How to create MASTER CALIBRATION frames required to Pre-process properly your deep sky images

Preprocess means to remove all the intrinsic CCD defects (hots pixel, biases), telescope optical field non uniformity, dust shadows from a raw image just coming out from you telescope.... To get rid off these defect and take full advantage of your CCD camera, the way to succeed is the use of MASTER CALIBRATION FRAMES.

Master frames are the calibrations frames which are used to remove the defects from CCD images. Typically, the real pixel value is affected by many effects/biases from the CCD itself. So the pixel value you get on your raw image is CCD[x,y] :

CCD[x,y] = (CCDreal[x,y] * FlatField[x,y]) + Dark[x,y]*t + Bias[x,y]

The idea is to retrieve the CCDreal[x,y] pixels values with a set of calibration frames. To preprocess your images, you must FIRST create the 3 MASTER CALIBRATION FRAMES properly (Bias, Dark,FlatField) from a set of images aquired from your CCD camera. Hereafter are described the steps :

1. Offset/Bias Master Frame

You have to have your CCD camera into complete darkness. PRISM has a feature to acquire offset/bias frames. (Menu-> CCD Camera/Offset Grabbing) This is recommended to acquire as much frame as possible (let's say 15 at least).

Once the process starts, it displays a status by the means of scrolling hints

These frames are automatically saved into files with the following scheme names :

offset-1.cpa
offset-2.cpa
offset-3.cpa
offset-4.cpa
offset-5.cpa
...
offset-15.cpa

To create the master frame which will be used to preprocess your images, you must combine them by using a median stack (Menu->Preprocess/Median stack)

You get an image which is free of any defect of the individual images: this image will be your MASTER BIAS FRAME (or OFFSET FRAME)

Bias/offset master frame

Save this image, we will call it offset.cpa for further use. So save it !

2. Dark Master Frame

You have to have your CCD camera into complete darkness again. PRISM has a feature to acquire dark frames. (Menu->CCD Camera/Dark Grabing) This is recommended to acquire as much frame as possible (let's say 10 at least). The CCD temperature MUST be stable during this phase.

The exposure time must be around the typical exposure you have used to acquire your images at the telescope.

These frames are automatically saved into files with the following scheme filenames:

Dark-1.cpa
Dark-2.cpa
Dark-3.cpa
Dark-4.cpa
Dark-5.cpa
...

Once the images have been acquired you have to combine them by using median stack, has you did for the bias master frame.
The next step is to subtract the master bias file offset.cpa file from the result of the stacked images (the previous image).
Now you have the MASTER DARK FRAME, call and save it as dark.cpa

Dark master frame

3. Flat Master Frame

The CCD camera is now attached to the telescope, and during twilight it's time to carry out flat field frames (Menu->CCD Camera/Flat Grabing)

You have to acquire at least 9 images to have a nice flat field master frame. The telescope should not track stars (otherwise stars will not be eliminated with median stack)

A panel shows you all the information of the flat field image acquiring process. In that case the mean of the image must be between 14000 and 24000 ADUs to get a proper flat field frame. The exposure time must range be set so as to avoid / neglect dark current contribution (Here between 12 and 1000 ms).

These frames are automatically saved into files with the following scheme :

flat-1.cpa
flat-2.cpa
....
flat-12.cpa
...

Hereafter, you must follow all these steps :

-> 1. Once the images are acquired, you must, first, subtract for each of them the offset.cpa file.
-> 2. All the images must have the same mean, for that purpose PRiSM has a special tool (Menu->Pre-processing/Same Frame Averaging):

Put the 6th image as a reference. The use of this is feature is to set the image with the same mean since they don't have the same mean due twilight illumination changes during the acquisition process. This aims to allowing them to be median stacked properly.

-> 3. Once all the images have been processed, you should stack them all by using (Menu->Preprocess/Median stack) as you did for getting the master offset and dark frame.

This is the result, a MASTER FLAT FIELD FRAME (or FLAT FRAME)

Congratulation !! You get all the MASTER files required to process properly your image. The dark.cpa and offset.cpa can be reused. A set of Flat field frame must be acquired and master flat frame recomputed each time you remove the CCD camera from the telescope, or have rotated it.

4. Ready to start to preprocess images

Now, you can use those frames as "Offset", "Dark" and "Flat" frames and fed the powerful preprocessing tool with them. (Menu->Preprocessing/Flat Dark Offset AutoRemoval). I recommend you to have qll the calibration files into the same folder/path.