Taking Flats?

Matty P (Matt) · #1 01-07-2008, 08:52 PM

I am now at the stage where I need to start taking flats to improve my images. The only problem is that I am not quite sure how to acquire them. I’ve read that there are a couple of different techniques on capturing flats but I am not sure which is the best to use. I know some people use light boxes or capture them in twilight.

What is the best technique to capture a flat?

TrevorW · #2 01-07-2008, 08:59 PM

As far as I'm aware the general concensus (from a lot of reading) is to go with a light box.

IMOFWIW

Zuts · #3 01-07-2008, 09:18 PM

Hi,

I do the following,

On a bright cloudless day i point my scope at the blue sky away from the sun. I stretch a grey or white bonds t-shirt across the end of the scope and take an exposure that gives a histogram spike about one third of the way across the range. Usually much less than 1/3 second is required, depending on the brightness of the sky and the thickness of the T-shirt.

Note, This will only work if you have left your scope focussed from the previous night with the camera and anything else (reducer etc) attached and in the same orientation. The orientation is easy. I just have a few small pieces of duct tape on the nosepiece and the focusser marked with a pen.

If you wish to take flats at any time though you will need a light box. Then you can take them right after your imaging session before you tear down.

The T-shirt method lets you get some good flats and allows you to practice your technique before going to the trouble of building a lightbox.

I find my flats last a few months, until I clean all my equipment then i take some more. I dont bother with a lightbox, though one day i may build one.

Usually I take 3 or 4 flats pointing at different parts of the sky. I just give them to Maxim or DSS and let it worry about creating the master flat when it calibrates my subs.

Cheers
Paul

Matty P (Matt) · #4 01-07-2008, 09:56 PM

Thanks for your detailed reply Paul.

Does it matter what Shutter speed or ISO setting I use?

Zuts · #5 01-07-2008, 10:52 PM

Not sure,

I would probably stick with ISO 400, not so much noise. You will know when you get it right as you will see a nice grey image with all your dust bunnies and vignetting clearly defined.

If your histogram is too wide or too far to the right your image will be pure white and useless, so just drop the shutter speed and watch the histogram move left, and those dust bunnies appear

Paul

Matty P (Matt) · #6 01-07-2008, 10:59 PM

Thanks Paul,

It's making sense now.

Matty P (Matt) · #7 02-07-2008, 01:33 PM

Another question,

Do you need to subtract darks from the flats?

TrevorW · #8 02-07-2008, 01:52 PM

matty

Refer to I think it's Jim Solomons Cookbook
http://www.saratogaskies.com/article...ook/index.html

Might help

[1ponders] (Paul) · #9 02-07-2008, 02:27 PM

Matty you only need to subtract darks from flats if they go for more than a few seconds.

As for ISO it is important to take your flats at the same ISO as your lights and darks. Flats don't just help to remove vignetting, dust and other optical aberations, but also help level out the quantum efficiency differential between pixels. i.e. say your chip has a QE of 40%. That is an average across the entire chip. Individual pixels will vary from that 40%. A flat helps to level out the playing field, so to speak. Now by varying your ISO you are basically changing the gain of the camera. Changing gain changes how many electrons are used for each Analogue to Digital Unit. If you do this you will affect how the flat influences the quantum efficiency differential. You will infact introduce more noise than what you are trying to remove (flats remove multiplicative "noise"which is why they are divided).

While I have used much of Jim Solomons information over time I'll have to disagree with him in this. As part of a project on testing noise, gain and other factors of different consumer grade cameras I found a noticable difference between using ISO, particularly if there was more than one ISO setting between the images.

Matty P (Matt) · #10 02-07-2008, 05:00 PM

Thanks for the advice Paul, I'm planning on making a Flat library with different ISO settings.

Tandum (Robin) · #11 02-07-2008, 05:59 PM

I've been looking at making a light box like this -> http://www.gregpyros.com/html/light_box.html

I finally found the white foamcore he uses ... it's called foamboard here and officeworks has it.

MrB (Simon) · #12 02-07-2008, 06:36 PM

Can confirm Officeworks in WA also has Foamboard if anyone was wondering.
Foamboard is a card-foam-card sandwich and I've seen various colours of card used.
BTW, the foam without the card is known as Depron sheet and can be found at foam suppliers in 6, 3 and sometimes 2mm thickness but needs special foam safe glue like UHU Creativ.
I've found spray adhesive works well.

Tandum (Robin) · #13 02-07-2008, 06:42 PM

I've no joy finding mylar drafting paper used in the 1st diffuser. Drafting paper seems to be a thing of the past.

MrB (Simon) · #14 02-07-2008, 06:58 PM

Dunno about mylar(I thought it was metalised plastic?), but all the newsagencies I've been to have drafting paper in the stationary section. Also try an art/stationary supplier like Jacksons.

Edit: oops sorry, Jacksons is WA only.
Spelling.

ozstockman (Mike) · #15 02-07-2008, 07:12 PM

I used a flat white roll blind evenly illuminated by white downlights.

For exposure length, instead of guessing what length to set for flats it's much easier to set a DSLR camera to AV mode.

P.S. Never tried with ISO different from light frames. I think it should be the same.

Tandum (Robin) · #16 02-07-2008, 07:40 PM

Quote:

Originally Posted by MrB

Dunno about mylar(I thought it was matalised plastic?), but all the newsagencies I've been to have drafting paper in the stationary section. Also try an art/stationary supplier like Jacksons.

Edit: oops sorry, Jacksons is WA only.

Cheers, I'll look further afield. He calls it Drafting Mylar, I figured he is talking about the opaue plastic stuff you used to get building plans on.

MrB (Simon) · #17 02-07-2008, 08:13 PM

Yeah sounds like the stuff I've got.
I use it for making PCB's, printing the artwork with a laser printer.
Used to get it in large A2 size sheets and cut it down.
Also found it available in a 'pad' of 6 sheets labeled as tracing paper, not the super-thin tracing paper I'm more familiar with, seems similar to the plastic stuff, more like a thick plastic impregnated paper. It looks opaque but is translucent when placed on any object.

iceman (Mike) · #18 02-07-2008, 08:21 PM

A flat library is only going to work if you don't change the orientation of the camera or the focus. Flats have to be taken at exactly the same orientation and focal point of your lights.

So to make a flat library, you really need a permanent setup where the camera stays on.

I use a light box, makes it really easy to do at the beginning or end of the session. Doesn't matter on the weather etc. Just do it after i've focused and set the orientation of the camera.

I didn't know about using the same ISO as the lights - I always used ISO100. Will do it at the same as the lights from now on.

Image quality really improves with the use of flats - made a big difference to my deep space imaging.

Tandum (Robin) · #19 02-07-2008, 08:23 PM

I see drafting mylar returning .au results in google so it's still about.

What would be easier is another busted LCD monitor I can pull apart. I'm kicking myself that I tossed the last one. It had exactly all the right stuff in it.

Tandum (Robin) · #20 02-07-2008, 08:24 PM

Quote:

Originally Posted by iceman

I didn't know about using the same ISO as the lights - I always used ISO100. Will do it at the same as the lights from now on.

DSS won't use flats if they are at a different ISO to the lights.