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Grab and Go Spectroscopy

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#1 Organic Astrochemist

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Posted 22 September 2019 - 05:53 PM

I don't have an observatory so I have been looking for a spectroscopy rig that I can set up very quickly to acquire data when the opportunity presents itself. I have gone through a couple iterations of this and I have learned a few lessons. To compensate for smaller aperture, I  pick brighter targets(mag <10), at lower resolution(Star Analyzer 100) and acquire for more time. To compensate for poor tracking (in exchange for ease of polar alignment and no guiding; iOptron Skyguider Pro camera mount) I use a light, fast, short focal length refractor (Borg 55 FL  250 mm f/4.5), I take short acquisitions (<30 s with high gain low noise ASI 290MM-cool camera) and stack many images ~100 (with Nebulosity). Finally, and not least importantly, to find and center my targets, I use a 35 mm f1.6 c-mount lens with a QHY174 camera. The tripod is a Manfrotto 190go! and the geared ball head (by Manfrotto) lets me point the telescope to the target. All images are acquired with SharpCap. They are stacked later with Nebulosity and the spectra are processed with RSpec. 

 

The whole thing looks pretty ridiculous, but I think it should be judged by the quality and quantity of spectra I can get from it.

Borg rig.jpg

Basically, I can roughly balance the rig in RA and that helps the tracking. Some points of sky are better than others, but with this system the whole sky is available (one half when the telescope points north and the other half when it points south).

 

It takes me less than a minute to polar align. This means I can move my scope in my backyard which is highly obstructed.

 

I acquire images with flats and darks calibrated in real time with Sharpcap. I usually re-use the same instrument response file in RSpec.

 

The helical focuser on the Borg is very nice and I do spend some time optimizing the focus, which of course is slightly impossible with the Star Analyzer because the blue end and the red end of the spectrum are not in the same plane. I usually concentrate on the blue end and lose a little focus on the red end. Here is an example of an A type star that gave showed many Balmer lines into the UV 

58 Cyg.png


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#2 Organic Astrochemist

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Posted 22 September 2019 - 06:07 PM

I acquired several spectra of planetary nebulae. I was happy with the details I could observe: H-alpha and H-beta and often at least the indication of two lines for [OIII] at 495.9 nm and 500.7 nm. I can see variations in these lines, but from my understanding, these spectra don't afford enough information to really calculate the temperature. Still, it is interesting to see how the spectra correlate with the known ages and temperatures of these planetary nebulae. I chose PN that were small (<10") so their light would be concentrated over a fewer number of pixels. Some spectra were acquired on terrible nights with very poor transparency and bright moon. Still I was happy to be able to see some details in PN as faint as mag 10.

NGC 6886.png

 

Attached Thumbnails

  • NGC 7027.png
  • NGC 6210.png
  • NGC 6572.png
  • NGC 6818.png
  • NGC 7009.png
  • NGC 6905.png
  • NGC6543.png

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#3 Organic Astrochemist

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Posted 22 September 2019 - 06:18 PM

I also had fun observing some Wolf-Rayet stars. I could easily distinguish between WN and WC types.

I snuck in this spectrum of P Cygni because it was right there in the constellation of Cygnus. I'm not sure those are truly P-cygni profiles.
P Cygni stacked annotated.png

I read about the Pickering series which is how the once ionized He+ (or He II for the astronomers) behaves similar to hydrogen with one electron (H I) and therefore forms a series similar to the Balmer series. I think I could see this most clearly in HD 192163 and WR 134. But I don't understand why the He II line at 468.6 nm. is so big? Apparently this line is used to identify Wolf-Rayet galaxies. Does anyone know why it is so big?

Attached Thumbnails

  • WR 140 HD 190918.png
  • WR 136 HD 192163.png
  • WR 139 HD 193576.png
  • WR 138 HD 193077.png
  • WR 134 HD 191765.png
  • WR 135 HD 192103.png
  • WR 137 HD 192641.png
  • HR 6265 WC7.png

Edited by Organic Astrochemist, 22 September 2019 - 06:33 PM.

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#4 Organic Astrochemist

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Posted 22 September 2019 - 10:52 PM

Another application of spectroscopy that I have always wanted to pursue is the observation of double stars. Although this slitless system will allow me to take two spectra at once, I think in the future I should optimize the gain/exposure time for the spectrum for each star.

 

61 Cyg (also known as STF 2758) is a fascinating system of nearby faint K type main sequence stars. Here's an annotated image:

STF 2758 AB.jpg

 

The primary compared to a K5V reference spectrum:

61 Cyg A with K5V.png

It's interesting to consider how cool and dim this star is. Ca II is barely visible and most of the metals are only weakly ionized. The G Band (CH molecules) is far weaker than in the F type stars. The only reason why this star is mag 5.0 is because it is so close, 11.4 light years (3.5 pc)! When I observe bright K type stars hundreds or more light years distant, I know that they are giants, not main sequence stars like this.

 

The secondary compared to a K7V reference spectrum:

This is even dimmer and cooler. I probably should have increased the gain and/or exposure time for this spectrum.

61 Cyg B with K7V.png

 

Often I platesolve my spectral images with astrometry.net This time I got an interesting result:

STF 2758 AB astrometry.jpg

The little green circles show where astrometry.net thought the pair should be. However, because the system is so close it has a very high proper motion and the system has moved from that position!

 

I also observed 31 Dra (STF 2241)

31 Dra AB annotated.jpg

 

I really like the G Band (CH molecules) at around 430 nm that I could see in the spectrum of the F type primary. This is the hallmark of the F type star spectrum. I think that doing spectroscopy like this will be the best way for me to learn stellar classification, even though I clearly won't be able to see many lines.

31 Dra A with F5IV.png

 

Here's the spectrum of the secondary with a K8V reference spectrum. Again, I think I have underexposed the secondary, but it's interesting to consider that the greater than one magnitude of difference between the stars results not only from the difference between F5 and F8, but also from the fact that the primary is probably evolving off the main sequence and becoming a more luminous subgiant.

31 Dra B with F8V.png


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#5 NigelR

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Posted 25 September 2019 - 08:28 AM

Hello from South Africa :-) Thank you for sharing, for a l-o-n-g time I have had capturing and analysing spectroscopy images on my bucket list but after dabbling in the astro photography part with very entry level equipment some years ago, I abandoned the goal and have stuck with visual observing. Recently I contacted Tom Field expressing an interest in the SA 100 / 200 and end result was whilst very keen to try, limited budget and equipment is a major factor for still 'sitting on the fence'. And now you share your experience and wow! Perhaps I need to prioritize obtaining a SA (and maybe the free BASS application as definitely do not have funds for both a SA and RSpec) and try with what I have?!? Thanks again for posting, I think I may be contacting you in the future for guidance and advice ;-) Wishing you clear skies and LOTS of FUN with your en-devours. Very best regards Nigel R



#6 Organic Astrochemist

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Posted 25 September 2019 - 06:39 PM

You’re very welcome.
Sadly spectroscopy is not the least expensive activity in amateur astronomy. However, you probably can use whatever equipment you have to get some results.

Without a tracking mount one could use the drift method
http://www.threehill...troscopy_11.htm
Or try taking short images and stacking with a very short focal length lens.

Either way, I think this will limit you to very bright stars. I don’t think this is the easiest way to start spectroscopy — having a tracking mount will be easier.

For me, it was a pain to find and center targets with the star analyzer grating in place. So a spotting scope or camera or filter wheel is also really helpful.

Whichever way you go with software there will be a steep learning curve for acquiring and processing images and spectra. I find RSpec intuitive and easy but I’ve used freeware too.

Just like visual astronomy, spectroscopy is a skill to practice and improve.

Good luck.

#7 Organic Astrochemist

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Posted 25 September 2019 - 06:52 PM

Although this setup cost nearly $2,000, it was easy enough to use that students could take spectra the very first time they ever saw (and touched) a telescope
https://www.cloudyni...n/?fromsearch=1

#8 NigelR

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Posted 26 September 2019 - 04:26 AM

Thanks for the feedback and encouragement - appreciated!

 

There is an 'observing challenge' organised by the Astronomy Society of South Africa (ASSA) called the ASSA 100 and whilst I have seen naked eye or with my ETX80 many of the targets from dark sky locations, attempting the challenge from home with lots of light pollution and limited sky view is difficult 'visually', even using my ST120 or SN 8".  So I am going to try 'imaging' with what I have and once I have acceptable results / acquired adequate 'skills', perhaps I will invest in a SA 100 / 200 (not sure yet which will be most appropriate for  the 'fast' instruments I have) and explore spectroscopy...

 

Part of the decision to try astrophotography with my 'low end' equipment is reading a book titled "Astrophotography on the Go: Using Short Exposures with Light Mounts (The Patrick Moore Practical Astronomy Series)" which is VERY encouraging.

 

Thanks also for the links.

 

Until we chat again remember to KEEP SMILING :-)

 

Wishing you clear skies.



#9 KLWalsh

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Posted 28 September 2019 - 06:47 AM

You can get a slightly flatter focal plane from the grating if you add a thin wedge prism to it. This is called a ‘grism’

Here’s a link:
http://www.astrosurf...3/userguide.htm

#10 robin_astro

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Posted 28 September 2019 - 09:54 AM

You can get a slightly flatter focal plane from the grating if you add a thin wedge prism to it. This is called a ‘grism’

Here’s a link:
http://www.astrosurf...3/userguide.htm

Indeed and a screw on version of one is offered as an accessory for the Star Analyser,

https://www.patonhaw...-page/3-8-prism

though I find the improvement is pretty marginal on the SA100 because the dispersion angle is small (A greater effect on the SA200) and it does have the down side of making the dispersion significantly non linear and making it difficult to use the zero order as a reliable calibration point as it is smeared into a tiny spectrum.


Edited by robin_astro, 28 September 2019 - 09:55 AM.

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#11 robin_astro

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Posted 28 September 2019 - 10:10 AM

The very fast f3.6 optics of the 55FL (faster than I normally recommend for the SA100) might also be contributing to the need to compromise between blue and red end resolution. As well as reducing focus errors due to the curvature of the focal plane, a wedge prism also reduces chromatic coma so it might be interesting to see what effect adding one might have on this fast system.

 

Robin



#12 Organic Astrochemist

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Posted 05 October 2019 - 08:07 PM

I didn't get the f3.6 astrograph version (because it was heavier) so this 250 mm FL 55 mm refractor is only f4.5.

 

My experience with the wedge prism is similar to Robin's and although it may solve some problems it does introduce others such as curvature in the spectrum (smile) and calibration issues.

 

My weather has been cruel; the clouds usually arrive just after I've set up although the forecast calls for a clear night. I even got rained on once. Very difficult to optimize the focus etc.

 

Here are a few first attempts with the grism. I chose some Be stars. Perhaps without the grism, the H-alpha might have been too flatted out to detect. I did choose Be stars with large H-alpha peaks. I need to improve focus and calibration is now more difficult. I'm not convinced it's an improvement, but maybe for hunting Be stars it is a good idea.

 

Attached Thumbnails

  • 48 Aqr with A0V reference.png
  • 26 Peg with A0V.png
  • 31 Peg with B2IV.png
  • Beta psc.png


#13 Organic Astrochemist

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Posted 05 October 2019 - 08:46 PM

Clearly I was having some problems with calibration and instrument response, so I tried simply rectifying the spectrum. This was pretty easy to do and gave pretty good results.

 

I got many Balmer absorptions with Beta Psc

Beta Psc rectified.png

 

I checked with recent spectra for 31 Peg which showed H-beta in emission. That's easier to see in this rectified spectrum.

31 Peg rectified.png




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