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I thought I would jump in with an example of how reddening affect can affect a spectrum. This is a spectrum of Brun 818. It is a B6 star behind the Orion nebula. It suffers from about 6.5 magnitudes of reddening in V, causing the slope of the spectrum in the optical to change direction!
To use such a star for response correction is tough. You either need a response corrected spectrum someone else has made, or you have to carefully deredden the spectrum.
If you have a good Miles spectrum that has not been dereddened, then this kind of scenario should be OK. Though with high reddening values you may end up with only a weak signal at the blue end.
On another topic that came up. When selecting the binning region for a spectrum, I stretch the image to make the full top and bottom edges of the spectrum visible. I then set the edge of the spectrum bin to include all of the signal, even the weak parts, while trying not to include any unnecessary background. I suspect this makes little difference, but if you set the binning region too narrow, then there is always a chance this will have a weird affect on the spectrum as you will be cutting out some of the signal.
Andy
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When selecting the binning region for a spectrum, I stretch the image to make the full top and bottom edges of the spectrum visible. I then set the edge of the spectrum bin to include all of the signal, even the weak parts, while trying not to include any unnecessary background.
Yes this can be particularly important if there is any chromatism or atmospheric dispersion which will change the width of the spectrum along the wavelength axis (or any residual tilt) so the fraction sampled varies. (ISIS has an optimal binning option which seems to cope with this while minimising the sky contribution. It is rather effective on faint targets, though I am not exactly sure how it does it). Similarly the sky background zones need to be set far enough away not to include any of the spectrum. Stretching also helps spot any weak spectra which might be in the selected sky background zones, a bigger problem with slitless setups.
Cheers
RobinSo how do you handle the blue end with the LHIRES then?
I don’t have a solution so run without a flat, making sure I put the reference star in the same position along the slit and try to include any flat effects in the response. (I don’t use the LHIRES at low resolution and 90% of the time I am working at the red end so it is not a problem there)
I have not investigated it fully but it may be mainly light from the built in lamp leaking round the slit holder. Running a flat lamp ahead of the telescope aperture might be a better solution as the beam will be more tightly controlled. The calibration setup in the LHIRES is rather crude with the lamps just swung in over the slit. The setup in the ALPY calibrator is much better designed.
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