Flat fields Dark frames

[Astroman (Andrew Wall)]

Does anyone have any info on what dark frames and flat fields do, also what they are and how to go about doing them? I have seen some people using light boxes and white t-shirts infront of their scopes but don't know why.

Also what effect they have on Astrophotos.

[gbeal]

Andrew.
When I used (for a short time) an ST-7, I made a "library" of dark frames, at exposure times I was using (eg: 5 minutes) and temperatures I was using as well (_25 degrees or whatever).
I usually made about 10 and then averaged them into one "MasterDark". Once I had captured the image I used the dark to reduce the noise by subtracting it. Well the software subtracted it, but you get the picture.
Flats I haven't tried, but any smoot uniform surface will do. I sometimes think that the inside of the dimly lit workshop near where I observe/image would be good, it has smooth painted walls.
Gary

[EddieT (Eddie)]

Hi Andrew,
Flats and Darks are used to calibrate digital images.
Dark frames are taken with the camera in darkness (hence the name) at the same temperature and exposure time as the images they will be calibrating. The dark frame is then subtracted from the images to remove any thermal noise that might have built up during the exposures.

Flat fields are images taken with the optical system pointing at an evenly illuminated light source and at the same focus and camera/lens orientation as the images they will be calibrating. The flats are then divided into the images to flatten the field. This removes vignetting and dust shadows from the images.

I'm not sure to what extent, but I think most consumer digital cameras have internal libraries of calibration frames that are applied automatically when an image is taken, but with astronomical digital cameras the calibration frames must be taken and applied by the user. Both steps are essential in my view, though many of the later chips have very low thermal noise characteristics and some manufacturers claim that darks are not needed with these.

[Astroman (Andrew Wall)]

Thanks Eddie that clears up the old t-shirt off the end of the scope picture then.

[EddieT (Eddie)]

no worries Andrew. I've never tried the T-shirt flat method but I hear it works!

[[1ponders] (Paul)]

Eddie would the taking of flat fields still apply to piggyback photography or only prime focus/eyepiece projections?

[EddieT (Eddie)]

Paul, I still take flats with a 50mm lens. There may be some vignetting, especially if you use a lens shade. In this case there may be a brightness dropoff around the edges of the frame which may or may not be noticeable depending on how bright the object is and how much low-level signal you try to tease out of it by adjusting the levels/curves.

Also, tiny dust motes on the inside of the lens, or on the inner-most lens, or anywhere near the chip, will cause shadows on the chip that look like donuts in the resulting image. The closer the dust is to the chip, the smaller and darker (more prominent) the donut will be.

Again these may or may not be noticeable for the same reasons as vignetting. Flat fielding will deal with these issues and remove the uncertainty, so I always do them.

[[1ponders] (Paul)]

I always seem to be the one doing this ....but.....how about a quick step by step "how to" for Mike to post on the "How To" Pages, it'd make life a bit smoother for us neophytes. I could problably dig around on the internet for a while and find stuff, but when there's obviously a master here I'd be crazy to not ask first.

[EddieT (Eddie)]

Sure Paul, I can do that. I've been up all night and I'm in "real slow" mode at the moment, so I'll put something together (at a snails pace) and post it a little later

[[1ponders] (Paul)]

You're a winner Eddie
Thanks

[iceman (Mike)]

That'd be great Eddie. You can email it to me as a word doc, or whatever form you like.

If you have a look at the IceInSpace How-To's page you can see what's there now to get a feel for the format etc.

Thanks

[EddieT (Eddie)]

Thanks Mike,
I'll check out the format and forward you a word doc.

Sorry for the delay. I fell into a gelatenous heap yesterday after a very long night Saturday when my observatory computer blew another set of ram. I got it going but last night was clear again which doesn't help when rain is forcast for the next few days

I will attempt to get something together today.

[iceman (Mike)]

No probs Eddie, whenever you get time.
For the pics in the article, try to keep them 60-80k in size and 800x600, and if you need a bigger/higher-res version just send them as well and i'll put a link to them in the article.