Astronomical Spectroscopy
The overall goal of the Astronomical Spectroscopy Project is to be able to collect and reduce data gathered from astronomical objects. This will be done using a Celestron CPC 800 GPS (XLT) telescope and a fiber-fed Ocean Optics USB2000+ spectrometer. The initial setup at the beginning of Fall term consisted of the telescope with a beam splitter and lens tube system attached to the back. The beam splitter attached to an eye piece on top and a lens tube at the back with a converging lens and the fiber attached at the end of the lens tube. The fiber was attached using a fiber adapter with thumb screws which gave three degrees of freedom for the fiber: 1 along the z axis where the length of the lens tube can be changed on the outside, and 2 and 3 along the x and y axis where the position of the fiber can be adjusted using the two thumb screws.
At the end of the 2012-2013 school year, the student group working on the project was able to collect a spectra of vega, however, the minimum integration time required to get a spectra that resembled Vega was 600 seconds. The group (Annika Gustafsson, Gerald Buxton, and Zach Small) concluded that the large integration time was necessary for multiple reasons: 1.) the fiber was not perfectly aligned on the back of the telescope which allowed for loss of light 2.) the use of a 50:50 beam splitter in the fiber adapter, which allowed for 50% light loss, was too much loss, and 3.) the Ocean Optics spectrometer might not be suitable (i.e. not sensitive enough) for gathering spectra of astronomical objects.
This term, the current students on the project, which include Annika Gustafsson, Justin Stockwell, and Gerald Buxton, are going to work on improving the spectrometer-telescope setup to rule out some of these factors. In the 2 weeks, the group ordered a flip mirror, machined an adapter for the lens tube to connect to the flip mirror, and began initial lab table setup.
The flip mirror replaces the beam splitter in the fiber adapter. This allows us to retain all of the light coming through the back of the telescope instead of dividing it 50:50 between the fiber and the eyepiece. With the fiber adapter assembled using the newly machined adapter between the lens tube and flip mirror, the group was able to begin designing the lab table setup. The goal is to have the simplest setup possible to eliminate excess opportunities for light loss.
Initially, the group used a red laser with known power and wavelength and aligned it directly into a fiber using a converging lens where the fiber was attached directly into the spectrometer. The idea was to gather a baseline sensitivity measurement which we can compare to the expected 61 photons/count at 600nm given by Ocean Optics. Then, we could have the laser go through our telescope system to gather another sensitivity measurement. Thus, allowing us to calculate the ratio of efficiency of our telescope system.
However, data measurements revealed a loss of light that was about a factor of 10,000 less than that emitted by the laser. The group used a hand held power meter in front of laser and found that there should be the expected 1mW of power from the laser going down the fiber. We also checked the numerical aperture of our setup to rule out loss of light due to that reason, however, our focal distance was large enough that it would not have played any effect. The group concluded that the lab table setup had too many places for light loss and there were not enough ways to do fine alignment on the fiber.
With the guidance of Bryan Boggs, the group changed the setup to allow for the possibility of doing fine alignment on the fiber. The setup consisted of the laser emitted through an iris to a converging lens, and directed into the fiber using two square mirrors with thumb screws for alignment. The focal distance of the lens was greater was consistent with the numerical aperture of our fiber. We were able to track power loss through the system using the hand held power meter and found that the power of laser light at the fiber was about 0.7mW. We took spectra of our data and gathered a bias frame which we would later subtract off of our data.
Moving forward, in Week 7, the group will first work on fine alignment of the fiber by first aligning the eyepiece to the finderscope, done in the atrium, and then the fiber to the eyepiece, done on the lab table. This needs to be done first before we can gather any data of the laser through our telescope system. Once we have our fiber perfectly aligned, we will use the same procedure as in the lab to gather data which we can then use to create an efficiency ratio of our telescope system.
Finally, we want to characterize the Ocean Optics Spectrometer. The overall goal for this term is to be able to calculate a sensitivity measurement for the spectrometer to determine if it is feasible to use this spectrometer for astronomical purposes.
