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Celestron AS-GT (CG-5 GT) GOTO EQ Mount

Mar 18 2005 12:48 PM | Tony Bonanno in German Equatorial

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One of the pleasures of amateur astronomy for me is sharing my views of the night sky with friends. I live several miles outside of Santa Fe, New Mexico and am fortunate to have reasonably dark and clear skies. I can haul my gear 50 feet from my front door and have a clear view of much of the night sky. My “star gatherings” were enhanced a couple of years ago by the purchase of my first GOTO system, a Meade LX90. I quickly became a fan of GOTO, and admittedly, have become somewhat spoiled by it. Like many amateurs, I’m always experimenting (to the degree the budget allows) with different types of telescopes and gear. I’ve had 10 different scopes and several different mounts over the past several years. I currently own two 6” Russian maks, a 10” Schmidt-Newtonian, and the LX90. My other scopes ride on a Losmandy GM-8 equatorial mount. I’m very pleased with the GM-8 mount, except for one thing. It doesn’t have GOTO. When I checked out the Gemini GOTO system from Losmandy, I found that the GOTO add-on by itself was as much as the GM-8 mount. This was more than I wanted to spend. I kept thinking of all the reasons it would be nice to have a reasonably portable GOTO equatorial mount; the ability to have GOTO for all my OTA’s, the ease of use for star parties, the amount of time I could spend observing rather than searching; etc. There was just one problem – finding such a mount at a price that wouldn’t break the bank.

I thought my quest was over when Meade announced their new LXD55 series with an equatorial GOTO mount and the same AutoStar controller as on my LX90. The price seemed too good to be true. WOW. I placed my order right away. Seven months later my LXD55 arrived. The Schmidt-Newtonian OTA that came with my package proved to be a fine performer. The LXD55 mount was a different story. I won’t go into all the details except to say that I have never been so frustrated with a mount in my entire life. I got rid of the mount and found myself right back where I had started. I still didn’t have a GOTO equatorial mount.

During the summer of 2003, I noticed advertisements for Celestron’s new AS-GT series telescopes with an equatorial GOTO mount. I began to wonder if this new mount might be the answer. I was a little gun-shy at this point. Perhaps, there was no suitable answer except saving my money for the Gemini system or perhaps a much larger mount like the Celestron CGE (doesn’t meet my criteria for portability). I cautiously began to research the new AS-GT mount and talked with several dealers. I was pleasantly surprised to hear encouraging remarks about the mount. I ordered one. Price including shipping was $724.00.

Initial Observations:

First let me qualify my comments by pointing out that I’ve not owned this mount very long at the time of this writing. The mount is a brand new model and I think I’m one of the first customers to take delivery of the “mount only” where no OTA was included (first week of November 2003). Also, I’m not going to attempt to review all the features of Celestron’s GOTO system. That information is readily available on many internet sites and I’m still in the learning mode myself. The following information is provided in the hope that it will be helpful in evaluating the basic suitability of the mount.

The bottom line:

This mount looks like a winner! I’m very pleased with the initial performance of the mount. With a 12.5 lb OTA, the dampening times are fast (generally under 2 seconds). I don’t experience any “jiggles” at the eyepiece - this baby is stable. The mount tracks smoothly without discernible vibration, even at relatively high power. The mount worked, as it should the FIRST time I set it up. After my experience with the LXD55, this was very refreshing! The first night out, I simply entered my site data, followed the prompts on the Hand Controller, did my 3 star alignments, and started a tour of various targets. WOW again! Target in FOV every time using about 60x at the eyepiece, usually near center portion of FOV.

My main objective in purchasing this mount was to obtain a relatively portable and stable VISUAL platform with GOTO that can be used with several different optical tube assemblies. At this time, I’m not planning on using the mount for imaging (I’ll use the GM-8 for that). I suspect, however, that the mount will prove to be a capable imager. It will be interesting to see what other users experience.

Do I think this mount delivers what Celestron said it would? YES. Does it appear to meet my needs and objectives? Most definitely. Would I recommend this mount to other amateurs looking for a cost effective GOTO that WORKS well? Absolutely!Tony Bonanno
Santa Fe, New Mexico USA

November 14, 2003
____________________
The author has no undisclosed interest in this product, its supplier, or its manufacturer. All photographs are the copyrighted property of the author.


UPDATE – December 1, 2003

A few more observations and thoughts about this mount.

First of all, I should correct the text in the above review where I state the “setup routine helps compensate for “cone error” (play in the gears)”. I was reminded by Mike Swanson that cone error and gear backlash are really two different entities. I should have said “cone error AND gear backlash”. The 3-star alignment using two stars on one side of the meridian and one star on the other side of the meridian compensates for the cone error. Mike helpfully explained that cone error is the difference between the optical axis (the line of sight of the optical tube) and a line that is perfectly perpendicular to the Dec axis. The use of the “up” and “right” direction arrows to center the alignment stars is specifically the routine that compensates for the backlash in the gears.

My Experience with the Meade LXD55 10” Schmidt-Newtonian OTA and two Maksutovs on the AS-GT Mount

I’ve had several folks email me about using this mount with the Meade 10” Schmidt-Newtonian (SN) OTA. I tried using the two together, but my bottom line impression is that it is not an ideal match. Its simply boils down to the weight and size of the 10” SN. It will work, but it’s pushing the limits of the mount. I think it will be fine for strictly visual use as long as one balances the OTA/mount carefully and makes sure the mount base and other fittings are tightened down appropriately. You will need 30-40 lbs of counterweights. The LXD55 10” OTA, with Telrad, eyepiece, dew shield, etc. weigh in at approx 35 lbs. The OTA itself is approximately 37 inches long and 12 inches in diameter. Add a little wind to the observing site and I think you’ll agree that it could be challenging. Everything is relative though. If you managed using the LXD55 10” SN OTA on the LXD55 mount, then you will be pleased to learn that, IMHO, the AS-GT mount is much more stable and a better platform than the original LXD mount. Dampening times are certainly improved.

For most of my observing with this mount, I’ve been using two Russian Maksutov OTA’s. One is a short, relatively lightweight (12.5 lb) Intes-Micro Alter 603 6-inch f/10 Maksutov-Cassegrain and the other tube is a relatively large (and lengthy) 22 lb. Intes MN61 f/6 6-inch Maksutov-Newtonian. Both of these OTA’s, even the larger and heavier Mak-Newt, appear to be VERY WELL MATCHED to this mount. With the Mak-Cass, it is necessary to extend the tripod legs to achieve a comfortable viewing position (similar to using an SCT). With the Mak-Newt, the tripod legs are fully collapsed. Interestingly enough, BOTH scopes show excellent dampening times with this mount. Generally between 1 and 2 seconds at the eyepiece ! I suspect that the fact that the tripod is collapsed when using the large Mak-Newt helps keep the dampening times similar. • A Few More Setup and Observing

Comments

Apparently the importance of centering the alignment stars using the “up” and “right” direction arrows is fairly important if you wish to obtain good GOTO’s. Several users on the Yahoo Celestron_AS group report that they’ve definitely noticed improved accuracy when strictly following this routine. I’ve also noticed that the leveling of the mount using the built-in bubble level seems to help the accuracy of my GOTO’s. Typically I will setup and do a tour of 20-30 targets during an observing session. I’m finding consistently good GOTO performance (generally within the central 2/3rds of a 26mm eyepiece). There have been a couple of instances when the GOTO slews did not seem to be working properly. I suspect a software glitch rather than a hardware problem. Hitting one of the arrow keys will stop the mount and then pushing the “enter” key seems to put everything back on track without having to re-align. • PC Computer Control

I can confirm that the mount appears to handle the ASCOM PC control platform just fine. Using the free Cartes du Ciel, the mount was responsive to the program’s commands. Be sure to use the proper serial cable for connection to the Hand Controller. The cable plugs into the base of the Hand Controller, NOT the mount itself. I had to also update the BIOS on my Dell 4150 notebook.• Cold Weather Operation

I’m not very tolerant of the cold, so about an hour at 18 deg (F) with medium wind was about all I could take for this test. The mount with the 22 lb. Mak-Newt performed well. Every GOTO target (about 20+) was in the FOV of a 26mm eyepiece. Not sure what the wind chill factor was during this session, but it sure felt cold. After about 30 minutes, I did notice that I had to hold the Hand Controller buttons down somewhat longer for the command to execute. The motors seemed to be okay, although a little slower. At one point, the Hand Controller seemed to be unresponsive. I then noticed that the cigarette lighter plug looked like it was not seated all the way in the power supply receptacle. Sure enough, I re-seated the plug and the Hand Controller came back to life. There is no question that the power supply is a critical factor with these electronics, especially with cold temperatures. I also realized that the power cord that came with my mount is ridiculously LONG - probably 25 feet of small gauge wire... I will replace it with something shorter and more substantial soon. I suspect that will also help in the cold weather.

Regarding the grease in the mount. I didn’t notice any problem on either axis as a result of the cold. I did notice a little grease along the edge of the DEC mount. It was black and did not appear to be affected by the cold temperature. I don’t think this is the same lubricant that used to cause problems in the older mounts.

IMPORTANT ASSEMBLY TIP

There are two threaded holes drilled in the tripod base in which the azimuth adjustment peg can be mounted. This is the peg that the azimuth housing fits over when attaching the mount on the base. It acts as a guide and a “stop” when using the two azimuth adjustment screws to polar align the mount. One of the threaded holes is located BETWEEN the two tripod legs. The other hole is OVER one of the tripod legs. My mount arrived with the peg installed BETWEEN two tripod legs which results in the counterweight bar and weights being situated between the two legs rather than OVER a tripod leg.

Celestron advised me that they drilled the base with the two holes so that the counterweight could be positioned between the legs instead of over a tripod leg. They said that this is sometimes necessary with certain setups at low lattitudes so as to avoid interference with the legs. BE AWARE, however, that with the counterweight positioned BETWEEN the legs it could lead to an unstable mount that will readily tip over if you are not paying attention (I learned this lesson the hard way). I found that with the legs fully RETRACTED (for example when using a large Newtonian), with counterweights attached, and WITHOUT an OTA on the mount, that the mount is pretty vulnerable to tipping over with a relatively small nudge or bump (typically while trying to attach your OTA in the dark). I would strongly suggest, just for stability, that you ensure that the alignment peg is installed OVER the tripod leg, not between them. Obviously, if you are located at a low latitude, then there may be some situations with certain OTA’s where you may not have a choice. The peg is easily moved simply by unscrewing it and threading it into the proper position. (See picture). In the current Celestron user’s manual that accompanied my mount, there is no mention of the two mounting positions for the alignment peg.
Overall, I continue to be very pleased with this mount.



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