Super telephoto (2.8/300) vs small APO thoughts?

[ollypenrice]

On 06/09/2024 at 15:56, vlaiv said:

My point is that any lens is far away from being diffraction limited optics and that any scope will outperform any lens on that front. Sure you can use lens to create wide field shots - but any image produced with the telescope will simply be sharper.

 

left M31 taken with Samyang 85mm lens and right M31 taken with 80mm scope ...

Point taken. However, if we were to use star removal and replacement software during the post-processing stretch, where would that leaves us?

Olly

[vlaiv]

16 minutes ago, ollypenrice said:

Point taken. However, if we were to use star removal and replacement software during the post-processing stretch, where would that leaves us?

Olly

Well it's not just stars that are affected by this - it is also target itself.

Not sure if this example will be representative or clear but here it is:

I simulated reduction in sharpness - again, those four stars as reference - with a block of original sharp data pasted above it. Now - ignore stars and look at dust lanes. Top refractor, bottom simulation of lens.

You can see this effect somewhat in the comparison image you referred to - but since I brought down 400mm FL data to only 85mm FL - that sort of hides the issues. More wide field you go with lens (coarser sampling rate) - less obvious lack of sharpness will be.

[Elp]

47 minutes ago, vlaiv said:

More wide field you go with lens (coarser sampling rate) - less obvious lack of sharpness will be.

Please explain if/how this applies to a wide+large aperture system like a RASA or Hyperstar, interested to know.

[vlaiv]

3 minutes ago, Elp said:

Please explain if/how this applies to a wide+large aperture system like a RASA or Hyperstar, interested to know.

Very similar.

RASA and Hyperstar are also not diffraction limited.

With faster system airy disk that certain aperture produces gets physically smaller. You can sort of see this from arc_second per pixel equation. Longer the focal length - less arc seconds per pixel there is and since pixel is certain physical size in micrometers - this shows that focal length is "conversion factor" between angular size and size in focal plane in micrometers.

Fast system will have the same aperture but shorter focal length. Same aperture will produce same airy disk size (or we could simply say star profile of a given FWHM in arc minutes - it does not need to be airy disk size). However - with shorter focal length you would expect that angular size - be that Airy disk or FWHM of a star to be smaller in micrometers - and hence that you'd need smaller pixels to record it properly.

However what happens is that you can't really use the smallest pixels on RASA and Hyperstar - you actually benefit from larger pixels because larger pixels "hide" blurriness.

Another way to come to this conclusion would be to look at spot diagram of said systems:

This is for 8" scope at 400mm focal length. 1um at 400mm is 0.51 arc second so 4.55um will be 4.55 * 0.51 = 2.32"

This is RMS of spot diagram, while Airy disk diameter (and not its "rms" part which is smaller) for 8" scope is 1.38" or about the half of that.

Black circle should be airy disk radius, but I'm not convinced it really is. For 550nm it is about 2 squares wide - or 2x 1.8um = 3.6um, but we have seen that it is 1.38" at 0.51" per micrometer which is 2.7um.

In any case - if you increase your pixel size - you will end up at the point where you start to properly sample for increased spot size of these systems and images will look sharp again - but if you try to sample at "normal" resolutions for 8" scope and regular seeing - say 1.5"/px - images made with these setups will show softness at 100%.