Otto Piechowski's: Owner's Review of the Astro-Rubinar 106/1000

Equipment Specification
Outward Appearance of the Astro-Rubinar system:
Optical Performance
Mechanical Performance
Storage and Use
Some Reviewer Background History
After-Purchase Service
Evaluating Oneself as an Observer
Addendum I: Obstructed Scope Type Comparison
Addendum II: Astro-Rubinar Eyepiece Selection
Addendum III: Buying Your Last Scope First
Addendum IV: 02/21/03 Otto Piechowski Update

System Specification

The Astro-Rubinar is a camera lens that, due to the nature and quality of its construction, functions well as a telescope.

Specifications:

Accessories: (included by ITE)

Other Components Evaluated:

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Outward Appearance of the Astro-Rubinar System

The Astro-Rubinar is a catadioptric design; but not a Maksutov or Schmidt telescope. The system includes three full aperture lenses, a primary mirror, a rear surface secondary mirror and small field flattener lens. The secondary mirror is affixed to the outermost aperture lens. A set of two full aperture lenses is situated between the secondary and primary mirror. The primary mirror is spherical. The field flattener lens is situated in the optical back, behind the primary mirror. The central obstruction of the secondary mirror is 13% of the area and 34% of the diameter of the clear aperture. Light passes through 18 optical surfaces and reflects off two mirror surfaces to arrive at the eyepiece. The reason for all of these optical components was to create an excellent flat photographic lens. However, it was discovered that the quality of the components created a good telescope that also has very good near focus capabilities.

Included with the scope is a certificate asserting that the wave-front error (p.t.v.) of the optical system as ¼ wave at 555 nanometers. This certificate is signed by by Mr. Bill Burnett (proprietor of ITE) and by Mr. Mike Palermiti (optical systems evaluator). That the end product is diffraction limited is amazing and a testament to good optical work on the individual elements and good mechanical work on the overall assembly.

The compound lens-mirror arrangement creates a light compact optical tube assembly. The scope is 7 inches long with the dew shield and diagonal removed. It weighs just over 5 pounds (2.5 kilograms). This solid metal OTA is painted black with etched in markings. With both attached, the unit is about a foot in length and roughly 5 inches in diameter.

The scope is attached to a camera tripod or equatorial mount by means of ¼-20 fitting. This fitting is attached to a ring of metal affixing to and encircling the scope near the visual back.

This particular unit has a number of cosmetic imperfections. The metallic ring has a roughened-up look; a number of striations/scorings are present which have been painted over. Further, the markings etched on the scope have a rough though uniform appearance. Finally, at the very edge of the field-flattening lens is a small chip/crack. This appears beneath and right next to the retaining ring. The chip is perhaps 1.5mm by 1mm and is seen reflected off the field-flattening lens. However, when light is blocked from reaching the chip, the reflection is lost to sight.

A very long baffle tube extends most of the way from the primary mirror central hole to the secondary. Within this tube are many baffle-edges. All interior surfaces have a dull black appearance.

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Optical Performance

The scope was tested by Mike Palermiti for ITE and a wave-front error of ¼ wave at 555 nanometers ascertained.

Appearances of intra and extra focal images of bright stars and point light sources are strange, even disturbing, to the observer accustomed to MK, MN or apochromatic optical systems. The intra-focal image is a sharply defined elliptical structure with a well-delineated ring forming the very perimeter. The extra-focal image is mushy in comparison, lighter in tone, and elliptical as well. The elliptical shape of the intra focal image is perpendicular to that of the extra-focal image. This indicates astigmatism.

The use of Cor Berrovoet’s Aberrator-software shows that the astigmatism of this particular scope creates a wavefront error of between .248 and .276 wave-front error. At a high degree of de-focus, both intra/extra focal images present nearly identically shaped and distinct double rings. Such astigmatism, according to expert opinion, does not compromise the diffraction limitation of this system; either its ¼ wavefront error or 80% strehl ratio.

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Aberrator Simulation of Astro-Rubinar Performance Under Perfect Seeing (.25 waves astigmatism - 1/5 wave SA)

NOTE: In this and the following simulation, the editor necessarily made a number of assumptions. First: The interferometric evaluation of the scope's .80 strehl rating was done at F10. Second: The rating was the result of combining .25 waves astigmatism and 1/5th wave spherical aberration (SA). The SA value was induced in Aberrator using +.25 waves 3rd order, offset by -.05 waves 5th order values. (This is typical of compound scope types.)

Focused Star Disparate Double Extrafocal Startest Planet Saturn Planet Jupiter

Aberrator Simulation of Astro-Rubinar Performance Under Perfect Seeing (Zero astigmatism - 1/5 wave SA)

There are no easily accessible means to collimate the scope. However, the collimation seems to be very close to dead-right-on with only a slight loss of the doubled aspect in the ring-pair of the greatly defocused intra-focal image.

The in-focus image of point-light-sources, when the air is stable and scope is thermally equalized, presents a round airy disk and single diffraction ring of uniform size, distance from the airy disk and brightness. Quite often, the continuity of the diffraction ring is disrupted by four tiny black notches, each 90 degrees away from the next. Sometimes, the diffraction ring is continuous.

The scope functions well on terrestrial objects, providing pleasant views of distant objects. It also shows sharp, clear, flat views of objects as near as 14 feet with or without the diagonal. This is a bit longer than the advertised near-focus capability of 4 meters (13 feet).

During times of ambivalent sky stability, with a limiting zenith magnitude of 4.5, the scope reveals stars of magnitude 13.0 with averted vision at 300X (75X per inch).

The Great Nebula in Orion gives a very satisfying appearance through a 17mm plossl (88x); as aesthetically pleasing as seen through an MK67. The appearance of the epsilon lyrae double-double is greatly superior to a Celestron F5 80mm achromat refractor and somewhat inferior to the view through the MK-67. Albireo and gamma Andromedae give color saturation views more like an F5 80mm refractor than the MK-67. In a somewhat stable sky, Castor’s two components can be seen at 50X, though the companion of Rigel is not visible until conditions improve significantly.

In a stable sky, eta orionis (listed as a double whose components are a full magnitude different separated by 1.5 arc secs), is easily resolved; the fainter of the two nestling on the diffraction ring of the former. Through a borderline-stable-sky, one can see detail within sunspot penumbra, easily see faculae and detect the presence of rice grain. In an unstable sky the Cassini division of Saturn is detectable - but not what would be called, visible. In a stable sky, the division becomes detectable at 90X and visible at 220X (55X per inch) as a faint vague pencil thin line. At moments of good stability the division can be detected through nearly the whole ring, and at moments of very good stability becomes readily apparent. Although the scope gets very good images of Saturn, and Jupiter in steady skies, image quality noticeably deteriorates as seeing becomes unsteady.

On the moon and planetary detail, best visual detail is seen at between 20X and 30X per square inch of aperture (80X to 130X in this system). Though higher magnification may provide more pleasing views of a few objects - due to an enhanced image scale, no additional detail is seen above 130X - even with good orthoscopic eyepieces. Only slightly more detail can be seen at 130X with a good quality ortho, than at 95X with a fair quality plossl.

EDITOR’s NOTE: Although obstructed scopes reveal detail comparable to unobstructed scopes of roughly 2/3rds aperture, obstructed scopes of high quality frequently offer an advantage in terms of image scale. Thus a 150mm Maksutov-Cassegrain (such as the Intes MK-67) is able to operate at significantly higher magnification than a well-refined 4 inch refractor - even when turned on low contrast Jupiter cloudtops.

There appears to be no chromatic aberration at all. None was observed at the terminator of the moon bright planets or stars.

Astronomical views are always dependent on atmospheric stability and thermal equilibrium within the optical system. Due to the number and arrangement of optical components within the Astro-Rubinar, up to 45 minutes may be required for the scope to "cold soak." Of course, should temperatures continue dropping, thermal-equilibrium may take longer or not be fully achievable.

The great virtue of the Astro-Rubinar system is that it provides completely flat field-of-view images. In all plossl and orthoscopic eyepieces the field is flat from edge-to-edge. Images are in focus across the field of every eyepiece - even through the (47x, one plus actual degree field of view) 32mm plossl. When looking at objects with a straight lined edge near the perimeter of the eyepiece field of view, the line seems to curve away at the ends of the line. This may, however, be an optical illusion of a line approaching a curved arc-shaped concave edge. Another very attractive feature for those stargazers who are also terrestrial viewing scopists, is that extremely sharp images can be obtained of nearby objects. At near-focus of 13 feet, the level of detail is such that the system may be characterized as a "long-distance microscope".

Another outstanding feature is that there is no glare in terrestrial views. This is probably due to the extensive baffling, to the flat black paint and to the basic optical design. Even at 100X per inch and above, no glare is seen. As a comparison, in the C-90, intrusive and distracting glare appears at 40X per inch and greater. The situation with astronomical viewing of bright objects is somewhat different, though contrast is very good. When gazing at a gibbous moon, a short distance from the terminator, the field is flooded with a decidedly bluish glare. Again, this does not impact the view of features on the terminator, but is noticeable. (It’s possible that this may be due to the plossl eyepieces used with this system, but it may be an artifact of the coatings and complex optical system.) The latter probably explains another curiosity. When a bright planet such as Jupiter is displaced some one or two full fields’ of view width from the eyepieces’ field of view, one sees the faint wide arc of a large circle whose focus is the planet so displaced. Finally, the telescope provides a very wide true field of view (up to 1.7 degrees with a 53 AFOV degree 32mm plossl when viewing occurs near the visual back). When used with a star diagonal, the true field of view drops to just over one degree.

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Mechanical Performance

Focus is accomplished by tube rotation. Those experienced at using a C-90 will find it easy to focus. Some scopists may require adjustment of the eyepiece position within the diagonal. Again, to those familiar with the focusing method, this is not difficult. The sweet-spot of best focus is very narrow. This may be a consequence of an optical design that promotes a flat field at shorter focal length. Or it may be the consequence of a modest amount of astigmatism. If the latter is the case, it may be that in this unit fine focus can be obtained, but at the cost of a bit more effort.

The field-flattening lens is very near the visual back of the scope. So close in fact that one could easily crack or scratch this lens by inserting a diagonal or eyepiece into the screw on visual back. One solution suggested by Mr. Palermiti, is to shorten the insertion tube on the diagonal so that it does not extend to the field-flattening lens.

The entire scope with Orion Mini-EQ tabletop mount weighs in at less than 15 pounds. The OTA is slightly heavier than the five-pound counter weight. In some positions, the counter balance cannot withstand the torque of the telescope’s weight. However, it does not put undue strain on the equatorial locking screw. Nor does it limit the effectiveness of the EQ-1M single axis drive. The drive is very smooth and accurate. Once the scope is situated relatively near the celestial pole, an object centered in a 4mm orthoscopic eyepiece remains in that field of view without adjustment two hours later. When the drive is on, images do not vibrate and therefore do not impact the view of close double stars. (An addendum: after three months of use, the RA axis has become a bit sloppy. It seems to tighten up by rotating the axis a few times without the scope or counterweight in place.)

By making a disk of wood 5/8 to ¾ inch thick and a few inches in diameter with a notch at one end and placing it on top of the cg4 tripod head, one is able to dismantle the screw-base of the Mini-EQ and mesh the rest of the Mini-EQ to a cg4 mount base. Between the use of the Mini-EQ tabletop mount and adapted cg4 unit, any study can be observed from a comfortable height and location.

With the telescope’s relatively large true field of view, an Orion Star Pointer provides adequate means for finding objects in the sky. Of course one needs familiarity with the position of objects in relation to bright stars or has reference to a good star chart. Another benefit of the star pointer is that it is very light, hardly affecting the balance of the equatorial mount. Further, the pointer has an easy-to-use mechanism for adjusting alignment. Finally, the switch that turns the pointer on and off (and adjusts brightness) has a definitive sound and feel. The only drawback to the pointer is that one often leaves it on inadvertently. It would be good if the unit had some type of automatic shutoff function.

The telescope comes with a screw on metallic dew cap/light shade. Care needs be taken when screwing on the cap to avoid stripping the threads. Also, in the dark especially, but at any time, care is needed to prevent cap slip. Unintended movements could allow its threaded edge to touch the surface of the forward full aperture lens. It is also possible to screw full aperture filters onto the telescope and screw the dew cap onto the filter - instead of the outer lens. Since there is little astronomical advantage to using full aperture filters (as opposed to ocular filters), one can disassemble them and replace the glass with Baader filter material, creating a very convenient solar filter.

The scope comes with a full aperture lens cap that covers the front of the scope when the dew cap is not installed. A nice modification of the current Rubinar design would enable the lens cap to fit on the scope with and without dew cap in place.

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Storage and Use

Alan MacRobert is correct; a small telescope will see more than a large telescope - simply because it is used more often. This due to the dearth of weight and general unclumsiness. Further, a consideration for aging stargazers is exposure to the cold. The reduced weight and size can eliminate the need for assembly outside or reduces the amount of time required. Such virtues are true of the Astro-Rubinar mounted on the Mini-EQ1 mount. A second advantage of low weight and reduced size is the small amount of space required for storage. With this particular scope, mount, drive, and all accessories are stored in two 13" plastic tool-boxes, the 15" soft padded bag, and Orion’s smallest eyepiece case - some two square feet of shelf space. Meanwhile, the entire kit can be transported easily in any vehicle or carried outside in a single pass.

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Some Reviewer Background History

I was once owner of an MK-67 on a cg4 (mount/drive) with a wide range of orthos and a stunning MK70 Konig (another good suggestion by Bill and Mike). After two years of very satisfying viewing, I grew dissatisfied as work, family obligations and age made me less and less willing to set the scope up and break it down. I also missed the terrestrial and near-focus views of the humble C-90.

Thus, began the journey of obtaining what I believe to be the perfect balance in a telescope: good optical quality, decent aesthetics, light weight, ease of transport, assembly and dis-assembly, robustness, good quality drive tracking, near-focus capabilities, and appropriate accessories.

To cover the cost of this purchase, I disposed of the MK-67, MK-70, and cg4. Because of the fine quality of the MK67, many professionals and experienced amateurs advised against this. Though the overall quality of the Astro-Rubinar does not meet the high standards of the MK-67, MK70, cg4 set up, it is better suited to my tastes and needs.

An experienced amateur asked if I was disposing of this in order to upgrade. With a slight bit of rye humor I could honestly say that "No, I am downgrading." Of course, a downgrade to what I believe is more suitable for me.

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After-Purchase Service

My desire was to obtain a scope that provided the ability to see objects only a few feet away as well as the Cassini division on Saturn. Bill assured me that this scope, under proper stability, could do just this as it was equal in optical quality to the Questar 3.5. As I studied advertisements, I especially noted the short focal length of 1000mm. This meant I could obtain a wide true field of view with commonly available eyepieces. It also meant that University Optics 4mm orthoscopic eyepiece would create a magnification in (what some consider the ideal high magnification) range of 50 to 60X per inch. Ads also showed that a 32mm plossl and right angle 90 degree star diagonal was included - along with lens caps for the visual back and the full aperture front.

With the order placed, I was told delivery would be in one to two weeks. Near the end of the two weeks I called to learn it would take somewhat longer. In the third week, an email response to a further query informed me that it should arrive soon. And soon it did; that very day. Bill, simply went ahead and had the scope shipped, from the Florida testing site, by second day air, without requesting or requiring additional payment.

Assembly of all the obtained components was a pleasure. Using it was a pleasure too, providing good views and easily meeting my desire for a unit that was easy to handle and achieved good near focus. It was also reassuring to have a certificate of optical quality. A further pleasantry was the inclusion of three full aperture filters in very substantial well-made housings at no cost.

Then, as always happens, imperfections became apparent: Scratches on the ring, a chip in the field-flattening lens, the shockingly long focal length with diagonal in place. This last resulted in the realization that I should not have purchased a 4mm ortho for this system as it provided magnifications of 100X per inch of aperture. Other issues crept up to: Astigmatic and visibly dissimilar intra/extra focal star and point source light images, that dangerous nearness of the field-flattening lens to the visual back and the fact that a 45 degree erect image diagonal was provided instead of the advertised 90 degree unit. (This was fortunate for me in that I possess a very good 90 degree diagonal but not an erect image unit).

All these issues occasioned a number of calls and emails to Bill and Mike. Bill returned every inquiry quickly. Both gentlemen talked at length over the phone when I called. Every query was responded to with full attentiveness to my concerns. One such interchange was to report the chip in the field-flattening lens, not so much to desire a replacement (since it did not seem to affect performance) but to put it on record in case it should enlarge through temperature changes or mechanical stresses.

A sign of good customer service is the willingness of a company to consider sensible suggestions. Bill was open to my idea that the advertisement on the website be changed to mention the focal length of the optical system with star diagonal in place and not just the focal length at the visual back, and that the owner’s manual contain a warning that care was needed when inserting the diagonal/eyepiece so as not to crack the field-flattening lens.

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Evaluating Oneself as an Observer

The ITE optical evaluator, Mike Palermiti said to me in a phone conversation that "Field tests" are not really tests of the telescope’s optical quality, but, rather, a learning session for the observer. All too often, amateurs spend too much time trying to evaluate an instrument, when the facts are that they are really evaluating themselves as observers."

Novice scopists and amateur stargazers can see more problems then might actually be present. We might overplay the negative and underplay the positive. We may become a bit obsessive, especially as it relates to systems-perfection. In this latter regard, I am grateful for my tutelage under a number of persons. These include the astronomer, Fr. Myron Effing (a student of Carl Sagan’s) who taught me the advantage of the "brute force" design; sacrificing cosmetics in order to put effort and money into optical quality; Andrea Tasselli who once wrote that what’s important about a telescope is that it is meant to be looked through and only secondarily to be looked at, Bill Burnett who once questioned my self-presumed ability to distinguish between a 1/6 and 1/8 wave-front corrected system based on lunar observations and Mike Palermiti for his comments immediately above. My mother-in-law who, in living that life common to us all in which we learn to deal with accommodation, once said, "You never get the whole pie. You have to learn to enjoy the piece you get." Though my MK-67 accustomed me to an absence of astigmatism and that ribbon-like appearance to the Cassini division; I find myself very satisfied with this telescope. One that is very portable, stores easily, gives very good views of terrestrial objects, performs as a long-distance microscope, produces good astronomical views, whose drive and mount function flawlessly, and is supported by two very knowledgeable and accessible professionals.

Otto Piechowski

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Addendum I: Obstructed Scope Type Comparison

There is no single optimal optical performance standard that all amateur astronomers and scopists would agree on. However, astro.geekjoy has its own ideas on the matter!

NOTE: For the 106mm Astro-Rubinar system, TLM@.7mm = 12.3@150x. At such magnification, a star should show a single, complete, and concentric first diffraction through an optical assembly displaying 80% strehl ratio performance. Brighter stars may display a second ring outside the first. Dimmer stars will show a smaller airy disk than calculated by the above formula.

Where does the Astro-Rubinar settle on this criteria? Tough question! The scope's strehl (encompassed energy) rating is roughly 80% (1/4 wavelength). However, the presence of a 34% central obstruction (CO) means that considerable light is thrown into the first diffraction ring. Thus a CO optical-quality compensation factor 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 the 106mm Astro-Rubinar is the equivalent of a 70mm unobstructed scope. Since the Astro-Rubinar is itself rated at ¼ wavelength (.80 strehl), there is no way to exploit a super-high strehl rating (such as in an MK-67) to adjust aperture upward from the 70mm mark. Thus we can expect that the scope probably performs like a Televue Ranger on low contrast studies - such as Jupiter - and when viewing widely disparate double stars.

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 bright stars anywhere in the field of view.

Otto reports that the Astro-Rubinar gives a very flat field - even over large expanses of sky. Though it may fall down somewhat in terms of low contrast resolution, the scope is no doubt quite admirable in presenting well-formed stars across one degree fields. As such, the Rubinar appears to offer a nice balance of rich field exploration along with the capacity to reveal a satisfying amount of detail on Moon and planets. All this without the chromatic problems inherent in fast achromats, or the outlay required to purchase a chromatically well-behaved fast refractor. However, this "rich field potential" quickly falls away when we realize that few observers will use this scope without a diagonal.

NOTE: This upper limit (of 425x) is only useful when 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 (265x in a 106mm scope) is probably the upper limit of magnification useful in achieving maximum image scale when viewing intermediate contrast detail.

Otto states that the Rubinar achieves maximum detail resolution at 130x. This amounts to .8mm’s of exit pupil. 34% linear obstructed scopes with smooth surfaces and well-corrected optics (spherical aberration in the 1/6th to 1/8th wave region) are able to push past this to about .7mm exit pupil. Unobstructed scopes of comparable figure are able to take magnification even further while continuing to resolve still more detail (.6mm).

Glass elements in the optic path absorb or scatter about 2% of the light passing through them. Mirror surfaces, 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 Astro-Rubinar possesses three reflective and six refractory elements plus diagonal. Thus the scope fails to meet the 80% transmissivity standard. For this reason, it is unlikely to approximate the magnitudinal reach of a comparably apertured MCT or SCT. A lot depends on the quality of such a compound scopes coatings and surfaces.

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’s (and accessories) merits. But the above factors are those that establish a baseline for optical quality and performance..

But just how well does the Astro-Rubinar stack up against the performance criteria established above? It can be seen from the earlier discussion that the scope is generally equivalent in low contrast feature resolution to a diffraction-limited, apochromat of about 70mms aperture. Due to the presence of a large number of mirror and lens surfaces, plus nearly a half inch cumulative aperture loss due to its 34mm central obstruction, the scope's effective aperture is equivalent to that of a 90mm refractor for most types of studies. Of course, that same central obstruction improves the scopes ability to resolve close matched-magnitude double stars. This to the equivalent of a diffraction-limited 115mm apochromat. Where doubles are disparate in magnitude, this unique feature is lost - especially if the companion falls near the first diffraction ring of the primary.

Given the scopes excellent color correction, larger aperture, and flat field, the Rubinar will undoubtedly outperform an ST-80 or 90 when operating in the 40 -120x range. However at F15 (astronomically), the scope does not share either short tubes expansive wide field capabilities. This is not a "rich field" scope by any means. But unlike the MK-67, inch and a quarter eyepieces are available to give the Rubinar true one degree fields of view.

The multiplicity of optical components in the Rubinar, does not bode well for collimation. In fact, it would appear that no effort has been made to correct the astigmatism the scope displays. Since the unit was tested interferometrically and returned a strehl rating of roughly 80%, the possibility exists for improved scope performance should efforts be made (if mechanically possible) along these lines. Likewise, due to the large number of components in the scopes architecture, quality assurance is likely to be an issue with the system. Otto did well to receive a test certificate along with this model.

The scope’s star test is not reassuring. The issue may have to do with the fact that the system was designed to operate at F10 (without a star diagonal). When scopes operate outside design parameters, they may display unanticipated spherical aberration. Under such conditions, elements in the optical path receive light cone incidences not well-matched nor optimized for proper handling.

Statements regarding an observers ability to distinguish between ¼ and 1/8th wave optics do not hold true for obstructed scopes. Even with unobstructed scopes, it is clear that an especially well-figured optical set will "get the most" out of marginal seeing conditions. Scopes barely "1/4 wave" corrected are practically indistinguishable from those that are 1/8th wave under the best of seeing conditions. But as conditions deteriorate, the combined effect of seeing and optics may be readily perceived. Meanwhile, 34% obstructed scopes of 1/8th wave correction just barely meet the .80 strehl cutoff. (This is the point where image degradation becomes evident to the eye as a sort of "softening" of details and perceptible loss of clarity.) As a result, even slight changes in seeing, tube currents, or poorly chosen accessories, can readily hamper definition and contrast in obstructed scopes of the highest quality.

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Addendum II: Astro-Rubinar Eyepiece Selection

Those considering the Astro-Rubinar for purchase, may wish to consider a proper range of eyepiece's. One might think that due to its published focal length, this would be a fairly straightforward task. - HOWEVER: As mentioned in Otto’s article, the scopes focal length increases by roughly fifty percent when used with a diagonal (astronomically). Thus the Astro-Rubinar is handicapped in ways similar to an MK-67 (1500mm vs 1800mm), so eyepiece selection could prove challenging.

Table of Suggested Astro-Rubinar Magnifications

PurposeMinimum FieldMagnification Focal LengthExit Pupil TLM*Resolution
Rich Field3 degree20x75mm 5mm11.69"
Navigator1 degree50x30mm 2.1mm11.74"
Framer30'100x15mm 1mm11.92.3"
Detailer15'150x10mm .7mm12.01.5"
Splitter9'210x7mm .5mm12.31.1"
Elongator6'420x3.6mm .25mm12.6.8"

* TLM calculated based on stars visible by an experienced observer to magnitude 5.5 on a 7/10 seeing stability night. No aversion used.

NOTE: Eyepieces of 75mm focal length are unavailable for use in inch and a quarter and 2 inch focusers. And based on the scopes astigmatism (if endemic to this model) it is unlikely that magnifications above 250x will offer any advantage in terms of elongating sub-dawesian matched pairs. For these two reasons, A range of eyepieces from 35 to 6mm is recommended for this model scope.

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Addendum III: Buying Your Last Scope First

The chances of any observer buying a single scope to satisfy their observing needs for a lifetime are extremely remote - some would say even "astronomical". In Otto’s case it became clear after over a year of observing through his 150mm Intes MK-67 that the scope fell down in terms of setup, portability, field of view, and near-focus magnification.

In my own case, and also after observing through an MK-67 over a similar period, I found that I needed only augment my observing kit with a small achromat to meet the unmet "rich field" requirement.

Had I set out to purchase my "last scope first" however, I would probably have purchased the largest affordable 1000mm focal length apochromatic refractor that could be carried as a single unit in my backyard or broken down for transport by car. (jeff’s mythical 140mm F7 apochromat.) But all this is now benefited by hindsight. And is purely hypothetical. Such a scope, well-mounted and with appropriate accessories would cost $6-8K to fund. Between my MK-67 and ST-80 plus all the trimmings, I’ve invested not more than a third of that amount.

As Otto said in his original MK-67 review: "Champagne taste - beer budget."

jeff barbour,

barbour@ihwy.com

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Addendum IV: 02/21/03 Otto Piechowski Update

Since the original review I have made additional discoveries about this fascinating telescope. Some of these are elucidations of previous comments, and some are corrections. These addenda are due to having had more use of the scope and having, with the encouragement of Jeff Barbour and Mike Palermiti, dissassembled the entire scope.

Otto

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