Focusing with an Off Axis Guider?

[Euan]

I've been looking in using a OAG, and there's one thing I can't fathom

If you are using the configuration similar to this picture:

Scope // OAG with Guide Cam // Filter Wheel // CCD

...if you are going between non-parfocal filters how to maintain focus on the guider?

Is it really a case of if you put it before the filter wheel you really need parfocal filters?

[old_eyes]

euan,

Hopefully someone with some real experience rather than my theoretical knowledge will answer, but my understanding is that the configuration you show puts the OAG ahead of the filters so that guiding is unaffected by the necessary changing of filters, and that there is the maximum light for you to pick off a suitable guide star.

Light then goes through the filter to the imaging camera, and if the filters are not parfocal you will need to refocus for each one.

Point is all this can be done without disturbing the OAG.

Hope I have understood your question.

old_eyes

[MartinB]

You are absolutely right Euan, if you don't have par focal filters and the off axis prism is in front of the filter, re-focusing the camera will take the guider out of focus. However, if the difference between filters isn't too great it isn't a huge problem. It isn't critical for the guider to be precisely focused, in fact, a lot of people deliberately de focus the guider a very small amount to increase FWHM and the accuracy of the centroid. I sometimes use an off axis guider with 1mm thick astronomic RGB and 2mm Baader Ha filters. I set the focus for the guider between the two. However, if using active optics the star has to be as bright as possible so I then stick with par focal filters or tweak the focus of the guider when using the different filters.

[Euan]

Thanks Martin, I thought that was the case

My plan is to try and get a parfocal set, as I notice that if I don't have the focus spot on for my guide camera, I can lose a bit of brightness with the really dim stars, which is what I will be needing with this type of setup!

I'm still getting a little bit of differential flexure in my system and hopefully this will solve it