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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