ZWO Filters - offset question

[g__day (Matthew)]

So I have a 7 position 36mm filter wheel and the standard ZWO RGB Ha OIII SII filters Bintel sold years ago - I have never check the offset required between the Luminance filter and all the others - nor created flats for each filter.

Is this very important with these filters? Do folk with these filters find them zero offset - or do you really need to set this up carefully - just curious!

[BlakPhoenix (Rohan)]

I don't use ZWO filters but I would always recommend testing the focus offsets between any group of filters. Even "parfocal" filter sets will often not really end up being so due to the telescope not perfectly focusing every wavelength at the same point. Dark Customs can do this automatically in NINA which also then inputs them as filter offsets for later use in AF routines.

[Drac0 (Mark)]

Like Rohan I always do it no matter the filters. I used to do it manually with a bahtinov mask, but now use APT & autofocus aid to calculate the offsets - though I still don't totally trust it so will check it occasionally.

Cheers,
Mark

[g__day (Matthew)]

Thanks guys - I will definitiely add it to my check list!

I like autofocus aid in APT - so I will try that - is there a video that shows you how to store your results per filter Mark?

So if I find the focus relative to say the Luminance filter for the other six filters - is it simple to tell either ASCOM ZWO filter wheel - APT or SGP what offsets to use for each filter?

Thanks, Matthew

[Drac0 (Mark)]

Originally Posted by g__day

Thanks guys - I will definitiely add it to my check list!

I like autofocus aid in APT - so I will try that - is there a video that shows you how to store your results per filter Mark?

So if I find the focus relative to say the Luminance filter for the other six filters - is it simple to tell either ASCOM ZWO filter wheel - APT or SGP what offsets to use for each filter?

Thanks, Matthew

If you use autofocus aid in APT, simply check the 'calculate filter offsets' box & it will run through the complete wheel & auto populate the offsets in the settings relative to the first filter used - I recommend starting with Ha, especially if you use the 'fimal inwards move' setting. You can then simply manually copy these across to SGP.

I know of no videos for this though I am considering making some APT related ones myself soon.

Cheers,
Mark

[g__day (Matthew)]

Mark,

Many, many thanks - I reckon those two tips will save me a few hours of effort!

Matthew

[g__day (Matthew)]

The APT per filter auto focus seemed to run okay for all 7 filters, but blue and green seem a bit out. Can anyone suggest optimal settings to find best relative focus - I tried 12 steps but I wonder if there is a better method.

One my first attempt it gave
Luminance : 0
Red : -25
Green : -5
Blue : -45
Hydrogen Alpha : -59
Oxygen II : -58
Sulphur III : -136

I am surprised that Green and Blue are that relative distance - given as a function of wavelenght of light I would expect it to follow the order of a rainbow Red, Orange, Green, Yellow, Blue, Indigo and Violet - so I would have expected the Green Value to be between the Red and Blue values.

But in DeepSkyStacker I can see tiny doughnuts instead of tight stars when I zoom in on the Blue and Green shots - whereas the Red and Luminance ones are tight. So I am wondering if the Autofocus per filter went a bit wrong on my first attempt...

[g__day (Matthew)]

So re-running the per filter auto-focus in APT gives different results again - will see how this goes with a 3 x 2min Image series on each filter:

Luminance : 0
Red : 5
Green : -13
Blue : -20
Hydrogen Alpha : -15
Oxygen II : -17
Sulphur III : -92

[Drac0 (Mark)]

Hi Matthew - I've replied to your post on the APT forums. Cheers.

[g__day (Matthew)]

Run 3 gave me:

Luminance : 0
Red : 50
Green : -7
Blue : -60
Hydrogen Alpha : 17
Oxygen II : -12
Sulphur III : -92

[g__day (Matthew)]

Hi Marc, saw you reply just now - thanks - yes focus inwards is selected; I am wondering if HFR is just a poor algorith with a subject like NGC 5139.

Run 4 - switching from HFR to FWHM gives settings of:

Luminance : 0
Red : 19
Green : -52
Blue : -94
Hydrogen Alpha : 0
Oxygen II : -33
Sulphur III : -99

Well that seems to have been the issue on this target - a small imaging run gave every filter is decent focus - so pack up astrolab and off to bed now!