40 replies to this topic

[Organic Astrochemist]

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.

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 

[Organic Astrochemist]

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.

 

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[Organic Astrochemist]

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.


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?

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

[Organic Astrochemist]

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:

 

The primary compared to a K5V reference spectrum:

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.

 

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

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)

 

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.

 

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.

[NigelR]

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

[Organic Astrochemist]

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.

[Organic Astrochemist]

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

[NigelR]

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.

[KLWalsh]

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

[robin_astro]

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.

[robin_astro]

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

[Organic Astrochemist]

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.

 

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[Organic Astrochemist]

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

 

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

[Organic Astrochemist]

Gamma Cassiopeiae is a very bright Be star that's a great spectroscopy target at all resolutions. It is a hot star surrounded by a gaseous disk. Atoms in the disk are ionized by the star and emit light.

 

Here's  a spectrum I acquired with my GRISM setup. H-Alpha is huge and H-beta is also in emission. I looked at higher resolution spectra and many other Balmer lines are also in emission presently though I'm not confident about my other Balmer lines (they're definitely NOT in absorption). Gamma Cas affords us a very privileged view, with each Balmer emission getting closer and closer to the ionizing star.

It's a good test of my GRISM setup - I have to optimize focus in both the blue end to detect the faint H-beta and also in the red end to maximize the H-alpha peak (and keep it narrow).

 

[robin_astro]

Nice work to pick out the quite faint H beta emission. Looking at Joan Guarro's recent high resolution spectrum in the BAA database  (Taken using his home build echelle spectrograph) H gamma has also been poking its head above the absorption from the star's photosphere and H delta is there but keeping its head down

https://britastro.or...legend_pos=none

 

Robin

[Organic Astrochemist]

Thanks Robin.

 

I had another night with clouds and poor transparency, but I managed to get a few spectra. They happened to be all post-main sequence stars.

 

Delta Cas is an A5III giant. Calibration and instrument response are now a little better under control with the GRISM.

 

R Cas is a Mira-type variable. Based on the AAVSO light curve it's just past maximum light and I thought I might see some emissions, but no luck with this system. The titanium oxide (TiO) bands are labeled in pink.

 

WZ Cas is a Carbon star. It's also supposed to be lithium rich so I was hoping I could see lithium at 670.8 nm and I think I did! This is probably another time where the GRISM made an observation possible. Richard Walker suggests the star's effective temperature is only 2500K.

 

Finally the planetary nebula NGC 7662 was nearby. It's a little big, 0.5 x 0.5 arcmin and the annular structure is clearly seen in the image of the spectrum, so the resolution in the spectrum isn't great and doesn't reveal many lines.

 

Edited by Organic Astrochemist, 22 October 2019 - 11:36 PM.

[Organic Astrochemist]

On the many crappy nights I continue to experiment with this system.
First a few failures. I put a 685 nm Baader filter in front of the GRISM, this only passes light longer than 685nm. I tried to learn what I might see of Paschen lines or the Calcium triplet. Short answer -- nothing.
Mu Per (M0III) should have large Calcium triplet absorptions (see the pink reference).


Gamma Cas should have Paschen lines and the Calcium triplet in emission, but I couldn't see anything.


So this led me to reconsider the strengths and weakness of this simple system. The small fluorite lens should have pretty good transmission in the UV. I rarely find interesting lines after H-beta (the Li I at 670.8 nm in WZ Cas was an exception). Maybe it is worth sacrificing the H-alpha line to concentrate on the UV.

So, as a proof-of concept, I removed the prism and extended the SA 100 by a distance of 100 mm from the sensor. My first try was on Delta Cas (A5III). Because of the clouds I couldn't see Polaris so I just pointed the mount roughly North. I took 100 2 second captures and this was the result:


Here is a zoomed in comparison to an A5III reference.
I never realized how lucky I was to start spectroscopy with a reflector, but I am happy with the performance of the Borg. I have a few tweaks to make but I'd like to try and sample stars from O through M with this system. I think the combination of Balmer lines and Ca II K and H lines should let me see something different in each spectral type.

Edited by Organic Astrochemist, 16 November 2019 - 09:48 AM.

[robin_astro]

 

...and extended the SA 100 by a distance of 100 cm from the sensor. 

?   100 mm ? 

[Organic Astrochemist]

Thanks Robin. I make all kinds of typos when I use my phone.

After I did this experiment I reread the article by Uwe Zurmühl that you posted on another thread.
http://spektroskopie.../Spektrum51.pdf

According to his equation 2 optimum distance = 24 mm x /f at a focal ratio of 4.5, I should be slightly below the optimum distance with most of the grating illuminated. Hopefully I will still be able to easily see much fainter stars than Delta Cas.

[Organic Astrochemist]

The point is, at the dispersion and resolution of the Star Analyzer at this grating-to-sensor distance, isn’t it worth sacrificing the “full” optical spectrum and H-alpha in order to observe with some detail this interesting spectral range from 350-500 nm.

There are lots of people out there with small APO telescopes and CMOS cameras. The price of a star analyzer and a few 1.25” spacers seems pretty compelling to get a spectrum like the one I posted above.

Edited by Organic Astrochemist, 16 November 2019 - 10:28 AM.

[robin_astro]

 isn’t it worth sacrificing the “full” optical spectrum and H-alpha in order to observe with some detail this interesting spectral range from 350-500 nm.
 

Indeed. The aberrations which limit the performance at the red end are much reduced this region and it is hardy a sacrifice as  the full spectrum can still be explored by a simple reconfiguration  (Going the other way to very low dispersions also has its applications too in cases where resolution is not critical, so very faint objects, eg supernovae, quasars etc can be recorded). There are some issues to be faced working in the UV region, some I outlined here 

https://www.cloudyni...ions/?p=9761195

but as you say it is a region worth exploring. 

 

Take care that you don't lose light through vignetting. Uwe's calculation assumes the star is on the optical axis, (Though his figure of 24mm is a bit smaller than the actual open aperture of the SA100, I think) If  you are not using the wedge prism and you move the zero order off axis to fit the spectrum in the camera field, the beam can overspill the edge of the grating. 

 

EDIT:  Just checked and 24mm is indeed about right for the Star Analyser aperture. (PH added a 1mm wide aperture ring compared with the original prototype to mask any edge defects on the grating) 

 

Cheers

Robin

Edited by robin_astro, 16 November 2019 - 12:59 PM.

[Organic Astrochemist]

Thanks Robin. It's good to know that I'm taking advantage of the area of the star analyzer. Unfortunately I think a lot of the light isn't landing on my small sensor (5.6 mm wide!).  But hey, the uncooled camera can be bought for $269!

ASI290mini

 

The quantum efficiency of the camera isn't optimum at these short wavelengths either.

 

I got spectra from 11 stars last night! The first thing I did was to buy myself a copy of Gray and Corbally because now I think I may be able to easily see some of the lines they use to classify stars.

 

I haven't yet made a good instrument response curve. I think I'm going to take the spectrum with the most flux furthest in the UV (early B type) and use that to make my curve.

 

Here are the spectra from hottest to coolest.

[Organic Astrochemist]

Mintaka is an eclipsing binary B0III and O9V, so I'm not sure which spectrum dominates. He I line at 402.5 nm is visible with an intensity roughly one third of the Balmer lines.

 

20 Ori (B5III) was nearby. The He I line is still visible, but now with an intensity much smaller than the Balmer lines. Things are cooling down.

 

Rigel (B8I) is burning brightly but I can see the Ca II line at 393.4 nm so this is even cooler than the previous stars.

 

The Balmer lines are looking good in 86 Ceti (A2V) but the He I line at 402.5 nm is gone.

 

The G-band is pretty hard to see in my spectrum of 11 Lep (F0I) but starts to build in cooler stars. The Ca II K and H lines dominate here.

 

The G-band seems clear in 9 Lep (G5III) but the Balmer lines seem very weak.

 

In the reference spectrum for G8III star the G-band and Ca K and H are starting to look of similar intensities. This is not seen in my spectrum and definitely a goal to work toward with a better instrument response.

 

The G-band is bigger than Ca I 422.6 nm in 23 Eri (K0IV).

 

In cooler 18 Eri (K3V) the G-band is getting smaller compared to the Ca I line, but not as much as is seen in the reference spectrum (need better instrument response correction).

 

Many of the atomic lines in Alpha Ceti are overprinted with absorptions from molecular TiO, but I'm happy that I can still see Ca I at 422.6 nm and Ca II K and H lines.

 

Mira is the coolest star (M5-M9IIIe) but it is not far from maximum light so Balmer lines are in emission due to shocks.

 

I got a late start last night, but it was a lot of fun.

Edited by Organic Astrochemist, 17 November 2019 - 11:40 PM.

[Organic Astrochemist]

Indeed. The aberrations which limit the performance at the red end are much reduced this region and it is hardy a sacrifice as  the full spectrum can still be explored by a simple reconfiguration  (Going the other way to very low dispersions also has its applications too in cases where resolution is not critical, so very faint objects, eg supernovae, quasars etc can be recorded). 

 

I didn't realize the versatility of the star analyzer. Maybe it is useful to change the grating to sensor distance depending on the target. 

Here are a few spectra I acquired with a distance of 100 mm. Because they were early type stars and had lots of lines, I didn't need the zero order spectrum to calibrate (it couldn't fit). Instrument response isn't perfect, but not bad for grab-and-go.

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[Organic Astrochemist]

One thing that I thought was interesting in my post #23 was that even in late type K and M stars I could see Ca II K and H lines, which meant that I could use these to calibrate the spectra the same way I did for the early type stars above: using the Show One Point Calibration Window feature in RSpec. I simply have a table with many of the lines seen in those early spectra and I just click on the one line that I want to use to calibrate the spectra. Very fast and easy. It worked above for the earlier spectral types, but it also worked for these G and K types by calibrating on the Ca II K line at 393.4nm:


I do feel like I'm cheating because I'm not referencing the zero order spectrum, but the results seem good enough to me. This allows me to maximize the dispersion and much more efficiently use my relatively small CMOS camera.

I was pretty hopeful that if I could work out an easy grab and go system, that I could take a lot of spectra even on crappy nights. I think I have succeeded in this. I also hoped that I would be able to learn more about spectroscopy. I think that is happening. I also hoped that by taking as many spectra as I could I would stumble across many interesting astrophysical phenomena. That hasn't happened too much yet because I've still been working out the kinks. Yesterday I did make a nice "discovery":



Alpha Camelopardis is an O-type supergiant star (the Balmer lines are marked in pink). I was surprised to see H-alpha in emission. The star is both a pulsating star and also has a ferocious stellar wind which result in significant mass loss. Apparently some of the hydrogen gets ionized while being blown off the star and that's what I saw. Often the H-alpha shows a P-cygni profile, but apparently the wind is highly clumpy and variable, so blue shifted peaks, like the one I saw, are not unheard of. There's so much variation during one night, from night to night and from year to year, that it's pretty hard to study.
Alpha Cam
So unless someone else was observing alpha Cam when I was, it might be hard to completely verify my observations, but I suspect that H-alpha is currently in emission.
One has to be a little careful with field stars superimposed on the spectrum, but my stars are significantly defocused so they have a distinct shape when seen in the spectrum.

Edited by Organic Astrochemist, 22 November 2019 - 08:42 AM.