Here! |
| Disparate Double Stars | 115mm | Point Sources | Mixed | Intermediate | Good Baffling and Image Coherence |
| Globular Clusters | 130mm | Intermediate | Extendo-Intermediate | Intermediate | Alas, more aperture! |
| Galaxies and Nebulae | 130mm | Faint | Extendo-Intermediate | Smoothly Gradiated | Needs 6.5ZULM Skies! |
In sum, the MK67 gives the overall performance one would expect from a 5 inch, diffraction limited, fully apochromatic F14 refractor. If there is anything that might be said in criticism of this level of performance, it is simply: "More light!".
Addendum: MK67 Eyepiece Selection
Those considering the MK67 as a "scope of choice", may wish to consider a proper range of eyepiece's. Due to the MK67's long focal length, the would be owner is a bit hardpressed to find 1 1/4 inch eyepieces capable of full one degree "navigational" fields. Orion B&T's 35mm Ultrascopic approaches this desideratum but suffers from field flatness issues. Televue makes a 40mm 4-element plossl, but its 40 degree apparent field approaches tunnel-vision. Though failing to come close to the one degree field ideal, the 32mm Televue Plossl meets the need for a low power eyepiece and gives a 50 degree apparent field. If the scope MUST be used for wide fields, certainly two inch eyepieces are in order. BUT since the MK67 has a largish central obstruction, perceptible "contrast loss" occurs at center in the low power field of view - especially during "daylight" (terrestrial) use.
NOTE: Based on my own relatively intense and diverse observing experiences with the MK67, I have concluded that, though it does many things well, it is NOT a "rich field" scope. So rather than purchase 2 inch diagonal and 50mm eyepiece, I simply added a short tube achromat to my personal kit to fulfill this requirement (at about the same total cost).
Setting aside this one issue of a good low power sweeper eyepiece, the field is wide open. For those of modest means, a set of 25, 15, and 10mm Ultrascopics - combined with a fine apochromatic barlow - can give a suitable range of deepsky, lunar-planetary and double star magnifications. A step up from the Ultrascopics is the Lanthanum series - also available from Orion B&T. Due to long eye relief (20mms) these eyepieces are especially useful when the observer must wear eyeglasses for astigmatic correction. Long eye-relief eyepieces are also warranted whenever the scope is used in a cold environment. (This helps prevent observer-induced eyelens condensation.) Due to focuser travel restrictions, some barlow lenses (such as Orion's Ultrascopic Barlow) may be exclusively used in 3x or 2x configuration exclusively. (Before or after the diagonal respectively.) This reality can have some effect on eyepiece focal length selection. Finally, though I have no experience with "zoom eyepieces", some exploration of this choice is in order - especially when sky conditions are quite variable and the observer must pick and choose magnifications carefully.
There are, of course, more expensive answers to the eyepiece question. The optics on the MK67 is worthy of extreme expense. Where funding is not an issue, it has been found that the 19mm 65 degree field Panoptic (at 95x) makes for well-framed deepsky views of Messier-class galaxies and open clusters. This magnification also assists detection of sub-one-arc-minute planetary nebulae possible (without nebula filter). Finally, this magnification and field of view combination can display the entire lunar disk and many larger open clusters.
Many small galaxies (1-2 arc minute in apparent size), intermediate sized planetary nebulae, open, and globular clusters, require mid-range magnifications. Thus the slightly higher and more aesthtically engaging "porthole-effect" of an 82 degree apparent field 16mm Nagler (112x / 44 arc-minute field) is in order. This magnification is also excellent for framing the entire lunar disk, visible nebulosity in the Great Nebulae, as well as viewing the gas giants under marginal (5+/10) seeing stability (the ultimate "star-party" magnification for Jupiter's belts and Saturn's rings!)
Since the Nagler's 82 degree apparent field at 112x covers more sky than the 65 degree Panoptic at 95x, it would seem that other factors must govern the choice of one over the other. Certainly, the Panoptic is less expensive. It also has greater eye relief (13mm as opposed to 10). However, where eye relief is not an issue, the Nagler gives a more breathtaking expanse of sky. (I've used both and found of these complex oculars very satisfactory in terms of image illumination, field flatness, and aesthetics.)
The MK67's superb optics and satisfactory light gathering capability easily supports bright globular cluster resolution at 200x. This same magnification also gives optimal views of the gas giants under 7/10 seeing stability conditions. Thus an 8mm Radian (225x) is an excellent choice given its fine image clarity and expansive 16 arc-minute field of view.
When the scope is properly collimated, and on truly superb seeing stability nights, both gas giants may be viewed to advantage at 360x. Thus a 5mm Radian (which includes a generous 20mm's of eye relief) is in order. Finally those who seek to partially resolve sub-Dawesian double star pairs of matched magnitudes (or very close disparates approaching Dawes limit) may wish to consider sub-4mm Naglers (or less expensive Vixen/Orion Lanthanums).
The following table is a rough guide to assist the would-be owner of the Intes MK67 in selecting appropriate eyepieces based on targeted uses. It can also be consulted in choosing an appropriate barlow or potential range in zoom eyepiece operation. It should be clear that no eyepiece is available to meet the criteria of "rich field" use. The selection of a good one degree field "navigational" eyepiece also requires some research on the part of the owner. The scope performs very nicely at 75x, providing a good sense of a wide dark background sky - "darkener". 100x adds the capacity to show the entire Moon or numerous "moonsize" open clusters in a single 30 arc minute field. 200x is superb for resolving bright globular clusters, showing details on the Moon, Jupiter's globe, and Saturn's ring system. Dawes limit separated doubles require 300x. 300x is also excellent for lunar-planetary studies on superb seeing occasions. 500x may be used for detailed examination of high contrast studies such as super-close double stars, Mars surface maria (near opposition), and Saturn's ring system - again under superb seeing conditions. It is needless to adhere specifically to suggested magnifications in each category of use. Certainly anything within one order of magnitude (+/-10%) is acceptable.
| Purpose | Minimum Field | Magnification | Focal Length | Exit Pupil | TLM* | Resolution |
| Rich Field | 3 degree | 21x | 86mm | 7mm | 12.4 | 7" |
| Navigator | 1 degree | 48x | 38mm | 3.1mm | 12.5 | 4" |
| Darkener | 45' | 75x | 24mm | 2mm | 12.6 | 2.5 |
| Framer | 30' | 112x | 16mm | 1.34mm | 12.7 | 2" |
| Detailer | 15' | 200x | 9mm | .75mm | 13.0 | 1.0" |
| Splitter | 9' | 320x | 5.6mm | .47mm | 13.5 | .77" |
| Elongator | 6' | 500x | 3.6mm | .3mm | 14.0 | .55" |
* TLM calculated based on stars visible to magnitude 5.5 on a 7/10 seeing stability night.
In sum, the MK67 OTA is deserving of the finest eyepieces and mounts. Due to its low initial cost (a bare-bones setup with CG-4 equivalent mount, three eyepieces, and barlow can be kept to under 1500 dollars american), it is easy enough to begin inexpensively and replace components over time - finances (and spouse) permitting. Of course, a fine eyepiece kit (mount, filter set, etc.) is not permanently attached to the scope - but may very well be...
Jeff Barbour,
barbour@ihwy.com