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My way of looking at this is that like screw drivers you need more than one type and size in your tool shed.
For planetary my choice was and still is a compound scope. At present and for the last 10 or so years is the C14 that I peltier cooled. It gives a great focal length that when combined with a barlow and nice sized pixels on a camera can mean I am imaging around 12m in focal length which in turn gives a really nice image size.
For DSO, well I have had a few telescopes but my main way of thinking is that you need to mix aperture/focal length with pixel size and image scale. I really like long focal length imaging, but you need big pixels to maintain a reasonable image scale. Something around .70-.90" per pixel I think is really good. Pixel size which is fairly close to f ratio in most circumstances produces the best results. For example I am using an f8 RC with 9um pixels. It makes for a nice image scale. Is it the best astrograph? No. It's slow photographically but does give nice sharp detail that does not require much sharpening. However it takes forever to gather very faint light from halo's around planetary nebula for example.
You can do something similar with focal length/image scale and a fast f ratio. At present I have another imaging system that has a fast f ratio of 4/3.8 with small pixels of 4.54um. This again gives a nice image scale of .82" per pixel. It's fast photographically, but the fine detail on some objects is not as nice as with a long focus scope. So almost the complete reverse of the above. Is this the best astrograph? Again the answer is no, although I can use this design to do both wide field and narrow field image, which is slightly better than the RC.
I have owned a variety of refractors but never really thought it worth looking at the image scale. I wanted these for imaging extended objects, so I was going for a field of view look. I know this does encompass a much larger image scale but it was not important to my decision. My decision was based on the size of the field and what I could afford to achieve that.
So again is there a best astrograph? I guess it all depends on what you are trying to achieve with your imaging. What you specifically want. Factors of focal length, f ratio, pixel size and resulting image scale are important.
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