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Comet Panstarrs


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Hello all,

not at all sure if i got this right, but managed a set of images of the comet with my 5 inch and the star analyser. I took 50 30 second images and stacked them. The comet was on the way down, so it looks a little noisy.

After calibration I divided by instrument response and then subtracted a close G type star (to account for sunlight - not sure if I had to do that)

post-1883-0-13938800-1366561947_thumb.jp

Is it close, if not how can I improve it or is there just too much noise?

Kate

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Hello all,

not at all sure if i got this right, but managed a set of images of the comet with my 5 inch and the star analyser. I took 50 30 second images and stacked them. The comet was on the way down, so it looks a little noisy.

After calibration I divided by instrument response and then subtracted a close G type star (to account for sunlight - not sure if I had to do that)

post-1883-0-13938800-1366561947_thumb.jp

Is it close, if not how can I improve it or is there just too much noise?

Kate

Hi Kate,

You should have divided by the solar spectrum not subtracted it. As a result you have a sort of inverse of the solar spectrum with the Fraunhofer lines appearing as emission lines. The strong Sodium emission seen earlier is much fainter now the comet is further from the sun so there may not be much to see in the spectrum after you divide out the solar component. More spectra of this comet on the ARAS forum here.

http://www.spectro-aras.com/forum/viewtopic.php?f=6&t=567

Cheers

Robin

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Well done for capturing it in the first place!

Can you post what your raw spectrum looked like (after stacking), especially when you are unsure of the processing yourself.

What features did you calibrate against (Sodium, CH and tellurics)?

cheers

John

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Hi Kate,

You should have divided by the solar spectrum not subtracted it. As a result you have a sort of inverse of the solar spectrum with the Fraunhofer lines appearing as emission lines. The strong Sodium emission seen earlier is much fainter now the comet is further from the sun so there may not be much to see in the spectrum after you divide out the solar component. More spectra of this comet on the ARAS forum here.

http://www.spectro-a...c.php?f=6&t=567

Cheers

Robin

Thank you Robin, have redone and divided this time instead of subtracting.Does this look better? I still think it is rather noisy and this may be leading to the hydrogen lines being emission and any extra lines.

post-1883-0-06567700-1366711322_thumb.jp

John, I calibrated an A class star first and then used one star calibration in rspec.

Kate

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Hi Kate,

I am not that familiar with RSpec. I assume the process applies the same dispersion as your calibrated A class spectrum and you line up the zero order. That would makes sense.

The balmer lines look weak (which is probably expected after them being cancelled out by division). The peaks at 420,750nm are very strong, any ideas (hydrocarbons)?

Could you kindly post the uncorrected image or spectrum? I am genuinely interested to see it and I think it might make it easier for Robin to know if the processing is right?

many thanks

John

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After calibration I divided by instrument response and then subtracted a close G type star (to account for sunlight - not sure if I had to do that)

Hi Kate,

The features in this latest version look interesting.

Just to clarify, is the close G type star spectrum (which you are now dividing by) an actual star you measured yourself or a standard spectrum eg the Pickles G2v spectrum in Rspec? If it is the former then there is no need to instrument response correct either the comet or the G star spectra, just divide the two raw spectra direct. (In this case any instrument response effect will appear in both spectra and therefore cancels out) If it is the latter, then, yes the comet spectrum should be corrected for instrument response before doing the division.

Cheers

Robin

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hello Robin and John,

I have some starnge results. I have divided the comet spectrum by both my G class star and pickles based on Robins advice.

post-1883-0-97481700-1366884650_thumb.jp This is the raw spectrum

post-1883-0-95826500-1366884681_thumb.jp This is it divided by a G2 star I imaged.

post-1883-0-92549600-1366884720_thumb.jp This is it divided by the Pickles G2v star.

Kate

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  • 2 weeks later...

hello Robin and John,

I have some starnge results. I have divided the comet spectrum by both my G class star and pickles based on Robins advice.

post-1883-0-97481700-1366884650_thumb.jp This is the raw spectrum

post-1883-0-95826500-1366884681_thumb.jp This is it divided by a G2 star I imaged.

post-1883-0-92549600-1366884720_thumb.jp This is it divided by the Pickles G2v star.

Kate

Hi Kate,

How about you divide it by the spectrum of the Moon? The Moon is just a giant mirror reflecting sunlight. From the raw spectrum it looks very much like the Solar spectrum so subtracting sunlight would be the best? Otherwise you could subtract morning twilight sky spectrum, you might get some sky telluric lines.

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Hi Kate, Starminder

Yes the moon or even the daylight sky can be used to remove (by division) the fine features from the solar spectrum but you would need to keep in mind that the broad continuum shape would not be quite right (Moonlight is redder and the sky is bluer than sunlight)

The division by the Pickles spectrum has left some solar features behind as spikes due to the resolution not being quite matched. The division by the actual measured G2 star is better in this respect but I am a bit surprised by the overall shape which still seems to have the characteristics of the instrument response. Were the comet and G2 star spectra both raw spectra ie not corrected for instrument response or was one corrected and one not?

I have not been able to identify the broad lines at the IR end. For example looking at Buil's spectrum of comet Lilin here, the IR end is rather featureless

http://www.astrosurf.com/buil/lulin/lulin_spectrum.png

from this page

http://www.astrosurf.com/buil/lulin/result.htm

Cheers

Robin

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