By Alessandro Odasso
The Rigel Telescope Transmission Grating Spectroscope (TGS) system has been made available to the SSON users thanks to the cooperation between Rich Williams and Dr. Robert Mutel (Iowa University). Quoting Rich’s words “The Rigel Telescope TGS system opens up a whole new field of inquiry for students, researchers, and curious people”.
I belong to the third group and I must say that when I first read his blog I was not only curious but also a bit puzzled. I was curious because I knew that the spectrum of a star is like a fingerprint, in a metaphoric sense it’s like the star’s own DNA. I was a bit puzzled because I was afraid that such a system would be too complicated to be used by a user with no specific experience.
During the Christmas holidays I decided to devote some time to read once again the blog and to pay attention to the user manual written by Dr. Mutel and I felt that, after all, it did not sound as complicated as I initially feared.
My first attempt in using the system was aimed at getting the spectrum of Merope, a star belonging to the Pleaides cluster. To do so required me to do two things:
1) I took an image of Merope(30 seconds exposure time)
2) I analyzed the FITS image with two different programs.
Phase 1 – Taking an Image
I decided to use the S filter, the one that has a slit to block out most light except for a narrow strip. You schedule to take TGS images on Rigel just as you do for regular images. However, you need to define two other parameters:
- The first parameter, “focus offest” is intended to tell the system which part of the spectrum you want to analyze with greater resolution. Having nothing specific in mind, I chose 402, a value that allows you to observe the H-alpha line with maximum spectral resolution.
- The second parameter, “RA offset” is a correction factor intended to have the star displayed on the left side of the image, just a trick to have the elongated black and white rainbow spectrum centered in the middle of the image. Again, not a problem: the manual gives you a very simple formula that calculates the RA offset, already expressed in RA seconds, taking into account the star declination.
Maybe, in the future, the SSON web page could be programmed so that this calculation is done automatically.
There is nothing different about the resulting FITS image. FITS is a standard so you can open the image with whatever FITS viewer, including Astrometrica (by H. Raab): you clearly see the spectrum spreading at the right of the slightly defocused zero-star and you can read the usual details in the FITS header.
Phase 2 – Analyzing the image and displaying the spectrum
My initial attempt to analyze the spectrum in my image was with RSpec, whose trial version can be freely downloaded from the vendor site.
I was impressed with RSpec’s very easy installation and very nice Graphical User Interface. Even without reading the manual, it is straightforward to open the FITS file and you are immediately rewarded by the system that displays a raw, not calibrated spectrum. In other words, the system displays a graph where the Y axis is related to the pixel brightness and the X axis represents the various pixels of the spectrum that you see in the FITS image.
The next step is to calibrate the graph so that the X-axis represents the wavelength expressed in nanometers (or Angstroms). The easiest calibration supported by the program is the one where you simply fill a field telling the system how many Angstroms / pixels must be taken into account. But here I was at a loss; I had no idea about the correct value!
The calibration technique is explained well in the video tutorials provided by RSpec. The idea is to identify a specific spectrum feature by simply clicking on it and by giving it the wavelength that you either know a priori or that you can get by clicking on the correspondent feature of a reference image of a star having a similar spectrum. The nice thing is that RSpec is shipped with a large reference library of star spectra.
As easy as this may seem, when I used the reference spectrum B6IV that should have worked with Merope… again I was at a loss! When I opened the spectrum of the reference image I was unable to compare it with my raw spectrum: the shapes were too different and I couldn’t determine which features matched in the two spectra!
At this point, to be honest, I felt confused. My learning curve was greater than I expected. While this learning process is certainly important, it is a bit frustrating, especially if you are a bit lazy as I am
But here I have good news! Another nice thing about RSpec is that there is a Yahoo RSpec User Group where you can ask for help! This is a place where many technical issues are discussed and clarified. So I joined the forum and I am pleased to say that they helped me immediately sending me a calibration file that was a good starting point.
I guess the reason why RSpec calibration process is not automatic is simply due to the fact that RSpec needs to be very general, the software is not tailored to any specific hardware platform.
Rigel TGS Software for Linux and Mac Operating Systems
When I contacted SSON and Dr. Mutel asking for help calibrating my TGS images, I was given software that is designed specifically for use with the Rigel TGS. Almost no calibration is needed. This software comes in the form of a Python script that requires a Linux installation. I was lucky in this case as I happen to have a dual boot system with Debian installed on my computer.
Before running the script, I had to install a few libraries according to the instructions that I received. The whole installation process took less than 30 minutes and it was straightforward.
The nice thing about the program is that it is able to generate a bitmap showing the calibrated spectrum plus specific spectrum features in a complete automatic way and in just a few seconds!
You do not have to bother about details like, for example, the graph title and exposure time: this information is generated reading the data from the FITS file. Furthermore, you can choose which spectrum features you would like to be highlighted. Finally, the program is able to synthesize a second bitmap image of the colored spectrum rainbow. There is one important input that you must be careful to identify and enter: the zero-star pixel position.
The image below is the Merope spectrum that I obtained a few minutes after the installation of Dr. Mutel’ script:
My experience proved to me that Dr. Mutel’s TGS software program is easy to use. It is only fair to note that when something like this is easy to use the reason is because other people spent a lot of time and effort to design and implement it. We must praise the Iowa and SSON teams for this!
Of course, this new tool does NOT transform us “curious people” into experts! However, thanks to this new tool and with appropriate guidance from professionals we may be in the position to give a little but not negligible scientific contribution in the framework of the so called pro-am collaborations or “citizen science projects”.
Alessandro has been an avid SSON user since November 2009. He participates in several astronomy programs and projects. I thank him for giving me his personal experience using the new Rigel remote TGS system and sharing it on my blog.
I also would like to thank Dr. Mutel for his continued excellent work on the Rigel TGS system. He wrote the code for the Rigel TGS Software and continues to enhance it with features such as TGS Stitch, which enables you to combine two or more spectrum data optimized for different wavelengths into a single data graph. It’s very cool and useful!
For those of you who want to run the Rigel TGS software on a Windows system you can install the Ubuntu Linux operating system on Oracle VirtualBox. Both VirtualBox and Ubuntu are free to download. I have this set up on the Windows 7 laptop I’m writing this with. It’s easy to do following the installation instructions for both programs. After setting up Ubuntu (or another Linux OS) on VirtualBox, download the TGS Software and install it according to Dr. Mutel’s instructions.
Here are some links to quickly get you started installing the Rigel TGS Software on your system:
– Rich WIlliams