Focusmax and narrowband filter focusing

[Paul Haese]

Something has occurred to me using focusmax. Every time the scope goes to focus it uses the luminance filter to focus. This is fine for LRGB imaging as the Lum image is the detail layer. However where this all comes undone is when doing narrowband imaging. I am yet to find that the lum filter is parfocal with the narrowband set.

Can you set the filter you want to focus with? I cannot seem to find it.

I know if I was using CCDautopilot I could set the filter I want to use for focus, but I am still integrating this into my automation plans and have not installed this yet.

Is there a solve to this problem?

[bert]

Ccdap has filter offsets. So you calibrate each filter compared to a reference filter. In my case I use the luminance filter to focus, then an offset of steps is applied to the luminance focus position.

[jase]

As does ACP. Most if not all observatory automation software has this capability these days. With ACP you simply run a script and it does it all for you, that way you can simply focus using the luminance filter once an while and freely switch between broad or narrow band filters and know you are hitting the CFZ.

You can also manually configure filter offsets in image acquisition tools such as MaximDL. Calculating the offset manually is not a fun exercise however as you really should make multiple passes per filter so you average the result. I believe ACP by default does 5 filter passes but its configurable to suit your needs.

[allan gould]

Paul
SGPro not only has a calibration routine for filter off sets but it will also temperature compensate as well as focus through each filter and obey instructions to refocus on a set number of subs or each x degree change in temperature.
Allan

[DavidTrap]

Not sure what you're meaning about Focusmax always using luminance.

I just manually select the filter in Maxim and the run Focusmax. Watching the log, it appears to me that it is focusing with the selected filter.

CCDC definitely lets you select which filter it will focus with.

It's not just the filter glass being par focal, but the whole imaging train - I'm using a fluorite doublet, so there will be more variance in focus point between wavelengths than a triplet. I have seen Mike comment that there are focus differences between filters even with his big AP scope - high end filters, high end scope.

DT