imcombine and the exposure time (exptime) header keyword
G. Grant Williams wrote on Dec 04, 2007
The documentation for imcombine implies that the header keyword for the exposure time will be modified to match the resultant effective exposure time. I'd like to sum n images and have the exposure time updated in the header rather than reflect the value of the first image. I've attempted many different ways but I can't get the "exptime" keyword to update. I'd like to use imcombine rather than other routines that can sum (or average) images because it records the combined images in the headers.
Can anyone provide suggestions or comment on your own success of having the exposure time updated.
Thanks,
Grant Williams
Can anyone provide suggestions or comment on your own success of having the exposure time updated.
Thanks,
Grant Williams
Francisco Valdes wrote on Dec 04, 2007
Hi Grant,
The feature of having IMCOMBINE update an exposure time keyword is no longer present and has been absent for some time. I believe you are referring to the one reference to this under the description of the "expname" parameter. I am removing this reference from the help page.
The whole question about what the "effective" exposure time of a combined image led to quite a debate and differences of opinion. That was when IMCOMBINE was change to essentially scale things so that the first image header is representative of the data. This is meaning full for the options that average or median. As you indicate, a sum is a different thing. The sum option was added at a later time and while it would make sense to include the sum of the times this is not done.
So I'm sorry that you will have to find an external (to IMCOMBINE) way to update the exposure time in the combined image header(s). One way to do this is
where <images> would be the same list of images you used with IMCOMBINE. The result of summing the exposure times is in the average.sum parameter which you can use as a variable in hedit as shown.
Yours,
Frank Valdes
The feature of having IMCOMBINE update an exposure time keyword is no longer present and has been absent for some time. I believe you are referring to the one reference to this under the description of the "expname" parameter. I am removing this reference from the help page.
The whole question about what the "effective" exposure time of a combined image led to quite a debate and differences of opinion. That was when IMCOMBINE was change to essentially scale things so that the first image header is representative of the data. This is meaning full for the options that average or median. As you indicate, a sum is a different thing. The sum option was added at a later time and while it would make sense to include the sum of the times this is not done.
So I'm sorry that you will have to find an external (to IMCOMBINE) way to update the exposure time in the combined image header(s). One way to do this is
ecl> hselect <images> exptime yes | average
ecl> hedit ("<combimage>", exptime, average.sum)
where <images> would be the same list of images you used with IMCOMBINE. The result of summing the exposure times is in the average.sum parameter which you can use as a variable in hedit as shown.
Yours,
Frank Valdes
Last post on Dec 04, 2007