Guide Scope focal length...

[daz]

So, following various threads on different scopes for guiding...

Is there a guide-line for the focal length of the guide scope?

I have an ED80 and an ST80 for guiding (at least, when I get the damned rig it will be), but if I change the ST80 for a 66mm in some form ( a) it looks nice, really wide field, c) it looks really nice ) will I be able to

a) Guide the 80mm with the 66

Guide the 10" with either?

Is there a rule of thumb calculation here??

[MartinB]

Yes Daz. As a rough guide the recommendation is that the focal length of the guide scope should more than 1/3 the focal length of the main scope. It also depends on pixel size of guide camera, and the software being used. There are plenty of people who manage to guide with a focal length guider of well under 1/3. The big advantage of a short focal length is, as you say, a wider field for selection of guide star.

Maxim is great for sub pixel guiding accuracy. It reports the corrections to be made and typically when guiding I try to keep these to less than 0.3 pixels.

Of course there's nothing to stop you guiding a 66mm frac with a 250mm newtonian!

[daz]

Thanks Martin

[MartinB]

Oh...thanks Daz, a pentax eyepiece would be nice

[Whippy]

Oh...thanks Daz, a pentax eyepiece would be nice

...

Tony..

[Greg]

Oh...thanks Daz, a pentax eyepiece would be nice rolleyes Wink

Sshh....

Don't tell anyone...

[daz]

Er, OK. Keep checking the post, eh!