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Alpy 600 and photometry

Ed Wiley_WEY's picture
Ed Wiley_WEY
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Joined: 2010-08-30

I am returning to variables after concentrating on binary astrometry for a while. At the Okie-Tex star party I had a nice conversation with Jim Edling about the Alpy 600. He directed me to Christian Buil who sent me a link to this documentation:

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

It seems that one can do two things: obtain a spectrum and reduce the data to BVR photometry using only the Alpy 600, yielding four results from one run, three of which (BVR) could be entered into the existing AAVSO databases. Since the entire unit costs less than a new CCD, I am definately interested. However, I am not sure of the actual flow of the data collecting. I assume that I must get my transforms from Landolt stars. May I then assume I can use the pseudostandards in any paticular AAVSO chart field? This would be a kind of differential spectrophotometry?


Anyone figure out a protocol for using this spectrograph in the spectrophotometric mode?

Photometry and Spectroscopy
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TCB168
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Ed

 

Christian's technique is certainly interesting and can be used as he is doing. The problem with it is that he is relying on using "known standard" spectra and a catalogue magnitude for these stars to calibrate the spectra and then the photometry from these spectra.

The standards are from the Miles catalogue and they are predominately northern bright stars. This is a real problem for me in the south.

I do not know how accurate the catalogue V mags for these stars is but they are certainly not Landolt standards. The standard also needs to be at the same airmass as your target to have an accurate measurement. Stars change airmass and a second correction needs to be applied to correct this. Christain has a technique for this as well but it is starting to get pretty complicated.

My work around is I have 2 scopes side by side. 1 has photometry filters and the second has a spectrograph. This allows simultaneous photometry and spectroscopy.

The process will improve but is still quite challenging.

Terry

Photometry and Spectroscopy
Ed Wiley_WEY's picture
Ed Wiley_WEY
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Thanks Terry,

So, if I have a star in the Miles catalog that is near the variable, the reduction should be fairly straightforward. Correct?

Ed

ALPY photometry
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TCB168
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Ed Wiley_WEY wrote:

Thanks Terry,

So, if I have a star in the Miles catalog that is near the variable, the reduction should be fairly straightforward. Correct?

Ed

Yes providing you can get an accurate V mag for it.

ALPY photometry
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Ed Wiley_WEY
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Thanks again Terry. I will do some more research on this and see where it leads.

Ed

URL ponts to AAVSO home page
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GTN
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Hi Ed,

 

    The URL in your e-mail points to the AAVSO rather than to Christian Buil's web page.  The text doesn't agree with the actual URL.  Here's the correct link:

http://www.astrosurf.com/buil/calibration2/absolute_calibration_en.htm

     Best wishes in your spectroscopic endeavors!

 

Cheers,

Thom

Synthetic spectrophometry
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GTN
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    Nice to see more activity in the unofficial "spectroscopic section" of the AAVSO!

    Christian's method is a variation of "synthetic photometry".  It's an interesting topic by itself, but it it is also of considerable importance to observational astrophysics.   A classic, and readable, treatment of the subject is given in a 1997 paper by Straizys.  His account may be more rigorous than you want to wade through, though!   (The link to it at ADS is http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1996BaltA...5..459S&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf)  Depends on what kind of accuracy you are aiming for.

    His technique is the basis of a big project I am doing, which is to determine the spectral energy distributions for a large population of very weird stars.  Bessell has made use of synthetic photometry, based on Straizys, and using the MILES data base, as have others.

    I will mention three potentially serious sources of error that can really screw up synthetic photometry measured from spectra.  First, extinction by the interstellar medium and by the atmosphere can be larger than one might think.   The true extinction at the time of observation, not that given by an assumed model of it, is different at different wavelengths on different nights.  Interstellar extinction (or, "reddening") for two stars at the same distance from us will be different for stars of different spectral types (i.e., different stellar temperature or B-V), which can introduce large errors into synthetic photometry.  Atmospheric extinction and interstellar reddening are wavelength dependent.  Straizys (and references therein) gives a very readable account of these, and other, sources of error and how to deal with them.

    Second, for broad-band photometric systems (e.g., UBV), the interstellar extinction, A, in any pass-band x, is not linear with wavelength, and the effect is not small.  There are ways to deal with that issue, too.

    Third, scattered light introduced into the spectrograph's optical path must be eliminated as much as possible.  I can't help you with how to correct the observations for that, but determining them is not trivial.  The effect is similar to light leaks in a CCD camera, except the spectroscopic effect is commonly to introduce a "pseudo-continuum".  Best just to eliminate the stray light and be done with it.


 Best wishes,

Thom

   

Synthetic spectrophometry
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Many thanks for the comments and link to the paper Thorm. I see the complexities. I wonder if there is some pipeline that professionals might use to extract such data from well calibrated spectra taken by amateurs. If so, amateurs could contribute spectra rather than photometry. Getting both at the same time is beyond my resources as I can set up for only one or the other on any given night.

Ed

Amateur Spectroscopy Reductions
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Glad to be of help, Ed.

 

Check out this report on some of the science end of what's being done in amateur spectroscopy and about some of the groups involved: http://astrospectroscopy.de/project%20overview.pdf  You will see mention of some software reduction programs in use, and you can try them on and see how they fit.  I use RSpec almost exclusively now. IRAF is the professionally used reduction system in photometry and spectroscopy, but the learning curve is very steep and runs under Linux (which can be installed on a Windows box as a second boot system).  The upshot is that you've got a lot of reduction software options available that can do everything that you want to do with your beautiful spectra.  There are also important pro-am spectroscopic collaborations, both past and present.  Some of those collaborations involve photometrists and spectroscopists, which is golden!

 

Cheers,

Thom

Ed, How are you going with
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Ken4optics
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Ed,

How are you going with the ALPY?

Any updates on progress?

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