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A few thoughts on OAG's.
1. For longer focal length scopes they (or an ONAG) or a imager with build in guider are a must. Every other solution finds it hard to combat residual differential flexure and /or mirror shift.
2. The selected OAG must be able to hold all your gear really firmly. Do all the attachment bolts up real tight or you will get coma and your camera/s will be sloped to the imaging plane! Check the tightness of all the bolts regularly!
3. Make sure your imaging train can take the weight! I think in the near future I will upgrade my Meade motor focuser connected to a large Lumicon OAG to a motorised, PC controlled Moonlight focuser into a ONAG. The Moonlight seems a lot better built than the Meade!
4. Make sure your imaging train has enough back focus. With an SCT you are probably fine - for everything else double check before you buy!
5. Nail the focus of both cameras! I used a bright star with a Bathinov mask to focus the main camera then slewed to the full Moon centred in my guide camera using the same Bat mask to check focus on the guider. This gave me sufficient light to properly focus the guide camera and haven't touched it since.
6. Get a really decent, mono guide camera. A colour Meade DSI won't do - the mono Meade II DSI is alot better but still just sufficient. Get a sensitive, mono camera and you won't regret it.
7. For you guiding learn your software and match it to your location's seeing and the arc seconds per pixel on your guide camera to ensure you don't chase the seeing and end up yo-yo'ing everywhere! For me I found setting a guide star must be 0.93 pixels displaced for a guide instruction to be issued to be optimal - given this equated to about an arc second - as I am over sampling and it I had it set too low - I would be ridiculously chasing the seeing (and for me 1 hour subs still look great - my guiding at 2.3 metre focal length is really good).
8. Balance and re-tighten your gear really well once again. If it is all set up right, you will be giving yourself every chance not to introduce stray factors that can ruin your imaging.
Hope this helps.
Matthew
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