TELLURIC and SKYTWEAK
Frank Valdes wrote on Jun 09, 1999
A: Is there any more information on either telluric or skytweak since they've been introduced in v2.11? I have the IRAF newsletter describing them, and I've read the help files about them. I have a few questions- Would typical spectrophotometric standard stars like F98, BD+33 2642, BD+25 3941, etc. also be usable as calibrations for this purpose? I assume when doing "telluric," (meant to remove telluric absorption lines) which divides by the calibration spectrum, sky subtraction still needs to be done before measuring emission line strengths. (I see that both telluric or skytweak can be done before or after dispersion correction.) I also assume that all spectra need to be treated identically; i.e. that the standard stars have to undergo the same processing before flux calibrating the object. Is this right? Regarding "skytweak," (meant to remove telluric emission lines) I assume that sky subtraction is included in this process and that no separate subtraction needs to be done. A: The help files and the newsletter is all that there is. Spectrophotometric standards would generally be suitable since they are also primarily hot stars with few intrinsic lines. What you are after is stars which are primarily continuum. Fast rotators are recommended because this also washes out lines. Yes you need to remove night sky before measuring emission line strengths. It is also correct that photometric standards need to be dispersion calibrated, etc. in the same way as the object observations. Normally, sky subtraction is done during the extraction step and before dispersion correction such as with APALL. However, it is also possible to not sky subtract at this point but instead treat a sky region as if it was an object. Then sarith or skytweak can be used for the subtraction. The purpose of skytweak is the sometimes the sky subtraction does not work well during extraction because the spectrograph was not well aligned or focused. Skytweak provides the ultimate flexibility in trying to optimally subtract strong night sky lines. If you are not trying to pull out information near strong night sky lines then it is probably not necessary to use the approach of extracting a separate sky and using skytweak.
Last post on Jun 09, 1999