DSLR imaging help pls - what all am I doing wrong?

[vineyard]

Hello,

As part of my continuing experiments in AP, I tried using an old DSLR (Pentax *istDS, no filters) for wide-field.  I took 15x60 sec lights, and also took 60 sec darks, and biases and flats (all at infinity focus and the same f/2.8 as the lights - lens cap on for darks & biases, and a light box for the flats at 1/180s exposure to avoid overexposure).  When I stack and process this is what comes out.  (PI for the stacking - but the same thing w DSS; these rings emerge when I apply ABE in PI).  I foolishly forgot to take the images as RAW (so these are all JPEG lights, darks etc).

Did I take my flats & darks & biases wrong, or is this b/c its JPEGs?  I like how many stars the image captured, but I wish I knew how to process it so that those wrings don't appear.  I will take them as RAWs again in the future, but what else should I do to avoid what I hope is user-error rather than the camera!

Thank you,

Vin

 

Edited March 6, 2020 by vineyard

[happy-kat]

Hi

Do you get the same result if you remove the flats from the stack?

[alacant]

1 hour ago, vineyard said:

this is what comes out

Hi

Post -links to- each of the master files you used. Then it makes it easier to find what's wrong.

Cheers

[vineyard]

Hello,

Thanks for this.

4 hours ago, happy-kat said:

Hi

Do you get the same result if you remove the flats from the stack?

I've posted that variation below - the rings aren't as distinct but they're there (as are the dust bunnies...which at least means the flats are removing the bunnies!). I'll iterate on other variations as well.

3 hours ago, alacant said:

Hi

Post -links to- each of the master files you used. Then it makes it easier to find what's wrong.

Cheers

Here are the links to a dropbox folder - I hope those are the master files you meant alacant?

https://www.dropbox.com/sh/s5s0ibkhmdqfbnd/AAA6CP4--E6_BG0hMm7XxUAma?dl=0

Thank you!

Vin

 

Edited March 6, 2020 by vineyard
wrong file uploaded before!

[alacant]

Hi. Sorry, my fault. We also need your light frame stack.

[vineyard]

So I iterated a bit more and the rings are there in all cases (even stacking lights only...does that mean there's something wrong with the DSLR chip or the lens - its a Sigma EX macro 105mm lens f/2.8 although I was using it at infinity focus).

Light frame stack (you mean the .tif file?) also added to the dropbox.

Thanks again!

[alacant]

8 minutes ago, vineyard said:

you mean the .tif file?

Hi

Yes, but not the one you uploaded. That file has been stretched. We need the linear file direct from the stacking software.

Cheers

Edited March 6, 2020 by alacant

[vineyard]

Oops apologies.  The unstretched one uploaded as well ("Autosave.tif") - thank you!

[alacant]

Looks OK. I'd redo the flat frames. Next time out, remove any filters from the imaging train.

E.g. set a white screen on your 'phone and place your lens face down on top of it. Use AV to shoot a few frames.

HTH

[Aramcheck]

I'd have thought it was because you used JPEGs, which are already non-linear?

[vineyard]

Thanks alacant - I really appreciate that (how'd you do that magic?).  Yes, will do that for the flats (from memory I'd just used B setting to take the flat).

And Aramcheck I hope you're right too - I hadn't used the DSLR in so long (smartphones!) that I forgot it was in JPEG mode until afterwards...

Cheers & clear skies!

Vin

[happy-kat]

You should turn off hot pixel detection in the cosmetic tab in DSS, that is causing your star centres I think. (though it might just be chromatic aberration in your posted image)

Edited March 6, 2020 by happy-kat

[happy-kat]

Before using AV make sure that you are in manual focus otherwise the camera focus will change. Personally I also use back button focus where available as it stops the shutter button altering focus.

[vineyard]

Hello,

So I tried with new set of flats (manual focus to infinity, same f-stop, AV mode) and unfortunately no joy.  I did manage to remove them during post-processing (see below), but unfortunately lost some detail in the orion wide-field shot (you could see a hint of the flame but that's gone).

Alacant I think you hit the nail on the head re the filters - my first thought was that I did not place any filters, but then I remembered I'm using an unmodded DSLR, and looking at my Pentax (istDS) specs, there is a fixed anti-aliasing filter on it.  So I'm guessing those are Newton's rings (this image of those looks v similar, and hence also being present in a stack of only the lights?)?

How to remove them though is my next topic of research.  I think an istDS can be modded but I'm not sure I'd trust myself (would anyone have any recommendations?).  Perhaps a tilt adjuster, but I think those have to be placed between the filter & the sensor since the rings are coming from the reflection between them?  Perhaps a different shaped dust shield on the end of the lens?

Ah well.

Cheers,

Vin

[happy-kat]

Could we see one of your new flats please? The iso would also need to be the same as the lights. And if you put a lens cap on that's darks flats I find them great for noise reduction.

Edited March 7, 2020 by happy-kat

[alacant]

41 minutes ago, vineyard said:

anti-aliasing filter

Hi

Not sure, whilst I think the filter you mention has something to do with both long wavelength red and ringing effects, I think its presence would be to minimise the effects rather than enhance them. It certainly looks however like the effect obtained when glass surfaces interact.

Could you take a flat frame with the same lens on a newer camera body?

Edited March 7, 2020 by alacant

[vineyard]

12 minutes ago, alacant said:

 

Could you take a flat frame with the same lens on a newer camera body?

Good idea - will try that with another DSLR (different dust bunnies I guess but let's see if the rings reappear).  Cheers

[happy-kat]

Ooo had a thought, I find if I use a phone as the light source for the flats I have to place the phone on the lens, if I place the camera on the phone it creates weird effects on the screen which might be an issue.

[vineyard]

Hello,

So I think it was the fact that I foolishly forgot to switch to RAW mode.  I think those rings may have been some weird compression artefact b/c I tried again (same camera, same lens) earlier this week - this time in RAW.  And no rings!  (The flats could be better in this image, and the old Pentax either has a lot of chip noise or there was a massive gradient because of light pollution (I didn't use a LP filter) so I've cropped it a bit.

Progress of some kind I guess (at least the stars are not quarantined!).