I’ve spent an awful lot of time recently critiquing the Celestron CGEM telescope mount and have said about all I can say about it. I thought that I would give a basic review of the some of the optical qualities of the matching 9.25 inch EdgeHD Optical Tube Assembly (OTA.)
Since the CGEM mount is not stable enough for precise astrophotography I will give my opinions on its suitability for terrestrial photography. There are a couple of things that I should point out; first is that most telescopes, unlike dedicated photographic lenses, are designed to be used exclusively at an infinite focus. Second is that the obstruction in the light path will also degrade the lens performance, especially out of focus performance. That being said I would not expect perfect performance of this telescope design for terrestrial photography.
The first test was to measure the scopes capability to provide a uniform image from corner to corner onto the image sensor. For this I used my Nikon D3s with a full frame sensor which is 36 mm by 23.9 mm, 43.2 mm diagonal. I also used Celestrons two-inch T-adapter. I am disappointed that there is definitely pronounced vignetting or light fall off, especially in the corners. Here is a light frame photo converted to eight-bit grayscale:
The two dots represent full black and full white, the eight-bit K values in percent were measured in Photoshop were as follows:
Center = 28%
Upper Left = 69%
Lower Left = 69%
Lower Right = 60%
Upper Right = 68%
Upper Center = 34%
Left Center = 37%
Bottom Center = 38%
Right Center = 38%
Black = 100%
White = 0%
This indicates that this scope is passable for 35 mm full frame sensors provided that any mosaics have plenty of overlap. It will have a fairly even light across a typical smaller consumer grade DSLR sensor. It is definitely unsuitable for any medium format film or digital sensor camera and just barely suitable for full frame 35 mm film or digital sensor cameras.
Next I took a picture of a brick wall of the building directly across from me. It is about 150 yards away. Note that the focus was obtained using the Live View screen. The focus is incredibly sensitive and simply touching the knob causes fluctuations in focus. This shot was the best focus out of several shots. I also used the mirror lock-up feature in the camera to prevent any vibrations from disturbing the image sharpness. The image was color corrected but no other adjustments were made to the image file other than resizing it for insertion into this post.
The focus is not as crisp as I would have liked, but also remember that ALL digital sensors require some sharpening, most consumer grade digital cameras add significant sharpness without the user’s knowledge. In this case the images require a very large amount of sharpening. The distortion from center to edge is very small and the pincushion distortion is minimal and is greatest along the upper horizontal portion of the frame. The focus is also very consistent across the entire frame showing that the EdgeHD optics are very good. I would like to see the sharpness of a standard 9.25 inch scope to compare, I’m wondering if the flatness comes at the expense of sharpness in the central portion of the frame.
Next two shots show a photo of a branch in a tree in front of the building. UGGGGGHHHH!!! As expected, due to the high percentage of light stream blockage, the out-of-focus areas or bokeh from this lens is absolutely horrible making it practically worthless for fine terrestrial photography: (the out of focus branches are not actually moving and are really quite annoying)
The depth of field is very narrow making focusing very critical. The chromatic distortion is also minimal as would be expected from a good mirror based lens, the field flattening optics are actually quite good:
Bottom line: This telescope is not at all well suited for terrestrial photography and given the sharpness is fairly bad when the focus is not set to infinity. It makes me wonder how crisp the image could possibly be for astrophotography. Even if I were to be able to place the scope on a stable enough telescope platform I am now fairly skeptical of the maximum potential of this lens, even at prime focus.
It would be nice if Celestron would release the reducer lens for the 9.25 EdgeHD as this might improve the sharpness, this may just be wishful thinking I simply can’t say for now. I’m also curious about the Faststar optical system. At around F2.3 this setup would probably have a ridiculously narrow depth of field.