Let's start at the very begining... (well the processing part anyway)

[Fay]

a lot of people say use AV setting for correct flat exposure. What do you think of that method?
I always find it hard to get the correct exposure for DSLR flats

[MarkS]

a lot of people say use AV setting for correct flat exposure. What do you think of that method?
I always find it hard to get the correct exposure for DSLR flats

I think the AV setting is the worst way of doing it.  On my Canon 350D and 400D this method gives a severely underexposed flat.  You also run the risk that the camera may change the shutter speed slightly from frame to frame during your sequence of flat acquisitions which will result in flats with different exposures.

My advice for taking flats is:
1)  Always use manual shutter speed
2)  Adjust shutter speed until it just begins to saturate part of the image (according to the "highlight" display) and then double the shutter speed (i.e. halve the exposure time) to produce your flats. Use the same shutter speed for the whole sequence of flats.  

This will give you well-exposed but unsaturated flats.

Mark

P.S.  If you want to be really an*l about this (and I often am) then take one set of flats to correctly expose the red pixels then another to correctly expose the green pixels and then another set to correctly expose the blue pixels.  Then using my favorite processing software (IRIS!!!) these raw images can be combined into one "super-flat" where the red, green and blue pixels are all correctly exposed.

Otherwise, typically the flat will contain green pixels that are correctly exposed and red/blue pixels that are underexposed.  This results in greater noise in your red/blue channels in your final image.  I'm giving away my trade secrets here!

[Ivor]

As vignetting isn't really a problem with my scope I'd not considered it, I've thought some more I've nearly got the equation bit wrapped up, so to recap if were to map the flat to a 3 x 3 grid it would look something like this

0.2	0.5	0.9
0.2	1.0	1.1
0.1	0.7	1.0

BTW How do you get the boarders on a table to appear?

In a 3D graph with a larger pixel map the stronger the vignetting the more the graph would look like a bell shape with a few dips for bunnies. This is scaled back to 0 - 1 range by the constant.

Now I've hopefully got that straight in my head I need to go back and understand the rest of this thread  

[Fay]

Have we got two Einstiens in collaboration here?

[Carole]

I was thinking the same thing Fay 

[Rick]

BTW How do you get the boarders on a table to appear?

You sit back and wait for the pirates!    Sooner or later there'll be a boarding party headed your way!  Probably with          and  for company. 

I can't see any options in the basic BBCode that might turn borders on or off. I'm afraid I just tend to fall back on the fixed format tags
Teletype
or

Code Select Expand

code for anything vaguely tabular...

[Ivor]

Going back to the practical acquisition of the flats

[I think the AV setting is the worst way of doing it.  On my Canon 350D and 400D this method gives a severely underexposed flat.  You also run the risk that the camera may change the shutter speed slightly from frame to frame during your sequence of flat acquisitions which will result in flats with different exposures.

My advice for taking flats is:
1)  Always use manual shutter speed
2)  Adjust shutter speed until it just begins to saturate part of the image (according to the "highlight" display) and then double the shutter speed (i.e. halve the exposure time) to produce your flats. Use the same shutter speed for the whole sequence of flats. 

This will give you well-exposed but unsaturated flats.

Mark

P.S.  If you want to be really an*l about this (and I often am) then take one set of flats to correctly expose the red pixels then another to correctly expose the green pixels and then another set to correctly expose the blue pixels.  Then using my favorite processing software (IRIS!!!) these raw images can be combined into one "super-flat" where the red, green and blue pixels are all correctly exposed.

Mark if you're saying the histogram on the camera is inaccurate what application are you using to capture the flats and review the histogram? I've tried doing this in nebulosity and it won't let me reduce the exposure time sufficiently.

Also now you're started you can't leave it there, how are you isolating the RGB channels when creating the flats?

It strikes me that approach above where effective and probably the best, like the others required a skilled eye unlike the other frames required. Surely there must be a more scientific approach to acquiring the flats especially if we consider the large scopes in observatories they clearly can't be using the approaches we are discussing.

[MarkS]

Mark if you're saying the histogram on the camera is inaccurate

I'm not saying it is inaccurate.  The histogram is definitely accurate - you just need to know how to interpret it.  I think most people assume the histogram axis on the camera is linear but it is not.

what application are you using to capture the flats

I don't use an application to capture them but I do review them later in IRIS

how are you isolating the RGB channels when creating the flats

IRIS - the split_cfa and merge_cfa commands - these act on a raw frame and will isolate the R, G1, G2, B channels and then recreate a raw frame from those channels.  However, I wouldn't recommend going to this level of sophistication when you're starting out.  There are far more important things to get right first.

if we consider the large scopes in observatories they clearly can't be using the approaches we are discussing.

They don't use DSLRs!!

[Mac]

They don't use DSLRs!!

[Ivor]

Clearly the don't use DSLRs, but wouldn't you need Flats for all CCDs as illumination noise could exist on any visual scope?

[RobertM]

Ivor,

Your term 'illumination noise' is misleading and incorrect - they are called Flat field frames.  Flat field frames are needed to characterise the illumination of a sensor by the optical system.  It is true they do contain all sorts of noise components but those are minimised by taking many sub exposures together with Flat Darks if you think they are necessary.  Ideally they should contain no noise.

Professional observatories will take Flat Field frames through each filter either at twilight or during the day as they can't afford to waste valuable observing time !

Robert