calibrate images

[Forum_2015]

Hello Group,
My name is Dick and I am new to this group having just recently acquired AA. I have a problem understanding the help file regarding "Calibration". (See below). There is no mention here of using bias frames but it does talk about F.Dark Frames. So what about the bias frames that are important? Have I missed a step somewhere in the help file? What are the Dark frames for the flat field images? I have never taken these type of frames and have seen no mention of them in any discussions on image calibration except here. Will someone please list the step by step procedure that I should use to successfully process my light, dark, flat and bias frames?

Thank you,

Dick Kirk


If you read a paper in astronomy journals, you'll often see the phrase "we applied the standard reduction procedure to our images." What does that mean? Let's suppose we have a raw CCD image, we have to:

1. Subtract a (master) dark frame.

2. Divide by a (master) flat field. (Flat field must be corrected with its dark frame too).

Creating these two master images with Astroart is very straightforward. You need three set of image, every sets should be composed of at least 5-10 image to decrease the noise.

▪ SET #1. Dark frames for the light image: be sure to take the set with a very stable CCD temperature and at the same temperature and exposure time of your light frames.

▪ SET #2. Flat field frames: they could be of a different exposure time and a different CCD temperature with respect with the light frames.

▪ SET #3. Dark frames for the flat field images: flat fields are like “light” frames so they need to be dark corrected. Again be sure to take them with a very stable CCD and with the same exposure time and temperature of the flat fields frames (SET #2).

Now, open the Preprocessing Window from the menu Tools > Preprocessing: in the Files Tab select from the Files Box (the lower left box) the first set of dark frames (you may use the multiple select shortcuts like [SHIFT]+click or [CTRL]+click), then drag & drop the set of frames into the Dark Frames Box. Do the same for the Set #2 (into the Flat Fields Box) and the Set #3 (into the F.Dark Frames Box)[/COLOR]

[Forum_2015]

Hi Dick, a dark frame has two components one is a bias frame, a 0 sec exposure (or as close to that as you can get) .The other is a thermal frame, this component changes as thermal noise increases. Many people just use a bias frame as a dark for flats because they are usually very short exposures and don't contain much thermal noise. So if you have an uncooled camera and take sky flats you may want to take true darks. Bias frames can also be used to scale dark frames so you can use a much longer dark frame with exposures of shorter duration. The bias is subtracted from the light and dark then the thermal portion of the dark can be scaled to match the thermal portion of the shorter light frame. Hope this helps.

Iver

[Forum_2015]

Thank you Iver and anon for your explanations. I now understand the proper work flow. I didn't take darks for the flats so will use the process shown in;

https://photos.google.com/share/AF1QipM ... ItUEJPWDdn

If this isn't good enough I will take darks for the flats in the future.

Dick

[Forum_2015]

Once I have the master frames, how do I finish the calibration and debayer my lights in AA? Also,how many frames will AA process? I have 64 lights. I think that I read somewhere that you can run into an "out of memory" error message.

Dick

[Forum_2015]

Just put your calibration frames in appropriate boxes and then in Options select CCD color synthesis (and choose correct debayer pattern).
What version do you have ?
Haven't tried new 6 version yet, but I've read it could handle large amounts better.