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Old 04-01-2009, 05:29 PM
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Quark (Trevor)
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Silly questions regarding flats?

Hi All,
Have never taken any flat frames before. I have "Introduction to Digital Astrophotography" by Robert Reeves, which talks about the need for flats and what they do.

I intend illuminating a projection screen in front of my scope, at least initially, to take my flats.

My question is how do I work out the length of exposure for my flats. I have seen write ups suggesting 2 to 10 secs, but what determines the time, how will I know that I have selected an appropriate exposure?

Also, do I need to take darks the same length of exposure as the flats to create a master flat?

Regards
Trevor
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Old 04-01-2009, 05:52 PM
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JohnG (John)
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Hi Trevor

I am going to asume that you have a DSLR....

The screen you have should be OK providing it is evenly illuminated. You must leave your camera in the same orientation and focus point as when you took your Light Frame subs.

You will need to experiment a little to get the right exposure, you check that by activating the INFO button on the camera and checking that the Histogram is between 1/3rd and 1/2 the distance to the right, just take a few JPEG's till you get it right, then take your RAW's, if you have a Canon camera, just look in Digital Photo Professional.

Cheers
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Old 04-01-2009, 06:22 PM
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Quark (Trevor)
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Hi Trevor

I am going to asume that you have a DSLR....

The screen you have should be OK providing it is evenly illuminated. You must leave your camera in the same orientation and focus point as when you took your Light Frame subs.

You will need to experiment a little to get the right exposure, you check that by activating the INFO button on the camera and checking that the Histogram is between 1/3rd and 1/2 the distance to the right, just take a few JPEG's till you get it right, then take your RAW's, if you have a Canon camera, just look in Digital Photo Professional.

Cheers
Thanks John,
I have a Canon 450D.

I realize that I will need flats for the lights taken each night, but after I establish what the appropriate exposure is will that always be the correct exposure for the set up used?

Regards
Trevor
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Old 04-01-2009, 06:29 PM
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JohnG (John)
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Should be close, I always check the Histogram, as I said, just take a couple of JPEG's to check it is over to the right by about 1/3rd then take your RAW's.

I also re-read your question, your Darks are always the same length, ISO and temp as your Lights, I think maybe you are refering to Bias shots, they are always taken at the shortest possible exposure with the lens cap on, they are not temp dependant but must be the same ISO as the remainder of your shots.

Not sure what program you use for combining, aligning etc, I use ImagesPlus 3.50 and everything is automatic.

Cheers
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Old 04-01-2009, 07:30 PM
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Quark (Trevor)
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Originally Posted by JohnG View Post
Should be close, I always check the Histogram, as I said, just take a couple of JPEG's to check it is over to the right by about 1/3rd then take your RAW's.

I also re-read your question, your Darks are always the same length, ISO and temp as your Lights, I think maybe you are refering to Bias shots, they are always taken at the shortest possible exposure with the lens cap on, they are not temp dependant but must be the same ISO as the remainder of your shots.

Not sure what program you use for combining, aligning etc, I use ImagesPlus 3.50 and everything is automatic.

Cheers
Thanks very much for the info John, I really do appreciate it.

I also have IP 3.50 but only got it recently and have not used it. I have been working through the instructional video's that came with it.

I have been using IP 3.60 camera control to take star trails with my 450D.
I intend to now try some wide field piggy back images with the 450D on my scope and then prime focus through my scope, steep learning curve.

All of my deep sky work in the past was on 35mm film.

Regards
Trevor
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Old 04-01-2009, 08:25 PM
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kinetic (Steve)
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Hi Trev,

Here is an old bookmark I have which gives a good, overall
theory of Darks , Bias and Flats , with some examples of why
you need to do them. (examples of Dust bunnies, vignetting etc)

http://www.aavso.org/observing/progr...manual/3.shtml


Hope this helps,

Regards,

Steve
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Old 05-01-2009, 10:01 AM
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Hi Trev,

Here is an old bookmark I have which gives a good, overall
theory of Darks , Bias and Flats , with some examples of why
you need to do them. (examples of Dust bunnies, vignetting etc)

http://www.aavso.org/observing/progr...manual/3.shtml


Hope this helps,

Regards,

Steve
Thanks Steve,

A very informative and graphic link, a picture really is worth a thousand words.

Regards
Trevor
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