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 IRAF spectra error
 03/12/2017 11:11PM (Read 470 times)  

Status: offline

Registered: 02/27/2017
Posts: 2

please can someone help me to understand how IRAF calculate errors of my extracted spectrum.

After I finish with spectra extraction in IRAF at the end I will get wavelength and flux calibrated spectra. This spectra is fits file where I have 4 extensions. When I am checking header of my fits wile name of these extensions are:

BANDID1 - spectrum - background fit, weights none, cleaned (if I understand this is my final spectrum of the object)
BANDID2 - raw - background fit, weights none, clean no (I think this is original spectra before reduction)
BANDID3 - background - background fit (background Smile )
BANDID4 - sigma - background fit, weights variance, clean yes (I think this is my error)

Can someone explain to me how IRAF calculate BANDID4 and please correct me if my understanding of these BANDID 1, 2, 3, and 4 is wrong I will appreciate this Smile

Thanks in advance


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 03/13/2017 03:38AM  

Status: offline

Registered: 09/30/2005
Posts: 3965
Frank probably has a more succinct answer but perhaps this will help: An explanation of the various bands, and which are computed based on what parameters are set, is given in the APSUM help page, specifically:

If the \extras parameter is set to yes then the above formats
become three dimensional. Each plane in the third dimension contains
associated information for the spectra in the first plane. If variance
weighted extractions are done the unweighted spectra are recorded. If
background subtraction is done the background spectra are recorded. If
variance weighted extractions are done the sigma spectrum (the
estimated sigma of each spectrum pixel based on the individual
variances of the pixels summed) is recorded. The order of the
additional information is as given above. For example, an unweighted
extraction with background subtraction will have one additional plane
containing the sky spectra while a variance weighted extraction with
background subtractions will have the variance weighted spectra, the
unweighted spectra, the background spectra, and the sigma spectra in
consecutive planes.

You might also have a look at the APVARIANCE help page for more details on the sigma calculation. Variations of this question have been asked in this forum before so you might also search for previous posts using the search box on the upper-left part of the page. Hope this helps.

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