Pixel scale - a graph showing calculations

[furrysocks2]

A while back, I put together this image to satisfy myself how to calculate magnification, exit pupil and field of view for an eyepiece in a scope:

Worth mentioning:

 

 

I've just tried doing the same for a camera sensor, after realising that only pixel size and focal length mattered when calculating pixel scale.

 

I guess that from here that it's pixel scale and aperture that determine exposure time... it's often cited that a "fast focal ratio" yields shorter exposures, I guess that's the same thing really.

Have I understood all this right?

[vlaiv]

8 minutes ago, furrysocks2 said:

I guess that from here that it's pixel scale and aperture that determine exposure time... it's often cited that a "fast focal ratio" yields shorter exposures, I guess that's the same thing really.

Pixel scale and aperture are ingredients to calculation of exposure time for target SNR, but many other factors come into play like:

Quantum efficiency, optical transmission / reflection coefficients (central obstruction if any), read noise level, dark noise level, LP levels, transparency / atmospheric extinction, number of subs, length of subs, ....

[furrysocks2]

2 minutes ago, vlaiv said:

Pixel scale and aperture are ingredients to calculation of exposure time for target SNR, but many other factors come into play like:

Quantum efficiency, optical transmission / reflection coefficients (central obstruction if any), read noise level, dark noise level, LP levels, transparency / atmospheric extinction, number of subs, length of subs, ....

Indeed! I guess I was thinking no deeper than "number of photons per pixel per second", or similar.

[vlaiv]

Oh that one is easy

It should go like this (for extended targets):

Take target surface brightness in mag / arcsec squared, apply extinction / transparency to it (depends on altitude and Aerosol optical depth, as well as local influence - similar to transparency setting in Stellarium), using magnitude logarithm calculate difference to mag 0 being around 880000 photons per second per cm squared to get target photon count per second per cm squared. Do the same for LP (also in mag/arcsec squared but without extinction) and add those together.

This is rough estimate of number of photons reaching aperture. Next you find effective aperture of scope - collecting area (for obstructed scopes subtract obstruction) in cm squared multiplied with optical losses in system (number of mirrors x reflectivity for each, or number of glass elements x transmission of coatings and glass). Multiply this effective aperture of scope with number of photons per cm squared per second - you will get number of photons falling on pixels covering 1 arcsec squared. Next you need resolution of system and find out surface of pixel in terms of arcsec squared - multiply with this to get number of photons falling on a pixel per second. Multiply with QE of sensor to get electrons per second. Add bias and dark electron level, multiply with ADU - and simple as that get signal level that you can compare to actual recorded image .