[Startrek (Martin)]

Selection of Guide Scope and Camera based on image or pixel scale ratios

The following formula is one of the best methods to ensure your matching your nominated guide scope to your main imaging scope and cameras


“Recommended” main imaging pixel scale to guide scope pixel scale ratio
No greater than 4:1
3:1 is better

Main Imaging Camera pixel scale

M is the Image scale in arc sec/pixel of Main Imaging Camera

Formula
M = ( imaging camera pixel size in microns x 206.3 ) / focal length of imaging scope in mm



Guide Camera pixel scale

G is Image scale in arc sec/pixel of Guide Camera

Formula
G = ( guide camera pixel size in microns x 206.3 ) / focal length of guide scope in mm


Ratio Formula
Divide Guide Camera pixel scale into Imaging camera pixel scale

Recommended Ratio 4:1

Example: My 10” f5 Carbon fibre Newt rig using ZWO2600MC imaging camera with Orion 60mm guide scope and ZWO290MM guide camera

M main imaging camera pixel scale - 3.76uM x 206.3 / 1250mm = 0.62 arc sec per pixel

G Guide Camera pixel scale - 2.9uM x 206.3 / 240mm = 2.49 arc sec per pixel

Main imaging camera pixel scale M = 0.62 arc sec per pixel

Guide camera pixel scale G = 2.49 arc sec per pixel


Therefore G divided by M = 4

Therefore pixel ratio is 4:1 which is Satisfactory !!

A ratio of 5:1 + will work but the bigger the ratio the more difficult it is for PHD2 to Guide especially in less than ideal atmospheric conditions

I’ve used the above formula for my 3 rigs , 6” Newt , 8” Newt and 10” Newt using guide scopes and all guide beautifully


Cheers
Martin