New to CCD with NB flters, trying to figure out focusing offsets


#1

I have a new Atik 383L+ with filter wheel. I have Baader LRGB, Ha, SII, OIII filters. This is on a 10" f/4 newtonian which is also a new experience for me. I’ve read through the instructions and searched through this and other forums but for me, setting up autofocus is not intuitive at all regarding which boxes need to be checked and so on. Autofocus works fine for the LRGB filters, it’s the narrow band that need offsets because the autofocus is not reliable with narrow band, at least for me.

So… What I have done so far is to figure out where the focus point of each filter is compared to the Lum filter using a mask. I have them recorded as a +/- steps relative to the Lum. I redid focus on Lum every time to compensate for temp changes as I get wide swings in my climate.

But the focus point in the filter setup window does not accept negative numbers and I see that it expects absolute focuser positions. Focus positions from night to night will be different due to temp changes, focuser slop or backlash, etc… so does it deal with these in a relative matter and the absolute number does not matter? If that is so, why not have it so Lum (or whichever you choose as the focus filter) is zero and we just put in the offset such as 25 or -40 or whatever?

But since it does need absolute position, can those numbers be arbitrary, say setting Lum at 100, Ha at 120, SII at 75? My focus position for Lum is around 18000 with the Rigel focuser and stepper and I can reset the position back to 18000 if I want to in it’s setup menu. But the next night, my focusing may start at 17790 due to a different ambient temp. Will it always run it back to 18000 at the start of the focusing run for Lum? It would start way off if it did.

Do I then check the “autofocus with filter” box and select Lum in the equipment profile manager? I was confused because the pop up tip says only do this if your filters are parfocal which mine are not. But then there is a different setting in the focus dialog of the sequence control panel which is “auto adjust focus per filter”. Is this the one I should use or is it both?

What I really just need it to do is switch to the Lum filter, autofocus, then switch to the desired narrowband filter and apply the focus offset every time it is supposed to autofocus (by temp, time, filter change, etc…)

Sorry for the long post but I’ve gotten fairly confused on how to set this up. 2 nights ago autofocus with narrow band worked fine all night but last night smartfocus went completely the wrong direction and all except the first couple of images were bad.

Thanks for any help,
Chris


#2

The focus offsets are relative. If you have your focus for LUM at 18000
in the filter wheel setup window, and your focus offset for Red is
18100, SGP will always add 100 when moving to the red filter (assuming
you are focusing with the LUM of course). So if auto focus runs and
your LUM focus comes in at 17800, then when switching to red it will
move to 17900.

The same thing applies to all filters, including narrowband. What I do
is I run AF on LUM, then run AF on Ha (which will require 30s-1min
exposures), calculate the difference and make sure the relative
difference is accurate in the filter wheel setup. Then I go back and do
AF on the LUM again, then move to the next filter and note the relative
difference. Etc. (The reason I go back to LUM each time is just in
case the LUM focus position changed slightly due to falling temps).

HTH.


#3

OK, I get that now. If that’s the way it works then I guess you could just set any arbitrary number for the Lum then set the others relative to it? It wouldn’t have to be 18000 since it will likely never be that again unless I reset the position through the driver. Or am I mistaken?

But… which of those boxes need to be checked? The focus tab in the equipment profile is different than the one in the sequence control panel. (v 2.4.3.7)

Thanks,
Chris


#4

That is correct Chris. I have my LUM set arbitrarily at 30,000 and
adjusted the focus points in the filter wheel setup. But for various
reasons the actual step number can be very different, in fact right now
it’s around 38000 for focus.

Also, the options you need are in both the equipment profile and control
panel. In the Control Panel/Focus tab tick “Use auto focus” and then
"set". Set whatever options you want but make sure to “autofocus with
filter: LUM”. Then back in the control panel tick “Auto adjust focus per
filter”.

Those settings are also in the equipment profile.


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