autoguiding guide scope focal length

[rikyuu]

I'm strongly considering using "Guidedog" for autoguiding and adding and ST-4 port to my EQ6 handset.

Currently I only have a 160mm lens to use as a guide scope and I was wondering if this is enough.

Both the imaging and guiding CCDs would be a similar size and resolution, my main scope with a reducer is F=600mm and the guide 'scope' F=160mm giving a ratio of 3.75. Would this be enough or should I have a longer focal length guide scope?

ps. the guide camera (Mintron 12V6) can do a 2x digital zoom, which will basically double the size of the guide star and hence I think double any movement it makes. Do you think this would help?

I would like to keep the zoom lens rather than swapping it for a refractor, apart from the cost, it reallty helps to find stuff, although a refractor would be nice for wide fields of course!

Any advice welcome, thanks again.

[ngc2403]

Hey,

First of all i think that the ratio of 3~4 is on the limit if both CCD's have the same sized pixels (resolution). A guider can accuratly determine the position of the star to sub pixel values. these vary on the conditions, but 1/4 is a reasonable value it think.

How about a guide camera like the QHY5, i has small pixels so can guide even larger scopes. 5.2um pixels means that a typical CCD with 7.4um pixels can be guided using scopes with a ratio of ~5.7.

The Mintron, does it have 12.6um pixels? if so then it's going to be easier to guide it but very difficult to guide a normal sized CCD with it, because you will need a barlow to make the image bigger.

The digital zoom not that important really, as it shows nothing new.

That lens is it f/0.21 at 16mm!!!! sounds unphysical. though i agree that it would be a good idea to keep it for a guide scope. However if the worse does come round then the SW 80mm Startravel is nice at ~£100 though there are plenty of other scopes around and am sure second hand one too

ally