PEP Manual

Part III
Getting Started

Types of Astronomical Photoelectric Photometry Equipment

Charged Coupled Devices (CCD)
The CCD cameras are becoming more popular everyday. The cost is plummeting and capabilities increasing. Even web cameras, selling for well under $100 have been used for astronomical purposes including photometry. The CCD camera is great for fainter stars when multiple stars are all in the same field of an image. Most CCDs are sensitive in the B, V, R and I regions. The ultraviolet band is out of reach of all but very specialized CCD detectors. CCD cameras excel in sensitivity in the Red and Infrared bands. Using CCD cameras for photometry involves more work than single channel devices before the data can be reduced, CCD cameras do not have the dynamic range, sensitivity and typically do not have the precision of photomultiplier tube photometry.

Photodiodes (PD)
Photodiodes are similar to the CCD devices, but are single channel. Instead of multiple small (pixels) detectors, the photodiode has just one large detector. This makes observing easier for stars that are not within the same image of a CCD camera. The photodiodes have about the same sensitivity range as the CCD cameras and most are not sensitive in the ultraviolet region. Like the CCD detectors they provide excellent sensitivity in the Red and Infrared bands. Photodiode devices produce extremely small currents and require very high amplification. This means extra special care to prevent signal leakage. As with the CCDs, photodiodes do not have the sensitivity or dynamic range of photomultiplier tubes and scale switching in the amplifier must be used for different magnitude stars. Calibration of the amplification stages must be made. The amplification factor must be taken into account when reducing the data.

Photomultiplier Tubes (PMT)
Photomultiplier tubes provide the most sensitive means of detecting light. They are capable of detecting single photons. Photomultiplier tube output is very linear with respect to input light. Typical dynamic range is on the order of 107. The tubes are glass vacuum tubes and are fragile. Unlike the CCDs and photodiodes, photomultiplier tubes can be damaged by exposing them to bright light when power is applied or from physical shock, e.g., dropping even if the glass does not break. Unlike normal vacuum tubes they contain no filament, do not get hot and do not burn out (with normal use). They require very little power. The standard photomultiplier tube is the side-window 1P21/931A. The 931A is essentially the same tube as the 1P21, but usually with a bit less sensitivity and with more noise. The 1P21 tubes are merely 931A tubes that have been selected for high sensitivity and low noise. Some 931A tubes are actually better than some 1P21 tubes, however.

RCA used to make these tubes, but no longer does. Hamamatsu currently makes them along with a wide variety of other photomultiplier tubes and associated devices and parts. EMI Gencom, Inc. (no longer in business) produced an equivalent side-window tube, the EMI 9781. Surplus tubes can usually be found at reasonable prices. A common tube is the 1P28. At first glance this seems to be a good replacement, but it has very high dark current and while it has the same spectral response as the 1P21 (enhanced for the ultraviolet region) and can be used, it is not recommended.

Some people are concerned that photomultiplier tubes require high voltage (around a -1,000 VDC). While this is true, the current required is on the order of a 1 or 2 milliamperes. Most photomultiplier tube high voltage power supplies are considerably less dangerous than the household wall socket with 110 VAC. While high current high voltage supplies could be used and would present a danger, they are not needed.

Why Single Channel Photometry
While CCD systems for photometry sound very enticing with their ability to image multiple stars within one image, for brighter stars this usually does not work. With brighter stars, finding suitable comparison and check stars within the same image area is rare. CCDs are great for fainter stars where the program star or stars have many similar stars in the same area and comparison and check stars can be found easily. This is especially true for star clusters. CCD systems are not as accurate as photomultiplier tube systems. At best and with a great deal of effort, CCD systems can approach 0.01 magnitude accuracy. Most of the time only 0.05 to 0.1 magnitude is obtained. The cost of a good quality CCD system can be considerable. There must be a computer connected to it. In extreme environments this requires environmentally isolating the computer. The CCD must have a good dynamic range which means the ADC (analog-to-digital converter) must provide at least 16 bit conversions (65,000 levels). While 8 bit systems can be used and the dynamic range increased by image stacking, this creates a great deal of extra work.

Single channel photodiode or photomultiplier tube photometry is much simpler and can be less costly than CCD photometry. Photodiode systems can be completely self contained battery operated units. This is great for student environments. Data can be read out directly on the device and they are not subject to bright light damage as photomultiplier tubes are. The photodiodes require amplification switching and cannot normally be used in the ultraviolet region. As noted before, they are excellent of the Red and Infrared bands, however.

Photomultiplier tubes have been a standard for photometry for many years. Most of the published astronomical photometry data were obtained with photomultiplier tube photometers. The photomultiplier tube system has several advantages. It's a simple system and building one is well within the capability of an amateur astronomer with modest building skills. Photomultiplier tube systems offer large dynamic range and linearity. They are very fast devices (can easily operate in the sub-millisecond range) and are great for occultations where short integration times are needed. With proper filters, they can produce data that closely matches standard photometry data. This means multiple worldwide observers can easily compare their data on a given star. Photomultiplier tube systems can produce very accurate data. With care 0.01 magnitude data are easily obtained and 0.001 magnitude data within reach. Perhaps the most fascinating characteristic of photomultiplier tubes is that they can indeed detect single photons. The following lays the ground work so you too can count photons.

	Hopkins Phoenix Observatory Astronomical Photoelectric Photometry Manual

	Part III Getting Started Types of Astronomical Photoelectric Photometry Equipment Charged Coupled Devices (CCD) The CCD cameras are becoming more popular everyday. The cost is plummeting and capabilities increasing. Even web cameras, selling for well under $100 have been used for astronomical purposes including photometry. The CCD camera is great for fainter stars when multiple stars are all in the same field of an image. Most CCDs are sensitive in the B, V, R and I regions. The ultraviolet band is out of reach of all but very specialized CCD detectors. CCD cameras excel in sensitivity in the Red and Infrared bands. Using CCD cameras for photometry involves more work than single channel devices before the data can be reduced, CCD cameras do not have the dynamic range, sensitivity and typically do not have the precision of photomultiplier tube photometry. Photodiodes (PD) Photodiodes are similar to the CCD devices, but are single channel. Instead of multiple small (pixels) detectors, the photodiode has just one large detector. This makes observing easier for stars that are not within the same image of a CCD camera. The photodiodes have about the same sensitivity range as the CCD cameras and most are not sensitive in the ultraviolet region. Like the CCD detectors they provide excellent sensitivity in the Red and Infrared bands. Photodiode devices produce extremely small currents and require very high amplification. This means extra special care to prevent signal leakage. As with the CCDs, photodiodes do not have the sensitivity or dynamic range of photomultiplier tubes and scale switching in the amplifier must be used for different magnitude stars. Calibration of the amplification stages must be made. The amplification factor must be taken into account when reducing the data. Photomultiplier Tubes (PMT) Photomultiplier tubes provide the most sensitive means of detecting light. They are capable of detecting single photons. Photomultiplier tube output is very linear with respect to input light. Typical dynamic range is on the order of 107. The tubes are glass vacuum tubes and are fragile. Unlike the CCDs and photodiodes, photomultiplier tubes can be damaged by exposing them to bright light when power is applied or from physical shock, e.g., dropping even if the glass does not break. Unlike normal vacuum tubes they contain no filament, do not get hot and do not burn out (with normal use). They require very little power. The standard photomultiplier tube is the side-window 1P21/931A. The 931A is essentially the same tube as the 1P21, but usually with a bit less sensitivity and with more noise. The 1P21 tubes are merely 931A tubes that have been selected for high sensitivity and low noise. Some 931A tubes are actually better than some 1P21 tubes, however. RCA used to make these tubes, but no longer does. Hamamatsu currently makes them along with a wide variety of other photomultiplier tubes and associated devices and parts. EMI Gencom, Inc. (no longer in business) produced an equivalent side-window tube, the EMI 9781. Surplus tubes can usually be found at reasonable prices. A common tube is the 1P28. At first glance this seems to be a good replacement, but it has very high dark current and while it has the same spectral response as the 1P21 (enhanced for the ultraviolet region) and can be used, it is not recommended. Some people are concerned that photomultiplier tubes require high voltage (around a -1,000 VDC). While this is true, the current required is on the order of a 1 or 2 milliamperes. Most photomultiplier tube high voltage power supplies are considerably less dangerous than the household wall socket with 110 VAC. While high current high voltage supplies could be used and would present a danger, they are not needed. Why Single Channel Photometry While CCD systems for photometry sound very enticing with their ability to image multiple stars within one image, for brighter stars this usually does not work. With brighter stars, finding suitable comparison and check stars within the same image area is rare. CCDs are great for fainter stars where the program star or stars have many similar stars in the same area and comparison and check stars can be found easily. This is especially true for star clusters. CCD systems are not as accurate as photomultiplier tube systems. At best and with a great deal of effort, CCD systems can approach 0.01 magnitude accuracy. Most of the time only 0.05 to 0.1 magnitude is obtained. The cost of a good quality CCD system can be considerable. There must be a computer connected to it. In extreme environments this requires environmentally isolating the computer. The CCD must have a good dynamic range which means the ADC (analog-to-digital converter) must provide at least 16 bit conversions (65,000 levels). While 8 bit systems can be used and the dynamic range increased by image stacking, this creates a great deal of extra work. Single channel photodiode or photomultiplier tube photometry is much simpler and can be less costly than CCD photometry. Photodiode systems can be completely self contained battery operated units. This is great for student environments. Data can be read out directly on the device and they are not subject to bright light damage as photomultiplier tubes are. The photodiodes require amplification switching and cannot normally be used in the ultraviolet region. As noted before, they are excellent of the Red and Infrared bands, however. Photomultiplier tubes have been a standard for photometry for many years. Most of the published astronomical photometry data were obtained with photomultiplier tube photometers. The photomultiplier tube system has several advantages. It's a simple system and building one is well within the capability of an amateur astronomer with modest building skills. Photomultiplier tube systems offer large dynamic range and linearity. They are very fast devices (can easily operate in the sub-millisecond range) and are great for occultations where short integration times are needed. With proper filters, they can produce data that closely matches standard photometry data. This means multiple worldwide observers can easily compare their data on a given star. Photomultiplier tube systems can produce very accurate data. With care 0.01 magnitude data are easily obtained and 0.001 magnitude data within reach. Perhaps the most fascinating characteristic of photomultiplier tubes is that they can indeed detect single photons. The following lays the ground work so you too can count photons.
	Part IV Return
	Present Page Version as of 23 March 2004 phxjeff@hposoft.com www.hposoft.com