Bad flats are ruining my hard work

[Legion Of Andromeda]

ok, my setup; Celestron 8"edgehd with no collimating issues, with focal reducer, unmodded Canon 6D with the correct back-focus, AVX mount, APT imaging suite, PHD guiding with minimono zwo and a half-decent laptop.

Post processing is DSS using all the recommended settings..(i might have over-looked some more obsure settings?) then out to Photoshop CC 2015 following astrobackyard tutorials.

Current object is Andromeda, focus is spot-on, guiding is spot-on. i recently got 200 subs - 30 seconds at 800 iso, 60 darks frames gathered at the same time plus 60 fresh bias/offset frames.

However, my Flats are causing me a right load of grief. I used the white t-shirt method pointing up to a bright sunny sky (not at the sun) and got 50 of each ISO (800,1600, 3200), a set with the focal reducer and a set without, i made sure that the histogram was in the middle and adjusted the shutter speed accordingly...but what i don't understand is when it is suggested that one puts the camera into AV mode to shoot flats... AV mode is unavailable when connected to a scope via the T-Ring, so i can only use Manual mode ...at least with that camera. i might be wrong on this...

I'm expecting that the flats will cancel-out the vignetting as long as the same camera/scope orientation and focus is used, so when the flats and lights are are good match that the vignetting is compensated. I don't expect it to be perfect but what i end up with are varying degrees of noticeable concentric rings, often colourful and sometimes saturated... so i'm currently trying to get away with not using my flats.

so, do i need to change the way i create my flats... maybe in mono?... move the histogram to the right or left?... use more - or less?,... do they need 'calibrating' - whatever than means...?..... should i ditch DSS?

I've had a good poke around the forums and found plenty on this subject but no conclusive guidance on best practice.

Thank in advance for any pointers.

i might upload some images later

[wimvb]

Flats contain a bias signal as well as dark current. The dark current is generally negligible, but the bias is not. Before you combine the flat frames into a master flat, you need to subtract the bias from them. When you calibrate the light frames, you also first need to subtract the bias as well as the dark signal. The stacking software takes care of this, as long as you provide the calibration files/masters.

[Legion Of Andromeda]

11 hours ago, wimvb said:

Flats contain a bias signal as well as dark current. The dark current is generally negligible, but the bias is not. Before you combine the flat frames into a master flat, you need to subtract the bias from them. When you calibrate the light frames, you also first need to subtract the bias as well as the dark signal. The stacking software takes care of this, as long as you provide the calibration files/masters.

Many thanks but i'm not sure what you mean by this, ive not been doing AP for very long so bear with me. how do i carry out the above procedures in DSS... literally step by step please.. or post a link to useful guide / video

[wimvb]

1 minute ago, Legion Of Andromeda said:

or post a link to useful guide / video

https://www.firstlightoptics.com/books/making-every-photon-count-steve-richards.html

http://deepskystacker.free.fr/english/faq.htm#lightdarkflatoffset

Also have a look here:

http://deepskystacker.free.fr/english/technical.htm#stackingmethods

Providing links to good information makes more sense than rewriting that information.

Also: get the book, it will be your best "bang for buck" in a long time.

If you have any more questions, or don't understand the contents of the links provided, don't hesitate to ask them.

[Legion Of Andromeda]

thats very kind of you thanks. i've just found an old laptop that i will use for my flats.. the t-shirt method is too variable

 

[Stub Mandrel]

AV mode works fine with a scope.

Have the histogram peak at about 25%.

I always rotate the scope and take flats at about eight different orientations to even out any unintended gradients. Variously I use a plain painted wall in diffuse light, a card in the garden or a light panel with a sheet of A4 paper over it.

For very long focal lengths the sky works better, but still rotate.

 

[Legion Of Andromeda]

9 hours ago, Stub Mandrel said:

AV mode works fine with a scope.

Have the histogram peak at about 25%.

I always rotate the scope and take flats at about eight different orientations to even out any unintended gradients. Variously I use a plain painted wall in diffuse light, a card in the garden or a light panel with a sheet of A4 paper over it.

For very long focal lengths the sky works better, but still rotate.

 

just to confirm that you are suggesting rotating the scope in relation to the light source and not the camera in relation to the scope?

[michael8554]

On ‎05‎/‎09‎/‎2019 at 22:43, Legion Of Andromeda said:

AV mode is unavailable when connected to a scope via the T-Ring, so i can only use Manual mode ...at least with that camera. i might be wrong on this...

I have a Canon 6D attached to a SCT with a T Ring.

I take Flats with the camera set on AV, no problem........

That's clicking on the manual shutter button in Canon EOS Utilities.

Michael

[Stub Mandrel]

28 minutes ago, Legion Of Andromeda said:

just to confirm that you are suggesting rotating the scope in relation to the light source and not the camera in relation to the scope?

Absolutely!