Otto Piechowski: Owner's Review of the ETX90-RA
General Comments Equipment Set Reviewed Comparative Scopology Astronomical Performance Terrestrial Performance Optical Performance Cool Down Finder Scopes Mount Considerations Aesthetics and Portability Price and Value
General Comments
Delightful and fun. Very good planetary performance. Great near focus (microscopic) performance. Good terrestrial views - only limited by narrow field of view (FOV). Limited on deep sky (Messier Catalogue primarily). Mechanically functional - even charming - for the experienced stargazer. The inexperienced scopist may find the mechanical functionality of the ETX discouraging in a manner similar to the ubiquitous department store altazimuth mounted 60mm refractor.
A full description of the appearance and functionality of the ETX can be obtained at the Astronomics.com website. The ETX instruction manual can also be downloaded from that site. Familiarity with the maksutov-cassegrain type scope in general and the etx/questar type configurations in particular, will facilitate understanding the following comments.
Equipment Set Reviewed
- ETX90-RA; second hand but in unused or nearly unused condition
- ETX90 Hardcase (Meade #774), second hand, used, functional, no keys
- 8X25 right angle finder (Meade #884), new
- Scopetronix Visual Back, new
- EZ-Focus cord, new
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Cost (including shipping and with equivalent accessories)
- As described, $330
- Retail RA model* ($380-$400)
- Retail EC/AT model ($700-$800) *limited retail availability
Comparative Scopology
| to a 100mm (F15) diffraction limited Rubinar on a table top equatorial mount | ETX90 has a slight optical superiority (contrast, detail, near focus)
ETX & Rubinar are equal on chromatic correction, FOV,cool down Rubinar is superior for photographic use (flat field) ETX is mechanically superior ETX is superior in terms of portability |
| to an 80mm ED (Orion) (F7.5) on a full equatorial mount | the Orion 80mm ED has a slight optical superiority in contrast
the Orion is greatly superior in terms of FOV and cool down the ETX & Orion are equal on detail and near focus the Orion is slightly superior on deep sky viewing the ETX is slightly superior on chromatic correction the Orion is mechanically superior ETX is superior in terms of portability |
| to an APM 102mm triplet achromat (F6) on a full equatorial mount | ETX is optically superior (contrast, detail, chromaticism, near focus)
APM is greatly superior in terms of FOV and cool down the ETX & APM are probably equal on deep sky viewing the APM is mechanically easier to use the ETX is superior in terms of portability |
| to a Celestron C-90 (F11.1) on a camera tripod with slow-motion device | ETX is optically superior (contrast, detail, near focus)
ETX and C-90 are equal in terms of FOV, cool down, deep sky ETX is mechanically easier to use ETX is slightly more portable |
Astronomical Performance:
Saturn: Wow! The cassini division is visible each time, every time (once thermal equilibrium has been reached) as a thin dark line nearly through the whole ring system at this time. The shadow of the disk of the planet on the obscured ring is clearly visible. Polar darkening, crepe ring against the planetary disk and the equatorial band are visible. The moons are visible but not spectacularly so. The ETX gives a beautiful crisp view of the ring system and cassini at 40X. The best views in terms of sharpness are around 100X. A nice crisp view continues to be delivered up to 210X (c. 70X per inch of aperture) with a concomitant three-dimensional appearance.
Jupiter: Nice sharp disk up to 210X. It is in the 100X to 135X range that the ETX performs best on the Jovian detail, revealing two distinct belts, with indications of belt splits, bars and fragments of belts elsewhere on the planet and some indications of festoons. The GRS and eclipse shadows should be easily visible. The features, though visible, seem to be slightly washed out. The use of a yellow filter helps. The moons appear as four beautiful bulls-eyes.
Venus: Well-defined gibbous disk at this time with no color (chromatic aberration) present.
Mars: At only six arc seconds in size, presents a definitive disk and a hint of a dark surface feature.
Double Stars: Bright matched pair Castor is beautifully presented at 100X as two headlights encircled by noticeable diffraction rings. The doubled nature of eta Orionis (1.5 arcsecs matched pair) is clearly seen.
Deep Sky: With a 32mm plossl providing a 1.3 degree FOV, the Great Nebula in Orion is attractively framed. At 100X the trapezium presents four perfect tiny airy disks with first diffraction rings. At 180X, in the clearest and most stable of skies, there is a hint of “e”. With the 1.3 degree FOV of the 32mm plossl, the entirety of the Pleiades can be taken in. Stars are pinpoint across the field, as is to be expected at this long focal length.
Terrestrial Performance:
At fifty feet, both through a window and without a window, views of juvenile blackbirds are excellent, revealing the detail of the coat as well as its iridescence. It is easy to see a sparrow work a seed’s covering apart with its thin red tongue to obtain the seed and leave the chaff. Though very pleasing, this scope is of limited “birding” potential because of its small FOV. Some nearer objects would not be properly framed.
When using attachments such as a diagonal and visual back in the rear port or an extension tube (2 inches) in the upright port, objects as near as three meters away can be focused on. Even at this near distance focus is sharp. Observations of a bubble in a window pane at ten feet illuminated by sunlight revealed numerous details that could not be seen at all upon close inspection with the unaided eye in full light conditions.
Optical Performance:
Sometimes point-light sources (sunlight reflecting off of crevices in ceramic or glass, stars) appear as tiny perfect airy disks surrounded by a bright first diffraction ring that is of uniform width but of slightly differing brightness at one end. I would call this 95% collimated. This level of collimation probably provides a degree of detail and contrast that in my eyes would be indistinguishable from 100% collimation. I obtain this degree of collimation inside the house at sun reflected point light sources, sometimes under the sky when the scope has reached full thermal equilibrium and at other times by pressuring the internal diagonal mirror in one specific direction (not advisable) or by loosening and tightening the meniscus ring. At other times there is a perfectly round airy disk with a discontinuous first diffraction ring. The ring is fully formed on one side going to a less well formed, fainter and at times disconnected appearance at the other side. Sometimes, more diffraction rings are noticed on one side of the disk than on the other. Slightly out of focus images of point-light sources reveal the central circle/point as slightly off center. I would consider this to be 90% collimated. Under most conditions this provides perfectly acceptable views such as those described above under Saturn and Jupiter, the descriptions of which were secured while observing the same under these conditions. Nevertheless, improvement of the collimation from what I am calling 90% to 95% collimated provides a noticeable difference in detail and contrast.
Intra and extra focal images of point light sources reveal a high degree of similarity. Though high optical quality can also be obtained in some optical configurations while also producing dissimilar intra/extra focal images of point light sources (of which the maksutov might be one), the presence of this similarity in a 90mm F13.8 maksutov cassegra-n is a good sign. The famed double dark ring that maksutov owners are familiar with in star testing is of nearly the same dark hue on both sides of focus; the extra focal being only slightly more washed out. Both defocused images nicely collapse down to the Berrevoet’s Dot, and then to the Barbour torus and finally to the airy disk with diffraction rings. There is no indication of astigmatism in the extra and intra focal images. There is a slight indication of astigmatism in focused point light sources in slightly unstable air or in thermal disequilibrium. However, this is well below diffraction limitation if it is present at all.
Cool Down:
A similar sized refractor may take 15 minutes to a half hour to equalize a thermal difference of fifty degrees Fahrenheit. Maksutov cassegrains of this size may take between thirty minutes and an hour for the same. Good views can be obtained between 15 and 30 minutes, with everything that can possibly be seen in a 90mm maksutov being within the visual threshold as one comes nearer to thermal equilibrium. I have adopted the technique of uncapping the meniscus lens, pointing it down, opening the rear port, inserting a visual back and a mirror diagonal and pointing it facing down to faciliate cooling and to minimize dust entering the system.
Finder Scopes:
I have not used the provided 8X21 finder. However, I suspect that it is nearly worthless for optical and mechanical reasons. The 8X25 right angle finder is adequate. It provides a more comfortable position for viewing. It also provides a nice apparent field of view. However, the limited FOV and the lack of light gathering surface somewhat limits its ability as a finder. If one is comfortable with using charts and star-hop-ing from bright objects, this should be adequate. Another challenge with this finder is that the use of the three double screws can prove challenging. This challenge is augmented by the fact that the #774 original carrying case was not designed with this finder in mind. The wall of the case jams against the finder. This in turns tends to change the direction pointed. One final negative about the finder is that under cold conditions, the glue holding the objective cell to the tube and the lens to the diagonal holder tends to deteriorate, resulting in these being pulled off. One other deleterious feature is that two dust caps were not provided with the finder.
Mount Considerations:
There are knobs that allow one to manually adjust both in right ascension and declination. There is a rod-attached-knob that can be used to tighten the declination setting and a knob provided to tighten the right ascension setting. In order to use the electric clock drive this knob must be tightened. The drive tends to drift a bit until it catches. Usually this drift is less than the FOV of a 32mm plossl and just about the FOV of a 12.5mm ortho. If one attempts to center the object by loosening the right ascension lock knob and using the other knob, the drift will be experienced again. I have learned to slightly adjust one of the table-top tripod legs and then re-adjust the declination. I have had little difficulty.
It is not difficult to open the back and insert the batteries. A suggestion given elsewhere is very useful; use the central leg fully screwed in and secured to pull the bottom plate off once the three screws are removed.
Aesthetics and Portability:
The metal tube of the OTA has a beautiful royal blue/purple sheen. Though invisible in lamplight, it is particularly attractive in daylight on the picnic table, on the office desk or a window table in the living room. For those who appreciate small complete packages, this scope is very ideal. The scope, its mount, its drive and needed accessories can be carried in one hand. Within its overhead compartment allowable hard case, everything one needs can be packed. This includes the scope (ota, mount, drive), eyepieces, diagonals, legs, filters, tools, etc. Everything one needs for a night of observing can be stored in this one convenient case. All the controls and easy to use and convenient. The addition of an EZ-Focus cord and a Visual Back increase the convenience. The plastic components, though they seem substantial enough, do cause one to treat things quite gently for fear of stripping some screw or cracking a housing. On the other hand, there are a number of ETX users who can claim up to eight years of use since the first ETX arrived on the market with no durability problems.
Price and Value:
Is the ETX worth its price? The comparison touchstone of small catadioptric maksutov-cassegrains is the Questar 3.5 (89mm). Any such comparison must be subjective and contingent on many unmeasurable factors. Yet, to construct a comparison is at least entertaining and perhaps somewhat useful. To this end, the following comparison attempts to be conservatively favored toward the Questar.
Value comparison of the ETX to the Questar 3.5
| Category | Percent of Questar |
| Optical Quality: | ETX 90% of the Questar |
| Mechanical Durability/Functionality: | ETX 50% of the Questar |
| Appearance and Aesthetics: | ETX 75% of the Questar |
| Portability: | ETX 90% of the Questar |
| Resale Value (percentage of purchase price): | ETX 50% of the Questar |
If these values are multiplied (i.e. 90% X 50% X 75% X 90% X 50%) one obtains a conservative-comparative value of .15 (.9 X .5 X .75 X .9 X .5). If we assume a new purchase price of $4000 to $4500 for a Questar, Standard Model (basic), a new ETX-RA with comparable accessories should cost $600 to $675. Currently, one can obtain a new ETX90-RA, with hard case, flexible focus cable and visual back for around $400.
Otto Piechowski