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 Post subject: Experiments in stellar spectroscopy
PostPosted: Wed Jun 22, 2011 7:21 am 
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I've now mounted a 500 lines/mm grating filter on the front of my 50mm lens:
Attachment:
File comment: A cheap diffraction grating fitted on the front of my 50mm lens
staranalyser500.jpg
staranalyser500.jpg [ 218.05 KiB | Viewed 6633 times ]

This will split the spectrum of point sources of light (i.e. stars).
Not to be put off by clouds (or daylight), I've made myself a synthetic star for testing:
Attachment:
File comment: Testing with a home-made "synthetic star"
cal_lamp2.jpg
cal_lamp2.jpg [ 2.74 KiB | Viewed 6998 times ]

which produces this spectrum:
Attachment:
File comment: Spectral lines from my synthetic star
cal_lamp2_lines.jpg
cal_lamp2_lines.jpg [ 1.56 KiB | Viewed 6998 times ]


And from the spectral analysis:
we can deduce that it's actually a compact fluorescent lamp, viewed through a pin-hole! :-)
Attachment:
File comment: Spectral analysis of my synthetic star
cal_lamp_id.gif
cal_lamp_id.gif [ 7.95 KiB | Viewed 6633 times ]

Identification of the characteristic Mercury emission lines (at 4046 and 4358 Angstroms) in its spectrum allows me to use it as a calibration source for my spectrographs.


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Fri Jun 24, 2011 12:15 pm 
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Rapid progress this week - I've now used a "proper" diffraction grating on a real star!

The Star Analyser 100 spectroscopy filter is designed to be fitted in front of the CCD chip, but it may also be used across the front of a lens:
Attachment:
File comment: Star Analyser 100 fitted across the front of a 50mm lens on my 1004x-JG camera.
And yes, it is possible to get spectra from bright stars without using a driven mount!

1004xspectrograph.jpg
1004xspectrograph.jpg [ 160.94 KiB | Viewed 6632 times ]

With a little IRIS processing, I extracted the spectrum:
Attachment:
File comment: Spectrum of Vega from 10x 5-sec exposures at f/22 in a twilight sky.
vega10x5stack.jpg
vega10x5stack.jpg [ 1.51 KiB | Viewed 6987 times ]

and then fed it into the VisualSpec program to do the analysis:
http://www.boltonastro.org.uk/Gallery/displayimage.php?pid=578&fullsize=1

The scale across the bottom of this graph is the wavelength in Angstroms (1A = 10nm), and the resolution is principally limited by my camera's 640x480 pixels.
With this set-up I get around 15 Angstroms per pixel, but I could improve this by a factor of four using my DSLR with a 200mm lens.


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Mon Jun 27, 2011 7:37 am 
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Well, I had another go at Vega last night, but this time with the Star Analyser grating on the front of a 200mm f/8 lens on my DSLR.
Although the extracted spectrum is now around than 4 Angstroms per pixel, the actual resolution of the spectral lines is around the same:
Attachment:
File comment: Spectrum of Vega with DSLR
vega2.gif
vega2.gif [ 6.95 KiB | Viewed 6970 times ]

and you can see that the response of this colour DSLR sensor is nowhere near as good as the monochrome one (Sony Ex-View) in my modified CCTV camera, particularly at the red end of the spectrum!


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Wed Jun 29, 2011 7:26 am 
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Last night I took some images of Vega with the £2 grating on the front of my DSLR with 50mm lens.
Even though the sky was not very dark, I was still able to see the "Balmer" hydrogen lines in the spectrum very clearly:
Attachment:
File comment: Spectrum obtained with camera tracking
vega500_gauss2.gif
vega500_gauss2.gif [ 6.62 KiB | Viewed 6962 times ]


And then I turned off the drive, to see what I could manage with a fixed camera:
Attachment:
File comment: Spectrum obtained with camera on fixed tripod
vega500_fix_guass2.gif
vega500_fix_guass2.gif [ 5.59 KiB | Viewed 6962 times ]
and yes, they are still visible! 8-)

For this test, I took care to align the grating so that the spectrum was at right-angles to the star-trails.
I'm sure that the noise could be reduced considerably with a dark sky, taking more longer exposures, and using dark-frame calibration.


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Fri Jul 01, 2011 1:35 pm 
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I'm back to working with the StarAnalyser100 filter now - this time with my 8" SCT (the filter is fitted just in front of my 1004x-JG camera). Since the field-of-view is very small, I've also got a flip-mirror in-line to help finding the target. But its CCD is sensitive enough that I can capture the starlight direct from the video stream (rather than needing its long-exposure facility).

My first target was Albireo, the famous yellow & blue double star in Cygnus. I used a single exposure setting for my images: so the blue component is fainter (and so its spectrum is more subject to noise).
Attachment:
File comment: Image of Albireo and the pair of derived spectra, showing some hydrogen absorbtion lines.
albireo.png
albireo.png [ 23.73 KiB | Viewed 6632 times ]

Attachment:
File comment: Analysis of the spectra using Visual Spec. The vertical axis is intensity and horizontal is wavelength in Angstroms (with blue on the left and red on the right).
albireo.GIF
albireo.GIF [ 19.07 KiB | Viewed 6949 times ]


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Fri Jul 01, 2011 4:20 pm 
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I've just found another interesting function in the Visual Spec program: the ability to synthesize a colour spectrum based on a calibrated monochrome intensity profile.
Here are the results for the two components of Albireo:
Attachment:
File comment: Synthesized spectrum of the yellow component of Albireo.
albireo1.jpg
albireo1.jpg [ 5.47 KiB | Viewed 6948 times ]
Attachment:
File comment: Synthesized spectrum of the bluecomponent of Albireo.
albireo2.jpg
albireo2.jpg [ 5.21 KiB | Viewed 6948 times ]

so now I can see the actual colours with my black & white camera!


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Sat Jul 02, 2011 12:17 pm 
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Having seen some absorbtion lines in a distant star, I thought I'd try to see some emission lines next, and with the famous planetary nebula the Ring (M57) high in the sky at the moment, it was too good an opportunity to miss:
Attachment:
File comment: M57 viewed through Star Analyser 100.
25x 15-sec with 1004x-JG camera on 8" SCT at f/10.

m57_sa.jpg
m57_sa.jpg [ 7.86 KiB | Viewed 6940 times ]

Notice how bright the first-order spectra are, compared with the "zeroth-order" (straight-through) image. But the difficulty is that this is an extended object rather than a point-source (like a star) and the spectra from different parts of the object overlap (I'd need to put a narrow slit in front of the grating to get a sharp spectrum). So I tried to select a narrow band around the brightest part of the nebula:
Attachment:
File comment: Selection of part of the image for spectrum analysis.
m57_sa2.jpg
m57_sa2.jpg [ 8.87 KiB | Viewed 6940 times ]

This produced the following spectrum profile:
Attachment:
File comment: Diffuse spectrum of the Ring Nebula
m57_spec4.gif
m57_spec4.gif [ 4.92 KiB | Viewed 6940 times ]
which is too diffuse to calibrate properly, but we can see the main peaks around 4861-5007 (Hydrogen-beta and Oxygen III) and 6548-6584 (Hydrogen-alpha and Nitrogen II).


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Sun Jan 15, 2012 1:45 pm 
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I've been doing some more experiments with our StarAnalyser100 filter, this time mounted between my 1004x-JG camera and a 200mm lens.
This spreads the light spectrum from each star across the CCD at around 2nm per pixel - here's an example using part of the Pleiades cluster:
Attachment:
File comment: Part of the Pleiades cluster imaged with a StarAnalyser filter and 200mm lens
m45spectra.jpg
m45spectra.jpg [ 8.43 KiB | Viewed 6588 times ]


The Visual Spec program can convert this spectrum into a frequency plot, which can then be calibrated (taking the Telluric absorbtion band around 759nm caused by oxygen in the Earth's atmosphere). Here's an example with Sirius:
Attachment:
File comment: VSpec graph of Sirius spectrum, calibrated for wavelength in Angstroms
siriusgraph.png
siriusgraph.png [ 17.34 KiB | Viewed 6588 times ]


Now this observation is filtered by the frequency response of the CCD detector, but if we know what this is (from the Sony datasheet) then VSpec can correct for it by division:
Attachment:
File comment: Sirius spectrum before (cyan) and after (red) calibration for the CCD response.
siriuscal.png
siriuscal.png [ 5.59 KiB | Viewed 6588 times ]


And finally, the original colour spectrum can be reconstructed by VSpec:
Attachment:
File comment: Colour spectrum of Sirius, synthesized by VSpec from data from a monochrome camera and diffraction filter.
sirius-col.jpg
sirius-col.jpg [ 14.5 KiB | Viewed 6588 times ]


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 Post subject: Re: Experiments in stellar spectroscopy
PostPosted: Sun Jan 15, 2012 2:01 pm 
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It's interesting to compare the spectra of three of the bright stars in the Southern sky - they are of different spectral types and so appear different colours.

Rigel, type B8:
Attachment:
File comment: Rigel spectrum from VSpec
rigel-col.jpg
rigel-col.jpg [ 14.44 KiB | Viewed 6588 times ]

Sirius, type A1:
Attachment:
File comment: Sirius spectrum from VSpec
sirius-col.jpg
sirius-col.jpg [ 15.22 KiB | Viewed 6588 times ]

and Betelgeuse, type M1:
Attachment:
File comment: Betelgeuse spectrum from VSpec
betelgeuse-col.jpg
betelgeuse-col.jpg [ 13.71 KiB | Viewed 6588 times ]


In each case the dark absorbtion band is caused by Oxygen in Earth's atmosphere (around 759nm).


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 Post subject: Re: Experiments in stellar spectroscopy - M42 emission lines
PostPosted: Mon Jan 16, 2012 3:24 pm 
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Whilst I had my spectrograph system pointed i the Orion region, I thought that it would be interesting to look for the emission spectrum from M42. As this is a diffuse object, I can't produce a proper spectrum without a slit, but we can still see the dominant emission lines with the diffraction filter:
Attachment:
File comment: M42 viewed through the diffraction filter
m42spec.jpg
m42spec.jpg [ 7.21 KiB | Viewed 6581 times ]

Isolating a narrow portion of the nebula (shown in the yellow box), I managed to extract the following profile:
Attachment:
File comment: Profile of M42 spectrum from VSpec
m42graph.png
m42graph.png [ 22.14 KiB | Viewed 6581 times ]

We can certainly see the main peaks around 4861-5007 (Hydrogen-beta and Oxygen III) and 6548-6584 (Hydrogen-alpha and Nitrogen II).
The re-synthesized colour spectrum looks like this:
Attachment:
File comment: Synthesized colour spectrum of M42
m42-col.jpg
m42-col.jpg [ 15.8 KiB | Viewed 6581 times ]


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