Banding in Flats

bodon

GDay All

I dont expect an answer with this but maybe your thoughts and experience would be appreciated.  I went from a ZWO2600mono APS-C to a brand new ZWO6200 mono full frame camera.  I also now have a larger new FW and 50mm unmounted LRGBSHO filters.  The APO is an Askar FRA500 petzval scope.  My scopes adapters screw straight into the filter wheel which secures the camera etc.  Ie 54mm straight to 68mm so no obstruction.  Dark rings came with the gear and they are also used. I used a mask and gloves to install the filters and to screw the FW to the camera so no smudges on the camera sensor or filters.  The adapter from the OTA does no hit the filters; which I tested prior to assembly.

I thought I do a light test with a 5min dark frame.  All good.  Then I took some flat frames to see how much vignetting I would get.  Well, looks as if I took X-Rays of some ones fingers.  More prominant in Ha and Sii.  So I thought light getting in here maybe; covered everything and no light leak.  I took flats with a trace pad; had to be the pad so I purchase a Giotto Flat Field generator..still weird patterns.  Ah lets get some sky flats; still weird patterns.

So, I turned the Giotto the other way up, still the weird pattern and in the same direction.  I also turned my scopes captain wheel around; took flats and the weird pattern is still in the same direction.  Tried different gains as well, still the same pattern.  Tad strange as the specs of this camera are basically the same as my previous camera and I never saw these patterns.  I did manage 5min subs with each filter aimed at Carina.  Due to weather I have not been able to get a few more.  I have contacted the shop and linked files and await an answer.  I also posted on the ZWO forum..must be in the too hard basket as no response.  My concern is will these flats corrupt a final image?  Flats taken at EAF focus. Id like to pick your brain if you have time.  Regret long winded post.  I know you are a busy man and dont expect a reponse.

I have one drive links to my Carina lights, sky flats and flat frames if you want to have a gander and have time.

Regards

Lights Carina Nebula Lights Carina

Flats with generator Flats Taken with Giotto 120

Sky Flats Sky Flats as at 4 Jun ADU 24 - 28000

BlockHead

OK... so a couple of things.

1. Note.. you turned the camera/optics and yet the pattern remained fixed. What does this tell you? Answer: it is the camera.

2. The "fingers" are a dead giveaway of a cyclic/repeating pattern. What is interesting is your pattern is actually what you would WANT to see in a text book example.

3. You did not say this- but I am going to guess you do not see the pattern strongly in Blue, Green, or OIII ... but in Red, Ha, and SII.

So all of this points to the very common effect of interference. At some wavelength the coating on the window of your sensor (or the sensor itself) has a particular thickness to cause an interference pattern. That is what you are seeing. This is physics! The typical coating thickness correspond to redder wavelengths of light. Fast telescopes in particular can help the interference pattern show up. 

The good news is that this pattern will be the same for any images (lights and flats). So it calibrates out. 

With my 61mm RedCat and Ha or SII...the flats are a bizarre squiggle mess of blotches. Your example means your coating was very evenly applied and is a similar thickness throughout.

The real test as you indicate is to calibrate your data with those flats. As long as you do everything properly... it should work out just fine.

Another name for the effect is "fringing" and professional astronomers deal with this all the time because they use thinned back-illuminated sensors.

-the Blockhead

bodon

I really appreciate your time and bright thinking mind.  You are correct in regards to the Red Ha and Sii; more prominent than the other filters. (I can see your grin here in AU).

Just one more; I took the Ha filter out, left the panel on and took a flat.  No 'fingers'.

I then thought the NB filters are poor and the culprit.

Just to confirm; the camera and filters are good?

Regards

BlockHead

Yes, this is a physics phenomena. 

If you want to complain, it would be with the coatings on the window of the camera not being uniform to within the wavelength of visible light. 

But the real test to answer your question is to take data, both calibration frames and light frames, and process them. 

-the Blockhead

bodon

This really drove me nuts.  To me it was like going down the rabbit hole. Is it the scope, is it light, reflections etc.  Well worth being a member on Adam Block Studios....even if it was not PI processing.

Appreciate your time in answering.

Regards

BlockHead

Did any of the other responses you receive agree with me?

(and what ratio? lol )

-the Blockhead

bodon

I wish I posted here two weeks ago.  I did not think it was an appropriate post as your videos etc are geared towards PI processing..which I enjoy watching.  Not so much for hardware issues.  But I thought Id give it a whirl as you have been in the game for some time.

You are absolutely correct and hit the nail on top of the head when you mentioned that the 'weirdness' will be in the Red, Ha, and SII; as that is exactly what I see in my flats. (also shows up in the PI Flat Contour Plot).

I also started a thread on Astrobin nine days ago and a member had the same filters, camera and flat panel but using a Takahashi FSQ106 and had very similar patterns.

https://www.astrobin.com/forum/post/196372/

Another member mentioned exactly what you explained and had a magic link explaining how it occurs  Link mentions (Optical Etaloning in Charge Coupled Devices (CCD))..Tad over my head...

Other members on Astrobin said they had patterns and some were using chroma filters.. 

If you had time and want a quick gander; link(https://www.astrobin.com/forum/c/astrophotography/equipment/flats-issue-with-zwo6200mm/)

Did not know what to think as I have only been taking photos for a short time.

But I am completely happy with what you posted; the guru convinced me.

Again appreciate your time and quick responses.

Regards

BlockHead

Well..if you want to do me a favor.. you can follow up on your post that I did properly answer the question and, to quote you, " Well worth being a member on Adam Block Studios....even if it was not PI processing.

Appreciate your time in answering."  :)

-adam

bodon

Well worth being a member on Adam Block Studios...Done!