Tried Taking Flats ...Am I seeing Vignetting or something else?

[Durrani84]

 

Hello Everyone:

I hope all is well.

My a newbie in Astrophotography and attempted to take a picture of the Dumbell Nebula. I took flats using a light box, stacked in DSS and started some processing on GIMP. I can see its darker on the sides. Is this a vignetting issue or something else (e.g gradient)? Any suggestions how I can tackle it and how I can improve my processing are deeply appreciated... thank you!

I live in Denmark in a Bortle class 4 zone. However, its summer so I have very little dark hours and even then its not totally dark. Also moon coverage was approx 60% at the time.

• Telescope: Orion 8 inch 3.9 FL
• CEM 40 Mount
• Canon 800D
• TS Optics GPU 1.0 Coma corrector
• ASIR AIR Pro for image capture

Image

• ISO 400
• Light: 180 Sec x 40
• Flat: 30 using a white box
• Dark: 30
• Bias: Unfortunately couldn't take it as battery died
• Stacking: DSS
• Processing: GIMP

 

Thanks everyone!

[The Lazy Astronomer]

Looks suspiciously like vignetting to me. Could you post up a copy of your master flat?

[Durrani84]

Attached is the master file @The Lazy Astronomer.

From your experience, does it look okay? I used an Light box ontop of my newt, which had a t shirt over the top.

Thanks for your help.

 

[The Lazy Astronomer]

1 hour ago, Durrani84 said:

Attached is the master file @The Lazy Astronomer.

From your experience, does it look okay? I used an Light box ontop of my newt, which had a t shirt over the top.

Thanks for your help.

 

The same vignetting is clearly visible in both the flat and in your earlier image, so the flats are significantly under correcting - it's almost like they weren't applied at all.

What settings did you use for shooting the flats?

What were your DSS settings?

Have you tried stacking without darks? (darks not usually recommended for DSLRs anyway)

[Durrani84]

@TheLazyAstronomer I used the recommended DSS settings but I believe my flats might have been on the bright side, Im using an ASI Air Pro and the histrogram was quite wide and around the half way mark.

So ideally, we dont want to see any vingetting in the flats, correct?

 

Thanks again!

[The Lazy Astronomer]

The purpose of flats is to correct uneven field illumination in the light frames. If you have vignetting in your lights (which you do), then you should see the same vignetting in the flats (which there is). The calibration process should use the flats to, er, flatten, the image.

However, your stacked image still shows vignetting, which shows your flats have not flattened the image. So something is not right. 

That's why l was asking about your settings, to get a bit more information as to why they're not working. While I think of it, how did you take the flats? I think with DSLRs you put them in AV mode and let the camera figure out the exposure time*. Is that what you did? 

*I have no experience using DSLRs for astrophotography, but I think that is right.

[Durrani84]

Thanks @The Lazy Astronomer, I get your point.. so is it expected to see vignetting in the flat image when I take then? If you have an example of a good flat?would you mind sharing?

I tried taking flats using a light box but since it’s connected to an ASI Air pro, the camera needs to be on Manual mode, hence I can’t use the AV method. So, I try taking exposures with different exposure lengths so the histogram more or less is in the middle.. I think I didn’t try hard enough

Thank you!

 

 

 

[alacant]

7 minutes ago, Durrani84 said:

can’t use the AV method.

With your light panel illuminated in a dark room, set the camera to Av, partly depress the shutter release and note the time it gives.

Set the dial back to M and dial that same exposure time in your asi app. Fire off, say 12 frames.

Or simply record to the camera's SD card without connecting to the app.

That's it.

HTH

[Durrani84]

Thanks @alacant! I’ll give it a go indoors to try before running a tea test (if the clouds ever go)

 

[alacant]

15 hours ago, Durrani84 said:

I’ll give it a go

Mild background issues such as these are easily corrected in modern software. Don't overthink it too much:)

Cheers

Edited July 3, 2021 by alacant