Its been a few weeks since my last post, I must admit that I have been busy. I researched and purchased a new telescope system with sufficient accuracy and precision to do some (hopefully) decent amateur astrophotography. The system I chose was driven by both price and performance, and this was a Celestron CGEM equatorial mount with a Celestron 9.25” EdgeHD Optical Tube Assembly (OTA).
I also purchased the two-inch T-adapter allowing for complete full frame exposures, without vignetting, with my full size sensor Nikon D3s. For precision guiding the scope for photographic exposures I chose the Orion Awesome AutoGuider Refractor Telescope package which consists of a 400mm focal length 80mm ShortTube refractor scope with the StarShoot AutoGuider CCD camera.
To mount both scopes on the CGEM I also purchased a second dovetail bar and 125 mm attachment rings to securely piggyback the Orion guide system to the 9.25 inch telescope with minimum flexure. Unfortunately with the camera and finder scope attached the entire scope setup weight is about thirty-five (35) pounds, ouch. This far exceeds the recommended ½ of the 40 lb. load limit of the CGEM for astrophotography, we’ll eventually see how this arrangement actually performs.
In short this system works quite well, for the money, and I am anxious to get to some dark skies to do actual photographic work beyond that of calibration and testing. In addition I chose to completely disassemble clean, adjust, and calibrate the mount in order to achieve the best results possible. The results are mixed and I found that it is true that you get what you pay for.
Let me explain, any astrophotography system will need some sort of compensation for both the inadequacies of the mechanics needed to accurately drive the telescope and/or some method to help compensate for the seeing or variations caused by atmospheric disturbances. However, the measure of a telescope mount performance can easily be measured by its distinctive Periodic Error (PE) curve, and this is what I used to measure the small amount of improvements that I was able to make above the stock telescope mount’s performance. A mount with low PE will allow for longer unguided exposures.
Here are some of my criticisms of the CGEM mount:
- Quality control of the manufacturing is entirely substandard for a such a precision instrument.
- Clutch design prevents precision tuning for minimal PE. Clutch application forces the rotational axis of the mount to differ slightly from the intended optimal rotational axis of the drive gear. Thus adding to PE error for long exposures. (period of about 24 hours, PP in 12 hours)
- The use of thick grease in the clutch mechanism creates friction thus hindering accurate balancing.
- Worm drive housing mount fastener holes have very, very weak threads requiring HeliCoil thread insert installation. (additional $50 for HeliCoil kit)
- Worm drive to axis drive gear mesh alignment adjustment is quite crude and inadequate for utmost precision, requiring numerous cycling of the worm drive housing mount bolts, see above bullet point.
- Motor drive to worm drive system has excess backlash, radial runout and overall play causing irregular meshing of gears and causing irregular worm drive rotational speeds compounding the PE.
- The counterweight bar is too thin causing flexure and is a vibrational nightmare in of itself, especially when fully loaded. (apparently this was improved for the CGEM DX mount)
- The motor circuit board has an excessive amount of leftover soldering flux, both normal and fully carbonized or burnt flux indicating excessive heat application, mostly around the motor and switch circuit board connections. This was a source of horrible audible noise, an acceptable, much less disturbing, sinusoidal inductor tone still remained after proper cleaning of the circuit board.
- In addition to the flux there was also an excessive amount of a white powder substance, possibly corrosion caused by the remnants of the acid based flux, on the back side of the circuit board.
- A complete lack of technical information available, either third-party or from Celestron, concerning maintenance, design, repair, or replacement part specifications.
- Lack of setting circles thus completely preventing any useful unpowered visual usage.
- The tripod circular bubble level is slightly untrue, true level is achieved when the bubble just touches the indicator circle in the northwest direction.
- Power and data connectors are lightweight and cheesy and not designed for repeated usage.
- Lack of modern data connections, no USB or Ethernet capability without third-party hardware.
- Housing casting had several places where there were fair amounts of casting voids, this may have contributed to the weak areas around the worm drive housing thread holes. Again a manufacturing quality control issue.
- Motor control feedback encoders are located on the motors themselves, rather than being located directly on the driven axis, thus introducing errors into the control system feedback loop due to the mechanical gear deficiencies causing variances of the entire gear-train system beyond that of simple backlash.
- This has a potential for compounding the effects on the PE errors. This also gives credence to the claim: ”1arc-second (pv) tracking capability with your existing mount without using conventional auto-guider or PEC software!!!” by MDA Telescoop, however, their solution is more expensive than the mount itself. http://www.mda-telescoop.com
I would like to point out that the next step up in quality would most likely cost well over twice that of the CGEM mount. If I had to do it over, and I can’t since I voided the warranty by disassembling it, I would probably have opted for the much, much higher quality Losmandy G-11 instead, or possibly even one from Astro-Physics Inc., which typically have mechanical PE errors of about only 7 arc-seconds when properly tuned. YOU GET WHAT YOU PAY FOR!
Now for the results of my initial testing. The best ten-run average PE for the factory stock mount that I could achieve was 22 arc-seconds peak-to-peak (PP). However I would like to point out something VERY seriously wrong with this average, The BEST individual run was 18 arc-seconds PP and the worst was 42 arc-seconds PP. The variation between runs was such that they averaged nearer to the smaller value, the maximum Standard Deviation across each run’s data points was initially over ten but after disassembly and adjustment was reduced to about seven, still not very impressive.
However, the ten run average PE did not change much after tuning and was still just under 19 arc-seconds PP with the worst runs having about 30 arc-second PP error. If anyone has a mount that performs better it is probably due to pure dumb luck in the assembly of components whose manufacturing variances counteract each other in a positive way. See graphs at the end of the post.
What this means is that even with PE corrections being played back the variations in the expected PE error will still be at least as high as 14 arc-seconds after tuning. Thus nearly ANY unguided astrophotography will be limited from perhaps a few seconds to maybe a few minutes in length with the CGEM. Fortunately tracking with a second guidance system will take care of most of this lack of quality control and substandard mechanical design.
I am not at all comfortable letting the electrical hardware and software makeup and hide the mechanical deficiencies and substandard quality control of this Chinese manufactured less-than precision instrument. I suspect that I am now seeing why and where Losmandy and Celestron parted their ways. Again, YOU GET WHAT YOU PAY FOR!
Items that I do like about the mount:
- Good large Losmandy D-type saddle plate.
- Hand controller and alignment is quick and simple
- Two star alignment with calibration stars provides for accurate goto movements.
- Setup and breakdown for transportation is quick and easy.
- Polar alignment scope works very well for accurate quick alignments.
- Price point. However, did I mention you get what you pay for.
- Ease by which counterweights can be adjusted and secured.
- DC servo motors for more precise positioning control, ideal for future hacking projects. (I experienced NO problems with DEC motor cogging so this problem may have been fixed)
- The use of PIC micro-controllers from Microchip Technology Inc. in both the hand and motor controller boards, again ideal for future hacking projects.
In short the Celestron CGEM can be an acceptable mount for long exposure astrophotography if and only if it is coupled with an adequate separate compensation or guidance systems to improve the tracking capabilities above the mechanically tunable possibilities of this price discounted, mass-produced, consumer grade telescope mount. Yet once again, you’ll get from it no more than what you pay for it.
I can’t wait to get out and do some real astrophotography and then I will have some insights into the EdgeHD OTA, I hope Celestron put more efforts into the quality control of their new flagship optical tube designed specifically for astrophotography, we will soon see…
The above data was collected using Celestron’s PECTool using its Batch Train and Drift removal features. The data was analyzed using GNU Octave open source numerical analysis software. Data files and Octave script can be found by clicking on this link: PECTool_data