Which Guide Scope?

[Oldtimer (Alex)]

I have two WO Uniguide guide scopes, a 50/200mm and a 32/120mm. I’m unsure which would be the better one to use with my Askar 400/F5.6 Refractor.
The guide camera is an ASI 178MM and I either image with an ASI533 MC Pro or an ASI 1600MM Pro.
I’ve checked out a couple of the online Astronomy calculators but I’m none the wiser.
Any suggestions would be most welcome.
Alex

[Swarvy (Adam)]

Hi Alex,

I was using the WO 50/200 guide scope with a 430/F5.9 refractor with no issues guiding at all.

You could always try them both and see which gives you the best results.

Cheers,
Adam

[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

[Oldtimer (Alex)]

Thanks Guys,
Using the formula in Martin’s thread there’s not a lot of difference between the two. 1.2 for the 50/200 & 2.1 for the 32/120, both well below the recommended ratio. When the clouds decide to clear I’ll give your suggestion a go Adam to see if there’s any noticeable difference.
Cheers,
Alex