Guidescope Suitability

[Tim Stevenson]

I have a Astro Tech AT8RC (8"  Ritchey-Chrétien) with a focal length of 1632mm and my imaging camera is a ZWO ASI2600MC Pro with a pixel size of 3.76 micrometers.  This combination yields an image scale of 0.48"/pixel.  For good guiding it's my understanding that my guide system needs an image scale ~1.5"/pixel or less.  I know many people either use an OAG or ONAG for guiding but I would rather stay with a guide scope.  That means the guide scope needs to be a long focal length guide scope (~500mm).  However, since we are looking at the imaging/guiding ratio, could I used a QHY-5-III-715C (1.45 micrometer pixel size) with a 240mm guide scope?  That image scale is 1.25"/pixel yielding a imaging/guiding ratio of 1:2.61.  While it meets the ratio requirement, is there any downside to using a small pixel size for a guide camera?  I'm aware that my image scale of my primary scope/camera will oversample and it's that reason that I'm trying to obtain the best guiding possible.

Tim Stevenson

[Brian Valente]

Hi Tim,

In general you have the right idea regarding the relative image scale of guiding-to-imaging. I don't see any problem using the smaller pixels.

However, it skips over the main point of using an OAG with a scope like yours: you will experience differential flexure when using a separate guidescope. It's not a question of 'if' but 'how much'. 

The reason to go with an OAG on a scope with a focal length as long as yours is to avoid DF. That is what I'd recommend.

If you do consider OAG, look for a guide camera that is modern and sensitive. An f/8 RC scope tends to have a lot of falloff on the outer edges, so there's less signal for the OAG. Using a more sensitive guide camera, having a larger pickoff mirror for the OAG, and adjusting the gain higher can all help here

Brian

On Sun, Jan 4, 2026 at 10:37 PM Tim Stevenson <tim.ste...@gmail.com> wrote:

I have a Astro Tech AT8RC (8"  Ritchey-Chrétien) with a focal length of 1632mm and my imaging camera is a ZWO ASI2600MC Pro with a pixel size of 3.76 micrometers.  This combination yields an image scale of 0.48"/pixel.  For good guiding it's my understanding that my guide system needs an image scale ~1.5"/pixel or less.  I know many people either use an OAG or ONAG for guiding but I would rather stay with a guide scope.  That means the guide scope needs to be a long focal length guide scope (~500mm).  However, since we are looking at the imaging/guiding ratio, could I used a QHY-5-III-715C (1.45 micrometer pixel size) with a 240mm guide scope?  That image scale is 1.25"/pixel yielding a imaging/guiding ratio of 1:2.61.  While it meets the ratio requirement, is there any downside to using a small pixel size for a guide camera?  I'm aware that my image scale of my primary scope/camera will oversample and it's that reason that I'm trying to obtain the best guiding possible.

Tim Stevenson

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Brian Valente

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