This information was firstly supplied by Rainer Kracht, who is one of the most successful SOHO comet hunters in the world. You can pay a visit here. However, I have another easier way in processing SECCHI's HI images.
You should firstly download STEREO/SECCHI images here. As you can see, all of STEREO/SECCHI images up to now are in FITS format that they cannot be opened with common program. These still are raw files so that you have to process them by yourself. In this case, you should have softwares to do this job such as IRIS, Fitswork, MaxImDL, etc. In my easier way here, program IRIS is strongly recommended.
Okay, launch the program and open one single HI image whatever you like (2007/02/05_221000 for example here). You probably have a window looking like this now:
Go to Processing - Subtract and select "File from d". Then select another HI image used for background. In order for good looking after the process you are suggested to select one of time close to the one you already had in the window.
Press OK and then you will see a much more beautiful and clearer image after adjusting contrast and brightness with Threshold. Of course, you can use another program to make it look good.
You can now choose any formats provided by the program to save this image.
You should have another way in processing COR2 images for there being no good result for them if you continue using the method above. Here, program FITSWORK (789KB) by Jens Dierks is strongly recommended instead of IRIS. First of all, you should download lots of COR2 images as many as possible. Note, the size of all the images should be the same.
Having finished all the preparations, you should launch the program. Then go to "Datei - Masterdark/-flat erstellen" and you'll see a newly-open window. Select the Flats tab and drag your saved COR2 images to the listbox at right. Select median, which is used for removing stars if far enough images are selected, and then click on "Erstellen". You will soon get the file Masterflat that contains the background image.
Now go to "Datei - Offnen" to open some COR2 images. I'm told that I will get good result if I treat them as triplets (you may probably find that the three images are typically seconds apart if you're careful enough). So open a triplet. Then go to "Bearbeiten - Bild addieren (mit Verschiebung)" to add them of each sequence. Leave the added image, close the rest of ones, and open the file Masterflat. Go to "Bearbeiten - (Hell-)Bild dividieren" and you will see a new window open. This image is the one you have just removed background. It may like this:
You can use "Histogramm" to adjust brightness and contrast or more in order for good quality. Sure you can save it now.