What's Causing This in My Oiii Flat?

[StevieO]

Hi all,

Since taking up imaging again after a long break I have found that my Oiii flats are exhibiting a circular feature on the left hand side (see the attached image). Doesn't happen with Ha and Sii. At first I suspected my filter wheel to be at fault, eg. not centring the filter in the FOV, but I've checked it and it's fine. It's not the shutter on my Atik 383L as the exposures are 5 sec.

I've stretched the image more than usual in order to show the feature clearly.

Any ideas would be gratefully received.

Cheers

StevieO

[wimvb]

Looks like a reflection of the glass edge. What brand is it?

You also have a band along the top and bottom. 

Edited September 14, 2019 by wimvb

[StevieO]

Hi Wim, thanks for your response,

They're Baader 36mm unmounted 8.5nm. I hadn't noticed the borders before either. I've now over-stretched the Ha and Sii filters and they also exhibit the same thing but to a much less noticeable degree. 2 possible causes  spring to mind:

I'll check that the retaining clips aren't too tight, causing tension in the filters

It may be that even 36mm is too small for the 383L chip.

Stange that I didn't notice this before my long break from imaging.

Cheers

StevieO

 

[wimvb]

Does the camera have a mechanical shutter that can cause the banding?

[Danjc]

This may be worth a read on CN

https://www.cloudynights.com/topic/582053-problem-with-narrow-band-filter-reflections/

Also internal reflections can also be of a concern, I found this a good read also 

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

Edited September 15, 2019 by Danjc

[StevieO]

41 minutes ago, Danjc said:

This may be worth a read on CN

https://www.cloudynights.com/topic/582053-problem-with-narrow-band-filter-reflections/

Also internal reflections can also be of a concern, I found this a good read also 

http://diffractionlimited.com/flat-fields-stray-light-amateur-telescopes/

Thanks for that. I think somewhere in there is the answer.  Looking at my flats I can readily believe it could be down to internal reflections, and I will try adding a 5mm spacer between ccd and filterwheel. I'd have to think long and hard before trying to paint the inside of the FW matt black, though!

Regards

StevieO