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Can very small telescope-spectrographs do useful work for AAVSO?

PeterOsman's picture
PeterOsman
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I'm new to astronomy but experienced with scientific instruments and keen to learn how to produce and interpret astronomical spectra. Eventually I'd hope to contribute to the AAVSO endeavour. I live in Sydney and am considering taking measurements in the local park and camping on sites near railway stations out of town. My proposed viewing station is an 80mm triplet refractor (ED80T) on an HEQ5 or NEQ6 tripod mount with an Alpy600 spectrograph and reflective slit for a guide. As I understand this would be capable of studying the brighter novae, supernovae and cataclysmic variables. Would a small low resolution system like this be capable of doing useful work for AAVSO activities? 

Thanks Peter

Small telescopes
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Matthew Templeton
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Hello Peter,

It's hard to say how many projects your telescope could be used for, but a small system would likely have best use for bright objects.  A detailed answer would require that we know the throughput of your system, and the resolution of the spectrograph.  Based on what you wrote, your system would likely be useful for confirmation of bright novae, since even a low-resolution spectrograph can detect the presence the broad hydrogen emission lines of novae.  If the telescope is capable of longer, guided exposures, and the resolution of the spectrograph is reasonably low, that would open up more and more targets for you to observe.

I'm reading the page Christian Buil has on the Alpy600 now -- he's able to reach down to 12th magnitude with an hour of exposure time on an 80mm scope, and that makes a system like this quite useful for nova work.

Matthew

Small spectrographs
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TCB168
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PeterOsman wrote:

I'm new to astronomy but experienced with scientific instruments and keen to learn how to produce and interpret astronomical spectra. Eventually I'd hope to contribute to the AAVSO endeavour. I live in Sydney and am considering taking measurements in the local park and camping on sites near railway stations out of town. My proposed viewing station is an 80mm triplet refractor (ED80T) on an HEQ5 or NEQ6 tripod mount with an Alpy600 spectrograph and reflective slit for a guide. As I understand this would be capable of studying the brighter novae, supernovae and cataclysmic variables. Would a small low resolution system like this be capable of doing useful work for AAVSO activities? 

Thanks Peter

Dear Peter

I have been taking spectra with a LISA for some time now but using a bigger scope than you have. I was using a 200mm scope but now use a C11. The issue is the light collection of the small scope.

I have confirmed nova and 1 supernova which was published in an astronomers telegram so it is possible to do useful work.

The small scope will limit your practical magnitude limit to something like mag 11. I can get down to about mag 14 with the C11 at least for identification purposed with 90mins of exposure. There are not that many nova or SN that are bright but when they do occur you can confirm them.

The other thing that you can do as well is to contribute to the BESS database of BE stars. I have been slowly surveying the dimmer southern stars from about Dec -45 and furrther south. This is just survey work really at the low resolution of a LISA or Alpy but will show hydrogen emission changes from year to year. No one else is doing it so you could help find changes to these interesting stars.

cheers

 

Terry

  Hi Matthew and
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Hi Matthew and Terry,

Thank you both for your advice and encouragement. I held off replying until I had absorbed your advice and had attended our club (ASNSW) annual Star Party. Over the last seven months I’ve been studying, planning and putting an equipment list together and came up with the following, still recognising that it may have a fair number of misconceptions

I’ll mostly observe two or three times a week from a park nearby that is completely screened from the Sydney city lights by trees but of course still with sky glow to contend with.

The Alpy spectrograph is a Littrow design with a 23micron fixed slit, 600 lines/mm reflective grating (grism), a resolving power of R=600 @ 650nm and f-number f/5, spectral dispersion 480 A/mm and a spectral range from 3700 A to 7400 A

The telescope options are either a 200mm f/5 Newtonian with its advantages of aperture size and no chromatic aberration;  I’m told that apertures greater then 200mm provide no advantage in the city. Alternatively a more transportable and robust refractor with optics that are more stable over time. The latter would be an 80mm to 100mm f/5.5 APO refractor depending  on budget and whether I need to buy a doublet APO or triplet to achieve acceptably accurate spectra. I’m asking for advice on the doublet triplet question at present.

The sensor would be an Atik 314L+ CCD camera based on a Sony ICX-285AL chip. It is a Peltier cooled temperature controlled array of 1392x1040 pixels, size 6.45x6.45u, readout noise 4e, full well capacity 17,500 e,  16 bit ADC, and linearity < 0.4% within the anti-blooming gate range.

The tracking unit would use the Alpy reflective slit guiding module with an Orion Star Shoot Autoguider and the tripod/mount is an HEQ6

Measurement time is expected to be four hours exposure each night for one to three nights a week about eight months a year for about six years depending on weather. We’re just entering an el Nino phase and in Sydney; during La Nina it’s much harder to find decent nights for astronomy.

Thanks Peter

ALPY 600
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Hi Peter,

While my APLY 600 has not yet arrived I feel I must correct a couple of things you mentioned. The ALPY 600 is not a Littrow design and uses a transmission grating not a reflective grating. The basic unit without the guiding unit will most likely be very diffeicult to use except in slitless mode. I am trying to develop an inexpensive fiber optic interface for the ALPY which would solve the finding and tracking of a star problem with the basic unit. The alternative is to purchase the guiding unit which then more than doubles the cost. 

BTW, I use a Lhires III which is a Littrow design and uses a 2400 l/mm reflective grating.

I will be discussing the ALPY 600 in detail along with much other spectroscopy information in my new book to be published by Springer the summer or early fall.

Jeff

Can very small telescope-spectrographs do useful work for AAVSO?
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Hi Jeff,

Thanks very much for the two corrections and I had planned to buy the guiding unit as well, considering the spectrograph uses a fixed slit. But even for adjustable slits think it would be useful for taking photographs to help confirm the star location. I also like the fact that the guide sensor is so close to the spectrograph and camera. I'll look forward to your book release!

I'd value your's and others opinion on whether low res spectroscopy is complementary to hi res spectroscopy for astronomical work? Or more of an introductory stepping stone?

Looking forward to your book release! Peter

Can very small telescope-spectrographs do useful work for AAVSO?
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Hi Jeff,

Thanks very much for the two corrections and I had planned to buy the guiding unit as well, considering the spectrograph uses a fixed slit. But even for adjustable slits think it would be useful for taking photographs to help confirm the star location. I also like the fact that the guide sensor is so close to the spectrograph and camera. I'll look forward to your book release!

I'd value your's and others opinion on whether low res spectroscopy is complementary to hi res spectroscopy for astronomical work? Or more of an introductory stepping stone?

Looking forward to your book release! Peter

Interested in espectrography
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rmu
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I am planning to put my first steps in espectroscopy. I've just ordered an ALPY 600 (wich is scheduled to be shipped in july), and I will order the tracking module.

While waiting to have the complete ALPY 600 unit I plan to get some knowledge and skills on how to make spectra with an star analyser attached to a ccd, trying to take well calibrated spectra however it is from bright stars.

ALPY 600
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Hi Peter,

It will be interesting to see how the ALPY 600 works out. I have had some success with a fiber optic interface for it. If it works out I will post the information.

While I said the ALPY 600 uses a transmission grating, it also has a prsim so the configuration is called a grism. From the User Manual there is a diagram that shows the slit, a collimating lens, the grism, an objective lens and then the CCD detector.The maunal is a draft and Olivier asked me not to distribute it so we will need to wait for hte final version.

Olivier said my unit should ship this week so perhaps later this week or next I shall have it.

More information can be seen at  http://www.shelyak.com/rubrique.php?id_rubrique=17

I'll let you know when my book is avaialble.

Jeff

Interested in espectrography
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The Star Analyser is a good way to get started. I suggest getting spectra of A type stars as they are easier to analyze. I also highly recommend RSpec for the processing. You can get a trial version that works for 30 days. RSpec is great for someone starting out in spectroscopy as it is easy to use and intuitive. It encourages experimentation without fear of crashing or getting lost.

Good luck with your projects.

Jeff

Spectroscopy Archives
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For those interested in spectroscopy, Olivier Thizy has made available an excellent pdf of a recent presentation he made. I highly recommend downloading and reading it.

http://thizy.free.fr/shelyak/slides/20130427_1_Introduction%20a%20la%20S...

Jeff

Spectral Classification for Small Telescopes
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Hi Peter,

    Terry's recommendation of Be-star monitoring is a great one (but I am a biased former Be-star observer), I'd add stellar spectral classification (a strong bias there, too). Spectral classification used to be done using photographic plates that had about the resolution of the Star Analyzer, and classification is still of scientific value.  For example, there are an appalingly large number of bright stars (V<7) that do not have modern (i.e., MK "two-dimensional") spectral types, and there are hundreds of stars in the Wahington Catalog of Double Stars without MK types.  (A WDS project may not work out for the closer doubles using the SA.)

     The  Pisgah Astronomical Research Institute (PARI) has an on-line citizen scientist web site called SCOPE that is for classifying stellar spectra.  The classification process is straightforward, and the classifications are put to use by pros.  The Pisgah spectra were taken on photographic plates by pros and, depending on the plate, generaslly have less resolution than the SA.   SCOPE is a great educational tool and I find it fun, too.  SCOPE and a copy of James Kaler's book "Stars and their Spectra: An Introduction to the Spectral Sequence" are excellent professional resources that I'd highly recommend to anyone interested in the subject.

     Dispersion is not the same as resolution and 60-80 A/mm is an optimal classification dispersion, but any dispersion is no good if your resolution is low.  480 A/mm is not good for much else than seeing strong emission lines, although that low dispersion has been used for wide-field spectral surveys.  Those surveys found many of the fainter Be and red emission-line pulsating stars that amateurs are now monitoring so successfully.

Cheers,

Thom Gandet

http://www.lhobs.org

  

Thanks Jeff, it's a very
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rmu
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Thanks Jeff, it's a very interesting presentation.

Can very small telescope-spectrographs do useful work for AAVSO?
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Hi Thom,

Thanks for the encouragement, suggestions for areas of work and the link to PERI. Fortunately the Alpy spectroscope reolution is much better than 850A/mm with a resolving power of R600 at 650nM ie about 1nM. I'd be keen to hear how that would affect the range of observation types possible and much appreciate the advice you and others have given me.

Peter

spectral classification
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GTN wrote:

 Spectral classification used to be done using photographic plates that had about the resolution of the Star Analyzer, and classification is still of scientific value.  For example, there are an appalingly large number of bright stars (V<7) that do not have modern (i.e., MK "two-dimensional") spectral types, and there are hundreds of stars in the Wahington Catalog of Double Stars without MK types.  (A WDS project may not work out for the closer doubles using the SA.)

 

Hi Thom,

The Star Analalyser in the usual  converging beam configuration is limited to R~200 at best so  not good really enough except for a very rough classification but the ALPY at R~600 in slit mode (or similar) could be up to the task. 

More info on the ALPY performance here on the ARAS forum

 http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=555

 

Cheers

Robin

spectral classification
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Hi Robin,

 

    Yes, you're right, R~200 would probably be a bit too low!  Thanks for the link to the ALPY - looks like a great instrument that neatly fills a niche.  Are there plans afoot for an R~1000 SA-type of instrument, or is that pushing things a bit far?

    You gave me an idea.  Making observations at R~600 with the ALPY to compare spectral type standards with "unknowns" would be an interesting experiment for someone to take on.  The results woud be very interesting to read about.


Cheers,

Thom

ALPY 600 User Manuals
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There are three user manuals for the ALPY 600 now downloadable as pdfs.

Basic unit:  http://thizy.free.fr/shelyak/alpy/DC0016A_Doc_Alpy_600_EN.pdf

Guiding Unit: http://thizy.free.fr/shelyak/alpy/DC0017A_Doc_Alpy_Guiding_EN.pdf

Calibration Unit: http://thizy.free.fr/shelyak/alpy/DC0018A_Doc_Alpy_Calibration_EN.pdf

Jeff

higher resolution from the Star Analyser
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Robin Leadbeater
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Hi Thom,

GTN wrote:

 Are there plans afoot for an R~1000 SA-type of instrument, or is that pushing things a bit far?

 

Sadly it is the aberrations generated when the grating is used in a simple converging beam arrangement which limit the maximum resolution to ~200 rather than the line density. This can be improved by placing the grating in front of a camera lens (like the old objective prism techniques used for wide field classification work)

 http://www.threehillsobservatory.co.uk/astro/spectroscopy_11.htm

but this is for bright objects only due to the lack of aperture. (The cost of gratings goes up rather steeply with aperture. I would hate to imagine how mich a decent grating for an 8 inch telescope say would cost !)

There has been quite a bit of thought over the past few years  into  how to bridge the gap up to a full spectrograph design and the basic ALPY 600 module is pretty much the next step in transmission grating development (a 600 l/mm transmission grism with a collimating and camera lens) but the quality of the additional optics needed to give a sharp spectrum across the full visible range add to the cost and as Jeff pointed out you then need to think about acquiring and guiding the target which is not trivial particularly if you are using a narrow slit. There are some DIY options around  to potentially  solve this particular problem though which could save money over the otherwise ideal  Alpy reflective slit guiding module solution.

 

Cheers

Robin

Peter, If you drop me an
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Peter,

If you drop me an email (kenm(dot)harrison(at)gmail(dot)com) I can assist you.

Re a R=1000 instrument.

On the Yahoo Group astronomical spectroscopy we have details of the MG80 spectroscope.

http://tech.groups.yahoo.com/group/astronomical_spectroscopy/

This uses a 300 l/mm reflective grating, a 80mm collimator and a 50mm camera lens for imaging. An adjustable slit can be used or one of the new multi slit reflective plates. In it's base configuration ist's an R=400 instrument.

The FC125 design, with 125mm collimator/imaging lens and a 600 l/mm grating will give R=1000

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