Guidescope Focal Length

[Startrek (Martin)]

Appreciate some advice on an appropriate guide scope focal length for my Newtonian reflector with 900mm focal length as the more you read books and forums the more subjective it becomes
Using the following universal formula -
Resolution = 206 x pixel size / FL

My Main Scope
Where my DSLR camera has a pixel size of 4.29 um
Main scope focal length 900mm
Main scope resolution = 0.98 arcsec/pixel

Suggested Guide scope Orion 50mm mini with a ZWO ASI120MM-S
Orion 50mm mini has 162mm focal length
ZWO camera pixel size 3.75um
Suggested guide scope resolution = 4.77 arcsec/ pixel

Therefore the pixel difference being around 4 arcsec/ pixel
Reading various AP articles the difference between main scope and guide scope should be around 2 pixels to minimise error

Therefore based on above I should be using a guide scope with focal length between 300 to 400mm to achieve a 2 pixel differential ???

And yet the Orion 50mm mini states it can be used on main scopes up to 1500mm focal length

Am I reading it all wrong ??

I welcome any advice on the subject
Thanks in advance

[doppler (Rick)]

A 50mm guider with the 120mm guide scope will work just fine. I try not to over think things, some of the maths is too much for me to worry about. I had a 50mm generic Chinese guider and that worked a treat with my zwo120mc and 10" 1200mm fl newt. The thing I would be more worried about is "differential flexture", which is anything a minute fraction loose in your imaging train, ie loose camera mounts, focusers, t mounts or adapters.

[Atmos (Colin)]

Don’t forget cooling differences between the newt and guide scope too!

As Rick says, mechanical issues are a larger problem that the image scale differences in this case. A guide scope should work up to about 10x imaging scale.

[Benjamin (Ben)]

Sorry to jump in on the thread, although it may be relevant, but I wonder if an Off Axis Guider would be of greater or leaser benefit. Specifically I wonder if a long heavy reflector on an HEQ5 would be better served from guiding at a finer image scale or the larger image scale? The finer image scale with on OAG (or long focal length guidescope) would magnifying seeing I guess and result in a steadier stream of fine guiding corrections, and the larger image scale with a finder guider would assist in nullifying the effects of seeing perhaps and result in more definitive guiding related to various PE and alignment errors? When the motors are being pushed by a heavy scope what would image scale would benefit guiding the most? Sorry for all the Qs and hope that makes sense.