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Spectrum of comet 62p/TsuChinShan

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#1 mborland

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Posted 04 February 2024 - 10:52 PM

While taking a spectrum of SN2024gy last night, I noticed in Stellarium that there was a 9th-magnitude comet not too far away in RA/DEC and decided to give it a try with my 120-mm APO and ALPY 600.

 

According to Wikipedia, 62p/TsuChinShan is a comet with a period of 59 years, discovered in 1965 in Nanjing, China at the Purple Mountain Observatory. I visited the Purple Mountain Observatory in Nanjing as a tourist many years ago, which made things a little more interesting. (I didn't get to see the telescopes, just some ancient astronomical instruments.)

 

My setup has the ALPY600 with the guiding and calibration modules attached to my Esprit 120, with a ASI120MM guide camera and ASI533MM Pro science camera. I used SharpCap to collect the data, largely because it allows me to specify the region of interest flexibly and also define lots of handy subroutines for taking calibration, flats, darks, etc. I also have an electronic finder consisting of an Esprit 50 ED and an ASI290MM; its well aligned to the main camera, so I can use it with NINA slew/center to bring the target close to the slit (alas, not close enough that I don't have to manually center it).

 

I collected five spectra of 720s each, using PHD2's comet guiding feature to keep the head of the comet in the slit. After processing the data with ISIS, I corrected the spectrum for instrumental response and atmospheric effects using HD97633, using data taken at an altitude of 36 deg. The comet data was taken at about an average altitude of 57 deg, so not a great match there.

 

In order to identify the many spectral lines, I used tables obtained for the Swift-Tuttle and Brorsen-Metcalf comets (M. E. Brown et al., Astronomical Journal, Vol. 112, No. 3, 1996). I identified peaks by hand in my spectrum, then used a script to scan for the peaks from this publication that most closely agreed with mine.  (I weighted the selection to avoid low-intensity lines.)

 

With all that said, here's the result

62pTsuChinShanLineID.png

I see lines due mostly to C2, C3, and NH2, along with a few oxygen lines. The data from Brown et al. contains over 2500 lines, so it is very possible that I am misidentifying some noise as a detected line. However, I think many of the stronger lines are likely to be correctly identified.

 

In the end I stayed up past 3AM, but it was well worth it even when my cat woke me up at 8:30 for his breakfast.

 

--Michael

 

 


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#2 robin_astro

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Posted 05 February 2024 - 09:10 PM

Nice result !  With the long slit ALPY you can separate out the various components and explore the distribution across the coma and for bright comets even look for ionised components in the tail (and sodium if  they get close enough to the sun)

 

eg here

 

https://britastro.or...e77e0992eb70386

https://britastro.or...6e35d7e76310596

https://britastro.or...a14ed7833fcd5cc

 

You might be interested in this video of a talk I gave on this at the BAA comet section meeting 

https://britastro.or...obin-leadbeater

 

Cheers

Robin


Edited by robin_astro, 05 February 2024 - 09:12 PM.

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#3 mborland

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Posted 06 February 2024 - 12:07 AM

Robin,

 

Thanks for the links. It's impressive how much information you teased from your spectra. I'll have to give this a try with my data.

 

I've been thinking about putting a motorized rotator upstream of my spectrograph so I can rotate the slit to a preferred orientation for extended objects. For example, the slit could extend from the coma and follow the tail (or one of them).

 

--Michael



#4 robin_astro

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Posted 06 February 2024 - 06:31 AM

 

 

I've been thinking about putting a motorized rotator upstream of my spectrograph

That would also be on my "wish list"  and would be potentially useful for running with the slit at the parallactic angle with stars, minimising the effects of atmospheric refraction. 

 

Cheers

Robin



#5 mborland

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Posted 06 February 2024 - 09:56 PM

I made an attempt at analyzing the width of the coma using various strong emission areas in the spectrum, two for CN, two for C2, and one for C3. The results surprised me.

 

For the two CN regions, the width is clearly different. That doesn't seem right, since presumably all regions containing CN will emit at all the CN lines. Although the weaker line at 4209 A is noisy, it seems clearly much more narrow than the strong line at 3879 A.

CNLineWidth.jpg

 

Similarly for C2 and C3. The two C2 regions have different widths.

C23LineWidth.jpg

 

Not sure what to make of this. There doesn't seem to be any trend with wavelength, which if present would have suggested some systematic error.

 

--Michael

 

 



#6 robin_astro

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Posted 07 February 2024 - 03:14 PM

Did you subtract the dust component of the spectrum first to leave just the emission component?

 

Robin



#7 mborland

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Posted 07 February 2024 - 08:26 PM

Did you subtract the dust component of the spectrum first to leave just the emission component?

 

Robin

Robin,

 

No, I didn't do that. I'll study how.

 

Thanks--Michael

 



#8 robin_astro

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Posted 08 February 2024 - 07:18 AM

Robin,

 

No, I didn't do that. I'll study how.

 

Thanks--Michael

It is a bit tricky. The way I do it is to start with the G2v spectrum from Pickles and from that produce a template which fits both the features and  the broad underlying shape of the dust continuum. I then scale and subtract it.  You can see the process in outline in my BAA Comet section talk here about 15 minutes in

https://britastro.or...obin-leadbeater

 

Cheers

Robin



#9 robin_astro

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Posted 08 February 2024 - 07:28 AM

For the distributions across the coma I used the non dust subtracted spectra, measuring the distribution of dust near the emission bands where the emission is low and subtracted that from the distribution in the emission band region, a bit like doing a sky background subtraction but in the opposite axis.


Edited by robin_astro, 08 February 2024 - 07:28 AM.

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#10 mborland

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Posted 07 March 2024 - 12:28 AM

I had an opportunity to get a spectrum of 62p/TsuChinShan a few nights ago using an ALPY200. The magnitude had dropped (according to Stellarium) from about 9 to about 11, a 6-fold reduction in brightness.

 

I was able to take 7x720s of data compared to 5x720s with the ALPY600. The ALPY600 results are superior, but only because the comet was much brighter then. I don't think I would have obtained a decent spectrum at magnitude 11 with the ALPY600.

 

62pTsuChinShanComparison.png

 

--Michael

 



#11 robin_astro

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Posted 07 March 2024 - 06:49 PM

Depending on how diffuse they are, a potentially better strategy with comets is to stick with the higher dispersion but use a wider slit. For example a 75um slit with the ALPY600 would still be higher resolution than the ALPY200 but would let more light though.

 

Cheers

Robin 


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#12 mborland

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Posted 07 March 2024 - 09:15 PM

Robin,

 

That's an interesting idea. I guess it would also work for planetary nebula and other largish diffuse objects. Shelyak only seems to offer slits up to 50 um, but I guess there are other sources.

 

--Michael



#13 robin_astro

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Posted 09 March 2024 - 07:31 AM

Robin,

 

That's an interesting idea. I guess it would also work for planetary nebula and other largish diffuse objects. 

Planetary nebulae are another interesting case. If they are larger than the seeing then yes a wider slit will collect more light but the emission lines tend to be narrow so the loss in resolution could reduce the detectability against the sky background. Exactly what is the optimum spectrograph configuration for PNe is an active area of interest for amateur spectroscopists looking to confirm faint possible PNe discovered by imagers. 

 

Cheers

Robin



#14 robin_astro

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Posted 09 March 2024 - 07:42 AM

If you are interested in hunting down and confirming PNe spectroscopically then this review article is a good read

https://arxiv.org/abs/1612.00167

In section 11 it mentions the role of amateurs and the team around french amateur Pascal Le Du who is doing this kind of spectroscopic classification work

 

Cheers

Robin


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#15 Inkswitch

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Posted 13 March 2024 - 10:44 AM

Nice result !  With the long slit ALPY you can separate out the various components and explore the distribution across the coma and for bright comets even look for ionised components in the tail (and sodium if  they get close enough to the sun)

 

eg here

 

https://britastro.or...e77e0992eb70386

https://britastro.or...6e35d7e76310596

https://britastro.or...a14ed7833fcd5cc

 

You might be interested in this video of a talk I gave on this at the BAA comet section meeting 

https://britastro.or...obin-leadbeater

 

Cheers

Robin

Robin,

 

Thank you for providing the link to your talk on dissecting comets.  It was fascinating.  I come to this sub-forum occasionally to see what you all are up to and I am amazed at what can be achieved with relatively modest equipment.  Well done!

 

Regards,

Lawen


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