Taking flat frames the morning after

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jazzin United States of America
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Taking flat frames the morning after

#1

Post by jazzin »


HI all,

The automation worked last night! I set SGP to take 40 - 6 min subs and went to bed. Woke up this morning and saw that it completed the sequence. However, because I went to bed, I wasn't able to shoot any flats last night and was going to just use the darks I took last weekend. So my thinking is I will shoot the flat frames this morning once the dew dries off my scope. How do you handle this when setting up a long sequence? Do you take the flat frames and dark frames the next day?

I know this is not as good as staying up all night and then just shooting them right after the lights but I need my sleep.
Telescopes: Celestron 6" Newtonian, Orion 8" Astrograph

Guide scope: 50mm guidescope

Camera: modded Canon T7i, stock Canon T1i

Software: PixInsight

I hate cold and rainy weather!
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bobharmony
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Re: Taking flat frames the morning after

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Post by bobharmony »


Flats are the tricky part of the equation. They need the optical train set up exactly the same way it was when the lights were collected. This includes any dust or other debris on the lenses and sensor not moving around. The way I do usually it is to the flats at the end of the session, but if you don't disturb the camera/scope connections it can be done later. I will normally not remove the camera from the scope until after I have processed the data to make sure the flats were effective, and that works pretty well for me. I have taken flats as much as a week after the imaging session with good success.

Darks are much more forgiving and there is no need to shoot them right at the end of a session. It is possible to build a dark library of various temperature darks taken at the same ISO and exposure time(s) you usually use for light, and keep them around for months at a time. When you calibrate the image just use darks that are same ISO and exposure time, and close to the temperature the lights were taken at.

Bob
Hardware: Celestron C6-N w/ Advanced GTmount, Baader MK iii CC, Orion ST-80, Canon 60D (unmodded), Nikon D5300 (modded), Orion SSAG
Software: BYE, APT, PHD2, DSS, PhotoShop CC 2020, StarTools, Cartes du Ciel, AstroTortilla

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jazzin United States of America
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Re: Taking flat frames the morning after

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Post by jazzin »


The way I did it for this session is I left the camera in the focuser set the same way as when I took my light frames. Then this morning I brought my scope inside and strapped a white sheet over the objective end and then shot 35 frames with my histogram showing halfway across the graph.
Telescopes: Celestron 6" Newtonian, Orion 8" Astrograph

Guide scope: 50mm guidescope

Camera: modded Canon T7i, stock Canon T1i

Software: PixInsight

I hate cold and rainy weather!
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Gordon United States of America
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Re: Taking flat frames the morning after

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Post by Gordon »


jazzin wrote: Sat May 15, 2021 3:39 pm The way I did it for this session is I left the camera in the focuser set the same way as when I took my light frames. Then this morning I brought my scope inside and strapped a white sheet over the objective end and then shot 35 frames with my histogram showing halfway across the graph.
That should get you pretty close. I use a cheap illuminated flat panel that is used for tracing, I put a couple extra sheets of paper just to help filter it down a bit. You can adjust the brightness of the panel. As long as I can keep the exposure times to more than one second it works great!!
Gordon
Scopes: Explore Scientific ED80CF, Skywatcher 200 Quattro Imaging Newt, SeeStar S50 for EAA.
Mounts: Orion Atlas EQ-g mount & Skywatcher EQ5 Pro.
ZWO mini guider.
Image cameras: ZWO ASI1600 MM Cool, ZWO ASI533mc-Pro, ZWO ASI174mm-C (for use with my Quark chromosphere), ZWO ASI120MC
Filters: LRGB, Ha 7nm, O-III 7nm, S-II 7nm
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Graeme1858 Great Britain
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Re: Taking flat frames the morning after

#5

Post by Graeme1858 »


I use NINA for imaging and it has a very good flat frame assistant. So I do them last thing at night after capturing the light frames. Doesn't take long. The flat frames are made using a 1m length of 12V LEDs in a cardboard and duct tape box diffused using a sheet of A3 paper!

Regards

Graeme
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Celestron 9.25 f10 SCT, f6.3FR, CGX mount.
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Re: Taking flat frames the morning after

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Post by bobharmony »


jazzin wrote: Sat May 15, 2021 3:39 pm The way I did it for this session is I left the camera in the focuser set the same way as when I took my light frames. Then this morning I brought my scope inside and strapped a white sheet over the objective end and then shot 35 frames with my histogram showing halfway across the graph.

That should work well. The only question I have is whether you are using the Canon back-of-camera histogram or something else. My experience with the 60D is that the in-camera histogram reports on a stretched image and not what is in the actual RAW data. This leads to the flats being somewhat underexposed. You may want to get the "hump" 3/4 of the way to the right side to get a proper exposure.

My other experience with the Canon is that it isn't that fussy about needing exactly exposed flats. Some other cameras are less forgiving, but the Canon isn't as affected by it. Can't wait to see what you've got!

Bob
Hardware: Celestron C6-N w/ Advanced GTmount, Baader MK iii CC, Orion ST-80, Canon 60D (unmodded), Nikon D5300 (modded), Orion SSAG
Software: BYE, APT, PHD2, DSS, PhotoShop CC 2020, StarTools, Cartes du Ciel, AstroTortilla

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Re: Taking flat frames the morning after

#7

Post by jazzin »


bobharmony wrote: Sun May 16, 2021 2:35 am
jazzin wrote: Sat May 15, 2021 3:39 pm The way I did it for this session is I left the camera in the focuser set the same way as when I took my light frames. Then this morning I brought my scope inside and strapped a white sheet over the objective end and then shot 35 frames with my histogram showing halfway across the graph.

That should work well. The only question I have is whether you are using the Canon back-of-camera histogram or something else. My experience with the 60D is that the in-camera histogram reports on a stretched image and not what is in the actual RAW data. This leads to the flats being somewhat underexposed. You may want to get the "hump" 3/4 of the way to the right side to get a proper exposure.

My other experience with the Canon is that it isn't that fussy about needing exactly exposed flats. Some other cameras are less forgiving, but the Canon isn't as affected by it. Can't wait to see what you've got!

Bob
Yes that is what I am using. I've always been not sure if I was doing them the right way but just doing them the way I read on here and other forums. Will have to take your suggestion into account next time.

Thanks
Telescopes: Celestron 6" Newtonian, Orion 8" Astrograph

Guide scope: 50mm guidescope

Camera: modded Canon T7i, stock Canon T1i

Software: PixInsight

I hate cold and rainy weather!
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