VV Cep flux calibrated
Posted: Fri Sep 30, 2016 2:20 pm
Bonjour,
Playing with flux calibration in my backyard, here is an attempt on VV Cep. This star should be fun to observe this way during the upcoming eclipse
I used 19 Cep (HD209975) as a reference for instrumental/athmospheric response and also for flux calibration.
I followed the spectro-phomometric procedure described on Christian's website :
- observe the ref star with the 23 um slit and reduce using normal Isis pipeline.
- convert relative intensities of the ref star spectrum to erg/cm2/S/A using catalog Vmag (taken from Simbad)
- observe the target star with the 23 um slit and reduce using normal Isis pipeline
- Write down the relative intensity in a low feature point of the target spectrum (6100A in my case, but I could have choosen another part of the tortured continuum ??) = Fa. Also write down the absolute flux of the ref star at the same wavelenght (6100) = F0.
- observe the ref star with the 200 um slit and reduce using a simplified pipeline (no flat, no instr. resp., deselect weighted additionnal mode, no normalisation).
- Write down the relative signal (ADU) at the reference wavelenght (6100) = Fb, and the exposure time = Tb.
- observe the target star with the 200 um slit and reduce using a simplified pipeline (no flat, no instr. resp., deselect weighted additionnal mode, no normalisation).
- Write down the relative signal (ADU) at the reference wavelenght (6100) = Fc, and the exposure time = Tc.
- multiply the target star spectrum (with 23 um slit) by C = (Fo Fc Tb) / (Fa Fb Tc) to obtain the flux calibrated spectrum.
It looks complicated but in fact it is quite straightforward.
I should now play with Landolt stars to convert the spectra in BVR magnitudes...
Meanwhile, some questions : During the observation, I realized that the photometric slit can be easily "contaminated" by a nearby star or even by the diffraction spikes. It is quite evident with eg delta cep and its close MILES ref star companion. I am not sure how to adress this.
Another point I am not sure how to address and whether it is a problem or not ; during the reduction of the photometric spectra, Isis uses the calibration lamp spectrum in the region of the large slit, resulting in a large RMS error of calibration (about 20 A against the 0.2 using the 23 um slit).
Advices and comment are welcome
Clear skies,
Jean-Philippe
Playing with flux calibration in my backyard, here is an attempt on VV Cep. This star should be fun to observe this way during the upcoming eclipse
I used 19 Cep (HD209975) as a reference for instrumental/athmospheric response and also for flux calibration.
I followed the spectro-phomometric procedure described on Christian's website :
- observe the ref star with the 23 um slit and reduce using normal Isis pipeline.
- convert relative intensities of the ref star spectrum to erg/cm2/S/A using catalog Vmag (taken from Simbad)
- observe the target star with the 23 um slit and reduce using normal Isis pipeline
- Write down the relative intensity in a low feature point of the target spectrum (6100A in my case, but I could have choosen another part of the tortured continuum ??) = Fa. Also write down the absolute flux of the ref star at the same wavelenght (6100) = F0.
- observe the ref star with the 200 um slit and reduce using a simplified pipeline (no flat, no instr. resp., deselect weighted additionnal mode, no normalisation).
- Write down the relative signal (ADU) at the reference wavelenght (6100) = Fb, and the exposure time = Tb.
- observe the target star with the 200 um slit and reduce using a simplified pipeline (no flat, no instr. resp., deselect weighted additionnal mode, no normalisation).
- Write down the relative signal (ADU) at the reference wavelenght (6100) = Fc, and the exposure time = Tc.
- multiply the target star spectrum (with 23 um slit) by C = (Fo Fc Tb) / (Fa Fb Tc) to obtain the flux calibrated spectrum.
It looks complicated but in fact it is quite straightforward.
I should now play with Landolt stars to convert the spectra in BVR magnitudes...
Meanwhile, some questions : During the observation, I realized that the photometric slit can be easily "contaminated" by a nearby star or even by the diffraction spikes. It is quite evident with eg delta cep and its close MILES ref star companion. I am not sure how to adress this.
Another point I am not sure how to address and whether it is a problem or not ; during the reduction of the photometric spectra, Isis uses the calibration lamp spectrum in the region of the large slit, resulting in a large RMS error of calibration (about 20 A against the 0.2 using the 23 um slit).
Advices and comment are welcome
Clear skies,
Jean-Philippe
