Addendum - Optical Components

Guide Scopes

updated: 2023-01-18

Rule of Thumb for Guide Scopes

Manual or visual Guiding

As a rule of thumb, the focal length of the guide scope needed to be at least 1/3 the focal length of the imaging scope when guiding visually.

Electronic Guiding

When guiding electronically, the guiding sensitivity of modern cameras and guiding SW can be precise as 1/10 of a camera pixel. The guiding accuracy or Pixel Error is roughly:



For good guiding results the resulting pixel error should be < 1, a value of 2 is acceptable. A better solution for the TSO RC203/1624 would be an off-axis guider (OAG)!

The pixel error can be reduced, for example, by using a 2x Barlow lens between the guide scope and guide camera, but this is at the expense of the aperture and field of view. 

I used the following guide scopes:

More information:

Selecting a Guide Scope and Autoguiding Camera for Astrophotography (

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