Determining Pixel Size based on Focal Ratio

[Uhhjoe82]

How do you determine the best pixel size of a planetary camera when using a large focal length telescope with the Barlow lens factored in? 
I have an edge hd 9.25 and a x2.5 Barlow and want to get the best match for planetary imaging.

Edited August 2 by Uhhjoe82
Misspelling

[CraigT82]

Generally the F ratio you should aim for is 5x the pixel size in microns.

Your F10 scope with 2.5x barlow will be around F25, so you want a camera with around 5 micron pixels. 

There are other things to consider but this general rule of thumb is a good place to start 

[Fegato]

Just beat me to it - yes pixel size  x5 = focal length

[Uhhjoe82]

Ok thanks, I wasn’t sure if the Barlow would count in the equation or not

[michael8554]

13 hours ago, Fegato said:

Just beat me to it - yes pixel size  x5 = focal length

= Focal Ratio.

[Fegato]

1 hour ago, michael8554 said:

= Focal Ratio.

ha ha - indeed!

[inFINNity Deck]

In general a ratio of 3x pixel-size should suffice unless you are imaging from Mount Everest or using the HST or JWST (thanks to lack of atmosphere), while going above 4x only is of interest in the blue end under those conditions. Please note that using 3x makes exposure times significantly shorter compared to 5x. I have written two articles about this, best to start with the second one (which directs you to the first):

https://www.dehilster.info/astronomy/optimal_focal_ratio_part_2.php

It includes two methods to check whether or not the images are oversampled (mine are when imaging using 3x pixel size and an 11" SCT @ f/20).

Nicolàs

[Xilman]

Here's what I do (and did).

Calculate the theoretical resolution of a telescope with the aperture of yours. The Airy disc size is easily precise enough.

Calculate the image scale in arcsec per pixel.

Apply the Nyquist criterion: sampling should not be less than twice the size of the minimum resolution desired. Even with very poweful deconvolution software you are unlikely to exceed this recommendation by very much.

All the above assumes perfect seeing. In practice, that will not happen and you will be seeing limited in almost all cases. Lucky imaging and speckle interferometry can often let you exceed this bound.

To give an concrete example:

The aperture of my telescope is 0.4m, leading to an Airy disk radius of 0.31arcsec

My scope has a focal length of 2614mm, leading to an image scale of 12.7 microns per arcsec.

So, the Airy disc is 0.31*12.7 = 3.9 microns in radius.

Seeing is very, very rarely less than 1 arcsec at FWHM, unless you live high on an oceanic island. A more reasonable figure for a rather good night is 2 arcsec, which corresponds to 3.9 * 2/0.31 = 25 microns. On a bad night, and I speak from experience, it can reach 20 arcsec; such nights are useless IMAO.

My camera has a native pixel size of 3.7 microns. The sampling over a 2 arcsec seeing disk is thus 25/3.7 = 6.8 --- way beyond the Nyquist criterion.

In my case, the radius of the Airy disk is very close to the pixel size, and the disk itself is sampled at very close to the Nyquist criterion.  Why do you think I chose that camera?

Almost all the time I run at 2x binning for other reasons. Only when playing with Lucky and speckle imaging is 1x binning used.

(All those numbers above can be checked with readily available sources. Finding and using them is left as an exercise for the reader.)