I would like to show a couple of photos using a Nikon 70-200 mm VR lens to show a comparison of the sharpness to that obtained with a Celestron 9.25 EdgeHD Optical Tube Assembly. I took two shots using the same Nikon D3s camera, one at 200 mm and the other with a Nikon 1.4x teleconverter giving an effective focal length of 280 mm. The Celestron’s focal length is 2350 mm. Both were taken at an f-stop of F/10 which is the same f-ratio as the Celestron 9.25 EdgeHD OTA.
I also cropped the two images such that it would show roughly the same amount of area and enlarged the images to give all three images approximately the same pixel density. No sharpening was applied and the only adjustments that were performed was the auto exposure in Adobe’s Camera Raw.
Here is the shot taken at 200 mm:
Here is the 280 mm shot:
It is fairly clear to me that using my Nikon 200mm lens with the 1.4 teleconverter results in an image that is not only on par but is actually superior to the Celestron 9.25 EdgeHD OTA. Superior in image flatness, distortion, and sharpness. The Celestron might have a slight edge in chromatic distortions but this has not been a serious problem thus far for my use.
To me this does not leave me with a very good impression of Celestron’s OTA for usage in astrophotography when I can simply use a much wider angle professional camera lens and simply crop and enlarge to produce a far better image. It should also be noted that when the above photos were taken there was light rain falling creating obstructions that were not present during the capture of the Celestron image.
For reference here is the full size 200 mm image: (note the smooth bokeh in the soft out-of-focus branches)
I would like to also show one definite advantage of the Celestron CGEM, at least its advantage over my Nikon 70-200 mm lens. This advantage is not nearly as high as one would expect from an instrument with an aperture of greater than three times it counterpart. That advantage is its resolving power. Even though the apparent sharpness is as good or better with the Nikon lens this next photo highlights the resolving power shown by less detail in the leaves and branches with the Nikon lens and 1.4 teleconverter. The depth of field is wider with the Nikon thus the background becomes more distracting than with the Celestron, but the Nikon’s bokeh is still far superior, and remember this is a highly cropped and enlarged image from a lens with an aperture smaller than one-third that of the Celestron EdgeHD:
Here we go again, the Celestron CGEM paired with the 9.25 EdgeHD OTA is a good combination for VISUAL astronomy but falls short of the necessary precision for serious astrophotography.