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A review of the Celestron CPC 1100 telescope
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A review of the Celestron CPC 1100 telescope
by Tom Bryant
by Tom Bryant
I've owned my CPC 11 for 2.5 years, and it has had extensive use. Hopefully this report will give you an idea what to expect if you get one.
The telescope is rather large, but not oppressively so. Its 63 pound (29 kg) weight is still not too much, even for a geezer like myself. Make no mistake about it. This is a large telescope, but anything bigger would be more of a problem. The difficulty of use of a telescope goes up with the cube of the aperture.The tripod, at less than 20 pounds (9 kg) is easily handled. There's even a strap to tie the legs together to keep them from flopping around during transport
I can have this telescope out of the car and ready to find its first deep sky object in about 20 minutes. And it easily fits in a small car. My Saturn SW-2 holds the telescope, observing table, computer, eyepieces, batteries, food, binoculars, observing chair, and a companion easily. Portability is one of the strong suits of this telescope.
Optical purists often excoriate the optics of the Schmidt Cassegrain. Huge secondary obstruction and excessive field curvature, (although recent designs from both Meade and Celestron address the field curvature problem) are most often cited. These are real errors, and they do detract slightly from the overall image. I emphasize slightly. Because of the field curvature, you have to focus between the edge and center of the field when using a wide field eyepiece, and a stars at the edge of the field appear a bit elongated. I don't find this bothersome when observing, however. The central obstruction creates a slight bit of contrast loss, especially on the bright planets. Planetary images in a large refractor or long focus Newtonian are slightly more contrasty than they are in a equivalent Schmidt Cassegrain.
This same large secondary, which tends to remove light from the central Airy disk to the surrounding diffraction rings also makes certain close, faint, doubles more easily split.
The most important factor in image quality, however, is aperture. The C-11 gives you this in relative abundance, and its images can be superb. If you have a great night, and the optics are in thermal equilibrium with the surrounding air, you can split doubles at the Dawes limit. The telescope holds 560x (a 5mm Pentax) and the centers of M 13 or M 5 are breathtaking at that power on a dark, steady night.
If wide field (greater than a degree or so) imaging or viewing is of critical importance to you, then this is not the telescope for you. At its widest, with a 41mm Panoptic in my 2" diagonal, I have a 68 arc minute field.
If you want to use this telescope for deep sky photography, you'll need to get Celestron's equatorially mounted version of the C-11.
The C-11, with its short tube and sturdy mount, is relatively impervious to wind, and slight bumps hardly affect it.
It's diminutive size makes it fairly easy to use, as the eyepiece is usually in a comfortable position. Objects close to the zenith will find you carefully maneuvering around the 12v battery, mount legs, and, for short eyepieces such as orthoscopics, the tube itself. Even so, the viewing is still fairly comfortable. The more comfortable you are, the more you see.
The telescope is fairly easy to set up. First, place and level the tripod, then carefully heft the telescope and base on to the tripod. This latter takes a bit of practice, as there is no obvious indicator as to when the base is on its pin until Thunk! it slips into place. Then you tighten the three hold down bolts, connect the battery and hand control, take off the lens cap, and you're ready to begin alignment.
I find the 63 pound tube to be a bit heavy, but still quite manageable. It sits in the back seat of my car as I drive to my dark sky site, seat belted in like any other passenger. The tripod is in the back, and it fits between the two rear wheels as if it were made to measure.
Once the telescope is well aligned, tracking is spot on, and very quiet. The slewing motors, however, sound like frantic sewing machines.
A well charged 12v storage battery (mine's a 22 amp hour capacity battery) can easily run the telescope, dew heater, and charge my computer from time to time for at least 6 hours. I've yet to run that battery down.
There is an ability to goto anyplace in the sky, by entering J2000 coordinates. I've seen other systems that do not have this feature, so this gains extra points for Celestron.
An idea of what the C-11 (and the C-8) can show visually can be found here. I use it mostly for galaxies from a quasi dark sky site near my home.
When the telescope is switched on, the switch itself lights up with a built in red orange light. It's very bright, so I cover it with a piece of duct tape. Once I'm really dark adapted, I often cover that with a handkerchief, as well. To the right is a photo of the telescope at the dark sky site before I applied the tape. The dome light of my car is on, creating a second light off to the left, and the sun just set 20 minutes ago. Still, you can see what a beacon that switch is!
The hand control, however, is on a rheostat, and can be dimmed to an acceptable level.
While moving the primary mirror gives you a focus range other designs have a hard time matching, it also introduces image shift as you focus. A star you're focusing on suddenly will jump to the edge of the field, or even out of it as you focus. Some catidioptric telescopes are well enough made that this is not a problem. My C-8 Ultima and Questar have almost no focus shift. My C-11 has it in abundance, and even after Celestron attempted to fix it (see below) the problem remains, unabated.
The "choke" capacitor about 10" (25cm) from the end of the power cord easily catches on the tripod legs. This can damage the power cord, I had to tape mine where the insulation pulled away after this happened the first time. You need to carefully watch that cord as the telescope slews.
The lens cap fits rather loosely to the end of the telescope. That piece of duct tape that covers the power switch is used to secure the lens cap so it doesn't come off as you transport the telescope.
At one point, the tripod centering pin came loose and made the telescope almost impossible to put onto the tripod. Once the problem was identified, fixing it was a simple matter of repositioning the pin and re-tightening the locking nut.
For the telescope to perform well, it must be collimated, and at the same temperature as the outside air. Take the telescope from a warm car to a clear, cold sky, and it can take upwards of two hours before the images are really good. It took me a while to figure this out, and I blamed the telescope's optics, for my C-11 wasn't nearly as sharp as a friend's when we did a side by side test. It turns out that he had transported his in the trunk of his car in an insulated case.
In a similar vein, please don't try to collimate the telescope until it has reached thermal equilibrium.
Sometimes, when you tell the telescope to find an object that is 5° azimuth from another one, it merrily turns 355° in the opposite direction, catching its power cord on the tripod legs if you don't watch it very carefully.
Early on, my C-11 lost lock on its carefully done alignment, and I had to do it all over again. This was downright frustrating, but was solved by a call to Celestron. It turns out that the center post of the base's power plug needs to be ever so slightly bent to make good contact with the power cord's plug.
This telescope, in a damp climate, dews like crazy. Once the telescope cools down and begins to give good images, the corrector plate at the front of the telescope begins to dew up on even a mildly damp night. Unless you live in the middle of the desert, a dew cap with a 12v heater is an excellent investment. I also keep a 12v "hair dryer" in my accessory kit to blow dry my eyepieces as they dew up.
Note that the mirrors inside the telescope rarely, if ever, have a problem with dew.
The initial alignment is often quite poor. You set up, say, on Sirius and Regulus, and then tell the telescope to find M 67, and it misses it by 10 degrees! Yikes. I suspect that the GPS hasn't locked on yet. You can either start over, or just manually guide the scope to M 67, and then replace Sirius with M 67. After you've done this for a few objects, the alignment improves substantially. Once it's working well, I can even slew to a new object and have it in the field of my 220x Orthoscopic, 12 arc minutes in diameter.
The user's manual is accurate for what it covers, but important things like how to goto a negative declination, how to slew the telescope quickly when the rate is set for much finer adjustments, are left out. I had to call Celestron or read it on the internet for how to do these things.
While the 40,000 items database is nice for my observing tastes in that it contains both the NGC and IC catalogs, its listing of doubles and variables is very poor. Celestial gems like STF 2804 or R Lep are not to be found. One wishes that some of the SAO stars could have been eliminated, or better, that more objects were in the database. As it is, I use Night Assistant to create my observing lists, as it sports nearly 166,000 objects, all of the 259,000 SAO stars, the WDS, GCSV, and of course all of the NGC/IC catalog. It also makes finder charts that show stars as faint as 16mv.
One sad day, I neglected to tighten down the 3 hold down bolts between the telescope and its tripod. A slight bump caused it to come banging down to the concrete garage floor, after which the drive was dead. The telescope would slew, but it wouldn't track. I called Celestron, got a shipping number, and sent it to them. It costs about $100 to ship the telescope from the east coast to California.
I was also told to expect an estimate within 4 weeks. After 4 weeks I called them, and was told they'd get right on it. After a couple more weeks I had my estimate. They insisted I fax (what OLD technology!) a reply with my credit card number on it. They wouldn't take it over the phone. Oh well.
Celestron estimated the repair would take a month. A month after I had authorized and paid for the repair, I was again on the phone with them, and they said they'd get it right out. Two weeks later, my telescope was sitting on my porch when I got home. I had wanted to have it delivered to a neighbor, who is retired and therefore able to receive a daytime delivery, but this was beyond the capability of Celestron's shipping department.
The telescope was still in its original box, very battered, but the telescope itself was unhurt. One would have hoped that after hundreds of dollars of repair cost Celestron could have given me a new box, but they did not.
The alignment the repair sheet claimed was done did not survive the cross country trip. No matter, I can align the C-11. Aside from that, the repair worked, and the telescope again tracks.
The primary mirror wobble when focusing was not fixed at all. Celestron's team either neglected to do this, or they were unable to. Their manufacturing acumen seems to have dropped. My 20+ year old C-8 Ultima shows only a minor mirror wobble when focusing.
Whenever I called Celestron, the person on their end was always polite and quite knowledgeable. It didn't hurt that I always had my repair number handy, so they could see that they were running a bit late in getting my estimate to me and my telescope repaired.
Overall I give the Celestron team a C-. They are very slow, and need to be politely prodded to get the job done. They couldn't deal with a secure SSH credit card transaction or a telephone card number transfer, (almost everyone else can do this) and were unable to ship the repaired scope to my neighbor, which should have been easy. They were also unable to fix the mirror wobble problem.
What I like about the CPC 11
- It's fairly portable for such a large telescope.
- The optics are diffraction limited.
- The mount is very steady, and tracking is both smooth and accurate.
What I don't like about the CPC 11
- The red light on the on-off switch is much too bright.
- Alignment is often inaccurate for the first few objects. This is fixed by re aligning on objects as they're found, and after three or four of them, the alignment improves to acceptable levels.
- The power cord is easily caught on the tripod legs as the telescope slews. This can damage the power cord and mess up your alignment. You simply have to watch it carefully as the telescope slews.
- Focusing at high power can be a bit of a challenge because of the primary mirror shifting by a couple of arc minutes. The star can bounce right out of the field. Celestron was unable to fix this, as noted above.
This telescope is a pretty good telescope. Bang for the buck, it's very hard to beat for what I ask of a telescope. The problems I've detailed above all have work arounds I can live with.
I'd like to thank both Kathie Bryant and Tom Corbin for their careful review of this report, which corrected many errors and improved its readability. Any errors that remain are my own.
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