What has happened to the stars help please

[Garethr]

Can anyone tell me what has happened to the stars in this image please 

Stacked in DSS and a quick stretch in PS

The stars seem to have turned into doughnuts for some reason

Any help much appreciated

Gareth

[pipnina]

My best guess is that the focus moved throughout the imaging session... Or, if the tracking wasn't perfect, some stacking method removed the innermost part of star's glare.

I think it's probably the former, though.

Check your individual subs.

Hope you get it fixed.

    ~pip

[carastro]

Yes I agree, it's to do with being out of focus.

Carole 

[Tinker1947]

Doughnut stars are out of focus stars normally, a Bahtinov mask should cure a focusing problem

http://www.firstlightoptics.com/bahtinov-focus-masks/starsharp-bahtinov-focus-masks.html

[Garethr]

Here is one of the subs saved as a JPEG not touched in processing

I might need better glasses if this is out of focus  

Gareth 

[nwcs]

Is there any chance that you had a bit of rotation? I found this type of behavior when using DSS when I had slight mount rotation. When I switched to Nebulosity I immediately got better results. Just a thought. Maybe use a trial of a different stacker?

[laudropb]

Definitely looks like a focus problem.

[Gazabone]

Looks to me like a focus issue, particularly if you're using a reflector or catadioptric.

It's still a nice photo anyway, lovely colouration.

[carastro]

Well if it that individual sub is in focus, then it is possible that focus slipped during your run but it's not always possible to tell from a single sub until you have stretched a data. However I think the nebula itself looks fractionally out of focus so am still pretty sure that's the problem.

Another slight possibility is the settings if you're using DSS for stacking, I have seen this happen a couple of times if you have cosmetic tab ticked to get rid of hot pixels, if it's anything higher than the lowest setting it has been known to eat holes in stars.

My bet is on the focus, Using a Bahtinov mask is a far better tool than using your eyes. 

Carole 

[Marky1973]

Yeah, check the hit pixel detection settings....if they are selected. Deselect and run the stack again. Worked for me!

[Garethr]

Carole,

You may have hit the nail on the head with DSS settings

I did go through and make some changes to the settings thinking I was re setting everything back to default

Will have to check all the settings.

Thanks for your thoughts

Gareth.

[Thalestris24]

Hi

Just to add - dithering should sort out any false hot pixel problems. Focus can, and does change, during a session. Maybe as a result of temperature changes, maybe as a result of focuser creep. An autofocuser is a useful piece of upgrade kit

Louise

[Garethr]

Thanks all for the replies,

I will check the stack again and check DSS settings and re do the stack and see what happens.

Gareth

[carastro]

Did you resolve this?

Carole 

[Garethr]

Hi Carole,

Tried re stacking with all settings at default but DSS started mucking about so not had a chane to try again

Will be having another go today.

Gareth.

[ArmyAirForce]

Once it is stacked in DSS, did you mess around with the darkness, midtone, and highlight etc in the stacked image controls? I've had that star effect if the Highlight slider is up around the 70 mark or higher. Not every stack has done it, but perhaps when the stars are quite highly saturated to start with.

[Garethr]

Hmmmmmm

Ran the stack again and after a few false starts ended up with the same result.  (DSS crashing as it saves the final image)

The final histogram looks weird.  Very thin peaks with a dotted outline where I would normally expect the normal outline of the peaks.

Must have some corrupt data in there somewhere.

Going to have another go at M42 but try the ASI 178 on lots of short exposures. Seen a thread here and it looks an interesting experiment weather permitting of course

CO says it should be ok till about 9pm 

Gareth