atmosphere independent instrument response
Posted: Thu Aug 25, 2022 11:04 pm
I finally got around to preparing a proper atmosphere-independent instrument response by following Christian Buil's combined method (Number 2. "Long method" in the link below, using a 3000K halogen light waved around in front of the telescope for the tungsten flat and a B8V reference star. I used 20 frames of 5seconds of waving. It doesn't take long to get tired of waving a lightbulb around, and i did manage to kill one bulb by accidentally breaking the filament when the bulb tapped the telescope.
http://www.astrosurf.com/buil/instrument_response/
So, big thanks to Christian for his detailed writeup, as always!
With this instrument response, I am able to tick the auto atmosphere box on ISIS and not need to view reference stars. It worked very well on a bunch of different elevation stars, giving very similar continuum slopes compared to reduction using individual, similar altitude reference stars.
I also made a wavelength calibration with about 13 different lines and 4th order polynomial with very low residual errors so I think I have that nailed down, although my L200 does occasionally move by a pixel, which is 5-10 A with my low dispersion 180lpmm grating, so of course the normal wavelength calibration images are necessary.
The star i used isn't a miles or CALSPEC star, so I do need to redo the method sometime with one of them but I'm really pleased with the results, as it will make munching through a bunch of Be stars and CV candidates, both in terms of telescope time and data reduction, a lot faster.
Worth a go if you haven't already done so. the lamp i used is a 3000K colour temperature 50W naked 12v halogen bulb running of a 12v lighting system switched mode supply. Wear sunglasses when doing the bulb-waving. i was half blind for half an hour afterwards!
it's of course possible that the 12v system might not be quite 12v, which could have a significant effect on the colour temperature. I took a while ferreting around the hardware stores until I found 3000K labelled bulbs, as most of them are 2600K to 2900K, and the extra temperature helps to get sufficient photons at the blue end. Even better would be a 3200K but they only seem to be available in 1000W theatre floodlight bulbs which i do not fancy having to deal with! Better would be a 240V 3000 or 3200K bulb, as at 240V there is much less likely for voltage to be out of spec.
It's also possible that there might be tungsten slide projector or photography floodlight bulbs available with 3200K colour temperature.
Beware not to use the little reflector type halogen bulbs as these often have dichroic reflectors which shunt IR and possibly some red out the back, so their spectra may be far away from blackbody radiators.
http://www.astrosurf.com/buil/instrument_response/
So, big thanks to Christian for his detailed writeup, as always!
With this instrument response, I am able to tick the auto atmosphere box on ISIS and not need to view reference stars. It worked very well on a bunch of different elevation stars, giving very similar continuum slopes compared to reduction using individual, similar altitude reference stars.
I also made a wavelength calibration with about 13 different lines and 4th order polynomial with very low residual errors so I think I have that nailed down, although my L200 does occasionally move by a pixel, which is 5-10 A with my low dispersion 180lpmm grating, so of course the normal wavelength calibration images are necessary.
The star i used isn't a miles or CALSPEC star, so I do need to redo the method sometime with one of them but I'm really pleased with the results, as it will make munching through a bunch of Be stars and CV candidates, both in terms of telescope time and data reduction, a lot faster.
Worth a go if you haven't already done so. the lamp i used is a 3000K colour temperature 50W naked 12v halogen bulb running of a 12v lighting system switched mode supply. Wear sunglasses when doing the bulb-waving. i was half blind for half an hour afterwards!
it's of course possible that the 12v system might not be quite 12v, which could have a significant effect on the colour temperature. I took a while ferreting around the hardware stores until I found 3000K labelled bulbs, as most of them are 2600K to 2900K, and the extra temperature helps to get sufficient photons at the blue end. Even better would be a 3200K but they only seem to be available in 1000W theatre floodlight bulbs which i do not fancy having to deal with! Better would be a 240V 3000 or 3200K bulb, as at 240V there is much less likely for voltage to be out of spec.
It's also possible that there might be tungsten slide projector or photography floodlight bulbs available with 3200K colour temperature.
Beware not to use the little reflector type halogen bulbs as these often have dichroic reflectors which shunt IR and possibly some red out the back, so their spectra may be far away from blackbody radiators.