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Old 15-02-2021, 12:48 PM
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gregbradley
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Its an interesting topic. Traditionally the answer would be aperture rules and its still true but perhaps to a lesser degree.

I think it comes down to what you want to image.

You can quote Craig Stark but try to image most spiral galaxies and you'll see the advantage of aperture and longer focal length.

With advances in sensitivity of cameras and higher quality modest aperture scopes I would say 70-130mm is good for widefield, 8-12 inches is probably a practical limit to galaxy and close in view imaging. There are quite few modest aperture APOs now that are not super expensive and are capable of a great deal.

I would class astro scopes into 4 categories:

Up to about 106mm aperture APO for widefield imaging.

Around 130mm APO for higher resolution widefield imaging.

Medium focal length wider aperture like up to about 12 inches and often F4 or faster. This for smaller objects but also can do fairly large objects. Its quite flexible. Focal length around 1260mm.

Longer focal length large aperture to get the small dim galaxies with decent detail.

There are tons of superb 10-12 inch Newts around F4 that show the power of aperture and a fast F ratio. A large aperture allows a faster F ratio whilst also maintaining a decent focal length needed for smaller dimmer objects.

As pointed out the latest CMOS sensors tend to have smaller pixels than the usual crop of CCD cameras which are typically 9 micron and as small as 4.54 micron. Whereas CMOS smallest is 2.3 micron and 3.76 is a common size now and 6 micron being one of the largest (also an older less sensitive sensor).

Plus the trend for these sensors is going to be smaller and smaller pixels so less and less suitable for astrophotography.
These later CMOS can be binned 2x2 but its not a hardware CCD-like binning but rather done after the fact of an image by software so the gains are less.

Greg.
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