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because of low S/N, I want to combine some images, but i don't know how to do it
If you know about it, please help me!! Thanks very much!

Hi,What you want to do is one of the more common tasks in data reduction and can also be rather complex. You need to start by reading the help for the IRAF task "imcombine". This is the tool you likely need.Yours,
Frank Valdes

Hi ,valdes
Thanks for your reply!
In general, before we use the package "imcombine", we have to match that images, using "immatch". But in that package, we should select one mode,such as
"xyxymatch","wcsxymatch","psfmatch". What puzzles me is which one may suitable for me. I want to use IRAF for multi-colour optical monitoring of blazars
Thanks again! Yours,Deng,wenguo

China,Kunming

Hello Wenguo,Yes some types of combining require you to register your images. There are three steps. The first is determining the registration coefficients. The second is resampling an image to register to another. The third is combining.To determine the registration coefficients you usuaually have to measure the positions of the same sources in each image. For some images there are automated methods such as XREGISTER. The simplest way is to use an interactive program to display, mark with a cursor and centroid. You need an image display (ximtool, saoimage, or ds9 are the common programs). You can use IMEXAM to measure positions using the 'a' or 'r' keys. These accurately measure the centroid of stars. Sometimes this amount of accuracy is not needed and you can just use the cursor and look at the x and y values shown in the display and write down the the positions and enter then in a file with an editor.Once you have some measurements the next choice depends on the type of misregistration. One common misregistration is x and y shifts. If you can compute x and y shifts you can put the values in a file and then use the "offsets" parameter in IMCOMBINE. This is the easiest method.With a shifts file you can also use IMALIGN.If the misregistration is more complex you can determine the registration using XYXYMATCH and GEOMAP. You can also use GEOMAP directly if you
match the stars manually in your text file. GEOMAP is the ultimate program for determining a complex transformation.To register the images you use GEOTRAN. This will make new images that are matched and ready for combining. Note that you only need to do this registration if your images cannot be registered with x and y shifts. Any type of resampling with GEOTRAN is generally undesirable because resampling degrades the resolution slightly. So if you only have x and y shifts and integer shifts are accurate enough then you should use the "offsets" option in IMCOMBINE and not do any resampling.Yours,
Frank Valdes

Dear Valdes Thank you very much for your detailed reply!
Happy every day!
Yours,
Wenguo