Odd focus discrepancy between consecutive frames? Any ideas?

[irtuk]

Hey folks. 

So I am starting my 2021 season  with the ZWO 1600 mono that I got at the tail end of last season, I never really managed to give it a good run out at the beginning of the year.

This morning I have  10 x 300 second exposures on M31 in L R G and B. I figured I would try something relatively easy to get the gear dialled in again as I have added a bunch of toys over the summer including a pier and a polemaster so I am happy with longer exposures,

Combining these in APP there's a horrid green halo around everything and if I look at the subs the G channel does indeed appear to be out of focus by some margin.

The problem is the last blue frame and the first green frame were taken back to back, the focuser is on auto-compensate, dew straps are on and most critically I am at this point asleep in bed grabbing some Z's and waiting on an alarm to tell me its done.

Any ideas?  I am going to pull the filter wheel out and look for smudges and grease on the green filter but are there any more experienced folks able to come up with more technical explanations as to why two back to back frames look so different?

I have attached the last blue frame and the first green frame for reference.

Cheers.

Ed.

Light_M31_300sec_Bin1_filter-Green_-10.0C_gain0_2021-08-29_001855_frame0001.fit Light_M31_300sec_Bin1_filter-Blue_-10.0C_gain0_2021-08-29_015857_frame0010.fit

[irtuk]

I have just finished removing and refitting the filter wheel, and to my surprise the channels were wrong, R,G and B in the filter software ( ASI ) did not map to R, G and B in the wheel, so that's obviously one problem, but more generally, do I need to be refocusing per-filter to account for the different wavelengths?

Ed.

 

[newbie alert]

What software program are you using to automate?

Yes, you should refocus on every filter change unless your filters are exactly  parfocal( most arent)

Did you say you've already done the offsets?

[tomato]

+1 for focusing after a filter change and after x number of subs and/or a change in HDR or FWHM exceeding a predefined level, if your control software can do this.

For me the time lost focusing far outweighs the risk of collecting a raft of subs that are not in tight focus.

[ollypenrice]

I think that most good filters are parfocal but that most optics are not. That's to say that even a top quality apo has slight variance in focal length for different colours. I don't think we know what scope you are using, but even reflectors will be affected by coma correctors, field flatteners, etc.

When I'm imaging for myself with Tak 106 or TEC140 I tend to focus in luminance and leave it there for RGB. When imaging with guests I do it by the book and refocus per colour.  If I found it made any difference in my rigs I would, of course, do it by the book all the time... 

What does make a difference is regular refocusing with temperature change.  I'm not much up on robotic focus but, if you use an offset, I assume it must be an offset from some focal point already defined in an initial focus. Wouldn't that mean that, if the focal position changed with temperature, the offset would now be wrong?

High haze can look very like soft focus...

If your file naming protocol is wrong, won't your offsets also be wrong?

Olly

[irtuk]

Thanks for the input all. 

What do we mean by "offsets" ? is that separate focus settings for each filter?

The optical chain I am using is an Altair Astro 102mm Refractor, then a field flattener then the filter wheel, then the ZWO 1600MM

The focus controller is a Pegasus unit and I have it set up for various temperatures and equipment, I was thinking that I should use these settings for R, G and B prime focus and set the auto temperature compensation options

I had set up auto temperature compensation on the last session by taking the motor position at 16c (25565) and the motor position at 13c (25385) subtracting the two (180) then dividing by 3 to get 60 steps per degree of change in temperature. Was this the right way to go about it?

Other than this, I don't think ZWO ASI Imaging can control the Pegasus focus motor, at least it does not see it in the options, not that I have spent much time looking into it.

 

 

[irtuk]

ah! I just found this....

[irtuk]

I just processed the "other" subs and I think I can see that at least one of the other two sets is out as well, not by as much, but noticeably.

 

I left everything as luminance in APP just to get an idea.

 

 

now I just need a clear sky to work out the correct offsets!

 

Ed.