Canon DSLR flats?

[Dealy]

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?

[rustigsmed]

Switch the mode to "Av" and start snapping away. try a variety of ISOs.

[RB]

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

:)

[glend]

I do as RB does. Build a library so you don't have to do them often.

[RB]

For 'flats', I do each session, because scope/camera orientation is different each time.
Darks can be done as a library.

:)

[troypiggo]

Mind if I ask - Is there a benefit to doing this over Av mode? Why do you need an odd number of flats?

[RB]

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

[troypiggo]

Cheers. Thanks mate.

[Dealy]

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 :confused2:

[RB]

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

[Dealy]

Thanks Andrew :thumbsup:

[Chris.B]

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 :-)