The spectrum of BL Lac object s5 0716+71
http://www.aavso.org/blazar-s5-071671-bright-outburst-0
is now calibrated in absolute flux using ISIS and the 2 spectrum method described by Christian Buil here (part 1 method 2)
http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm
See attached spectrum
The Star Analyser has an advantage over a slit spectrograph in this instance as there is no risk of lost of light at the slit.
I believe this may be the first time this has been attempted using the Star Analyser. The result looks encouraging with a V magnitude of 12.3, (estimated from the flux in the spectrum at 5500A) which is consistent with measurements in the AAVSO database around this date.
Note that conditions were not photometric at the time so there is potential for error in the relative flux between target and reference spectra measured some tens of minutes apart and no correction has been made for the small difference in air mass between target and nearby reference.
Robin
Robin,
Was there any particular reason that you chose that wavelength range?
Tony Mallama
Hi Tony,
In principle the spectrum could cover the full range of the CCD sensitivity (3500-9000A say)
Above ~7500A there is the risk of 2nd order contamination, but the main constraint with this relatively faint object for a slitless setup in my moderately darks skies is the sky background which was of the same order as the signal even at the maximum sensitivity region around 5000A and considerably higher at the extreme wavelength ranges where the camera sensitivity is low. This means the SNR and the accuracy of the flux measurement starts to deteriorate rather rapidly outside these limits.
Also the accuracy would potentially be compromised in this particular case specifically at the blue end due to the lack of extinction correction so I erred on the cautious side and cropped the extreme ends of the wavelength range.
I have not attempted any error analysis though as this was really just a proof of concept.
Robin
Robin,
That's very nice. I also found it interesting that you were able to estimate the V magnitude from the flux and 5500 and that it agreed with recent photometry. Quite impressive.
Tony Mallama