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Asi1600mm pro - focus and flat frames advice


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Hi, I used my asi1600mm pro for the first time last night, mostly to test the water and make sure everything was set up and working. 

I shot about 3 hours of luminence on M63, then thought it a good idea to do some test shots on the ring nebula with each of the LRG&B filters just so I had a full set of data I could look at today (just 5 x 1minute subs in each filter, didn't bother to adjust focus between each filter as it was gone 3am and I just wanted to use this data to practice stacking each colour and combining into a colour image. It didn't look as though the focus changed massively anyway). 

I've got a couple questions: 

- Out of interest, how much does focus change between filters? In practice obviously I would check focus when changing filters, but it sort of leads onto my next question

- With flat frames for each of the filters - how critical is it that the focus of the flat frames exactly matches the focus of the individual filters, or is approximate adequate? Is there an acceptable tolerance in focus between the light and flat frames? I was thinking of taking flat frames this evening for the L R G& B filters as I haven't removed or changed camera position since bringing in the scope last night, and there wasn't an obvious noticeable difference in focus between the filters in the subs of the ring nebula. 

- Still on the subject of focus - it took me a while to get focus last night and I tried using the focus aid and bahtinov aid in APT but I'm sure they were both giving difference results. I went with bahtinov aid in the end. Is one better than the other?  Also, something I've never really thought about before - what sort of star is better to use for focusing? I tried focusing using arcturus but feel that this could be too bright a star to use for focusing with this camera?

 

Thanks in advance 

Adam

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1. Depends on the ota in my experience. The zwo lrgb and ha filters are pretty much parfocal but the sii and oiii are not. In an ideal world focus per filter (or even better calculate offsets) using electronic focus.

2. Not very, if you are somewhere close it should be fine.

3. I get mixed results with a b mask, i tend to use the hfr measurements and mimic an auto focus run by starting one side of focus, moving in small increments to the other side and then trying to equal the best measurement moving it back. This is easy to do on the zs73 because the focuser has the wo logo so you can use that as a reference for your movements. 

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I'd say Arcturus is too bright. It's ok for a b mask maybe. The thinner the lines though makes it easier to centre them so a very bright star might bloat the middle out. If using the HFR approach with focus aid it's better to loop short frames and try and take an average reading. Also, turn off the screen stretch in APT if using focus aid as you can see the effects of focusing better.

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If you use something like nina (free) or sgpro ($99) the hfr calcs are based on the entire field so you don't need to worry about bright or dim stars. It also gets you best focus across your entire frame as opposed to a single star at a singe point of the frame.

Edited by jimjam11
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If you are using the ZWO 1.25" LRGB Filter Set Optimised for ASI1600, then these should be par-focal for your initial testing purposes.

I have tried these, and using SGPro to go through the focus routine for each filter and so far, there has been maybe 10 - 20 steps difference on the focus motor.

Didn't see anything at 100% on the images with a Bahtinov mask

Caveat - My glasses are now nearly two years old, and the focus range go the old MKI eyeball may be off

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