PEP Manual

Hopkins Phoenix Observatory

Astronomical Photoelectric Photometry

Manual

Part IX
Stellar Photometry Observing Technique

Introduction
For stellar photometry, there are two types, All-Sky and Differential. When calibrating the system it is necessary to do a All-Sky photometry. There may be other time you will be doing All-Sky photometry also. For the most accurate work on program stars, Differential photometry should be done.

Red Leak Measurements
Sometimes red leak measurements are made for stars that are very bright in the R and I bands. This is to compensate for red light leaking through the U filter. The red leak is measured using a special red leak filter, sometimes a V filter plus U filter. Usually the red leak is negligible since the photomultiplier tube's response drops rapidly with longer wavelengths. If the red leak is to be measured, treat it as just another filter in the Basic Procedure and subtract the counts from the U counts.

All-Sky
All-sky photometry is the measurement of stars without a comparison star. It is very important to know the system's color-transformation coefficients and zero points. Careful measurement of the sky's extinction coefficients for that evening's observations must be made. This is done by measuring standard extinction stars near the zenith and near the horizon (at least 30 - 60 degrees from the zenith). From these measurements the extinction and zero points can be calculated. This must be done for each filter.

Differential
Differential photometry has the advantage of being much more accurate than all-sky photometry. Effects of equipment drift and extinction are minimized and the photometer system's zero points cancel when doing differential photometry. It is fairly easy to have standard deviations of 0.01 magnitudes, or better, using differential photometry.

Check Stars
Sometimes a check star is used. Check stars are used in case the comparison star is not known for sure to be non-variable. Later, if the comparison star is shown to be variable, the data may be salvaged by using the check star as the comparison star.

Sky Readings
Sky reading can vary greatly. The Hopkins Phoenix Observatory is located on the west side of Phoenix, Arizona which has an area population 2 -3 million people,. On a clear and moonless night, with a 93 arc second diaphragm the sky visual and blue filter reading are around 2,500 counts per second and ultraviolet around 800 counts per second. This is close to the zenith and well past twilight. With a full Moon 45 degrees away, the similar condition produce counts that rise to around 4,000 for visual, 5,000 for blue and 1,500 for ultraviolet. Since most of the work here is done on stars brighter than 5th magnitude, the sky counts are not a significant problem.

Differential Photometry Observing Sequence

Minimum basic sequence requires sets are 1, 2 and 3. A complete sequence requires sets 1 through 7. Set 8 is for the check star and optional for comparison stars that have a high confidence of non-variability. If unsure of the comparison star's non-variability, use a check star. V= Visual filter, B= Blue filter and U= Ultraviolet filter. Comp is comparison star, Prgm is program star and Chk is check star. Program star data sets should always be bracketed by comparison star data.

Set 1 (Basic set)

* Find Comparison (Comp) star, use the maximum sized diaphragm
* Find and center Comparison Star in smallest diaphragm used
* Comp V (3 ten second readings)
* Note Universal Time for set
* Comp B (3 ten second readings)
* Comp U (3 ten second readings)
* Move telescope to view just sky next to Comp star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 2 (Basic set)

* Find the Program (Prgm) star, use the maximum sized diaphragm
* Find and center Program Star in smallest diaphragm used
* Prgm V (3 ten second readings)
* Note Universal Time for set
* Prgm B (3 ten second readings)
* Prgm U (3 ten second readings)
* Move telescope to view just sky next to Prgm star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 3 (Basic set)

* Find Comparison star, use the maximum sized diaphragm
* Find and center Comparison Star in smallest diaphragm used
* Comp V (3 ten second readings)
* Note Universal Time for set
* Comp B (3 ten second readings)
* Comp U (3 ten second readings)
* Move telescope to view just sky next to Comp star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 4 (Expanded set)

* Find the Program star, use the maximum sized diaphragm
* Find and center Program Star in smallest diaphragm used
* Prgm V (3 ten second readings)
* Note Universal Time for set
* Prgm B (3 ten second readings)
* Prgm U (3 ten second readings)
* Move telescope to view just sky next to Prgm star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 5 (Expanded set)

* Find Comparison star, use the maximum sized diaphragm
* Find and center Comparison Star in smallest diaphragm used
* Comp V (3 ten second readings)
* Note Universal Time for set
* Comp B (3 ten second readings)
* Comp U (3 ten second readings)
* Move telescope to view just sky next to Comp star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 6 (Expanded set)

* Find the Program star, use the maximum sized diaphragm
* Find and center Program Star in smallest diaphragm used
* Prgm V (3 ten second readings)
* Note Universal Time for set
* Prgm B (3 ten second readings)
* Prgm U (3 ten second readings)
* Move telescope to view just sky next to Prgm star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 7 (Expanded set)

* Find Comparison star, use the maximum sized diaphragm
* Find and center Comparison Star in smallest diaphragm used
* Comp V (3 ten second readings)
* Note Universal Time for set
* Comp B (3 ten second readings)
* Comp U (3 ten second readings)
* Move telescope to view just sky next to Comp star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Set 8 (Optional)

* Find Check star, use the maximum sized diaphragm
* Find and center Check Star in smallest diaphragm used
* Chk V (3 ten second readings)
* Note Universal Time for set
* Chk B (3 ten second readings)
* Chk U (3 ten second readings)
* Move telescope to view just sky next to Prgm star
* Sky U (1 ten second reading)
* Sky B (1 ten second reading)
* Sky V (1 ten second reading)

Differential Photometry Data Logging
A typical observing session can produce a fair amount of data. Depending on your skill, a session doing differential photometry on just one program star can take 30 minutes to an hour. This is to get just one data point per color (UBV). For a typical session without a check star, a minimum of 91 numbers are used with some of those numbers with values in the hundreds of thousands range. When a Check Star is added, the total can be 100 numbers. All this to produce just one data point for each color. All-sky photometry does not use comparison or check stars, but extinction must be determined so All-Sky photometry can take even longer.

Manual Data Logging (Form)
Using forms and writing in the data is the simplest way to log the data. Ten second integrations of the counter will provide ample time to record the numbers and even take a sip of coffee now and then. Doing manual data logging has some advantages. You will get to know the numbers more intimately. This will enable you to tell if there is a problem. A thin invisible cloud or contrail may pass in front of the star, The star may have drifted out of the diaphragm. High Voltage may have drifted or any number of other possible things could happen. Recording data under those situations will produce bad data. It is better to wait, fix the problem or just concede for the evening, close up and go to bed. Hard copy forms also provide an excellent back up of the data. You can enter the data into a software program at another time. Because of the sometimes harsh conditions in the observatory, computers do not always work well. Pencil and paper always work. Figure 38 shows a raw data form used at the Hopkins Phoenix Observatory.

Figure 38
Sample Data Logging

Automatic/Computer Data Logging
Automatic logging of data into a computer program is not difficult, but requires the computer to be connected to the pulse conditioner output and the observer to control what is logged where and when. As noted above, the conditions may not be suited for a computer as it will be far more sensitive to extreme cold than the other electronics. A warm box may be used to help provide a more friendly environment for a computer during a cold evening's observing. One drawback to the auto logging is it is very easy to get detached from the data and allow bad data to enter. Also, if the computer should crash, you may lose your data. For high speed photometry, e.g., occultations, the automatic logging is required, but for most variable star work, it's optional.

Tips
Determine the center of the diaphragm you plan to use, as seen by post-viewer eyepiece. Make a mental note of the center and always try to put the star there. Use a large enough diaphragm. This allows maximum movement of the star. Also, for high accuracy the same position of the star in the diaphragm should always be used.

Dew can be a killer, particularly with Schmidt-Cassegrain telescopes with their corrector plates. You may not even see it on the corrector plate, lens or mirror, but it can still cause a significant reduced reading. If the humidity is 50% or more, be aware of dew. If your readings suddenly drop 10% or more and the sky still looks clear, it may be dew. Use a dew shield. In areas with heavy dew, consider a heater with the dew shield. For an 8" Schmidt-Cassegrain telescope a commercial ice cream container can make a nice inexpensive dew shield. Some resistors around the inside bottom powered by a 5 or 12 V (AC or DC) supply can add heat.

For manual logging, unless more precise timing is required, record the time (Universal Time) at the start or end of the Blue measurements. This puts the time in approximately the middle of the set of readings. Alternatively, record the time at the start of the session and at the end of the session and use the time midway as the time for the session.

Get to know your equipment. Know when something is not right and what to do about it. Make sure the high voltage is stable. Drifting high voltage can change the photomultiplier tubes sensitivity. Voltage should remain constant to within 1 volt. Again, watch for dew. With small diaphragms, the clock drive periodic error can cause problems. If reading drop during a set, find out why.

Readings that vary by 1% or less during a three-10 second set indicated a very good night and equipment working well. Readings that vary 5% or less indicate a good data set. Readings that vary more than 10% may have a problem. If no problem can be found, the sky may just be variable that much and while the data may still be good, the accuracy may decrease considerably. Readings that vary 20% or more and all seems fine, may indicate it's time to wait for a better night.

If doing All-Sky photometry, be sure to determine the extinction during the session. A star will appear brighter the higher in the sky it gets. Plotting the star's brightness over several nights at different heights or even the same heights will give the appearance of the star being variable, when indeed it is constant and just the extinction has caused the star's brightness to vary. Extinction is very important! Do not neglect it.

Observe sober. This may sound strange, but even a glass of wine or two at supper can have a profound effect on your ability to do photometry. What you need to do in the dark requires more coordination and concentration than one might think. Any impairment due to alcohol or even drugs, e.g., cold medicines, can hamper your coordination and concentration.

Part X

Return

Present Page Version as of 23 March 2004

phxjeff@hposoft.com
www.hposoft.com

	Hopkins Phoenix Observatory Astronomical Photoelectric Photometry Manual

	Part IX Stellar Photometry Observing Technique Introduction For stellar photometry, there are two types, All-Sky and Differential. When calibrating the system it is necessary to do a All-Sky photometry. There may be other time you will be doing All-Sky photometry also. For the most accurate work on program stars, Differential photometry should be done. Red Leak Measurements Sometimes red leak measurements are made for stars that are very bright in the R and I bands. This is to compensate for red light leaking through the U filter. The red leak is measured using a special red leak filter, sometimes a V filter plus U filter. Usually the red leak is negligible since the photomultiplier tube's response drops rapidly with longer wavelengths. If the red leak is to be measured, treat it as just another filter in the Basic Procedure and subtract the counts from the U counts. All-Sky All-sky photometry is the measurement of stars without a comparison star. It is very important to know the system's color-transformation coefficients and zero points. Careful measurement of the sky's extinction coefficients for that evening's observations must be made. This is done by measuring standard extinction stars near the zenith and near the horizon (at least 30 - 60 degrees from the zenith). From these measurements the extinction and zero points can be calculated. This must be done for each filter. Differential Differential photometry has the advantage of being much more accurate than all-sky photometry. Effects of equipment drift and extinction are minimized and the photometer system's zero points cancel when doing differential photometry. It is fairly easy to have standard deviations of 0.01 magnitudes, or better, using differential photometry. Check Stars Sometimes a check star is used. Check stars are used in case the comparison star is not known for sure to be non-variable. Later, if the comparison star is shown to be variable, the data may be salvaged by using the check star as the comparison star. Sky Readings Sky reading can vary greatly. The Hopkins Phoenix Observatory is located on the west side of Phoenix, Arizona which has an area population 2 -3 million people,. On a clear and moonless night, with a 93 arc second diaphragm the sky visual and blue filter reading are around 2,500 counts per second and ultraviolet around 800 counts per second. This is close to the zenith and well past twilight. With a full Moon 45 degrees away, the similar condition produce counts that rise to around 4,000 for visual, 5,000 for blue and 1,500 for ultraviolet. Since most of the work here is done on stars brighter than 5th magnitude, the sky counts are not a significant problem. Differential Photometry Observing Sequence Minimum basic sequence requires sets are 1, 2 and 3. A complete sequence requires sets 1 through 7. Set 8 is for the check star and optional for comparison stars that have a high confidence of non-variability. If unsure of the comparison star's non-variability, use a check star. V= Visual filter, B= Blue filter and U= Ultraviolet filter. Comp is comparison star, Prgm is program star and Chk is check star. Program star data sets should always be bracketed by comparison star data. Set 1 (Basic set) * Find Comparison (Comp) star, use the maximum sized diaphragm * Find and center Comparison Star in smallest diaphragm used * Comp V (3 ten second readings) * Note Universal Time for set * Comp B (3 ten second readings) * Comp U (3 ten second readings) * Move telescope to view just sky next to Comp star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 2 (Basic set) * Find the Program (Prgm) star, use the maximum sized diaphragm * Find and center Program Star in smallest diaphragm used * Prgm V (3 ten second readings) * Note Universal Time for set * Prgm B (3 ten second readings) * Prgm U (3 ten second readings) * Move telescope to view just sky next to Prgm star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 3 (Basic set) * Find Comparison star, use the maximum sized diaphragm * Find and center Comparison Star in smallest diaphragm used * Comp V (3 ten second readings) * Note Universal Time for set * Comp B (3 ten second readings) * Comp U (3 ten second readings) * Move telescope to view just sky next to Comp star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 4 (Expanded set) * Find the Program star, use the maximum sized diaphragm * Find and center Program Star in smallest diaphragm used * Prgm V (3 ten second readings) * Note Universal Time for set * Prgm B (3 ten second readings) * Prgm U (3 ten second readings) * Move telescope to view just sky next to Prgm star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 5 (Expanded set) * Find Comparison star, use the maximum sized diaphragm * Find and center Comparison Star in smallest diaphragm used * Comp V (3 ten second readings) * Note Universal Time for set * Comp B (3 ten second readings) * Comp U (3 ten second readings) * Move telescope to view just sky next to Comp star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 6 (Expanded set) * Find the Program star, use the maximum sized diaphragm * Find and center Program Star in smallest diaphragm used * Prgm V (3 ten second readings) * Note Universal Time for set * Prgm B (3 ten second readings) * Prgm U (3 ten second readings) * Move telescope to view just sky next to Prgm star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 7 (Expanded set) * Find Comparison star, use the maximum sized diaphragm * Find and center Comparison Star in smallest diaphragm used * Comp V (3 ten second readings) * Note Universal Time for set * Comp B (3 ten second readings) * Comp U (3 ten second readings) * Move telescope to view just sky next to Comp star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Set 8 (Optional) * Find Check star, use the maximum sized diaphragm * Find and center Check Star in smallest diaphragm used * Chk V (3 ten second readings) * Note Universal Time for set * Chk B (3 ten second readings) * Chk U (3 ten second readings) * Move telescope to view just sky next to Prgm star * Sky U (1 ten second reading) * Sky B (1 ten second reading) * Sky V (1 ten second reading) Differential Photometry Data Logging A typical observing session can produce a fair amount of data. Depending on your skill, a session doing differential photometry on just one program star can take 30 minutes to an hour. This is to get just one data point per color (UBV). For a typical session without a check star, a minimum of 91 numbers are used with some of those numbers with values in the hundreds of thousands range. When a Check Star is added, the total can be 100 numbers. All this to produce just one data point for each color. All-sky photometry does not use comparison or check stars, but extinction must be determined so All-Sky photometry can take even longer. Manual Data Logging (Form) Using forms and writing in the data is the simplest way to log the data. Ten second integrations of the counter will provide ample time to record the numbers and even take a sip of coffee now and then. Doing manual data logging has some advantages. You will get to know the numbers more intimately. This will enable you to tell if there is a problem. A thin invisible cloud or contrail may pass in front of the star, The star may have drifted out of the diaphragm. High Voltage may have drifted or any number of other possible things could happen. Recording data under those situations will produce bad data. It is better to wait, fix the problem or just concede for the evening, close up and go to bed. Hard copy forms also provide an excellent back up of the data. You can enter the data into a software program at another time. Because of the sometimes harsh conditions in the observatory, computers do not always work well. Pencil and paper always work. Figure 38 shows a raw data form used at the Hopkins Phoenix Observatory. Figure 38 Sample Data Logging Automatic/Computer Data Logging Automatic logging of data into a computer program is not difficult, but requires the computer to be connected to the pulse conditioner output and the observer to control what is logged where and when. As noted above, the conditions may not be suited for a computer as it will be far more sensitive to extreme cold than the other electronics. A warm box may be used to help provide a more friendly environment for a computer during a cold evening's observing. One drawback to the auto logging is it is very easy to get detached from the data and allow bad data to enter. Also, if the computer should crash, you may lose your data. For high speed photometry, e.g., occultations, the automatic logging is required, but for most variable star work, it's optional. Tips Determine the center of the diaphragm you plan to use, as seen by post-viewer eyepiece. Make a mental note of the center and always try to put the star there. Use a large enough diaphragm. This allows maximum movement of the star. Also, for high accuracy the same position of the star in the diaphragm should always be used. Dew can be a killer, particularly with Schmidt-Cassegrain telescopes with their corrector plates. You may not even see it on the corrector plate, lens or mirror, but it can still cause a significant reduced reading. If the humidity is 50% or more, be aware of dew. If your readings suddenly drop 10% or more and the sky still looks clear, it may be dew. Use a dew shield. In areas with heavy dew, consider a heater with the dew shield. For an 8" Schmidt-Cassegrain telescope a commercial ice cream container can make a nice inexpensive dew shield. Some resistors around the inside bottom powered by a 5 or 12 V (AC or DC) supply can add heat. For manual logging, unless more precise timing is required, record the time (Universal Time) at the start or end of the Blue measurements. This puts the time in approximately the middle of the set of readings. Alternatively, record the time at the start of the session and at the end of the session and use the time midway as the time for the session. Get to know your equipment. Know when something is not right and what to do about it. Make sure the high voltage is stable. Drifting high voltage can change the photomultiplier tubes sensitivity. Voltage should remain constant to within 1 volt. Again, watch for dew. With small diaphragms, the clock drive periodic error can cause problems. If reading drop during a set, find out why. Readings that vary by 1% or less during a three-10 second set indicated a very good night and equipment working well. Readings that vary 5% or less indicate a good data set. Readings that vary more than 10% may have a problem. If no problem can be found, the sky may just be variable that much and while the data may still be good, the accuracy may decrease considerably. Readings that vary 20% or more and all seems fine, may indicate it's time to wait for a better night. If doing All-Sky photometry, be sure to determine the extinction during the session. A star will appear brighter the higher in the sky it gets. Plotting the star's brightness over several nights at different heights or even the same heights will give the appearance of the star being variable, when indeed it is constant and just the extinction has caused the star's brightness to vary. Extinction is very important! Do not neglect it. Observe sober. This may sound strange, but even a glass of wine or two at supper can have a profound effect on your ability to do photometry. What you need to do in the dark requires more coordination and concentration than one might think. Any impairment due to alcohol or even drugs, e.g., cold medicines, can hamper your coordination and concentration.
	Part X Return
	Present Page Version as of 23 March 2004 phxjeff@hposoft.com www.hposoft.com