Using longer focal length scope for guiding

[that_guy]

Hey gang, just been doing a bit of thinking. Currently I image with the ed127 but sometimes I miss the widefield view of the ed80 (which has been demoted to a guidescope). Would I have any issues switching the cameras and using ed127 as a guidescope while imaging with the ed80? I've always been told not to guide using longer focal length than your image scope but I can't see why not :shrug:.

Thanks for the help
Tony

[kens]

There's no law against it. The things to watch for are when you have a small guiding pixel scale and differential flexure is always a problem.
If your guiding pixel scale is very small you can alleviate that with either a focal reducer and/or by binning the camera.

[peter_4059]

Ken,

Why is the the small pixel scale an issue for differential flexure? I always believed flexure would be an issue regardless of the pixel scale.

I also thought binning the guider would reduce the resolution placing more reliance on the (sub pixel) centroid calculation.

Peter.

[kens]

Poor punctuation on my part. Small pixel scale is one issue and differential flexure between the two scopes is another.
With a small pixel scale, binning wont adversely affect the centroid calculation but can improve the SNR. Calibrating with very small pixels is problematic as PHD2 always moves 25 pixels. With small pixels any noise can impact the accuracy of the calibration.
In your case you'll be guiding at around 1000mm so not likely to be a problem but just something to be aware of.

[peter_4059]

I chose the ASI120MM for the small pixels because I wanted to use a lightweight mini Borg as the guidescope. That has a fl of about 250mm so I'm guiding at about 3 arcseconds per pixel. My main imaging combination works out at about 1 arcsecond per pixel. I was thinking binning the guide camera would not be ideal in my case.

[kens]

Sorry - thought you were the OP. The OP was talking about using the ED127 as a guide scope. It has a focal length of 950mm