Mobility and Versatility- the Celestron C90
Written: Jul 24 '00 (Updated Mar 27 '07)
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Product Rating:
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Pros: Small size, durability, light weight, ease of use
Cons: limiting f/ratio, limited to .96 devices.
The Bottom Line: The C90 is a useful little telescope. If you want a lot of capability to stash in a backpack, it's smaller than a set of 10X50 binoculars.
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| Pirich's Full Review: Celestron C90 Mak (38x88.9 mm) |
NOTE: This Opinion is being posted both under the C90 and under the G3, since both are versions of the exact same optical device. If there were a mechanism for cross-posting without doing this, I would have rather taken that approach. More general information on getting a telescope is in my article on Picking a Telescope.
I am dividing this review into several sections since it turned out I actually have some ground to cover on this product:
Background
Versions
Performance
Use as C90, G3
Issues with Design
Fixes for some things
General Comments
Background:
The C90 first appeared in the 1970s as the smallest of Celestron's catadioptric telescopes. The C90 is the only Maksutov telescope Celestron produces, and has significant differences from Maksutovs produced by other manufacturers (Meade, Questar, Orion, misc. relabeled Japanese made scopes). With the exception of the Questar standard (which dates from the 1950s), the C90 is the most time-tested Maksutov design in production in the US. The extremely compact design immediately lent itself to use as a telephoto lens as well as a telescope. Over the years, several variations on this optical system have been produced including variants with focal lengths of 300mm, 500mm, and 1000mm. Today, the sole survivors in production are all the 1000mm variant. Although they are called C90 and G3, all of these are actually the same telescope optics with a few variations in the accessories and the telescope body.
The spotting scope, pictured with this category, is the variant I own. I chose this version because the body is covered with rubber armor for knock-resistance, making this the largest aperture telescope I am aware of with that feature. The G3 and regular spotting scope versions come in gloss black paint. The optics are all fully coated. Early examples from the early 1980's and 1970s will have uncoated optics which means in practice that the image contrast won't be quite as good if you aren't using a lens shade of some sort. Variants with coated optics can be recognized by the violet or green sheen reflections on the corrector lens at the front of the telescope.
The older 500mm and 300mm variants will have inferior optical performance to the 1000mm variants for astronomical use (due to their large central obstructions from the secondary mirror size required to get a focal length that short). An astronomical version of the C90 was produced for a few years with a single arm fork mount which had a separate secondary mirror cell in the center of the corrector plate. There are quite a few of the old C90s out there, but judging from the number coming up for sale, their owners aren't giving them up. The C90 variants in current production are all readily usable for astronomy, use as telephoto lenses, or for terrestrial viewing as spotting scopes.
The design of the C90 uses a helical focuser arrangement where the entire front half of the telescope rotates to move the corrector and secondary mirror forward and back to focus. This makes the body extremely compact and eliminates a complicated internal focusing mechanism as found on all other designs. The C90 can focus between 5 ft. (that is not a typo- you can focus on things that close) and infinity. As a result, the C90 has been very popular as a spotting scope and for nature photography since it can make close-up observations from a safe distance. The only real nit I have on the focusing mechanism is that touching the barrel invariably moves the telescope slightly, so getting sharp focus takes a bit of technique and some practice.
The Maksutov geometry has a strong following because it deals with several persistant problems. The design originated in Moscow during WWII at a time when producibility for high image quality optics was a primary concern. First, the reflecting design eliminates the chromatic abberation problem refractors experience. Second, the corrector plate design eliminates the curved field and coma problems many newtonian designs have. Third, the spherical geometry of the corrector eliminates many of the out-of-figure problems which frequently degrade images in Schmidt- Cassagrains (SCTs) because it is easier to machine. As a result, Maksutovs are known for having true color flat field images across their entire visual fields.
Performance
The C90 is best viewed in terms of its intent. The telescope is a modestly priced, small aperture general purpose telescope designed to be very easy to move about and use. Compared to the Meade ETX-90 and the Questar Standard, this telescope has a significantly lower focal ratio (1000 mm or f/11 compared to the ETX-90's 1250mm f/13.9 or the Questar's 1300mm f/14.6). This lets the C90 see a wider view of the world. The largest practical eyepieces for the C90 are in the size and view angle range of the Celestron 40mm Plossl which gives a field of view about 1.84 degrees, which would be about 32 meters at 1000 meters (96 ft. at 1000 yards). With the same eyepiece, the Meade ETX 90 can only see a 1.47 degree field of view while the Questar Standard can see a 1.42 degree field. What this means in real life is a C90 pointed at the Pleiades (you'll need to look in fall-winter) can just fit this entire star cluster into the field of the 40mm eyepiece while the Meade or Questar would be unable to capture the full field at one time.
Now, there are some important limitations to the C90 one should be aware of. This telescope, although the same approximate sixe as the Questar, should not be confused with a $3,500 telescope. No expense is spared in the Questar, and it shows in slightly better image quality. On the other hand, if something catastrophic happened to my C90, I would be upset, but I know I could replace it. The C90 gives good looking views up to around 133X (7.5mm eyepiece). I find the most pleasing views are from 25X (40mm eyepiece) to around 80X (12.5mm eyepiece). Going above 80X requires really clear atmospheric conditions. In use as a spotting scope, the 40mm eyepiece and the 38X 26mm eyepiece which came with it are the clear standouts. The views they deliver are bright, wide, and sharp from edge to edge.
At the moment I think I really need to get this telescope to an area with darker and clearer skies than are available along the gulf coast to push it to its ultimate performance limits. For the timebeing, it has worked quite well as a quick-use telescope.
Use as C90, G3
The G3 is a C90 mounted on a lightweight equatorial mount and tripod. The setup has the option of attaching a motor to the mount to counteract earth's rotation. The mount and telescope are available as a set for about $450 from sites such as www.astronomics.com and others. The motors often come bundled with the telescopes as a combo deal, so look around.
Using the spotting scope version with a camera tripod doesn't work very well for astronomical viewing since they can't do precise movement unless you get a special head for it, and even then they tend to shake a lot and distort the image. However, the C90 works just fine for daytime viewing. Just remember to never, ever, under any circumstance point it anywhere near the full sun without a solar filter over the front end (these are available from orion at www.telescope.com and are very economical at about $70). A C90 cannot be used for eyepiece projection imaging of the sun because there are internal components which will be destroyed by the heat of the sun. You may run across a type of solar filter which threads into an eyepiece. If you find one (they look like a piece of very dark glass in a small threaded ring), take a hammer and smash it so no one attempts to use it. These are no longer made because relatively minor flaws cause them to shatter without warning and let through concentrated sunlight which will instantly and permanently blind whoever is observing at the time.
Between the spotting scope C90 and the G3, the main difference is the deluxe has rubber armor, an erecting prism, and accepts 1.25" eyepieces. One verion of the spotting scope has no rubber armor, and is sometimes sold with just a camera adaptor and no eyepieces. I personally feel the rubber armor and the high quality plossl eyepiece is well worth the cost of the deluxe version. One version of the deluxe comes with a carrying case (see www.telescopes.net for their special on these). I have personally found the case to be a great addition- it has room for all of the basic accessories plus a camera body and some extra stuff like more eyepieces.
The G3 is the sole remaining astronomical variant and comes with a mirror diagonal which gives an image which is upright but reversed from right to left. the mirror gives a better image than a prism, but the plastic housing it is in is rather cheap looking, clunky to use, and I'd dump mine in an instant if I could find a version as well made as the housing for the Celestron erecting-image diagonal that came with the telescope. The 25mm SMA eyepiece is Celestron's bottom-of-the line eyepiece in that size. I would strongly recommend going to the far superior plossls.
The mount with the G3 version of this telescope is the same one as used with the larger G5 telescope. Since I had purchased the rubber armored verion of the telescope, I bought the tripod separately with wood legs (the G3 variant has aluminum legs). The mount uses a simple spring-loaded screw thread for the declination axis (north to south) which allows about 15 degrees of travel in that axis. I have found I really don't like this method that much (I'd rather have a real worm gear) and I have put a marking on the screw shaft with a magic marker so I can judge when I'm about to run out of travel. To use the mount, you need to point the hub roughly at the pole star and then release the two axes to move the telescope to a viewing object.
In practice, this is a lot easier to do than it sounds at first. The telescope mount needs to have its axis tilted to your observing site's lattitude. To use the telescope, simply walk outside with the tripod and set it down with the hub pointed roughly north (eyeball it). This will be good enough for visual observations. If you want to do photography, you will need to follow a procedure for precision polar alignment. Celestron tells how to do this on their website at www.celestron.com. You will also need to have a motor to do this. However, the amount of drift error while visually observing an object is usually pretty low on the time span of a few minutes of viewing before going to another object.
The mount is very easy to use. The equatorial axis has a worm gear with steel gears. The main mount adjusts via simple tightening knobs. To point the telescope at an object, I have developed a technique which seems to have overcome the usual problems with sighting on an object. For this to work, the finder scope must be aligned with the main telescope as outlined in the instruction. What I have been doing is looking at the object I want to see in the telescope with both eyes. Then, I look through the finderscope with one eye and keep the other eye open looking at the object. It is surprisingly easy to tell where the cross hairs are pointed while doing this. What I do then is simply push the telescope until the crosshairs appear to move to the obect. The object will pop into the finderscope's field of view just as the crosshairs approach it. Then all I have to do is tighten the axle releases on the mount and then use the worm gears to fine-tune the pointing at the object. This technique actually works well enough to center objects at 100X. So, it is generally very easy to find objects with this telescope setup, and the mount is actually pretty easy to use this way. If you are looking for a dim object, select an easily visible star nearby and sight in on it.
The mount has an interesting feature where the counterweight arm rotates in synch with the declination head. As a result, it should be possible to attach a camera to the arm in place of the counterweight to make a pretty neat photo-guide scope setup that is very light. However, the arm on mine was loose when new and tightening it up proved to be somewhat difficult since it has a threaded end with three set screws from the sides holding it in place (yeah- it's really strange. I have no idea why it is made this way). The possibility of mounting a camera is still there, although I haven't seen a mounting shoe for doing this (there must be a part out there that does this, though- maybe for something made to mount on a lab ring stand). The tray included for holding eyepieces and other parts is split up between .96" accessory holes, a couple for 1.25" oculars (one of which is blocked by the tripod attachment tab). In practice, the tray, as supplied, is worthless since items set on it can easily be knocked off, and lenses with caps on both ends can't fit into the holes.
All in all, this mount is easy to use. The quality is OK, although I am prompted to wonder how much it would cost to do the few small extras like a full declination gear, a better weight arm attachment, and maybe a simple polar alignment scope.
Issues with Design
There are a rew quirks and limmiations of this design to be aware of. First, the small chassis does not leave room in the back of the telescope for large eyepieces or other attachments. The ETX-90 and the Questar Standard both have large back-end attachments with provisions for installing other hardware behind the main mirror. On the C90, there is a threaded ring on the outside for something to thread on to in addition to the set screw receptacle for .96" diameter fittings. As a result, the C90, though far more compact and lighter than the ETX-90 or the Questar Standard, gives up capabilities such as having an eyepiece installed while a camera is mounted on the back (where it is possible to flip a mirror between the two devices). On the other hand, the C90 isn't permanently weighed down with equipment most people never use.
The other problem is one which is faced by all catadioptric telescopes with glass corrector lenses in the front- dew. The large glass corrector lens cools down and makes an ideal spot for dew to form. In practice, a dew shield is essential for preventing the formation of dew on the telescope optics which can bring your evening viewing to an abrupt end.
Fixes
Telescope:
The only serious problem I've had with the telescope itself has been dew fogging the corrector lens in front. So, I made a dew shield for it. As it turned out, the lightweight 4" inner diameter yard drain pipe sold at Home Depot has a bell-end cuff which fits right over the rubber armor like it was made for it. A 10' length of pipe with a bell fitting is $2.00, so I got one and cut it to an approximate 7" length beyond the bell mouth. I spray-painted the dew cap black to match the telescope. A piece of automotive .5" rubber tubing slit in half lengthwise and fit into the bell helped it stay straight on the telescope. I spraypainted the interior from a distance to get a velvety super-black finish on the interior to absorb stray light. This modification has worked quite well as a glare shield and so I have been using it in daylight to improve contrast. The corrugated tubing is quite light and doesn't weigh down the telescope, but is sturdy and with its shape looks like it was made to go with the armored telescope. The corrugations appear to limit dew formation by slowing air flow. This has made the telescope extremely resistant to dew formation. It has now become common at observing events for it to be the only telescope on the field that doesn't need to be cleared with a blowdryer every few minutes.
Mount:
The biggest problem I had with the mount was the rather pathetic tray it came with. I made a tray enlargement out of a gray plastic utility box which I cut to fit with a Dremel tool and then drilled holes in the tray and mounted the utilit box using its original screw-holes. The box is just big enough to fit all of my eyepieces into, with their caps, and the depth of the tray doesn't let them topple out while carrying the assembled telescope.
The only other modification I made to the mount was to add a marking to the declination axis axle so I could tell if it was at one end or the other of its range.
General Comments:
The C90 does what it is supposed to, and does it well. To put it simply, you're going to have to spend a lot more than $450 to find something that will outperform one, and even then you won't find something as easy to move, or able to focus so close, or fill so many roles.
Recommended:
Yes
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Epinions.com ID:
Pirich
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 in Electronics |
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Member: Rich W.
Location: Tucson, AZ
Reviews written: 137
Trusted by: 40 members
About Me:
Dad, Engineer, Scientist, Astronomer, Traveler; order may vary.
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