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Old 09-02-2016, 03:58 PM
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Dealy (Kev)
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Canon DSLR flats?

Anyone know how to measure the correct exposure time for flats with a Canon 1000d using a light panel?

Can it be determined simply by looking at the histogram, or is it more complex than that?
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Old 09-02-2016, 04:18 PM
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rustigsmed (Russell)
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Switch the mode to "Av" and start snapping away. try a variety of ISOs.
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Old 09-02-2016, 04:51 PM
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RB (Andrew)
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Put the DSLR in P (Program Mode).
Set ISO to 100.
Take an exposure and check the histogram is 1/3 rd of the way along from the left.
Take a lot and an odd number of flats.
Also I move the camera/scope slightly around the Flats Screen/Light Panel every few shots to avoid any hot spots.

Can't miss.
RB

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Old 09-02-2016, 05:13 PM
glend (Glen)
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I do as RB does. Build a library so you don't have to do them often.
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Old 09-02-2016, 05:22 PM
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RB (Andrew)
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Quote:
Originally Posted by glend View Post
I do as RB does. Build a library so you don't have to do them often.
For 'flats', I do each session, because scope/camera orientation is different each time.
Darks can be done as a library.

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Old 09-02-2016, 05:42 PM
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Quote:
Originally Posted by RB View Post
Put the DSLR in P (Program Mode).
Set ISO to 100.
Take an exposure and check the histogram is 1/3 rd of the way along from the left.
Take a lot and an odd number of flats.
Also I move the camera/scope slightly around the Flats Screen/Light Panel every few shots to avoid any hot spots.

Can't miss.
RB

Mind if I ask - Is there a benefit to doing this over Av mode? Why do you need an odd number of flats?
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Old 09-02-2016, 06:43 PM
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RB (Andrew)
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Quote:
Originally Posted by troypiggo View Post
Mind if I ask - Is there a benefit to doing this over Av mode? Why do you need an odd number of flats?
Hi Troy,

It's been years since I've done any astro photography, I'm pushing the old brain to remember, it's been discussed many times on here though I'm sure.

I use P mode and stick to 100 ISO (giving lower noise, better signal) because it initially gives me a histogram close to what I'm after, 1/3 to 1/2 way across.
If it's not close to that I can switch to manual and tweak the setting I got from the P settings to get the ideal histogram.

I use an odd number because I 'median combine' the flats and the 'math algorithm' used in 'median combine' works better with an odd number of frames, from memory.

RB
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Old 09-02-2016, 07:05 PM
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Cheers. Thanks mate.
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Old 09-02-2016, 10:08 PM
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Thanks. The next question is of course how many?

I read 2 articles today - one said to take at least 50 each of dark, flat & bias shots, the more the better, and that the number of calibration shots is not related to the number of lights.

The other said to take 3-4 times as many calibration shots as the number of lights.

I've also read in posts here on this forum that some people take an equal number of darks/flats to lights.

Is there a right answer to the "how many" question?

I've never taken flats before, and have only used approx the same number darks as lights and thrown in a dozen or so bias shots. I want to start doing things the right way but there seems to be many opinions out there.

Kev
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Old 09-02-2016, 10:32 PM
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RB (Andrew)
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Forget bias frames for DSLRs, the bias/offset is included in your Darks.

Do 35-55 flats.
And do flat darks for your flats too.

Do as many darks as you do lights 'if possible', and at same/close to ambiant temp of your lights.


RB
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Old 09-02-2016, 11:08 PM
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Thanks Andrew
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Old 10-02-2016, 07:02 AM
Chris.B (Christopher)
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Sounds like I have a lot to learn and a lot to do.
I'm just beginning my astro-photographic journey.
Only dabbled with single frame shots till now.
Will keep reading and learning :-)
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