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Hi, I want to flux-calibrate spectra using noao.imred.kpnoslit package. When I look at onedstds$README, I only see ctioextinct.dat and kpnoextinct.dat files available. I didn't observe at either ctio/kpno, but my standard star is in the onedstd$spec16cal so I could use it. So I went ahead and ran standard, sensfunc, and calibrate by setting the parameters for extinction file to " " and extinct = no. My questions are:1. Is this an acceptable way to calibrate? If not, can someone suggest what can be done?2. I got the spectrum "calibrated" but there are no units on the y-axis. Does someone know?Thanks for your opinion.Siti

Hi,Doing very rigorous spectroscopy can be a little challenging. The key is having good data in photometric conditions. The extinction function can be derived from your data taken at a range of airmasses. See the documentation for that. Even for data taken at CTIO/KPNO a correction to the extinction function is required for the best spectrophotometry.In the absence of sufficient and good quality calibration it is probably better to use one of the extinction functions from the wrong mountain than to ignore it entirely.So it is ok to ignore the extinction or use extinction from another mountain. This can be ok for some projects depending on relative fluxes, etc. For absolute flux calibrations of high quality, you must take calibration data in good conditions and derive the extinction for the night at your site.Yours,
Frank Valdes

Hi Frank,I am still learning about observations, and I don't understand what you mean by
"The extinction function can be derived from your data taken at a range of airmasses. See the documentation for that." I am not sure which documentation you are referring to. I had computed the air masses using 'setairmass' and what shall I do next?Is 'extinction file' the table of atmospheric extinction (mag) vs wavelength? If we have such data for our site, then how can we upload it to use with the tasks standard, sensfunc and calibrate? Thanks for your response.Siti

Hi,You should do[code:1:f9c958a814]
phelp sensfunc
[/code:1:f9c958a814]for a description determining an extinction function from the data using sensfunc. Basically what is done is look at standard star spectra at different airmasses and see how the response varies with airmass. Since these are spectra the result is a function of wavelength; hence the extinction as a function of airmass needed to convert the observed standard star spectra to the tabulated absolute fluxes.I hope this helps.Frank

Hi again,You are talking about the extinction correction derived from the standard star spectra which is observed on the same night as the program star. Thanks for explaining that. But there is also something else required by sensfunc which is the extinction file. The choices are onedstds$kpnoextinct.dat and onedstds$ctioextinct.dat which are not applicable to me. In fact I didon> lcalib ext | graph and enter onedstds$kpnoextinct.dat just to see how it looks like.I got a plot of something (mag, optical depth?) vs. wavelength. So probably I need something similar for my observing site. If so, how can I apply it to IRAF?Many thanks so far. Siti :wink:

Hi,It is also permissible to use "" for the extinction file; that is, no extinction file. This is what is done to then derive a new extinction file from standard stars on the same night or run. Note the reason there are ctio and kpno extinction files is that is often not possible to derive this from a night (because of weather, etc) and the extinction at a site is assumed to be approximately constant. The main way this fails is if there are nearby fires or volcanos. Note that some world wide volcanos, such as Pinatubo in the Phillipines left notable signatures in extinctions functions all over the world.So, I would recommend just using one of the existing extinction functions if you don't have sufficient data for a new extinction function or else you use "" for the input extinction and then derive a new function.Frank

Dear Frank, Thank you so much. Astronomy is interesting.Siti