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GRO Instrumentation -
The GRO Star Spectroscope
In 1835 Auguste Compte, a French philosopher in his “Cours de Philosophie Positive” stated with respect to the stars in his theory of knowledge in astronomy “whereas we would never know how to study by any means their chemical composition or their mineralogical structure” he added further “I persist in the opinion that every notion of the true mean temperatures of the stars will necessarily always be concealed from us”.
One sunny Sunday afternoon I was putting a few finishing touches on my newest instrument, a grating spectroscope that I had designed and built utilizing a couple of off-the-shelf achromatic lenses, a 1200 line/mm reflective grating and a lovely little adjustable slit. The rest of the instrument was assembled from mostly ¼” thick model maker’s plywood, miniature brass hardware, two machined aluminum tube holders and a modified 2.0” adapter to allow the instrument to be installed on the Vixen A80MF achromatic refractor piggy-backed on the mighty 6.0” Holland. I took these pictures on my workbench because it is very close to completion lacking only some internal baffles, black flocking material to line the interior surfaces and a nice gloss white finish.
To be sure the alignments were at least in the ballpark I set the instrument on my work table in our living room and pointed it towards a south facing window. I raised the blinds and picked a wall on the apartment building across the street that was as about as reflective as any other part of the building. After a slight rotational adjustment of the grating to bring the green portion of the spectrum to the center of the FOV and a touch up of the Borg helical focuser, the Magnesium triplet snapped into view surrounded by a multitude of additional lines including Cr and Fe not the least of which was the very dark line of Hβ.
All of this from a reflection of sunlight (not the sun itself) from a building across the street as cars rolled by occasionally blocking the image.
Sitting there on a chair in my living room, looking into the eyepiece of an instrument I built with my own two hands, with all due respect to Auguste Compte, I was able to determine that the sun contains Magnesium, Chromium, Iron and Hydrogen. By adjusting the rotation of the grating bringing more and more identifying lines into view we can then with a few calculations determine that the sun is a G-type star with a surface temperature of ~5800K.
I realize this is quite a bit simplified but the basic premise is sound. We can determine the composition of the stars without ever traveling to them and we can do it with a simple instrument you can build on your kitchen table.
The basic construction is pretty straightforward. It is based on a “classic” 38° design with a simple collimator and grating. Thorlabs supplied the adjustable slit with a metric micrometer that adjusts the slit aperture with a one-to-one read-out. The star diagonal also acts as a telescope utilizing a 25.4mm diameter, 125mm focal length achromat. I looked high and low for a relatively low profile helical focuser and settled on a Borg with a simple
1 ¼” adapter (also Borg). It works like a charm, so well in fact that I bought one for the solar spectroscope as well.
There are no requirements for matching parameters, calculating optimum slit gaps, including some kind of calibration source, determining integration times, etc., etc. That is the beauty of visual observing, just look in the eyepiece, adjust the slit and focus as necessary, rotate the grating to scan the entire spectrum and simply enjoy!
The grating is from Edmund Optics, 25mm square, 1200 lines per/mm, blazed at 500µ working in the first order. The grating turntable is made from an aluminum disc set into a simple bronze bearing, pre-loaded with a spring pulling a tangent arm against the micrometer head. There is no magic to the relationship between the micrometer read-out and the grating position, it is a simple visual instrument after all, built to display enough information to allow me to *see* the differences between the various stellar classifications. Resolution and absolute positioning are simply not important, the stars have been categorized to far greater detail than I could ever accomplish with my simple instrument but the beauty and wonder lie in the fact that I can actually see the differences with my own eyes
Once again, imaging and computers have their places and to those of you who have chosen that path I hope it brings you joy, my place is still at the eyepiece.
In keeping with my determination to do all of my observations with a pair of soft-warm-eyes, being able to see the chemical absorption lines in not only the solar spectra but in the stars as well, provides an aperture into the reality of physics and the fact that it is the same everywhere we look. To say one is humbled by this knowledge and the evolution of its discovery is to admit the wonder of science and how important it is to create that sense of awe and curiosity in our future as well as our current generations.
The GRO Star Spectroscope - an Update
Since the first iteration of the Star Spectroscope I have incorporated a few changes and completed the assembly. This included lining the interior surfaces with black flocking material and where flocking could not be installed painting the surfaces with Krylon Ultra-Flat Black.
The exterior was sanded smooth and painted with three coats of Krylon Gloss White with just a bit of polishing done on the brass hardware. The overall appearance is just what I was looking for and it matches the cosmetics of the Holland refractor perfectly.
The modifications include switching the 1200 lines/mm grating to a 600 lines/mm but keeping the blaze wavelength at 500nm and still imaging the first order. I found that the 1200 lines/mm grating due to the increased dispersion positioned the outer limits of the spectrum beyond the rotational limits of the micrometer/grating platform. My criteria for the Star Spectroscope was to be able to image the entire stellar spectrum in a single view without having to run the micrometer from one limit to the other. Of course a bit of adjustment to center a particular feature was also in the design but unlike the solar spectroscope where “resolution” was the major consideration the star spectroscope was to be used primarily for being able to recognize stellar classifications.
The final requirement was to be able to rotate the grating enough to bring the zero order into view (the zero order is the reflection of the slit) thereby allowing the star image (in this case the star LINE due to the installation of a cylindrical lens) to be focused and centered prior to resetting the grating for viewing the spectrum.
I have also included in the spectroscope accessories a Meade Astrometric eyepiece. This eyepiece displays a variety of reticles and has an adjustable red illuminator. By “calibrating” the linear scale to known spectral positions and separations the identification of an unknown feature becomes quite possible.
Here also are two new pictures of the Star Spectroscope, one showing the completed instrument with both the normal eyepiece and helical focuser shown in the foreground, the Meade Astrometric eyepiece on the spectroscope and a copy of J. B. Hearnshaw’s book on Astronomical Spectroscopy, I found it to be a great read. The second picture shows the GRO SS installed on the Holland achromatic refractor ready for a nights work.
The GRO SS fully assembled
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The GRO SS installed
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