IntroductionMany institutes offer a modern physics course which mayinclude experiments in nuclear and atomic physics. Hence,most physics departments carry standard photon detectingdevices. These may include photodiode photometers from acommercial company, or Geiger counters, often homemade!Both such devices are similar to what astronomers use tomeasure photons from celestial objects - Charge CoupledDevice (CCD). The efficiency of the photon detectors plays asignificant role in every apparatus, as do photon statisticsand noise. The goal of these experiments is to take thestandard lab equipment and consider its astronomicalcounterparts.

Conclusions

• Given the large expense of astronomical equipment,it is imperative to use every day physics labequipment to illustrate the principles of astronomy

• The average astronomy student in a second year labwill have had enough exposure to physics labequipment to be able to better relate the principles ofthe equipment to an astronomical setting. Asopposed to a first year student being bogged downwith fundamental equipment questions.

• Such an astronomy lab course is a good complimentto an “applied” modern physics course and is often ofgreat interest to physics majors and future scienceteachers alike.

Stellar Photometry in the Physics Lab RoomZachary Smith and Shawn Langan

Departments of Physics and Astronomy, University of Nebraska-Lincoln, 855 North 16th Street, Lincoln, Nebraska 68588-0299

References:

Curtis, L. J., 1975. Simple formula for the distortions ina Gaussian representation of a Poisson distribution.American Journal of Physics 43: 1101-1103.

Aperture Photometry Tool:http://www.aperturephotometry.org/

Physics 223 Lab Manual – Spring 2015 – University ofNebraska-Lincoln

AbstractThe focus of our workshop will be in reusing standard modernphysics equipment with the intent of showing its application toastrophysics. Most every physics department offers a modernphysics lab, which includes experiments in such topics asquantum mechanics, and atomic and nuclear physics. Theequipment for such classical experiments is quite standard.What is not standard across many physics departments, is theuse of such equipment to conduct meaningful experiments in anastrophysics lab course. Such labs attract a large portion ofSTEM and non-STEM majors alike. Given this popularity, andthe likelihood for such courses to be taught as an elective in aphysics department or high school, it becomes essential torecycle equipment from mandatory courses, such as a modernphysics course.

The workshop will include photometric reductions of celestialobjects. The presented photometric experiments will begin withtraditional radiometry, including its fundamental applications witha Geiger counter and Gamma button source, or light source anda Photodiode detector. Special attention will be paid toastronomical procedures and data processing (e.g., dark currentand bias subtraction, etc.) as they apply to the equipment athand. After finishing the traditional experiment, we will proceedwith applications of photon statistics with regards to imageprocessing and CCDs. We will make heavy use of AperturePhotometry Tool software, supported by NASA and JPL.

Lab ExperimentOnce in the classroom, it is easiest to review photon statistics withradiation counts. Button sources and Geiger counters are readilyavailable through most commercial vendors, and often can be madewith existing equipment. Above, sample data from studentsmeasuring gamma radiation from Co-60 are presented. A review ofGaussian vs. Poisson statistics can be quite helpful (reference #1).

Considering the equipment, it is worth discussing with students thecomparison between the chosen photon detector and CCD’s. Forexample, a CCD is like an array of photodiodes or Geiger counters.It is also important to point out the distinction regarding multiplemeasurements of the Co-60 source and using a Gaussian fit for thedata, vs. measuring a star once and using a Gaussian fit as well.

Software AnalysisAfter making the connection between lab photon counters andCCD’s, one can have students move into actual photon statistics ofstellar images with Aperture Photometry Tool (reference #2). Thisportion of the lab can be catered to the preferences of the instructor,however our preferred approach is to begin with a single star andreview the procedures for measuring sky brightness and instrumentgain. Then, moving on to Gaussian fits of the star brightness as afunction of distance from the center of the star image. Lastly, it isimportant to have students check the instrumental magnitude of thestar from actual photon counts.

MaterialsIn order to conduct such an experiment the main tools will bea photon source, a photon detector, and a computer forimage analysis.

Figure 3 - Photo credit:http://www.astro.virginia.edu/class/oconnell/astr511/im/QEcurves-vardetec.jpg

Figure 5 - Screen shot of Aperture Photometry Tool

Figure 2 - Standard Geiger counter needed for photon statistics

MethodsPrelabBefore touching on photon statistics, it is important toreview/explain the limits of the chosen photon detector. Thisincludes discussing the efficiency of the detector at variouswavelengths, the dark current and the bias of the detector.

Figure 4 – Sample Co-60 data from a student – Spring 2015

Figure 1 – M57 taken from the 30” telescope at Behlen Observatory