Celestron CM-1100 Telescope

The title of this review isn't an exaggeration. There aren't many optical instruments average people can afford capable of outperforming this telescope. This is the sort of instrument to get if you live somewhere where you have dark skies. If you live in the country or somehow have access to a spot in the country where you can leave this telescope set up, you will have your own observatory. More general information on getting a telescope is in my article on Picking a Telescope.

I've arranged this review into the following sections:

Description
Optical Performance
Mount Performance
Summary

Description

The C-11 is the second largest telescope Celestron makes. It essentially replaced the C-10 in the late 1970s and has optical fittings compatible with all of the accessories any other Schmidt Cassegrain Telescope (SCT) can use. This lineage includes the venerable C-8 (aka G-8, NexStar8, Ultima2000-8), the portable C-5 (aka G-5), the highly capable G 9 1/4, the mighty C-14, and the entire Meade line of SCT telescopes. As a result, the user can configure a C11 to do just about any optical astronomy activity there is. Spectroscopy, photography, planetary viewing, nebula viewing, galaxy viewing, and just about anything else is well within this telescope's grasp. About the only constraint is the fact this is a 280mm diameter telescope a f/10, which gives it a focal length of 2800mm, which is pretty long (more about this under Performance).

The telescope tube itself is quite large and weighs about 30 lbs. It is a big telescope. However, compared to anything else of this diameter, it is very compact from front to back. The fit and finish are superb. Celestron appears to be quite aware that this is the sort of telescope a person buys for a lifetime and it is built like it. The focus mechanism is smooth, the fittings look good, and the tube is very sturdy. With basic care, this telescope can have an indefinite lifetime. The only constraint it may face is it may need to have its mirrors stripped and recoated every few decades and eventually may need to have its corrector plate recoated.

The current mount is kind of like a Celestron version of a Losmandy mount. The drive action is extremely smooth thanks to large carefully machined bronze gears. This system is built to last, and is built to perform well. The mount isn't really built to do GOTO tracking of astronomical objects like the Ultima 2000, but has a few new tricks up its sleeve. One of the biggest ones is it can swap out optical tubes. If, for whatever reason, you decided to mount another telescope, then unlike a fork mount, this mount would readily accept it. The C-11 is configured with a dovetail rail which actually holds it on the mount so the telescope can be slid either forwards or backwards and locked in place to preserve its balance, even if heavy cameras and other equipment is mounted on it.

I think it is important to note here there is a persistent rumor out about a new Ultima telescope coming out- sort of an Ultima 2000 using the C-11 tube. The C-ll has been sold as an Ultima before, and with a fork mount, but without the degree of drive sophistication found with the Ultima 2000, which has been a unique product for its entire life sofar. The previous Ultima C-11 was more like the Celestar 8 telescope assembly.

Optical Performance

Have no doubt- the C-11 was designed to be a flat-out performer. Where the apochromatic telescopes try to make 102mm or so of aperture do something fairly impressive, the C-11 simply steps into another catagory of performance with its full 280mm aperture. This telescope can easily split all of the famous star doubles. The four stars of Epsilon Lyrae are very obvious as four distinct and separate points of light well away from each other when you move in them with the C-11. On the Orion Nebula, the cat's paw of the Trapezium reveals a sprinkling of new tiny stars in amongst the four visible in 90mm class instruments. On Jupiter, it gives the impression of being in orbit. And this is where the telescope shows how powerful it is- even at 250 power, both Jupiter and Saturn are so bright that they leave spots in an observer's eyes. As a result, it's a good idea to keep eyepiece filters handy to dim down the image so it's easier on your eyes to see the detail. Dim blurry images are not part of the C-11's domain. The detail is amazing- the delicate festoons around Jupiter's waist are quite obvious and the Cassini division is obvious in Saturn's rings. Even on Saturn's infamously low-contrast cloud tops, I could see at least one secondary polar band as well as texture in the pale equatorial stripe. In addition, several moons of Saturn entirely invisible to smaller telescopes readily appear in this instrument.

I didn't get a chance to look at Mars through a C-11, and so I'm going to have to pay attention and see if I can get a peek when it is at its next close approach about a year from now.

There was some focus shift thanks to the heavy primary mirror. The way SCT telescopes focus is by moving the primary mirror back and forth inside the optical tube. The only problem with this is the mechanism often will shift slightly when the direction is reverse so you have a dead space where when the knob turns the focus doesn't change, and then the mirror will suddenly overcome friction and move, thus causing an abrupt shift in focus and slightly change alignment. This is almost always an overshoot from where you wanted to be and makes it difficult to focus if you are trying to rock back and forth to get it to focus. So, the usual practice is to back way off of the focus in the direction that will lower the primary mirror and then approach the focus where the mirror is being lifted against gravity. This prevents the image from having a sudden shift from the mechanism being reversed, but requires you to stop as soon as it reaches focus. A trick for doing this while looking at planets and other objects is to look for a moon or a star in the same visual field and stop when it becomes a pinpoint. Another solution is to attach a separate rack and pinion focuser like those found on refracting telescopes to the back of the SCT. In this configuration, the main mirror is left in one spot and the rack and pinion takes on the role of focusing so there is no image shift. This type of focuser does let the observer rock back and forth across to make sure they are at the best focus. These are available from several places such as Apogee (www.apogeeinc.com)

The only problem is the 2800mm focal length necessarily makes this a high magnification telescope. The Celestron .63 focal reducer can be used to drop the focal length to about 1800mm, and today there are 2" eyepieces with focal lengths all the way to 100mm which can handily drop the magnification for a telescope such as this one to illuminate dim galaxies and nebulas covering wide patches of sky. However, there is a price to this- the telescope really needs to operate from a rural setting to get a sky dark enough to perform to its potential. In the city, the pinkish glow of light pollution from gas stations, parking lots, and other sites which shine lights up in the sky makes observing much more difficult. The C-11 is such a good light amplifier that it brightens up the background and so washes a lot of the contrast out of the image. Light pollution filters work OK as a defense, but in my experience, they don't seem to be able to get rid of all of the glow.

All in all, this telescope is capable enough to satisfy even a very finicky observer.

Mount Performance

The mount sold with the CM-1100 version of this telescope is one of the better German equatorial mounts around. This piece of hardware is not only capable of handling larger payloads. but is a step ahead of the capability of mounts such as the CG-5, Great Polaris, or other similar mounts. The drive is very smooth and the tracking is nearly perfect. This mount is built to do photography and to maintain smooth tracking for a very large telescope. All of its components are supersized from the automotive scale ball bearings and shafts to the oversize worm gear drive for super-smooth operation. The C-ll had very little shake after I had touched the focusing knob, although I would suggest an electric focuser motor to make the operation completely jitter-free. The mount needs to have the Celestron Astromaster computer added to make it computer guided, but actually already has everything an observer would need to track objects and take photographs.

Besides having digital setting circles and the like for computer control, this mount also has backlash compensated drives. That means it automatically controls its motors so it doesn't lag or jump when you press the button for it to move in one direction, and then reverse directions. This is really a neat experience because it makes it very easy to precisely point the telescope at something. All in all, I could easily imagine this being someone's "One mount." If this were mounted on top of a permanent pier instead of a tripod inside an observatory dome, this would be as capable an observatory as most amateurs would ever desire.

The mount does have a built in polar scope to use to get it lines up whe used as a portable telescope, though. Although this is a fairly heavy telescope system, it is still fairly wasy to move it in a modest sized car. You should plan to get to an observing site before dark, though, and get set up while there is still light because all of this telescope's parts will need to be brought out one at a time and put together. The amount of effort needed to do this will likely cut down observng to just occasional use unless you are in an area with great weather most of the time and have some gung-ho friends to help.

Summary

The CM-1100 is all the telescope most people would ever need. I tend to wish more people could have a chance to look through a telescope as capable as this one on a regular basis. The images are absolutely first-rate. The three-dimensional quality of live image is in many ways more revealing than the higher resolution, but flat images large observatories such as Hubble can make with cameras. I have found every time I have had a chance to look through a C-11 to be a real treat. To put it simply, the performance is there.

The only problems I can name are associated with just how much telescope this is. The counterweight for the mount weighs 22 lbs. by itself- more than an entire lightweight telescope and its tripod. At the same time, the incredible performance of this scope asks for good skies away from the city to free it to do its best. As a result, this is really a telescope for a serious observer. When compared to other high ticket purchases, though, it should be remembered that a telescope like this, unlike a computer sold for the same amount, will hold its value.

I think perhaps the best use for one of these would be for astronomy clubs or for a school. Such an effort would need a few people who knew how to operate it and someone to keep up with its maintenance, but it could be expected to pay large dividends. One thing which should be remembered is the high light gathering capability of this telescope combined with its high resolution images and excellent mount make it a very good candidate for use in hunting for new comets and asteroids, or getting a look at known small bodies such as these to track their orbits. Many objects which appear to be little to get excited about in small telescopes reveal themselves for the C-11.

I can't recommend the CM-1100 for a beginner, or for most intermediate amateurs simply because of its scale. To do so would be kind of like recommending a Lambourgini Diablo for a new driver. However, for an advanced amateur or a group with collective advanced skills, this telescope can really deliver. Compared to what $3500 will buy in any other telescope type, this telescope is a great value.