A question regarding guiding and image resolution

[rogerg]

I have always figured it logical that if I am to accurately autoguide a photograph, I need to be guiding at the same or higher image resolution as the imaging chip.

That is, if I am imaging at 1.04 arcsec/pixel, I need to be guiding at 1.04 arcsec/pixel (or 0.86 if I felt like it, but 1.04 is already asking for trouble).

Practically, this implies that the guide scope would usually need a longer focal length than the imaging scope. You could of course have a shorter focal length scope and smaller pixel size on the camera, but practically, you really need a longer focal length for the guider.

As an example, there would be no point guiding a scope that has a focal length of 2160mm with a scope that has a focal length of 900mm, when both scopes have cameras of the same (or near) pixel size.

Correct?

[EzyStyles]

hi roger. your correct . the guidescope has to be twice the f-ratio than the imaging scope. eg: F5 imaging scope = guidescope F10 or there abouts.

[jase]

This all sounds good in theory, but you also need to take into account your seeing conditions and mount capabilities. If your typical seeing is only 2.5 or 3 arcsec/pixel and/or mount has erratic worm/worm gear, guiding at long focal lengths can give erroneous results. Shorter focal lengths can level out the seeing, though again depends on the arcsec/pixel combination.

[Dennis]

On those long, boring nights watching CCDSoft auto guide the ST7E, I often notice that guiding appears to be at a sub-pixel accuracy? I think this allows the guide scope to be a shorter focal length than the main imaging 'scope, yet still produce nice, tight, round stars.

Cheers

Dennis

[JohnH]

As has already been mentioned this should be about relative image scale in "/pixel and not just fl there is dependency on the chip and pixel sizes.

As we are talking autoguiding there is no need to be at the same fl as the main scope if you are autoguiding. Typical autoguiding software will do sub-pixel guiding to +-0.2 or less pixels.

In my case the FL of my guidescope is 388mm, my main scope is 1800mm, my guidecam is an Orion DSI (one shot colour) and the imager is a Canon20D.

So my guide resolution is 3.42" per pixel and imaging is 0.73"". This is in fact marginal at 0.2 pixel accuracy.

One discovery I have made recently is that one shot colour cams really should be avoided for guiding - they are less sensitive, and the debayering process requires processing and can cause loss of precision.

The second issue is the type of guiding setup you intend to use, some of the software out there is MUCH better at guiding than others (thus you may be able to get to 0.1 pixel precision or better).

Finally it will depend on your mount and its accuracy.

I have a long boring thread on this elsewhere.