Celestron CGEM Review

I think that this may be the final installment of my Celestron CGEM telescope mount reviews. I have been fairly hard on Celestron for the lack quality and quality controls in the design and manufacturing of their CGEM mount for the purposes of astrophotography. I finally got out for a few nights of mainly visual observing with this mount and I must say that for visual use this mount does actually exceed my expectations. Having the ability to quickly setup and align the portable mount for visual use can easily be done within fifteen minutes of arriving at a site. Using the Celestron hand controller to guide the scope to one of many predefined objects is a breeze and the numbers of objects one wishes to see is only limited to the time one wishes to put into the observations. After a good alignment all objects were in view with 9.25 EdgeHD and a 20 mm eyepiece. Two thumbs up for visual use.

Back to astrophotography for one point, I used the Two Star alignment along with four additional calibration stars as well as the polar alignment of the mount using the hand controller. After alignment the hand controller showed that I was polar aligned to within a few arc-seconds of a degree with the North Celestial Pole (NCP.) So how good was the actual polar alignment? Well I can assure you that it was NOT within a few arc-seconds, in fact after doing a thirty-one minute series of ninety second exposures I can tell you that the scope was only within about nine arc-minutes of the NCP. Much better than when only aligning with the polar alignment scope but still an improvement of less than a half. I’ve been told that the polar alignment is an iterative process and the more one repeats the whole procedure the closer one will get to the actual NCP.

This may be fine for a permanent mount but not very useful for a portable mount. So with a single polar alignment, taking about an hour from setup to being ready for snapping photos. This means that for deep space astrophotography I was limited to exposures of about ninety seconds. Here is another photo of M57 using seventeen separate exposures and combined and aligned using DeepSkyStacker with its default settings and further processed in Photoshop:

Here is a blowup of M57, note the slight star elongation with ninety second unguided exposures:

Here is another quick shot of the Andromeda Galaxy, M31, which is the closest galaxy to our own Milky Way: (note the trade off is for longer exposures through high sensor gain of ISO 6400, if one had the ability to expose a single frame for a full thirty minutes at a lower gain (ISO) the “nebulosity” would be much, much more detailed)

I think that a comment on astrophotography in general is in order here. The popularity of amateur astrophotography relies heavily on post processing. From aligning between frames for poor scope mounts, to “stacking” many frames for reducing noise, to widely varied statistical and forcing algorithms in addition to very destructive processing techniques. There is a great amount room that must be given for “artistic licensing.”

The final image, while may be quite beautiful to the eye, may not have one bit of data that actually represents what was actually “captured” by the photographic sensor. The “artists” may merely be injecting his/her own subjective artistic touches in order to bring the final image into what they may have seen in another artists rendering or in other photos. Even NASA uses “artistic” measures in how they present many of their incredible Chandra and Hubble photos containing wavelength emissions outside of the visual spectrum.

The longer one is able to expose each frame the closer one is going to be able to present actual information and data that ends up looking spectacular without all of the deeply destructive processing and artistic licensing now in common (over)use.


One final word, the polar alignment scope is a venerable pile of junk. In perfect concert with the poor quality control in the rest of the design this one also hits its stride. It took me over an hour and a half of trial and error to get the center of the polar OTA tube to be within 1/4 degree of rotating around the center of the mount. (as close as I’m going to get) I’m not talking about the reticle cross hair, I’m talking about the polar scope itself.

The jam nut must be loosened adjusted and after many tries you may or may not get it centered. This may not be redly apparent if one never points the RA axis at some terrestrial targets by contorting the mount to be able to point low enough. Try it for yourself, if you spin your mount and the crosshair stays on a star you might not actually be aligned. Rotate the axis around a terrestrial object and you might be amazed at how much the center of the crosshairs itself rotates around while it sticks to the object as it also rotates around. PISS POOR QUALTY!!