Photoelectric Photometry Basics

Introduction

Photoelectric photometry is a measure oath brightness of an object, usually stars, using techniques that involve the photoelectric effect. Photomultiplier Tubes (PMTs), Pin Diodes and Charged Coupled Devices (CCDs) detectors all use the photoelectric effect and can all be used for photometry. Each type of detector has is strong and weak points. Pmts and Pin Diodes are usually considered single channel detectors meaning they measure just one star at a time. CCD detectors have the advantage of bang multiple channel detector able to observe multiple objects in a single frame. Wale CCD detectors may seem ideal, they have their drawback. For one they are actually too sensitive for stars brighter than 6th magnitude even in an 8" telescope. While it is possible to use a smaller telescope, even just camera lens, or to stop down the telescope it usually still requires 1.0 second or less exposure, which causes problems due to atmospheric scintillation. Normally at least 10 seconds is needed to average out the scintillation. There is also the the problem that suitable comparison stars are hard to find in the same field of view for bright stars.

To be scientifically useful photometry measurements must be done using standard filters and with the data reduced using calibration coefficients that standardize the data. Differential photometry if preferred as it is typically easier and more accurate. With differential photometry a comparison star of known magnitudes is measured along with the program star of interest. The comparison star should be of similar magnitude and color and not variable. If the comparison star is not a well know star, a third star is sometimes used as a check star. If the comparison star is later found to be variable, the data can be saved by going back and using the check star as the comparison star for the data reduction.

Because single channel and multiple channel photometry involved considerably different techniques they will be discussed separately.

Single Channel Photometry

Single channel photometry uses either a photomultipler tube or pin diode as a detector.

Photomultiplier tubes are glass vacuum tubes and have been around since the early 1940's. They are extremely sensitive and can be used in a photon counting mode to count individual photons. They have a thermal dark count that can be reduced to zero by cooling the tube with dry ice. Typical Quantum Efficiency (QE) is around 30%.QE is a measure of the probability that a photon will be counted. In other words if the detector is exposed to 100 photons in 1 second, a count of 30 would be produced for a QE of 30. While this may not sound very good, it is not a problem because the QE is very stable and the input verses output of the detector is very linear over a large dynamic range (in the millions). Pmts do not wear out. There is no filament or heater in the tube and it does not get hot. Photomultiplier tubes can be damaged mechanically via shock and if strong light (brighter than star light) is applied with power on the tube. The standard system for star magnitudes was devised using a 1P21 photomultiplier tube.

1P21 Photomultiplier Tube


	Photoelectric Photometry Basics

	Introduction Photoelectric photometry is a measure oath brightness of an object, usually stars, using techniques that involve the photoelectric effect. Photomultiplier Tubes (PMTs), Pin Diodes and Charged Coupled Devices (CCDs) detectors all use the photoelectric effect and can all be used for photometry. Each type of detector has is strong and weak points. Pmts and Pin Diodes are usually considered single channel detectors meaning they measure just one star at a time. CCD detectors have the advantage of bang multiple channel detector able to observe multiple objects in a single frame. Wale CCD detectors may seem ideal, they have their drawback. For one they are actually too sensitive for stars brighter than 6th magnitude even in an 8" telescope. While it is possible to use a smaller telescope, even just camera lens, or to stop down the telescope it usually still requires 1.0 second or less exposure, which causes problems due to atmospheric scintillation. Normally at least 10 seconds is needed to average out the scintillation. There is also the the problem that suitable comparison stars are hard to find in the same field of view for bright stars. To be scientifically useful photometry measurements must be done using standard filters and with the data reduced using calibration coefficients that standardize the data. Differential photometry if preferred as it is typically easier and more accurate. With differential photometry a comparison star of known magnitudes is measured along with the program star of interest. The comparison star should be of similar magnitude and color and not variable. If the comparison star is not a well know star, a third star is sometimes used as a check star. If the comparison star is later found to be variable, the data can be saved by going back and using the check star as the comparison star for the data reduction. Because single channel and multiple channel photometry involved considerably different techniques they will be discussed separately. Single Channel Photometry Single channel photometry uses either a photomultipler tube or pin diode as a detector. Photomultiplier tubes are glass vacuum tubes and have been around since the early 1940's. They are extremely sensitive and can be used in a photon counting mode to count individual photons. They have a thermal dark count that can be reduced to zero by cooling the tube with dry ice. Typical Quantum Efficiency (QE) is around 30%.QE is a measure of the probability that a photon will be counted. In other words if the detector is exposed to 100 photons in 1 second, a count of 30 would be produced for a QE of 30. While this may not sound very good, it is not a problem because the QE is very stable and the input verses output of the detector is very linear over a large dynamic range (in the millions). Pmts do not wear out. There is no filament or heater in the tube and it does not get hot. Photomultiplier tubes can be damaged mechanically via shock and if strong light (brighter than star light) is applied with power on the tube. The standard system for star magnitudes was devised using a 1P21 photomultiplier tube. 1P21 Photomultiplier Tube Created 20 June 2006 Modified 20 June 2006 RETURN