Methods for convert spectra intensities in flux unit

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Re: Methods for convert spectra intensities in flux unit

Postby Christian Buil » Fri Apr 04, 2014 12:02 am

Hi Paolo,

Excellent initiative!

All seem right

Concerning BVR photometric coefficients evaluation, I have used some
Landoldt star observed in photometric mode. See part 3 of this
page:

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

You have yourselve the answer about flat-field (not necessary because division of two
affected terms). But ok, is the two spectrum (ref and target) are not in the
same place on the CCD you can have some pseudo noise - but of low importance, I think.

While this does not seem to me strictly necessary here, you can
calculate a "plane" flat-field containing the map of pixels response non-uniformity (PRNU correction).
This solves the problems of high frequency response dispersion.

Look second item here (PRNU):
http://www.astrosurf.com/buil/isis/eshel/tuto1.htm

After computation of PRNU map, give the name of this image
in the standard "flat" field of "General" tab of ISIS.

I do not recommend however a classic flat-field for photometry mode, because
I think it causes more problems than it resolve (to be confirmed) and remember
it is a double flat-field correction.

Christian
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Re: Methods for convert spectra intensities in flux unit

Postby Paolo Berardi » Sat Apr 05, 2014 2:35 pm

Hi Christian!

Thank you very much for your suggestions. I was too hasty in reading the spectrophotometry page for magnitude extraction... I'll observe some Landolt standard stars, then perform parameters calculation. I also have to prepare a PRNU map, very useful here.

See you soon!
Paolo
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Re: Methods for convert spectra intensities in flux unit

Postby Paolo Berardi » Sun May 11, 2014 11:13 pm

Hello Christian and all, a few night ago I had the opportunity to observe some Landolt stars (Lhires III 150, metal slit opened at 350 microns). I think the sky was sufficiently good for the purpose. I observed three stars, two in the same AR sequence, another in the next (I waited until the star was at same height above horizon). Including Miles catalog reference star, all stars was between 40 and 43 degrees above horizon during observation. All spectra taken with 4 x 120s exposure.

Landolt stars profiles:

Image

Relevant data after reduction:

Image

Equations and errors:

Image

There is some dispersion in B band, perhaps due to noise and some de-focus in the blue part of spectra (not an easy target, especially with Lhires III 150). Maybe I should observe other more assorted stars to confirm parameters. Anyway it seems to me a "miracle" to extract so accurate magnitudes from our spectra!

I wonder if, using the Shelyak photometric slit (I plan to buy), I must repeat the observation (if I'm not mistaken slit aperture is not in air but light have to pass through the glass).

Clear sky!
Paolo
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