Forums / The Science / Photometry / Creating a flat image according to the IRIS tutorial
Creating a flat image according to the IRIS tutorial
Posted: April 21, 2011 - 7:24am
Hallo - and another beginner problem:
After having finished the IRIS tutorial successfully with the given data, I eagerly started to process my own images. My attempts now have com to a sudden stop at Step 3 "Processing", at 5.1.
I already created an saved a master-bias and a master dark. The first two commands beginning with "sub2" and "opt2" seem to work well. After "ngain2 i 1000 i 3" there is an " ERROR: Determination impossible
What might have gone wrong?
Is there maybe another way of creating flats that can be recommended to beginners. As I have learned elsewhere in this blog the answer seems to be "No"
Thank You for any help
Marko
Hi Marko, I think the best is to ignore that 5.1 "Note: if you do not have flat images... " !!! This is expected to be an "alternate" way making a master-flat when you do not have made"true" flat images but I am afraid this is a mistake of the tutorial! I actually do not see how it could generate an acceptable flatproviding the vignetting correction of the lens that is essential for making good photometry (vignetting at large aperture needed for getting a maximum of electronsis often as high as 50% !! ) I strongly recommend to make true flat images and then apply the standard flat processing of IRIS to generate the master-flat. To see: " 3. Do the same for flat-fields, but... " The input images (generic name "flat" in the tutorial) shall be made by shooting a uniformly lit white paper. The camera shall be set atthe same stop and focus, (ISO ?) than being used for photometry. The exposure should be set to get an output about 50% of the ADU range on the green channels. The very difficult part of it is to get a target luminance uniform within 1% on the overall image field. The best way I know is to use a "non-glossy" good andlargephoto paper (any specularreflection shall be avoided)Then I put it on the floor. Next I illuminate the whiteceiling just on top of the paper using a stronghalogen lamp. The setup shall be such the illumination path is well symmetrical and normal to the paper. A large spot on the ceiling ensure that the paperillumination is well uniform. The camera is set on a tripod and shoots the center of the paper, the operator should be away (using retarder) Any other light source, reflexion,should be eliminated. The camera could be setat the vertical of the paper center, at leastmid way to the ceiling. If the spot on the ceiling is large enoughthe camerashadow should be negligible. It would be a good idea to make two sets of image with the camera rotated 180° . Then the set of images ( eg. 2 x 3...5) is to be processed with IRISto generate the master-flat that will be used to flatten your sky images. That master-flatremains usable each time you workat the corresponding camera shooting conditions. Yours truly, Roger