Difference between revisions of "Astronomical Spectroscopy"

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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 eyepiece 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.
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[Images]
  
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 eyepiece 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.
 
  
This term, the current students on the project (Annika Gustafsson, Justin Stockwell, and Gerald Buxton) set goals of working to improve the spectrometer-telescope setup to rule out some of these factors. The first goal was to improve the T-adapter by replacing the beam splitter with a flip mirror. In the first 2 weeks, we 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 replaced the beam splitter in the fiber adapter, allowing 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, we were able to begin moving forward and design the lab table setup. We wanted to calculate an efficiency measurement for the simplest system possible, which we would design on a lab table, and compare that to an efficiency measurement of light through our entire telescope system. The purpose of having the simplest lab table setup was to eliminate as many opportunities for light loss as possible and to calculate a maximum efficiency. This number would be equivalent to the efficiency through our telescope system if we had 100% efficiency.  
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'''Links:'''
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[http://www.oceanoptics.com/products/usb2000+.asp Ocean Optics Spectrometer]
  
Initially, for our lab table setup, we used a red laser with known power and wavelength and aligned it directly down a multimode fiber using a converging lens with the fiber attached directly to the spectrometer. We wanted to calculate the total energy received by the spectrometer using the conversion 61 photons/count at 600 nm given by Ocean Optics. Then, we would compare our calculated energy with the 1mW power given off by the laser to find our efficiency. However, data measurements revealed a loss of light that was about a factor of 10,000 less than that emitted by the laser. We went back and checked the power emitted by the laser, which we confirmed as 1mW. 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. Thus, we 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.
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[http://en.wikipedia.org/wiki/Spectrometer Wiki Spectrometer]
 
 
With the guidance of Bryan Boggs, we changed the lab table 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 fine alignment. The focal distance of the lens was large enough to be in agreement 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 at the fiber was about 0.7mW. We also took spectra of our data and gathered a bias frame which we would later subtract off of our data.
 
 
 
Next, we wanted to work on calculating the efficiency of the entire telescope system. However, we first needed to align the system before we could take any measurements. We began our alignment process by first aligning the eyepiece to the finderscope. We did this alignment in the Willamette basement hallway by placing the telescope at one end of the hallway and aligning the telescope on an Oregon O which was over 100 feet away. Once the O was placed directly on the crosshairs in the eyepiece, we adjusted the finderscope so that it was also perfectly aligned in the crosshairs. Then, we needed to align the fiber to the eyepiece. We began with a similar setup in the Willamette basement hallway. We placed the telescope at one end of the hallway and a hydrogen spectral lamp at the other end. Again, we aligned the hydrogen lamp on the optical axis of the telescope so that it was perfectly in the crosshairs of the eyepiece. With the fiber connected to the spectrometer, we had a nice hydrogen spectra to view in SpectraSuite.
 
 
 
We used this spectra to check the alignment of the fiber. By changing the x, y, and z axes we can maximize the number of counts in the main hydrogen spectral peak. When the counts are maximized, we know that our system is perfectly aligned. We began by adjusting the z axis. The distance of the fiber is changed by altering the length of the lens tube. We started with the lens tube completely screwed in and then slowly unscrewed the lens tube, increasing the length. We noticed that each full turn we would get a max counts and they were roughly the same each full turn. We decided to settle on a distance that was close to the focal distance of our converging lens where we had a max counts. Then, we proceeded to adjust the x and y axes by rotating the thumb screws on the fiber adapter. We found that max counts was when the thumb screws were all the way screwed in. However, we noticed that we had a difficult time returning to a specific max counts number. Even if when we were super careful and tedious, there was variance in the max counts number when we would move the fiber and then try to return it to the previous position. We also noticed how the flexure in our fiber system changed the counts by a couple hundred counts just lifting the fiber up and down.
 
 
 
We decided to change our procedure after talking with Bryan Boggs. Bryan recommended that we move our setup to a dark room, such as Willamette 100, where we have an LED on one end of the room and our telescope system on the other. We can measure the power given by the LED that has gone through the back of the telescope. Then, we can calculate the power through the entire telescope system out of the end of the fiber. This power should be 70-80% of the power received through just the telescope. We can fine align the fiber setup until we get a power that is 70-80% of the initial power.
 
So, we went forward with the recommended procedure. We first ran into trouble when the power meter would not pick up our LED. After messing around with the power meter and calibrating it a couple of times, we finally got some measurements. We measured a power of 4.8 microwatts through the back of the telescope and 17 nanowatts through the entire telescope system with the fiber as best aligned as we could get. We calculated that we were losing 99% of the light through our telescope system. We were very surprised by this, so we decided to try to determine where the problem was. We took the T-adapter off of the telescope and used a piece of paper to find the focal point of the light. We found the focal point was 14.5cm from the back of the telescope. On our T-adapter, 14.5cm is right after the flip mirror, so the light is being focused way too fast. We then tried to replace our converging lens with one that converged much slower. We chose a 100cm converging lens placed as close to the flip mirror as possible and the light still focused far before the fiber. Next, we tried to use a diverging lens with a converging lens to see if we could get the light to focus slower, but we were unsuccessful, and the light still focused far before the fiber. The best solution to this problem would be to use Optica and determine exactly what the light is doing in our T-adapter because we need to change the setup so that we can have the light focused directly onto the fiber.
 
 
 
Going forward next term, we propose that Optica should be the first goal. Once we have a visual of what is going on in the T-adapter, we can get the light to focus on the fiber. Then, we can return to our previous procedure where we want to align the fiber to the eyepiece using the power meter. Once the system has been successfully aligned, we can calculate an efficiency ratio of the entire telescope system and continue to characterize the spectrometer. The overall goal of the project at this point in time is to determine if it is feasible to use this spectrometer for astronomical purposes.
 

Revision as of 12:48, 7 December 2013

[Images]


Links: Ocean Optics Spectrometer

Wiki Spectrometer