Flats Primer

[George]

Flats Primer

This is not a complete guide but rather a how-to that works for me

In this primer I’m concentrating on flats for use with a DSLR, flats IMHO have contributed more to my images becoming better than either darks or bias frames. I use DSLR Focus http://www.dslrfocus.com/ for actually taking the images, DSS http://deepskystacker.free.fr/english/index.html for stacking my images and combining flats, darks and bias frames. And Adobe CS3 for processing

So what is a flat and what does it do?

Just as a dark frame removes the camera system noise from an image, a flat-field frame removes optical sources of noise. This includes dust that casts shadows on the CCD chip; vignetting in the optical systems; uneven lighting from internal reflections; etc. When imaging under very dark skies with a non-vignetted optical system, flat-field frames are less necessary. The brighter the sky, the more critical it is that you take high-quality flat-field frames. Unlike darks, flat fields are not subtracted from an image. The flat is scaled to the background level of the image, and then divided into the image to remove the effects of optical noise.

A typical image train comprises of the following:

Camera

Filters

Telescope

Anyone of these surfaces no matter how clean you try to keep them will be contaminated with dust and airborne crud.

Right the important stuff:

A flat frame is only good for the orientation of the optical train for when it was taken, therefore if you move any of the optical train new flats will have to be taken, e.g. if you rotate your camera or add/remove filters.

DO NOT touch your optical setup once the flats have been taken until you have taken your lights (actual subject images).

Technical stuff

Flats should be taken with the same ISO setting as was used with your lights (image subs), temperature is not important.

White balance (WB) needs to be set at whatever setting you are using for taking your lights (actual subject images) usually this is set to auto.

What we are aiming for now is to get all the data of the flat image in the centre of the histogram (Diagram 1), this will depend on how much light enters the image train and the speed of the shutter.

Exposure

We need a even illumination to get a perfect flat, there’s different ways of doing this and its up to you how you do it:

Light box

Point your scope at a white PC screen

White tee shirt in front of scope

Twighlight sky

My way

My secret weapon of choice for diffusing light is A4 lamination pouches, 2 or 3 stacked on top of each other make a perfect even diffuser when pointing your scope at the sky, and these are simply held flat against the end of the scope while you take your flats. Using this method enables you to-do your flats at any time of the day whether it’s cloudy or not.

Histogram

The histogram is the key to getting it right, you need to get the light curves as central as possible without clipping the data (see image 1) choose an exposure time length that gives a median value of 2048, the center of the camera's 12-bit A/D converter's numerical range (0–4095) if you get halfway in the histogram, this equals around 2048 ADU . Trial and error will soon get you where you want to be.

Don’t use the histogram feature on the camera readout as it gives a false reading for our needs.

If using the sky as your light source you need to take your sequence of flats ASAP as the light changes by the minute especially in our northern lattitudes.

The blue peak in my histogram is caused by the blue tint in the CLS filter its also slightly ahead of the other colours, if your not using a CLS filter the colors will be more tightly bunched.

Image 1

Open the image in CS3 and you should get something like Image 2 you can just see the contamination of my imaging train in this image.

Image 2

Doing an auto levels reveals the full horror J see image3 ,also visible is the vignetting, the darkening of the corners in the image. This is another reason to be using flats when stacking your images as the master flat will subtract all the imperfections including vignetting from your final image.

Image 3

Master flat

DSS creates a master flat from the flats you include in your stacking list which in its raw form looks like Image 4.

Image 4

Auto levels is applied and you get something like Image 5 Notice how smoother everything is now.

Providing you don’t change anything in your image train the master flat can be used over and over again. If you start noticing imperfections in your images it’s probably time to take a fresh batch of flats.

Image 5

How many flats to take?

There’s no hard and fast rule though consensus says you should take a minimum of 20. I take 51, DSS for some strange reason prefers odd numbers when stacking flats, bias or darks hence 51 J that BTW came from DSS’s writer J

The more flats, bias and darks you use the smoother the image, I now take a minimum of 51 of each.

Final words

I have only been imaging for around 18 months so I don’t claim to be an imaging guru J

I also don’t claim to understand all the techno speak you find in lots of other tutorials and for that reason have kept this primer as simple as possible to make it as painless as possible to take good flats.

Useful Links

http://www.saratogaskies.com/articles/cookbook/index.html

http://www.covingtoninnovations.com/dslr/EOS300Dastro.html

http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM

Copyright George Edge 2008

This text may be freely distributed without charge to anyone. For

That purpose it may be put on a web site, provided that this footer is

Included and the contents are not changed. If this text is used in a publication I will expect substantial gift vouchers for use at my telescope dealer of choice (ha ha ha)

[Psychobilly]

Cheers George

Billy...

[jgs001]

Thanks George, that's great and clearly shows just how much the flats clear up the image.

[MikeP]

Thanks George. You've put in a lot of work to educate us, which is appreciated.

To go the extra mile and deliberately dirty your optics, what can I say.

Mike

[dph1nm]

How many flats to take?

Always remember that flats have shot noise (and read noise - but this is less of an issue) in them. If you flatten an image with 1000 counts in the sky with a flat with 1000 counts you will increase your shot noise by sqrt(2) - equivalent to throwing away half your exposure! So a rough rule for flats should be to have about 10 times as many counts in your stacked flats as in the total sky in your stacked images.

NigelM

[steppenwolf]

Cheers George!

[beamish]

Cheers for that George, the next step

Karlo

[MartinB]

Excellent work George, I'm sure lots of members are going to find that very useful.

One extra thing you can/should do with flats is to subtract the bias particularly if you are also calibrating with darks. Otherwise the bias gets subtracted twice!

[Psychobilly]

I bought a pack of25 A5 sized 80 micron laminating pouches yesterday from staples for under a £5 they are 160mm across the narrowest edge... so will be making up a holder later to try tehm- thanks again George and Martin for adding the info about dark subbtracting the flats - do you do something similar with bias as well ?

Reading the DSS Help files I think it takes casre of this for ouu.

Billy...

[Psychobilly]

Ok been out with the cardboard, parcel tape and soem laminating pouches and just knocked this together as a first off lash up prototype... if it works i'll do some machining over crimbo... and make a pukka one...

(click to enlarge)

(click to enlarge)

Cheers George

Billy...

[Reggie]

Very nice tutorial George, I reckon I might be popping down to staples in a minute

[Starflyer]

Many thanks George, great tutorial and the next step for me too.

I don't have a permanent setup and at this time of year it's often dark when I set up. If I mark the camera and focuser with strips of tape to get the camera angle repeatable will this be accurate enough to use flats taken at an earlier date? I realise that the crud will move around to a degree but it's the vignetting that I struggle to process out with software.

Ian

[barkis]

That's a nice Xmas prezzie for us George. Thanks a lot mate, it's very much appreciated. A good tutorial, that not only says how to, why you need to.

Excellent.

Ron.

[MartinB]

thanks again George and Martin for adding the info about dark subbtracting the flats - do you do something similar with bias as well ?

If you subtract a dark you are automatically subtracting bias. You can create darks to match the flat exposure time but since these are normally very short the dark current is minimal so a bias will do the job.

[Psychobilly]

Fom what I have read DSS seems to sort it all out for you anyway just give it the light, dark, bias(offset) and flats and it does the rest...

Cheers Guys

Billy...

[George]

Fom what I have read DSS seems to sort it all out for you anyway just give it the light, dark, bias(offset) and flats and it does the rest...

Cheers Guys

Billy...

Thats how I have read the DSS FAQ's too

[Reggie]

I crafted myself a nice diffuser ala george yesterday, knocked out 51 flats and am very pleased with the results, thanks very muchly George, Astronomy tips aren't just for christmas, they're for life

Cheers for the heads up on the A5 laminating pockets PsychoBilly, I'll get myself some A4s from work for the 6SE.

@Mods, can we get this tutorial stickied please?

[philj]

Excellent how-to George.

Phil

[Psychobilly]

Here a Re-sized Stretched flat from my Moded 350D taken using the prototype George inpired "Mushroom" diffuser.... I think the senor needs another wet clean by the look of it....

(click to enlarge)

Billy...

[Reggie]

for a moment there I thought you'd been imaging the sun and got some superb activity captured

[Psychobilly]

Tried to use flats with DSS for the first time last night.

Firstly it complained that the ISO of the flats didn't match that of the lights... and secondly of the 31 flats it only used 14 of then the first 14 had 4s exposures, the next 7 had 6s exposures and the last 8-9 s exposures as the sky background was darkening as they were being taken any ideas?

Billy....

[Reggie]

That's odd, I took george's advice and did my flats at ISO 100, it did mention that the ISOs didn't match but it still did the Job in hand, I only had a few lights though. 51 flats, 51 Bias, 15 darks and about 10 lights I think nothing unused.

[Psychobilly]

Yeah it did the job provided i had plenty of light frames made a pigs ear of one where I only had 2.......

Reggie...What was the exposure range of your flats? I took mine by pointing the diffuser at teh twilight sky last night and settign the camera to Av mode... exposures varied from 4 to about 9s over the few minutes it took to take the flats...

BIlly...

[Reggie]

mine were done indoors, just the halogens in teh kitchen indirectly, 1/60th of a sec was long enough, you've seen the results though so something must've been right about them I did a test shot in auto mode on teh camera and copied what it did, worked out well I think.

[Reggie]

I'm going to do some more tomorrow during the day time