SPECTROSCOPY Section
By Chris Schur 
Updated 12/1/13

Introduction

As an astroimager for 4 decades now, I am always expanding my studies of the field of astronomy, In this branch - A grating or prism is used in conjunction with the telescope and camera to record the signature imprints within the light itself of the chemistry of galaxies, stars, planets and other solar system objects. While I have been doing objective prism images of comets for decades and have had numerous comet spectra images published, I am now starting to get more serious on the equipment and techniques used by professional astronomers. I will also be exploring a method using the sky drift method for a spectroheliograph of the Sun in narrow band wavelengths.

Spectroscopy Section Layout. Like deep sky imaging, spectroscopy is tremendously varied and catagorized. You can be dealing with the spectra of quasars billions of light years away, or the surface of Mars right next door astronomically speaking. This makes it not only interesting, but one can never see everything in a life time of study. As I have examined other amateur spectroscopists web sites, I found that the best are project oriented. That is how we are going to present this section, as an ongoing project catagorized basis.

Section started October, 2012

Instrumentation

 There are several methods one can obtain spectra of objects in the sky, such as the Sun, Comets or Exploding Stars. The simplest is the use of an "Objective Prism" in which a thin glass prism is placed in front of the camera lens and the subject which is offset at an angle is recorded with very high efficiency as a spectral strip. Trailing the spectrum perpendicular to the dispersion will result in the traditional appearance of a strip with lines. Here is an example I did of a comet:

 Comet Austin

May 6, 1990, 
Objective Prism Spectra
 400mm f/6.3, 15mins on
 Ektar 1000, Payson
 Here in this image for example is Comet Austin with a flat wedge shaped prism mounted in front of a 400mm lens. You can see that the spectrum has multiple images of the comets coma in it which for the most part represent ionised molecules such as CN and Swan bands present. This comet was quite active and photographed with a very green coma. Comets with plenty of dust tend to mute the gasses spectrum and are primarily that of reflected sun light. This appears as a bland spectrum with dark lines instead of bright ones. You will also notice that the red part of the spectrum is rather shortened, but the blue is drawn out. This is the non linearity that prisms posess because they work by the principle of refraction, and blue wavelengths are refracted more than red. (Think rainbow).
  Left: A shot of the 15 degree surplus prism I use when shooting comets in this manner. You can see Ive mounted it on a coffee can jar lid which fits over the lens snugly, and Ive painted it black. Also a small sighting tube is mounted on the top to aim the prism at the comets. It is adjusted to compensate for the angle of the prism.

   Another method for obtaining spectra of celestial objects is with transmission gratings. These are a resin film mounted on glass or on a plastic sheet with grooves ruled into thier surfaces to disperse the light. Those rainbow reflectors you see on products or credit cards are an example of this, as are the rainbow pattern you get with the reflections off of a CD or DVD disk. I use a professional grade blazed grating in an 1.25" filter ring called the "Star Analyzer" by Patton Hawksley, Inc. For cometary spectra, a focal length is chosen such that the image is small and point like. For example, a tiny 12th magnitude average comet is small enough to allow direct imaging through the SA unit and obtain a resonably clear spectrum.

 Here is an example of this in effect. A very faint comet ISON when it was dim and small in the 12.5" scope made a suitable target for the transmission grating. In this image at the left, you can see both the standard sharp stars image, and to thier right side is the drawn out short spectrum. ISON is to the left of center, and you can see its tiny nucleus and short tail. To its right is its spectrum which can be analyzed with appropriate software for signatures of molecules.

 Here we have mounted the grating on the front of the camera lens, which when zoomed to 200mm allows a large full spectrum to be imaged. This is useful for small point like celestial objects that are not too large to blur the spectra out into a mush. This is suitable for small objects like stars fields and small but bright comets. The next panel shows an image taken with this setup on the stars of the belt of Orion:
 Click on this thumbnail for a whole page of some wide field spectroscopic survey images with the above setup! Each star is a point source and will be visible dimly on the left side, while the spectra of each star is a colored streak to its right. This grating is 100 lines per millimeter ruled, such low dispersion is great for dim objects as it does not spread the light out too much and make it hard to see.
 Threaded into a camera adapter for my Imaging Source DMK cameras, this allows me to install the unit at the focal plane of any telescope and take low resolution spectra of dim stars and small planetary nebula. The idea is to be able to get the primary image and its spectra in the same shot so measurements and calibrations can be easy to obtain. With my 12.5" I get a dispersion of 7.9 angstroms per pixel with this 100 line per millimeter grating. This is how the comet ISON image above was taken, as well as many of the spectra of assorted stars in the data below.
 Here is an example of the star Sirius's spectrum as imaged with the 12.5" scope. Lines can be seen in its spectrum which represent the chemical elements in the stars atmosphere, in this case the hydrogen balmer lines. Cooler stars yeild different chemistries, and for the very cool M stars, you start to see many dim bands instead of sharp lines from molecules making thier impressions. Hot stars do not normally contain the spectra of molecules because they are too hot and the energy is too high for a molecule to stay together.
Projects
 MAIN PAGE: Higher Resolution Spectrograph / Spectroheliograph Project
   
Low Resolution Spectroscopic Series - OBAFGKM
Be Stars (B stars with emission lines) B0 - B9
Additional Main Sequence Normal Dwarfs to Super Giants OBAFGKM
  Carbon Stars C Class N Class R Class
Planets and Moons -- Europa vs Io
INDIVIDUAL TOPICS -- SS Cygni Observations -- Stars in M45 graphic --Stars in Cassiopiea Graphic
 NOVA -- Nova Delphi 8/17/13
 Comet ISON 10/12/13
    
   
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