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"But it doesn't have to be in Focus to guide. It is better if it is slightly out of focus."
I have heard that idea before. I am not sure that is true.
I have found I get lower guide errors when the guide stars are sharply in focus. As the software calculates the centroid of the star, it seems to me it would be a more accurate computation if the bright central area is smaller than if it were larger.
Also, I find with autoguiding that if I get larger than expected guide errors, simply picking another tighter looking star gives instant improvements.
Also, often guide stars are double stars. If they are a bit out of focus they will look ellipsoid in shape. This would have to mean an inaccurate computation of the centroid as it is an ellipse not a circle. It certainly is good practice not to pick a double star as a guide star nor a guide star that has another brighter or similar brightness star nearby that could confuse the software.
Another point as well is sometimes when guiding you get a brief cloud getting in the road. A well focused star is more likely to shine through a partial cloud cover than an out of focus one.
What have you found with regards to having guide stars slightly out of focus and what is the theory about that?
This is an interesting thread. I'd like to hear what guidescopes people are using that they are happy with.
I suppose the requirements are:
1. wide enough field to be able to pick up guide stars easily. Also to have the guide stars bright enough to be useable.
2. Nice solid and easy to use focuser. No slop or too large an opening. The eyepiece holder has solid and powerful locking screws or compression rings so
you don't get a rocking motions.
3. Long enough focal length to give accurate guiding but not so long as to make it hard to pickup a guide star. Not sure about the ideal focal length
here - I'd love to hear what others say about this. AP's guide scope is 900mm and about 60mm aperture or so. I assume that is considered by them
to be ideal. Too high an F ratio is probably not a good thing either as guide stars become too dim and harder to find.
4. It has to be light and easily mounted.
5. No flex in the rings or the focuser or the rotator if it has one. Enough backfocus to be able to mount a guide cam without extension tubes.
6. Not too expensive.
The little AT66ED that cost US$200 2nd hand fufills the above quite well except for focal length as I think its only about 340mm or so. But as I say the Vixen 95LMC
with nearly 1 metre focal length was very hard to use as I had trouble finding a guide star. It is also F10 I think.
Greg.
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