Focus Set Point Questions

So, I’ve been thoroughly enjoying SGP’s AutoFocus goodness, but you know, it isn’t fast when you’re using Narrow Band Filters. (That’s not SGP’s fault, of course.)

But, I’m thinking that I could increase the efficiency and speed of the whole business if I set up filter offsets.

Can I just use the data from FITS Headers captured in a recent set of light frames to populate the required information in the filter settings? Does that make sense as a scheme?

I’d appreciate any insight anyone is willing to share.


Hi Karl,

There are surely better ways to do this but just working things out for my self I spent at least half a night to come up with the offsets that worked for me.

I let temperatures settle…both air temp and scope temp. I then did a focus run with Lum …noted the focus position, then did a focus run with the NB filter (much longer exposures as you well know)) and noted the focus position for the NB filter…then went & did the Lum again to make sure nothing had changed there. (Sometimes it had - as the NB focus run took a long time and things were not sufficiently settled - so did that one again all again).

Did the same for each NB filter. Thus I had a good offset for each individual NB filter from the Lum in the wheel.

Took quite a while simply because I started before things really settled…but in the end…if it takes half a night…it will be worth it in the end to have good offsets. Then you save a bunch of time for each imaging run.

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Thank you!

I agree with Brendan, it took a good while for me to get my offsets sorted but worth it, surprisingly, my 3nm S11 is quite a bit different from my O111 filter despite claims of being par focal…



I used a Bahtinov Mask + Bahtinov Grabber (software) and an artificial star to set my narrowband filter offset. It went quite easily.