Guiding per-filter with an OAG or Guide Scope?

May 24, 2019

I ran into a funny situation with my off-axis guider and one particular filter, an Optolong 685nm longpass filter. The filter itself gave me some amazing shots of star clusters and even did well with IR emissions from nebulae and galaxies. I run a set of mid-range Astronomik narrowband filters in the ZWO EFW I have attached to the ZWO ASI1600MM-Pro mono camera, and within reason, they’re parfocal. In SGP I’m running the autofocus process every filter change anyway, but I don’t see much difference. The IR filter glass on the other hand is thick enough to move the focus far enough to affect the focus of the guide camera in my OAG. I had to go out to the scope and adjust the guide focus in PHD2.

One solution is to go with the guide scope when using the IR filter, but I don’t know if PHD2 can support this automatically, or if I have to stop, disconnect the OAG camera when switching to IR, and connect the camera on the guide scope. It will be an interesting workflow. I’m wondering if ASCOM/SGP can work with three connected ASI cameras? ASCOM’s approach seems to be to add a number after the device name, like ASI Camera [1], ASI Camera [2]. Will there be an ASI Camera [3]?

Although this sounds like a fun experiment, I’m going to stick with my OAG, and swap out the near IR filter at some point--maybe for a tighter Ha filter, a 3nm? Up to now I’ve never paid attention to the thickness of the glass for filters, but I’m going to look into that now.

Image: I got one frame into the IR sequence and PHD2 wasn’t happy: