I'm new to flats and darks - pls help.

[Davo3960]

I understand dark frames and how to take them but flats are a bit confusing. Do people take them at the start of each session or create them and re-use? I guess if you did this you would need to have the camera oriented exactly as it was when the flat was taken. Any comments much appreciated.

[rogerg]

Your flat image needs to be taken in the exact imaging configuration that your image is taken.

That is, if you pull apart your gear between imaging runs then you will most likely have to re-do your flats. This depends how accurately you can re-create the setup each time. If your gear is permenantly mounted you should only have to do a new flat if you change the configuration of your setup.

I typically only re-do the flat when I swap focal reducers or the like.

If I use the F/6.3 for a month or 2, then swap to using the F/3.3 for a month or 2, then swap back to the F/6.3, I would take a total of 3 flats. I may be picky, but I don't trust the original F/6.3 flat when re-using the F/6.3 reducer later after swapping, just because so many things are variable. That's for my permanent setup.

Edit: when I say 3 flats, what I really mean is "3 sets of flats" because I take 10 flat images and stack them (average) just like any other image. I do the same for dark frames (average however many I can be bothered taking).

Roger.

[Davo3960]

Thanks Roger. How exactly do you take your flat image?

[JohnG]

You don't say what equipment you are using so I will mention 4 ways that you could take flats.

You could use a Light Box and take them immediately after your Lights and Darks, you could use the twilight method, using a clear twilight sky, you shoot your flats before you image, you need to be reasonably quick as the sky darkens and also not image any stars or you could use daylight flats taken with a white T-Shirt over your objective. You can also shoot against a light coloured wall although I think this method could lead to uneven flats.

I use method 3, I have a refractor and DSLR and use a known setup and camera orientation, I shoot my flats usually in the afternoon with the telescope pointing away from the sun and into a blue sky, I try to keep my histogram towards the left and shoot about 20 or so around 400th to 600th of a second. You then stack the images together to make a Master Flat. I have three Master Flats taken at Prime Focus, With Reducer and with Extender on my Takahashi FS-102, I normally use mine for quite a long period before making new ones.

Cheers

JohnG

[jase]

I'd like to also add to Roger's great explanation that "exact imaging configuration" also includes maintaining correct focus. If your focus is different between light and flat frames the dust donuts and other optical anomalies will be different.

A good starting point is a flat should be approximately 30% of your well capacity. For example, the STL11000 has a full well capacity value of 50,000 so an exposure should be long enough for the CCD pixel well count to reach 15,000. A too short exposure will not reduce the light frame efficiently; too long will have the opposite effect. There are no hard and fast rules, simply experiment with what works best for your imaging train and processing techniques.

[seeker372011]

The simplest way to keep on top of flats is to take them after every imaging session. In fact I now use a light box and take flats after every target so if I have two objects imaged in a night-which happens very rarely,I'll take two sets of flats

There are so many other things that can go wrong this just reduces one more risk
It's worth making a light box too for the same reason-check the article here in IIS for a how to

[Adrian-H]

flats are kind of like darks, cept the main difference is that

darks are for maping the noise, to subtract them from your image

flats are for usally mapping the dust on the sensor and other defects ,to subtract them from your image

noise cant physically move

while dust can

and the rest is history.

[Davo3960]

Do I stack before or after I apply the darks and flats?

[Adrian-H]

after you apply dark/flat to each individual image

[seeker372011]

Yes you must subtract darks and divide the flats before you stack