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You can evaluate your own image scale ratio with the following formula rather than on line calculators -
A) Main Imaging Telescope Image or Pixel Scale
Main Camera Pixel size in microns x 206.3 / Telescope focal length in mm
B) Guide Scope Image Scale or Pixel Scale
Guide Camera Pixel size in microns x 206.3 / Guide scope focal length in mm
Image Scale Ratio is B divided by A
Either use a longer focal length Guide Scope or smaller camera pixels
You can experiment with various combos to achieve a tighter image scale ratio
Ideally 4:1 is a good number which is what I image at with my f5 longer tube Newts
Cheers
Martin
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