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Old 10-11-2019, 02:33 PM
JimIrish (Jim)
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airglow photometric spectroscopy

Hi all,

Has anyone used a telescope and grating to obtain spectra for airglow? I have an eleven inch Celestron SCT, which I'm planning to point at the South Celestial Pole (so I can leave the mount off, while permanently observing a narrow field of view in which there are few stars.

I'd expect to see the usual suspects for airglow line emissions and hopefully be able to get relative intensities for these and the continuum throughout a night and to compare nights. I expect a cooled camera will be needed to manage thermal noise.

I could be persuaded, however, to use an f/5 refractor.

Or am I dreaming? Will I only get faint spectra from the dim stars? To stack, or not to stack? Exposure times?

My copy of the recent Cambridge University Press book on astronomical spectroscopy hasn't been returned, so I'm in the dark, so to speak.

The alternative is extremely costly narrowband filters for each spectral line and/or standard photometry for at least B and V filters (possibly using Johnson-Cousin filters and Sloan filters in an electronic filter wheel to get some of the airglow emission lines by subtraction).

I'd rather not have to go the fully Monty with calibration every night using standard stars. One alternative is to measure the extinction coefficient at several wavelengths in the daytime with solar photometer.

Thanks in anticipation of your thoughtful reply.

Jim Irish (near Warwick, in SE Queensland - badly affected by bushfire smoke for more than a month, and counting)
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Old 10-11-2019, 03:44 PM
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Merlin66 (Ken)
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Jim,
I record airglow and background street lights etc. every time I collect data!!!!

Not sure how successful you would be with just a grating in the telescope.
You'd certainly pick up a background, but at a very low resolution.

I use a slit spectroscope on a C11 @f10 and usually 10 min exposure subs with an ATiK 314L camera. Due to the slit the emission for lamps etc appear as definite emission lines. For my work I just remove them using the background removal tool in BASS project.
Walker in his "Spectral Atlas", Section 31 shows a typical "night sky" spectrum.
His example is dominated by Hg, Na and [O I] at 5577A. The airglow at 6300A is missing...
I would think light pollution may be a worse factor than airglow, but I'm no photometric expert.
https://en.wikipedia.org/wiki/Airglow

In Henden & Kaitchuck "Astronomical Photometry, Chapter 9.6, p 229 they discuss airglow and light pollution.
Their conclusions (p231) ""...the light of the night sky is not constant...and therefore will always limit the accuracy of your measurements....
...measurements redward of the V filter are strongly affected by the varying night sky and should be avoided until experience is gained with the UBV system.
...The night sky variations will probably never be noticeable in your photometry, but knowledge of the possibility of these errors should be filed in your mind for later reference.""
Budding & Demican "Introduction to Astronomical Photometry" don't seem to even mention airglow......

Not sure if this helps but if you do need spectra let me know.
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Old 25-11-2019, 05:13 PM
JimIrish (Jim)
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Quote:
Originally Posted by Merlin66 View Post
Jim,
I record airglow and background street lights etc. every time I collect data!!!!

Not sure how successful you would be with just a grating in the telescope.
You'd certainly pick up a background, but at a very low resolution.

I use a slit spectroscope on a C11 @f10 and usually 10 min exposure subs with an ATiK 314L camera. Due to the slit the emission for lamps etc appear as definite emission lines. For my work I just remove them using the background removal tool in BASS project.
Walker in his "Spectral Atlas", Section 31 shows a typical "night sky" spectrum.
His example is dominated by Hg, Na and [O I] at 5577A. The airglow at 6300A is missing...
I would think light pollution may be a worse factor than airglow, but I'm no photometric expert.
https://en.wikipedia.org/wiki/Airglow

In Henden & Kaitchuck "Astronomical Photometry, Chapter 9.6, p 229 they discuss airglow and light pollution.
Their conclusions (p231) ""...the light of the night sky is not constant...and therefore will always limit the accuracy of your measurements....
...measurements redward of the V filter are strongly affected by the varying night sky and should be avoided until experience is gained with the UBV system.
...The night sky variations will probably never be noticeable in your photometry, but knowledge of the possibility of these errors should be filed in your mind for later reference.""
Budding & Demican "Introduction to Astronomical Photometry" don't seem to even mention airglow......

Not sure if this helps but if you do need spectra let me know.
Thanks, Ken. I may use narrower band filters than G, B and R, with flatter spectral response. And point at the South Celestial Pole, so there's little "contamination" by starlight, though there'll be a calibration issue. I'm 12 km from the nearest town (popln. 12,000) and Brisbane is in the opposite direction so hardly affects the SCP. Nearest neighbours are well-behaved and more than 500 metres away. Surprised you only get low intensities for airglow lines with a C11. If not a grating, what might improve my resolution, ie, S/N?

I was aware that Budding and Demican didn't mention airglow, but will consult the other text you quoted.

My copy of Walker has wandered. I may have to buy another.

Will you be attending NAACA at Easter?

Jim Irish
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Old 25-11-2019, 05:23 PM
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Merlin66 (Ken)
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Jim,
I donít intend to be a NAACA next year.
At this stage Iím not sure what you really want to achieve.
Obviously a slit spectrograph would help with the resolution.
Ken
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