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I'm using an f4, 130mm guide scope. This is being used on scopes of focal lengths of 600mm - 950mm. I suspect this is too small for scopes of these focal lengths, and a longer fl would give 'better' guiding..

Any advice on guide scopes would be very much appreciated.

Thank you

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This will give you a ratio but does not give any advice. I think the general rule is no greater than a 4:1 ratio of pixel scale. I have used a 400mm guide scope (ST80) with a 1600mm FL and it was OK - although at this FL I really should be looking at OAG.

The ST80 is a pretty common choice for guiding as they are reasonable quality and can be picked up pretty cheap.

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Maybe look at guiding resolution vs mount rather than guiding resolution vs imaging resolution.

What mount will you be guiding? What is guide RMS that you realistically hope to achieve with that mount?

Say you want to guide HEQ5 and you think your model is capable of doing 0.8" RMS (for tuned stock model this is not unrealistic).

In order to accurately measure such guide error - you need to have your centroid accuracy at about 1/3-1/4 or less of that. You want your centroid accuracy to be about 0.2".

Centroid algorithms can get accuracy down to 1/16 of a single pixel so your guiding sampling rate should be 16 * 0.2 = 3.2"/px

Say you are using ASI120 (or any other guide camera with 3.75µm pixel size) - you need about 250mm of focal length.

Key points here 1/4 of expected best RMS and x16 to get sampling rate - which we can simplify to: sampling rate should be about x4 of expected guide RMS.

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