Flats from near full disk?

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MAURITS
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Flats from near full disk?

Post by MAURITS » Mon Jul 01, 2019 2:11 pm

How do you take flats from a near full disk?

When I do this with my 150mm scope with the Quark / IMX174 combination, then I go to the surface of the sun where I can't see the edge (defocus), I take a flat and load it in AutoStakkert.
So no big deal ... so far all good.

Now I have a little 60mm scope and with the same Quark / IMX174 combination I have near full disk (except a part above and a part below).
So I always see the near full disk AND the edge when I take a (defocused) flat with the little scope.

When I go to AutoStakkert, than I see strange things outside the disk if I load the masterflat.
How can I solve this problem?


This is my FLAT.
Flat .jpg
Flat .jpg (95.19 KiB) Viewed 396 times

Next is a "unprocessed" image from my 60mm scope (and the above combination) done in AutoStakkert WITHOUT flatframe.
Sun_111108_g3_ap5230.jpg
Sun_111108_g3_ap5230.jpg (92.26 KiB) Viewed 405 times


This is a result from AutoStakkert WITH flatframe.
With flat.jpg
With flat.jpg (91.3 KiB) Viewed 405 times
All the black color outside the sun is gone ...???

What happend on the edge? :?
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Maurits

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Re: Flats from near full disk?

Post by marktownley » Mon Jul 01, 2019 7:30 pm

The unfocused flat is bigger on the chip than the focused image, so when the flat is applied its affects go beyond the limb of the sun.
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Re: Flats from near full disk?

Post by MAURITS » Mon Jul 01, 2019 8:45 pm

Ah so Mark, how can I fix such a problem?
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Re: Flats from near full disk?

Post by marktownley » Tue Jul 02, 2019 4:41 am

I'm not sure you can effectively, the full disk flat is a holy grail for solar!
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Re: Flats from near full disk?

Post by PDB » Tue Jul 02, 2019 6:50 am

The holy grail indeed ...
I am surprised it even worked good, except for the solar edges. There is also some limb darkening effect that will "ruin" the flats (but maybe it helps to remove the gradient towards the limb?)

Now I would try either one of the following:
- Take the flats with the plastic bag in front without defocusing (probably will not really work)
- in PS or Gimp or whatever try to reduce the flats size by a small amount so the disc size is the same as on the sun image. This might be a tricky job, because:
1. the flats total size should remain the same to be applied in AS!3 (so only disc should be smaller)
2. the image of the Sun on the video for stacking should not move (and in same place as on the flat) or the applied flat will again make it worse.

Looks a bit impossible, but you neve know ....

Regards,

Paul

PS: The gradients you can probably remove with an artificial flat, but for the dust motes only cleaning helps. But these always seem to come back. (Even seconds after cleaning)

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Re: Flats from near full disk?

Post by Montana » Tue Jul 02, 2019 9:11 am

It is impossible if you have black edges. I always recommend the drift method. Let the image slowly drift a bit in capture and ASK3 will reduce all the dust bunnies away.

Try to clean the camera as best as you can :)
Alexandra

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Re: Flats from near full disk?

Post by MAURITS » Tue Jul 02, 2019 6:22 pm

I try to take a FD capture with a cleaner sensor and without flat frames.
Thanks all for the info.
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Re: Flats from near full disk?

Post by marktownley » Tue Jul 02, 2019 7:20 pm

Give it a go Maurits but I don't think you will get rid of the uneven illumination due to variation in bandpass across that quarks FOV, the variation in bandpass will always give a variation in detail. Doubt you will stitch images together for this reason (to get a full disk). My advice if you're after a full disk is get one of the dedicated 40-60mm solar scopes.
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Re: Flats from near full disk?

Post by MAURITS » Tue Jul 02, 2019 7:45 pm

Well Mark, I have a very little Lunt LS 35THA scope, I give it a try with these if it will work with my ZWO 174MM camera.
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Re: Flats from near full disk?

Post by marktownley » Tue Jul 02, 2019 8:11 pm

I think you will get better results with a camera with smaller pixels, the IMX174 sensor is best suited up at around f25 in Ha.
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Re: Flats from near full disk?

Post by MAURITS » Wed Jul 03, 2019 4:59 am

Thanks Mark.
Regards,
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Re: Flats from near full disk?

Post by RodAstro » Wed Jul 03, 2019 8:37 pm

Hi Maurits
Hope you don't mind but I had a play with your image above as I am interested in the same thing, removing variations in brightness.
It sort of works but as Mark says you can't remove the bandpass variations.
What I did was copy your image, then I saved two copies.
One copy I used gaussian blur in PS6cs set at 9.8 to make a flat.
then loaded the suns image into AutoStakkert and then the new flat, then saved the image.
I then increased the new images size by about 2.5% in PS6cs and cut out the suns image with edge blur set at 10 and layed it over the original suns image.
I then just used RegiStax wavelets to bring out a little detail to see how the bandpass zones are.
Not perfect but some way towards getting a flatter image.
I had two or three goes trying different parameters, the hardest part is selecting the solar disk for cropping.

Just an idea cheers Rod
Attachments
flatern3smallwavecrop.tif
flatern3smallwavecrop.tif (11.61 MiB) Viewed 289 times

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Re: Flats from near full disk?

Post by RodAstro » Wed Jul 03, 2019 8:39 pm

Oops put the Tiff on sorry
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Re: Flats from near full disk?

Post by marktownley » Wed Jul 03, 2019 10:42 pm

Yup, I thunk that is the best you will get with that method Rod, good result.
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Re: Flats from near full disk?

Post by RodAstro » Thu Jul 04, 2019 4:56 pm

Hi I had another play with the image.
Shadows and highlights and added colour.
Not to bad, maybe a little over processed in places but does show some even detail.
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Mautris Colour flat.jpg
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Re: Flats from near full disk?

Post by Valery » Fri Jul 05, 2019 3:06 am

The only method which is correct and which will help is: place a white cloth or white piece of paper in front of the objective and record a flat field movie with about 60% histogram fill. One will need to increase the exposure and slightly increase the gain to fill the histogram. If it will be too noisy use more frames (about 500 frames movie) to stack for FF.
This FF movie must be recorded when a telescope was focused and pointed at the center of the sun.

Gamma setting must be the same for the focused images and for the FF.

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Re: Flats from near full disk?

Post by MAURITS » Sun Jul 07, 2019 6:36 pm

Yes Rod, this is a lot better than before.
With the added color even more better ...
This is a very nice explanation Rod.
A thank you for Mark, Paul, Alexandra, Rod and Valery.
Regards,
Maurits

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Re: Flats from near full disk?

Post by RodAstro » Mon Jul 08, 2019 5:13 pm

Hi Maurits
Yep the second image shows the info is all there and easy to bring out once it has been flattened.
I will be up for trying Valery's method when I get my full disk setup it does sound good.
One tip I use for deep sky flats is a white drum skin over the objective pointed at the sky or a light source.
They come in all sizes, are cheap, easy to get and very durable.
With the lip they are also easy to put on and off without moving the scope when pointed at the sun.

Cheers Rod

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Re: Flats from near full disk?

Post by MAURITS » Tue Jul 09, 2019 1:26 pm

Thanks for the effort Rod.
Regards,
Maurits

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