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I
have been intrigued by the stellar spectrum one sees in many magazines and books
describing the composition of stars, nebula and galaxies. I had looked
'gratings' around the web but found only professional-type at professional
prices! But then I came across Rainbow
Optics and purchased their Star Spectroscope from Adirondack
Video Astronomy (who shipped item from USA to UK in three days from order).
The package from Rainbow Optics comprises the diffraction grating, a lens cell and a small manual.
The blazed diffraction grating is supplied to screw into the barrel of a 1.25" eyepiece (as per any filter) whilst the cylindrical lens cell screws on to the top of the eyepiece and provides magnification in one dimension only - this spreads the spectrum to provide easy optical views.
First light on Meade LX200GPS 10" at f/10 with 15mm eyepiece gave immediate bright, coloured spectrum of Betelgeuse with some dark bands visible. Good for the first view with a new piece of equipment!
I then mounted the grating into the nose of my Starlight Xpress MX7C CCD
camera for a quick test. The spectrum was immediately visible as a bright line
across the whole of the CCD (as the telescope tracked). I tried to capture a
spectrum by putting the telescope in 'Sleep' mode with little success (this will
be followed up another time) but noticed that if I slewed the scope whilst
camera was taking 1 second images I could immediately see the absorption lines!
Then the clouds arrived.......
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Spectrum across complete Starlight Xpress CCD on Meade
LX200GPS 10" scope at f/10 |
Spectrum, showing absorption bands, smeared across complete Starlight Xpress CCD
by slewing during exposure on Meade LX200GPS 10" scope at f/10 |
On the second night out with the Rainbow Optics Grating, I was able to image Betelgeuse with the grating pointing north and the spectra imaged across the width of the chip on my Starlight Xpress MX7C camera.
With my setup on a Meade LX200GPS 10" SCT (with the Meade focuser and flip-mirror system plus a f6.3 focal reducer before my Starlight Xpress MX7C camera mounted with the Rainbow Optics spectroscope grating) when I get the star centred on the chip the first order spectrum is not visible; I slew N and it immediately appears and stretches across the complete width of the chip.
I held the grating up to a light, rotated the grating to get the spectrum vertical and then scratched a mark on the rim to indicate the position I needed for the grating within the telescope. I fully thread the grating into the nose of the camera and then back it off until the spectrum mark is parallel to the long side of the chip; then comes the careful task of pushing the camera with loose filter into the telescope mount so that spectrum will be in N-S orientation and grating is not twisted relative to the camera. (I must find a way of locking the grating in the nose thread for the imaging session - no bright idea has flashed into my mind so far - ideas gratefully received!).
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Betelgeuse |
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Star Spectrum Meade LX200GPS 10" Spectrum analysis using AIP4WIN Dorridge, UK |
I found I could get a good smeared spectrum which looked attractive by suspending the tracking on the telescope for part of the exposure. I have since found that a simple short, tracked, exposure on a spectrum gives the same analysis so smearing is only used to make things look pretty and to actually view the lines in the real spectrum.
As usual, focussing is difficult; I focussed on the star through the grating and then slewed to bring the first order spectrum into view - maybe I ought also to adjust focus for the spectrum itself?
The size of the spectrum on the chip is dependent upon the length of the lightpath from the grating to the chip;
To measure the spectrum I used the spectroscopy analysis tool in AIP4WIN, this produces a text file which I imported into Excel to produce the chart shown above. I used a 2 moving average function within Excel to smooth the raw AIP4WIN data.
My first attempt at spectra with my Canon EOS 10D digital camera was targeted at the sun.
The
first requirement was to mount the Rainbow Spectroscope onto the camera. I
purchased the cheapest filter I could find at the local camera shop - the UV
glass was held in with an expansion ring which I removed. The glass was then
used as a template to cut a thick plastic card to size to replace the glass in
the filter ring. I drew around the Rainbow Spectrograph and cut a hole to match
in the centre of my circular card. The grating was then 'screwed' into the
centre hole as shown to the left.
I could then mount the grating onto my lens and see if I could see spectra - amazingly well. I adjusted the orientation of spectra by rotating the grating until the spectra were spread across the long length of the camera chip. (Most lit edges in any image will show spectra through the camera at this stage)
I now wanted to photograph sunlight through a slit (as in school spectra experiments). I had read that reflecting the sun from a needle gave the same result as a slit so I tried both!
I mounted a simple sewing needle in a plastic 'cork' at angle to make it approximately normal to the sun - see image to left. I took images of needle and spectra shown below. I found it easy to see the bright reflections from the needle by eye but when looking through the eyepiece the eye of the needle was regularly bright but I could not get complete needle at its maximum reflection. The spectrum below shows brighter spectra at the two bright spots at the top and bottom of the eye of the needle. This technique shows potential and needs more work.
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Solar Spectrum |
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Solar Spectrum: Zero + 1st Order |
I further tried to manufacture a crude slit by cutting through plastic card with a craft knife. The poor quality slit, professional ones are very narrow but cost about £75 and are on my wish list, did show that mounting the slit in a cardboard tube 4" in front of the grating mounted as above would work - see below.
I
found Alan Schwabacher's page at UWM irresistible - and promptly assembled a
basic spectroscope from some card and an old CD.
Alan provides full instructions and a downloadable drawing for printing to card. I think the pictures of my progress through assembly are self explicit. I found it immediately interesting to view the spectra of the various types of light bulb in the house and the various types streetlights. The respective spectra are demonstratably different with lines clearly visible after very little practice.
Click the logo for link to Alan's page. 
The dark lines that we see in the spectrum are known as Fraunhofer (1817) lines.
Click the spectrum for a larger image and more detail..
I am enthused with my first attempts at spectroscopy - for other sites I have found particularly of interest and help see the table below:
| Click entry in this column | ||
| Worcester Park Observatory, UK |
Maurice Gavin's site for a real treat with lots of information on Amateur
Spectroscopy. |
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| S P E C T R A | Information on the analysis of spectra |
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Stellar Spectra |
More information on the analysis of spectra |
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How to start in spectroscopy with an old CD or DVD, some card and a pair of scissors. I really was excited when I first found Alan Schwabacher's page. |
Try these books to help you on your way (click on book icon for more information):
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Practical Amateur
Spectroscopy |
This book is the one to get you started in
spectroscopy! Stephen Tonkin has collected chapters from experts in the field. Recommended as the first book for beginners. |
STARS and their Spectra |
A comprehensive review of the history and
science of stellar astronomy. The OBAFGKM sequence explained in detail with many diagrams and examples. Not your first book on the subject but the reference book once you need more depth. |
Click here to go to my Spectroscope Observations page
