units for seeing

[alacant]

Hi everyone

I'm using this. How does the seeing unit arc sec relate to the camera telescope combination? Say I have a 1200mm focal length reflector and a dslr...

Cheers

[Demonperformer]

I don't think it relates to the camera/scope comination at all, it is a representation of how clear/steady the atmosphere is. (I made the same mistake!)

[alacant]

47 minutes ago, Demonperformer said:

(I made the same mistake!

Phew, yeah. I can see it's one of those things I'll never understand. No worries! Thanks.

[vlaiv]

Well it relates in the following way:

For two seconds exposure (to give enough time for seeing to do its thing, but short enough so that mount tracking error is small enough) on relatively bright star, depending on the scope aperture, it will give you FWHM in pixels related to pixel scale you are using.

So if you are using for example 1.5"/pixels and combination of seeing in arc seconds and scope aperture gives you FWHM of 3" - your FWHM in pixels will be 2 pixels.

To figure out how aperture comes into equation, you need to know two things:

1. FWHM to sigma of gaussian - that is ratio of ~2.355, so FWHM of 2.355" will have sigma of 1". (sigma being of course standard deviation of gaussian).

2. Airy disk approximation with gaussian and it's sigma. And that is ~0.45 * lambda for airy disk radius being 1.22 * lambda, so it is ~1.22/0.45 smaller than airy disk radius.

Add two sigmas together and you will get resulting sigma (that is combination of seeing and aperture).

To give you an example, let's say you have 1" seeing, 10 inch aperture, and wonder what will be your FWHM in pixels (DSLR pixel size 4.3 and FL 1200 mm).

1" seeing equates to sigma of ~0.42463". 10" aperture has airy disk size (in green) of ~1.03" which translated in sigma is 0.38".

Total sigma will be 0.80463" and FWHM of that will be: ~1.895"

In pixel units that will be (sampling of 0.74"/pixel): 2.56 pixels

This of course assumes - seeing can be approximated by gaussian (fairly good approximation), same for airy disk, and of course perfect focusing and perfect optics.