Would someone mind please checking my flats?

[BrendanC]

Hi all,

I'm not even remotely sure I'm doing the right thing with my flats.

I've tried two methods: one with the laptop white screen against the scope, using a Flats plan in APT; and one with the same method, but using a white t-shirt between the laptop and scope.

However, I have zero idea whether either method has 'worked', or how I would even check this.

So, would anyone who even slightly remotely has an inkling of what they're doing, please take a look at an example of each one and let me know what they think?

They're here: https://1drv.ms/u/s!AqovBuVZMwj3iZl0sdW ... A?e=uN3cAA - one RAW sub taken directly from the screen (.CR2 format, from a Canon EOS1000D), and one using the t-shirt method, and the master TIF for each method, created in DSS as a result of stacking 50 of each.. The t-shirt versions seem quite dark to me, but I know that the actual colour of a flat isn't that important. Still, it would be good to know whether one, or both, or neither, are any good!

I used the AV exposure setting on the camera because that's what APT uses (and I've seen it recommended elsewhere too, not least by AstroBackyard). My understanding was that this means the correct exposure time is automatically calculated without needing to manually check the histogram.

The exposure time was 1/500s for each.

I've only just started taking flats. The past few times I've used them, they've just produced weird results in StarTools. So, I think I must be doing something wrong, but I'm 'flying blind' because I don't know what a 'good' flat or master flat should look like!

Thanks!

[admin]

Hi Brendan,

Have a look here for some tips.

Looking at both masters, they appear really quite dark and underexposed.
You will want to have the peak of your histogram approach the middle.

As a result of the under-exposure, the frames seem quite noisy and show a fair bit of banding.
They do, however describe vignetting which is a good sign, while I can also make out some dust donuts.

Let us know how you go!

EDIT: the "screen" master looks bad actually, the horizontal tapering off at the bottom is not something you'd expect.

[BrendanC]

Hey Ivo, thank you so much!

I've been relying on the Canon's AV setting too much, I think. Time to start doing this manually with more attention paid to the histogram.

My reason for using AV mode was actually based on the advice on that AstroBackyard page you recommend: "Luckily for DSLR astrophotography shooters, there is a mode on the camera that is ideal for shooting flats. This mode is called “AV“, or Aperture Priority mode. This means that the camera will decide on the correct shutter speed to properly expose the image."

Not for me, it would seem!

Again, thanks a lot for looking at this for me.

[admin]

Hey Ivo, thank you so much!

I've been relying on the Canon's AV setting too much, I think. Time to start doing this manually with more attention paid to the histogram.

My reason for using AV mode was actually based on the advice on that AstroBackyard page you recommend: "Luckily for DSLR astrophotography shooters, there is a mode on the camera that is ideal for shooting flats. This mode is called “AV“, or Aperture Priority mode. This means that the camera will decide on the correct shutter speed to properly expose the image."

Not for me, it would seem!

Again, thanks a lot for looking at this for me.

No problem!
Once you get your head around what flats are meant to do, you'll get a better idea as well as to what looks (roughly) correct.
If you shoot without flats and always have the upper left corner looking darker than the rest of the image, then you will want to see the same thing in your flats.
And vice-versa; if you see something in your flat you will want to be able to see the same sort of behaviour in your uncalibrated lights.

If there is a clear discrepancy between your lights and your flats, then something might be wrong.

[BrendanC]

Great advice, thank you again Ivo.