Darks

[jaygpoo]

SO I hear alot about taking darks and flats. I have a Cannon 500d . How do you take darks or flats . Some info to open my mind would be great.

[Tinker1947]

Look in DSS Help file, there's all you need to know......

http://deepskystacker.free.fr/english/index.html

[ollypenrice]

The easiest way to take flats is with an EL panel. Expose to put the histogram peak about a third of the way over to the right. Shoot at least fifteen and, if you want to make a master flat to keep for several images, average combine them using a master bias as a dark. I find flats far more reliable if I shoot them in the dark because, against all logic, flats shot in the daylight seem often to introduce gradients.

Olly

[planetdnb]

its definately worth investing the time in working darks and flats, it can make all the difference!

[Bizibilder]

Using your Canon 500D: First decide what exposure time you want to use for you "Lights" (ie your images) and at what ISO setting you will be using. You don't list your kit but it may be 1min at ISO800 for unguided DSO imaging or 5mins at ISO 800 for guided. Once you have that information you are ready to go.

Darks: You need a set of images (25-30) at the same exposure and ISO and temperature as your lights (Notice the introduction of temp!) taken with the lenscap on your camera. They should be in RAW format. (I take RAW and JPEG so I can see what i've taken - then delete the JPEG's). If you take a series of these (over several nights throughout the year) you can build up a library of Darks at different temperatures. Every 5°C will do - and you can use them over and over - but replace them after maybe a year or so (camera sensors change over time!). I do mine on cloudy nights and put the camera in the garage either using the Canon utility program on the lappy or a timer so I don't have to stand there all evening pushing the button!! Save them on your computer and you will soon have a "darks library". Doing it this way means you are not taking darks during clear imaging weather !!

While you are about it take some (50-100) "Bias" frames as well - simply set the camera at the ISO you have used for the Lights and Darks, put the lens cap on again, and use the fastest possible shutter speed. Save these as well (RAW images) - but you don't need to bother with temp, so one set will do.

Flats: Totally different process! You will need an even light source - I made one for under a tenner: http://stargazerslou...box bizibilder (Modest blushes!! - there are several others in the DIY section). To take your flats you need the scope/camera set-up for imaging assembled and focussed (I take my lights at the end of the session). The optical path, including focus, must be exactly the same as used for your lights. Then put the light source over the end of the scope (I set the scope vertical and just plonk the light box on the dew shield) - set the camera to Av mode and start taking pictures - the camera will set the exposure automatically for you. Again take RAW images. You will need about 30 flats and this will not take very long as the exposures are typically less than one second. You must take a new set of flats every time you take a set of lights and keep them together so you know which go with which.

You can then (assuming you also have a set of light images) put all the appropriate sets into Deep Sky Stacker (freeware) and sit back and wait for it to produce a calibrated and stacked final image ready for you to do the final processing on.

Hope this helps.

[Jannis]

I find flats far more reliable if I shoot them in the dark because, against all logic, flats shot in the daylight seem often to introduce gradients.

Olly

interesting... I had the sme problem, and couldn't understand why my flats made my pics a lot Worse.. My guess was that the OTA wasn't completly sealed and was letting in light here and there, causing the gradients, but i'm not sure if this is what causing it?

I was able to create 1 set of flats right before sunrise that seems to work well, and i've been using only those for most of my processing recently. I've marked my camera's position on the scope, so camera get's into roughly same position every time (+/- 1mm), but it's just a matter of time before dust on the sensor will force me to either take new flats or clean my sensor though