atmosphere independent instrument response

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Hamish Barker
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Joined: Thu Jun 20, 2019 12:11 am

atmosphere independent instrument response

Post by Hamish Barker »

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.
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: atmosphere independent instrument response

Post by Hamish Barker »

It seems the cheapest, most readily available 3200K tungsten bulbs are 55W 12V DC, car foglight bulbs with 3200K colour temperature. All the 3200K 240V bulbs I can find start at 650W and beyond and probably need special fittings to hold them and deal with the heat.

So I'll try a 3200K car foglight bulb.
Robin Leadbeater
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Re: atmosphere independent instrument response

Post by Robin Leadbeater »

Hi Hamish,

I would guess lamp temperature is not too dependent on voltage. Power ~ V^2 and T^4 so T~sqrt(V) giving ~3% or 100K at 3000K for 10% voltage

Is your AOD constant between the seasons or do you plan to use a different one at different times of the year? If I were to use this method, I think I would probably at least take one reference star per night (perhaps at high-ish air mass) to check all is well.

The killer with this method for me though is chromatism in my telescope optics which means the lamp spectrum is not the same as the star spectrum through a slit which also varies with focus so I have to use a contemporaneous reference star measurement. It could be useful to use the atmosphere model to correct for any difference in air mass between target and reference though, expanding the range of possible reference stars. This could be a useful feature in software. Francois Teyssier's reference star spreadsheet calculates this, though ISIS does not do it explicitly.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
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Re: atmosphere independent instrument response

Post by Robin Leadbeater »

There is also the potential problem of atmospheric dispersion unless you always run at the parallactic angle (How many amateurs do this in practise?) At least by using a nearby reference star this effect would be cancelled to some extent.

Another practical point is that to calculate the air mass, ISIS looks up the coordinates of the target in SIMBAD. How does this work if the object is not catalogued? There does not seem to be any way to manually enter the coordinates.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: atmosphere independent instrument response

Post by Hamish Barker »

Good point re atmospheric dispersion.

regarding airmass lookup of uncatalogued objects, if it cannot lookup an object on simbad, ISIS asks if it should use the coordinates which have been entered on the misc/airmass lookup tab. note that the date entered on the airmass lookup tab are not used, airmass is calculated separately for the middle of each sub exposure time.
Hamish Barker
Posts: 226
Joined: Thu Jun 20, 2019 12:11 am

Re: atmosphere independent instrument response

Post by Hamish Barker »

My telescope is all reflecting (newtonian) so no chromatism problem.
Benjamin Mauclaire
Posts: 258
Joined: Thu Sep 29, 2011 10:14 am

Re: atmosphere independent instrument response

Post by Benjamin Mauclaire »

Hi,

An other method to get the intrinsic instrumental response and then for each target the corresponding instrumental response taking care about atmospheric absorption is described in the following tutorial:

http://spcaudace.free.fr/training/corratmo/

It based on Spcaudace software (http://spcaudace.free.fr/) wich purpose is the use of powerfull pipelines without any peculiar setup related to the spectroscope.

Benji
Spcaudace spectroscopy software: saving you hundred hours of frustration.
Robin Leadbeater
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Re: atmosphere independent instrument response

Post by Robin Leadbeater »

Hamish Barker wrote:
regarding airmass lookup of uncatalogued objects, if it cannot lookup an object on simbad, ISIS asks if it should use the coordinates which have been entered on the misc/airmass lookup tab. note that the date entered on the airmass lookup tab are not used, airmass is calculated separately for the middle of each sub exposure time.
I did not know that, thanks !

Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
Robin Leadbeater
Posts: 1926
Joined: Mon Sep 26, 2011 4:41 pm
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Re: atmosphere independent instrument response

Post by Robin Leadbeater »

Benjamin Mauclaire wrote:
An other method to get the intrinsic instrumental response and then for each target the corresponding instrumental response taking care about atmospheric absorption is described in the following tutorial:
Hi Benji,

Yes I agree. Measuring two reliable standard stars at different air mass on the same night as the target is the most accurate method.

Cheers
Robin
LHIRES III #29 ATIK314 ALPY 600/200 ATIK428 Star Analyser 100/200 C11 EQ6
http://www.threehillsobservatory.co.uk
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