Experimenting with a Star Analyser (200 lines/mm)
By Peter Gudegon
The Star Analyser looks like an ordinary 1.25″ glass filter, but is a diffraction grating that splits-up the light coming from a star into its various colours. Many stars show absorption lines in their spectra where they have certain colours missing, while some others may have bright spots on their spectra where they emit most of their light at particular wavelengths. Suddenly each of those otherwise bland normal stars start to reveal their own individual identity…. some people refer to it as looking at a star’s fingerprint.
The analyser can be used in a variety of ways:-
- You don’t need a telescope. If you have a camera (preferable a DSLR) you can mount the star analyser in-front of the camera lens, either by mounting it on a piece of black card, or by buying a special filter holder. This limits the lens aperture size, but is fine for bright stars.
- Those with a telescope that uses standard 1.25″ dia eyepieces can simply screw it into the eyepiece as you would a normal filter, then observe the star and it’s spectra through the eyepiece as normal. Additional spacers between the analyser and eyepiece can be used to increase the spread of the spectra, making it easier to see any detail.
- But the most versatile way of using it simply, is with a telescope and camera (with no lens) and mounting the analyser just in front of the camera (most telescope/camera adapters already have the thread there for mounting filters).
The cost of these analysers is around €130-155 (August 2016) and they come in two versions:- 1. A 100 lines/mm (this is the standard type usually recommended), B. A 200 lines/mm, intended more for work with CCD cameras, and has a lower profile to allow it to fit inside filter-wheels.
What makes these so good is that they use a “blazed” grating. A normal diffraction grating has most of the light passing straight through it, with a relative low percentage of light being directed into 1 of several orders of diffraction, creating a rather dim image. A “blazed” grating uses a method that increases the amount of light being directed to one particular order of diffraction, in some designs up to 70% of the incident light may appear in the preferred diffraction.
For my own use, I used a (large) normal camera lens in place of a telescope (Sigma 150-500mm zoom), then a slim EOS lens to T2 adapter, followed by a Filter-Drawer (in which sits the star analyser), in front of a QHY9 (monochrome) CCD camera.
Unlike a normal (and very expensive) spectroscope that passes the light through a very narrow slit, this relies on the star image being as small and stable as possible (ideally a stationary point source). Too high a magnification (with a telescope) amplifies any movement of the star due to the atmosphere, which greatly reduces the resolution of the end result.
Although a zoom lens might be frowned upon by most astronomers, on a mount that has to be erected/dismantled every night it provides a very neat, portable solution, and makes initial alignment of the mount very easy before zooming in to use the full diameter of the objective. The relative low magnification means the motorised drive easily allows exposures of 30 seconds without the need to set up an auto-guide, and to my surprise has already allowed me to record spectra from stars down to magnitude 10.
After a quick test on some of the stars in Lyra, what I really wanted to try this on are some Wolf-Rayet stars. These stars have very strong emission lines, but unfortunately they are quite rare, and in the Northern hemisphere the best known/easiest to observe are a group in Cygnus. However they are all quite faint, the brightest being magnitude 6.7 (ie. below naked-eye visibility).
Below is a highly enlarged part of a picture showing the star WR136 on the left and on the right its resulting spectra, in which you can clearly see some bright spots.
But to analyse the results you really need to turn the spectra image into a graph and calibrate it (which turns out to be much easier than it sounds). Using RSpec to do this created the following graph. Although determining the significance of each line is where it becomes interesting…..and involves a lot of “Googling”.
Next another couple Wolf-Rayet stars, this time WR135 and WR137 which are known as Carbon rich stars….The difference in their spectra from the above is immediately obvious. These type of WR stars are known for their strong C [III] & C[IV] lines at 5690-5820 Angstroms. But I was surprised that with this relatively simple set-up I could look at these two stars, about magnitude 8.5, and even record the differing amplitude of the C [III] line and that they are noticeably narrower for WR135, which distinguishes it as a spectral type WC8, compared to the spectral type WC7 of WR137.