Capturing Flats

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snubem
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Joined: Mon Jul 09, 2018 3:45 pm

Capturing Flats

Post by snubem » Sat Jul 14, 2018 11:09 pm

Hi Robin.
I need help with SharpCap FLATS. I checked out the forum but a lot of the stuff is over my head. I’m sorry for the basic questions.

I ironed a clean white T-shirt, put over the scope, smoothed out and rubber banded it.
Place the light box on top of the shirt. Capture Flat Frames. Uncheck Create Monochrome. Select 20 Flat frames.
I lower the Gain and adjust EXP so BLUE histogram spike is at 50% and start FLAT capture.
Should the Blue spike be at 50% on the histogram or another color spike at 50%?
What is the correct # of FLAT frames to take? I take about 75 light frames at 5 secs/frame for DSO's.
Should I Save as TIFF or PNG?
I read that ZWO imager Flats should be a light green color. Mine were blue.
I changed the EXP to make the screen GREEN. This moved the histogram spikes to the far right though.
Is there a correct color the FLATS should be? Or should I only care about where the histogram spikes are?
How do I check the FLATS to see imperfections on the imager? Is this how you determine if your FLATS are good?

Thank you,
Greg

Celestron Evolution 8 on an Alt-Az mount using Star-Sense. ZWO 183MC, 6.3 Reducer, SharpCap 3.1
Dell laptop with 512 SS HD, 16G of Ram, USB 3.0
From the Old Town Observatory in the light polluted skies of downtown Chicago.
Amazon link below to lightbox.

https://www.amazon.com/gp/product/B018R ... UTF8&psc=1

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admin
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Re: Capturing Flats

Post by admin » Sun Jul 15, 2018 7:41 am

Hi Greg,

the goal when setting up a flat capture is mainly to get the histogram to be mostly in the right-hand half of the graph but ***without*** touching the right hand side. If you have to choose between the two then allow it to be somewhat in the left hand half of the graph but don't let it touch the right hand side. Here's an example of what I'd think of as a decent histogram for capturing a flat:
Capture.JPG
Capture.JPG (78.52 KiB) Viewed 163 times
The colour of the image depends on the sensitivity of your camera *and* the type of light you are using to illuminate, so don't get too worried about the colour being green - perhaps your light box gives off quite a blue light. You could just as easily get an orange/red hue if you took a flat under old-fashioned tungsten incandescant lights. I would suggest not playing with white balance or gamma controls to try to adjust the hue.

It's probably a good idea to check the 'Create a monochrome Flat Frame' option when creating your flats - this will take the colour information out of the saved flat, meaning that applying the flat will *not* adjust the white balance of the camera image - it will leave the white balance as the camera sees it to be adjusted finally in post-processing.

hope this helps

Robin

PS I separated your post into a new thread on creating flat frames.

snubem
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Joined: Mon Jul 09, 2018 3:45 pm

Re: Capturing Flats

Post by snubem » Sun Jul 15, 2018 2:29 pm

Thank you Robin! That helped a great deal.
How many Flats should I take if I'm acquiring 75, 5sec/frame lights of a DSO? Is there an equation for the # of Flats to take?
I Focus, set Gain and EXP on a DSO then put the cap on and take Darks. Then I apply the Darks prior to acquisition without changing parameters.
Is it best to take FLATS at this time too and apply so the Focus is the same, or does Focus on Flats not matter?
My post processing skills are still beginner level, so I'd like the Darks and Flats applied to imaging on the front end if possible.

What's the difference between a Flat and a Master Flat? Doesn't the dust change from session to session and a new Flat is needed?

Thank you,
Greg

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Re: Capturing Flats

Post by admin » Sun Jul 15, 2018 3:17 pm

Hi,

a flat is a single frame that is captured with flat field illumination. A 'master' flat is an average of many such frames. SharpCap will automatically create a master flat for you by capturing the number of frames you specify and averaging them.

The number to use depends on the gain you are using - if the individual frames are noisy due to high gain then capture lots (20-40). If the individual frames have little noise then 10-20 would probably suffice. For messing around and testing 5 would be fine.

You typically need to create a new flat if you change anything in the optical path - ie

* adjust focus
* rotate the camera
* remove/replace the camera
* add/change filter
* etc

You can possibly get away with ignoring flats changing when you adjust focus in most cases.

cheers,

Robin

snubem
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Re: Capturing Flats

Post by snubem » Sun Jul 15, 2018 4:21 pm

Well, all that makes a lot of sense!
Thank you.....Greg

desper0
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Re: Capturing Flats

Post by desper0 » Wed Jul 18, 2018 5:42 pm

Robin, can this be adapted to using a DSLR / Folder monitor in the 3.2 Beta? Or should I create Flats outside Sharpcap?

Thanks.

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Re: Capturing Flats

Post by admin » Wed Jul 18, 2018 8:57 pm

Hi,

it's untested with the folder monitor camera in 3.2. If you leave the option to auto-subract a dark/bias then it will likely fail as it will want to control the camera exposure and can't. If you pre-set-up your dark subtraction then you may be OK.

cheers,

Robin

snubem
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Re: Capturing Flats

Post by snubem » Tue Jul 31, 2018 1:40 pm

Robin,
Your advice on Darks and Flats have increase the quality of my images tremendously!
f/u question:
I image DSO’s at 250 Gain.
Is it best for Flats to have the same Gain as my lights?
When making Flats, I’m keeping the same Gain and lowering the EXP to get the histogram close to the right side.
But I could also lower the Gain and increase the EXP time on Flats to get the histogram to the same spot.
Which way makes a better Flat?
Thanks,
Greg

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Re: Capturing Flats

Post by admin » Tue Jul 31, 2018 4:17 pm

Hi Greg,

good to hear that the suggestions worked :)

For a flat you want as little noise as possible, so the best bet is to reduce the gain to minimum and then adjust the exposure to get the correct histogram shape. That way with lower gain you should get lower noise images.

cheers,

Robin

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