In my blog “Robert Mutel – Remote Astronomy Pioneer” I talked about how the University of Iowa students and Robert developed a transmission Grating Spectrograph (TGS) system on the Rigel telescope for education and research projects. This system is now available to SSON users. This is a ground breaking opportunity for our users. Now you can do your own spectroscopy project using SSON!
A TGS is basically a high quality piece of glass with rulings or lines that refract and disperse the incoming light to produce a spectrum of the object of interest. These gratings are measured in lines per millimeter. The more lines per millimeter in the grating the higher the resolution is of the resulting spectra. You might think you want to use transmission gratings with the most lines per millimeter possible (or affordable) to get the highest resolution possible. However, it’s not that simple of a decision. Because the grating disperses the incoming light spreading it out to create a spectrum on the receiving CCD chip the signal (light) is attenuated (dimmed). The more attenuated the signal the longer the exposure times you need to record the spectra to get a usable signal to noise ratio to analyze the data.
The more lines per millimeter in the TGS the more the light in the spectra are spread out. This has two effects: the length of the spectrum increases and the signal decreases. For the first effect you need to make sure that the CCD chip that gathers the light from the spectra is wide enough to capture the entire spectrum. That is easy to solve if you can find a big enough CCD chip to fit the resulting spectra. The second effect can only be overcome by increasing the size of the telescope aperture (more light gathering capability) or increasing the length of the exposure times for gathering light. And it gets even more complicated. As you increase the number of lines per millimeter you start cutting off the light at the lower frequencies, decreasing the transmission efficiency curve. Because the grating transmits less effectively at lower frequencies the gratings need to be mounted in a tilted angle (instead of flat like most filters) to give a more linear signal throughout the length of the measured spectra.
The University of Iowa Rigel team took all of these constraints into consideration in designing and testing the system that is currently on the Rigel telescope. The system automatically points to the desired object, centers it, and adjusts the focus for their remote system. The system has both a 300-line TGS and a 600-line TGS installed. The end result is an excellent lower resolution spectroscopy system for interesting and powerful projects.
When you receive your TGS data from SSON you will need a software program to analyze the spectra data in your FITS files. The students at the University of Iowa use RSpec (http://www.rspec-astro.com/). It is a powerful, easy to use program that only costs $99. You can download a free fully functional trail version for 30 days to check it out. It’s amazing to see the chemical makeup of stars, nebula, and planets when you analyze your data.
So what can you do with the TGS on the Rigel telescope? It turns out quite a lot. You can check out some of the projects being done by the University of Iowa students. You can measure the spectra of stars down to 12th magnitude or so with the 600-line TGS with exposure times < 300 seconds. Low resolution TGS systems are not sensitive enough to measure the doper redshift effect of exoplanets, but the system has been used to measure the redshift (and therefore the distance) of a quasar.
I’m just getting started with remote spectroscopy and it fascinates me. This capability opens up huge potential for new innovative projects by SSON users.We are making plans to add this same type of system on other larger SSON telescopes increasing the range and depth of the potential projects to try out.