Maxim & Flats A black art!

[Penfold]

Probably not just about Maxim this.

I have nearly got to grips with most things now but flats have always alluded me so I bit the bullet and bought a flat panel. Sunday night I imaged M81/82 brought the scope inside with out altering anything camera, focus etc. and took numerous darks and bias images.

Then I come to take flats and no matter what settings I have nothing seems to work.

In Maxim I set the type to flat, I place the panel over the scope and then open up the information windows and proceed to take a succession of shots from 0.1 of a second up to 20 seconds and all images show a value of 65536 (approx) when clicking on them.

I am now utterly confused in what I should see and how to know what shutter length to set.

[Merlin66]

Hmmm

certainly over exposed.

Try even shorter exposure times???

Dim the panel?

[Ibbo!]

you have overexposed the flats

I think mine are something like 0.03 s to get 23000 adu

Steve

[Penfold]

Ahhhhh 0.03s OK , I was expecting it to be higher for some reason.

Cheers Guys.

[RobH]

Make sure you subtract bias from them before applying them to your lights too....if you don't, they won't work properly.

Rob

[Merlin66]

Rob,

Why subtract the bias from the flats?

The lights also contain the bias?? So subtracting just the flat from the light does both?

[RobH]

Rob,

Why subtract the bias from the flats?

The lights also contain the bias?? So subtracting just the flat from the light does both?

It doesn't work like that.

The software calculation that's performed when flats are applied needs bias to be subtracted from the flats or they will either under or over correct.

If you are subtracting darks, then you don't subtract bias from your lights. If you do, then you've done a double subtraction as the bias information is already contained in the darks.

Flats need to be considered, in some ways, as a type of light. The calculation isn't done in the same way as with, for example, darks as they are not simply subtracted, but divided when applied.

I spent ages trying to get flats to work until in some obscure part of the web I found a mention of all of this. Once I applied bias to my flats, bingo...no problems anymore. Every experienced imager I know does it this way as it's the correct way to do it.

Beyond that...I didn't write the software...I just know how to use it

Cheers

Rob

[Merlin66]

I use AstroArt, and by subtracting the associated Dark from the Flat before applying it to the Light I probably get the same result.......

So, I end up with:

Corrected Flat = Raw Flat - Flat Dark

Corrected Light = (Raw Light - Light Dark)/ Corrected Flat

No Bias frames involved....

[Penfold]

Cheers for the explanation Rob... I'll try and give it a bash tonight.

[RobH]

Hi Ken.

Yes, if you're subtracting flat darks, there's no need for bias. The same job is done and your flats will apply correctly. If you don't apply flat darks or bias to them though, they won't.

I don't bother with flat darks as the durations are normally so short for the flats that bias does effectively the same job without me having to shoot a bunch of flat darks to match each set of flats....as you know, a master bias (and master dark for a given temperature) will be good for 6 months or so.

Cheers

Rob

[Penfold]

Right so I've taken all mine at -15, that means as long as I keep the set point at that temperature I can reuse them? If this is then the case is there an optimum amount of each to take.

[Merlin66]

I take 8-10 each for darks/Flats/Bias to make masters....

[martin_h]

Right so I've taken all mine at -15, that means as long as I keep the set point at that temperature I can reuse them? If this is then the case is there an optimum amount of each to take.

30 -50 would be good.

Don't forget that the darks and bias can be re-used, flats need to be re shot if you alter the image train in any way.

[RobH]

I use 30-50 also....for bias, as they're so quick and easy, I shoot 100.

Too few and you'll end up adding noise.

Rob

[Penfold]

Thanks all that's great info.....

[BlueAstra]

I think we've been here before, but can't remember the answer. Anyway, here we go. If I take lights(target image), flats, darks, bias as separate sets of images is it OK to just load them into MaximDL or DSS (in both cases correctly labelled) and just let it get on with it, or do you manually have to subtract the bias from the flats?