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STARMAX 90mm EQ



Background:

I have been observing the heavens for the past 36 years from my home in Beatrice, Nebraska, and have been a member of the Prairie Astronomy Club of Lincoln for about 30 years (I have been President of the club for about the past 4 years or so). I currently own five telescopes (60mm f/11.7 refractor, 80mm f/5 refractor, 90mm Maksutov-Cassegrain, 8 inch f/7 Newtonian, and a 10 inch f/5.6 Newtonian). I have participated in most of the activities of amateur astronomy (including astrophotography, telescope making, variable star observing, ect.) but my favorite activities are Solar H-alpha observing, Deep-sky observing, and public outreach. I hold a B.S. in Physics/Astronomy from the University of Nebraska, Lincoln, and have designed and built several telescopes including two 8 inch Newtonians, a 10 inch Newtonian, and a 3 inch fully-enclosed (ie: idiot proof) solar rear projection telescope for Hyde Observatory. I currently serve on the Board of Directors of Hyde Memorial Observatory in Lincoln, as well as acting as an Observatory supervisor to open the facility for special groups or public nights. For the past three years, I have helped put on the Beginner's Field School at the annual Nebraska Star Party, and have served as the Field School's Coordinator for the past two years. I have 10 articles on the Cloudynights web page (not counting this one), and have had one article published in SKY AND TELESCOPE magazine (not counting the letters to the editor which got in).

The Maksutov and its variants have been famous for their compact nature and often good optical performance. I was looking for something which offered both for my solar H-alpha filter, which currently requires me to lug my big Newtonian out of the basement, only to be stopped down to a mere 3.5 inches for solar work. It was with some trepidation that I decided to "bite the bullet", and order Orion's little STARMAX 90MM EQ telescope to see if it could meet my needs. What I got was a fairly capable small telescope with a lot of optical performance in a small and portable package.

Optical Tube Assembly

The StarMax 90 is a "90mm" 1250mm focal length Maksutov-Cassegrain telescope on a small German Equatorial mount. The reason I put quotations around the "90mm" is that this is the diameter of the primary mirror inside the telescope, but is NOT the clear aperture of the telescope! Catadioptic designs usually require a primary mirror which is slightly larger than the corrector plate's clear aperture, and the StarMax 90 is no exception. Its measured clear aperture is only 87.5mm, (3.44 inches), a fact which might be somewhat misleading to newcomers who run into this. I feel that if the telescope is stated to be 90mm, the opening at the unit's front end had darn well better be 90mm (and the primary mirror be larger still)! Still, this only means a 3 percent drop in effective aperture and resolution from the "90mm" advertised level, which is fairly slight.

The telescope is about 11 inches long, which is lengthened to about 13 inches when the included 90 degree mirrored star diagonal is in use. The tube is about 4 inches in diameter and is painted a reddish Maroon color, with black ends. On the side of the tube is a dove-tail mount for the finder, which will accept most of Orion's single-arm finder mounts. The included finder is a tiny low 6x20 achromatic "correct-image" straight-through finder, which puts your eyeball a mere 2.5 inches off the surface of the tube. This low awkward position and the tiny size of the finder sometimes make it rather hard to use (I ended up installing its finder mount backwards to give my head a little more room). The back base of the tube has a standard camera tripod mount block, which allows the telescope to be easily removed from its equatorial mount for storage or for use as a long telephoto lens on a camera tripod. The back end of the telescope has standard T-adapter threads, so the scope can be mated to a camera with the proper T-ring. The telescope itself with finder and diagonal installed weighs in at a mere 3 pounds, which should be light enough to be handled by most camera tripods. On the back end of the telescope is also the rubber-covered focusing knob which turned easily and fairly smoothly. Not documented in the manual are three tiny recessed Allen screws 120 degrees apart which control the fine collimation of the instrument.

The optics appear to be close to a Gregory Maksutov Cassegrain design, using the usual concave meniscus corrector plate out front and a 90mm spherical primary mirror inside the scope. At least the front surface of the corrector plate appears to be multicoated. The secondary mirror appears to be a standard aluminized spot on the back of the concave mensiscus corrector, with a diameter of about 27.5mm. This amounts to about a 31 percent central obstruction, and while large, is not as large as the obstructions found on many SCT's. The obstruction gives the telescope about the same light gathering ability as an unobstructed 83mm aperture telescope, although the resolving power would still be that of an 87.5mm aperture one. The small secondary mirror is also surrounded by a short cylindrical baffle to reduce light scatter. The light collected by the primary mirror and bouncing off the secondary then proceeds down towards the hole in the primary mirror, and is also baffled by a long slightly conical baffle tube. Like many Catadioptrics, the StarMax 90mm focuses by moving the primary mirror towards or away from the secondary, changing the effective focal length of the telescope and thus moving the focal point. This does give the instrument considerable focusing range with just a twist of a small knob, although it is somewhat of an optical compromise. The telescope is supplied with a rather minimal manual which helps mainly in assembling the mount, with a few minor tips on using the completed telescope and mount. Also included is a 25mm Sirius Plossl eyepiece yielding 50x and about a degree of true field of view.

StarMax 90mm EQ-1 Equatorial Mount

The mounting is the Orion EQ-1 German Equatorial mount with dual-axis slow motions on an aluminum box-channel adjustable tripod with a small plastic accessory tray underneath. The telescope and mount together weigh about 15 pounds, so it is light enough for many people to carry fully assembled, although this can get a little awkward with the tripod legs extended. The tripod legs allow an eyepiece height of between 3 feet and 5 feet off the ground when pointed at the north celestial pole, so there is more than enough range for either standing or sitting use. The tripod has some plastic parts on it, including some fragile-looking locking knobs for the legs, but with care, it should last a while without needing repair. The equatorial head is heavy, with a 8 inch long counterweight rod holding a 5 pound counterweight. In some cases, this weight or rod length has proved slightly inadequate for heavy items placed in the focuser. The telescope attaches to a "U"-shaped adaptor which then bolts onto the regular attach flange of the declination shaft. This adaptor allows the telescope to be quickly removed from the mount with just a few turns of the attachment knob on the bottom. The right-ascension slow motion is at the top of the polar axis shaft rather than the bottom, which can make for a somewhat awkward locations for the slow motion cables, which are each about 6 inches long and do not flex very much. An optional clock drive is available which attaches to the right ascension drive in place of the R.A. slow motion cable. I purchased it and found that for certain locations in the sky, the tube hit the clock drive, so it might be somewhat of a restriction for its use. The drive is a simple DC motor powered by a 9 volt battery (internal) and is manually controlled with a simple speed knob, so it doesn't have a lot of torque. I had little trouble getting the speed control of the motor adjusted to a proper tracking rate. The manual states that the 9V battery for the clock drive should last for 35 hours, but this might be a bit liberal, as the drive could no longer keep up with the tracking load after about 31 hours on the supplied battery at near 75 degrees F. This number would be a lot lower under much colder conditions, so thought should be given to using a 9 volt "battery eliminator" power supply in the winter.

The mount is fully adjustable for latitude with a latitude scale included. The mount has small analog "setting circles" with primitive sharp metal pointers, but these circles are more decorative than useful. They are far too small and inaccurate to function as a serious finding device. Still, the mount is fairly easy to use and stable, although it would have been better to have the tripod legs spread out somewhat more than they are. The telescope/ mount combination damps out vibrations in about three seconds, which isn't too bad at all. The bearings on the mount were just a bit on the stiff side, as even with the locks fully backed off, it took a bit more than a finger's touch to move the telescope on either axis (especially the declination axis). There appeared to be no provision for lubrication of the bearing surfaces, nor did the manual provide any information about maintenance of the mount.

Performance

Optically, the instrument performs fairly well. After about an hour and a half of assembly, I finally got it out under a moonlit night (1 arc second seeing) for a good first look at how well things worked. The star test (195x, Meade 6.4mm Super Plossl) showed the stars diffraction patterns and revealed that telescope was very slightly out of collimation, but it didn't effect the views of extended objects very much. I noted three large and three small sets of Allen screws recessed into the back end of the StarMax 90. I guessed that the small ones could be collimation screws, and as it turned out, I had guessed correctly. The manual did not provide any information about whether the scope could be collimated by the user or how it could be done. However, after a high power look and a slight tweaking of one screw, it became obvious that very slight adjustments of these three screws would bring things back to proper alignment. If you see gross misalignment of the optics of this scope, or you are unfamiliar with Maksutov collimation proceedures, I would recommend the telescope be sent back to the manufacturer for proper adjustment. However, if you are a little "brave" and know how to align something like a Schmidt-Cassegrain, the proceedure for the StarMax 90 is similar, except that you are adjusting the primary mirror and not the secondary.

Further testing over about 5 or 6 late evenings revealed a bit more about the optics of the telescope. Even before collimation, the high power star images showed fairly round diffraction disks with a fairly bright 1st diffraction ring and much fainter outer rings. This is fairly typical for most Catadioptric instruments with large secondary mirror obstructions, so it was no surprise. Careful examination of the system using the star test revealed a small amount of astigmatism and spherical aberration, which is also sometimes seen with these small low-cost instruments (I have seen worse). The astigmatism was the more significant of the two aberrations, and its presence was just a little bit disappointing. The best-focus diffraction patterns were fairly symmetrical, although the outer rings tend to seem a bit broken or irregular in brightness, probably as a result of the system's astigmatism along with a little surface roughness in the mirrors (often seen in machine-polished optics). However, the aberrations were not enough to prevent the telescope from functioning at least reasonably well. The focusing action was smooth and easy to do, with no sign of any significant image shift. The star diagonal is a mirrored one which did work fairly well, but was ever so slightly inferior to my Tele Vue mirror star diagonal. The 25mm Sirius Plossl performed in a satisfactory manner, but was not my first choice, as my 24mm Koenig was a bit better overall. My 30mm Ultrascopic began to show the first hints of vignetting at the field edges due to the limited physical size of the baffle tube and the hole in the primary mirror. These physical limits impose a maximum true field of view of about one degree before vignetting occurs. A few additional shorter focal length eyepieces will be needed for the user to take full advantage of this telescope's moderate to high power capability.

The StarMax 90 resolved double stars well down to near its diffraction limit, including fifth magnitude Epsilon Arietis (1.4 arc second separation). Epsilon Lyrae showed hints of its "Double-double" nature at only 50x, although it took about 83x to get a hint of dark sky between the stars. The little scope really showed its stuff on the moon, with razor-sharp detail and very good contrast. In fact, I pushed the scope up to 250x, and still got a very passable lunar image, although it was somewhat better below 200x. With the moon just outside of the field of view, there was some slight glare as if some light was glancing off some component of the instrument which had not been properly baffled, but with the moon in the field, the background was nice and dark. The moon was a nice neutral grey or greyish-white in the Maksutov, in contrast to the slightly yellowish view through my 80mm f/5 Achromat. There was no hint of chromatic aberration in the StarMax 90, and I found it quite pleasing to view the moon seated in a small chair while still getting a sharp view of it. Early in the evening around sunset, Venus was similarly quite sharp, while much later on, Saturn showed Cassini's Division all the way around the rings' span, as well as the diffuse equatorial belt and moons Titan, Rhea, and Iapetus (125x, 195x).

A few nights later under a dark moonless sky far from city lights (zenith limiting magnitude 6.6), the StarMax 90 peformed quite well once again. M11 showed its triangular grainy mass of stars at 50x, and resolved nicely at 125x into a mass of between 100 and 200 faint stars. M13 began to show hints of resolution at 50x, and many very faint stars at 125x, although the cluster was not totally resolved. The view was, however somewhat better in the StarMax 90mm EQ than at the same power in my 80mm f/5 Achromat, as a few more stars were visible and the high power star images seemed more point-like. I had little trouble seeing the 'smoke-ring' form of M57, and could just barely glimse the 13th magnitude star just east of the nebula with averted vision at times. M31 and M32 were also quite easy, with the fainter M110 also visible. Bright open star clusters were well shown in this telescope, with the Double Cluster at 52x being particularly spectacular. The maximum one degree true field of view of this instrument might limit this scope's use on some of the larger or fainter deep-sky objects, but overall, it should still be fairly useful for viewing all the Messier objects and many of the brighter Herschel 400 objects. Indeed, I had the scope out at my rural observing site under dark skies and was picking up quite a number of fainter and more challenging targets.

The biggest obstacles to finding and viewing the fainter objects were the tiny inadequate 6x20 finder and the stiffness of the mount's bearings. Although stars were visible in the finder (and the image was correct in orientation), the finder is simply too small and way too close to the tube to be easily usable. In my opinion, it would be best for the purchaser to immediately replace the tiny finder with Orion's 6x30 right-angle correct-image finder before attempting to use this telescope extensively.
Solar observations are possible with the StarMax 90mm EQ, but only with full-aperture solar filters. Attempting to do unfiltered solar projection is probably not a good idea, as the intense solar heating may cause the baffle around the secondary mirror to fall off if the glue gives way (I have seen this happen on an ETX-90). For my H-alpha viewing, I built a cell for my DayStar H-alpha system's Energy Rejection Filter which goes over the front of the corrector plate. Using Tele Vue's 2.5x Powermate to achieved the f/35.7 ratio needed for the DayStar T-Scanner 0.7 Angstrom H-alpha filter, I easily observed the sun's chromosphere in all its fine detail It worked just as well as the T-Scanner system had in my stopped down ten inch Newtonian, and it was a good deal more convenient to sit down while viewing, as well as being a lot more portable.

In summary, the Orion StarMax 90mm EQ Maksutov-Cassegrain is a fairly good small telescope. It isn't exactly a high-end instrument which performs to perfection, but it does perform well enough to serve as a fair beginner's scope, It might also serve advanced amateurs seeking a quick "get and go" instrument, or, if equipped with the proper filters, as a good solar scope.




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