Photoelectric Photometry PEP

A stellar photometer is an electrical device that measures the amount of light received from a single star. The process of using a stellar photometer to measure the light intensity of a star is called photoelectric photometry and is abbreviated as PEP. People who make photoelectric photometry measurements are sometimes called "Peppers."

There is much interest within the amateur community regarding PEP. For example, the International Amateur-Professional Photoelectric Photometry (IAPPP) group was formed in June 1980 with the goal to facilitate collaborative astronomical research between amateur, student, and professional astronomers. IAPPP provides a medium for the exchange of practical information not normally discussed at symposia or published in other journals. Dr. Douglas Hall, Dyer Observatory, is the publisher and he is well known for his interest in amateur/professional collaboration.

The AAVSO Photoelectric Photometry Program began in 1983 with three observers who in that year contributed 219 observations on 28 stars. Standards were established on how to observe, i.e. taking three measurements of the variable star and one of a check star to monitor constancy of the comparison star. These standards have not changed and the reduction program used today is the same one developed when this program began.

As with using a CCD, there is a learning curve when you begin using PEP methods but it should not be viewed as an obstacle. For more advanced interests, I recommend Photoelectric Photometry of Variable Stars, by Douglas Hall and Russell Genet; and Astronomical Photometry, by A. Henden and R. Kaitchuck.

Make no little plans; they have no magic to stir men's blood and probably themselves will not be realized. Make big plans; aim high in hope and work ...

Daniel H. Burnham

"What variable stars are observable by me at my particular geographical location, during each season, and with my particular equipment?" are also concerns for variable-star observers. A well-developed plan will assist you in answering these questions, and many more, while at the same time directing your activities so that you can begin to prepare to observe variable stars with confidence and little wasted time. Without a plan, you'll struggle while making preparations. And without a plan it's easy to get sidetracked, distracted or waste time looking for stars that are just not in a good position for you to observe. If you believe that you're not the type to become distracted, imagine exploring the sky between Scorpius and Sagittarius, perhaps on a clear night letting your eye wander over near p Ophiuchus, or depending upon the season, looping through Orion "the Hunter", across the line of belt stars and on to the Great nebula. If you don't believe that you can't be seduced, at least delayed, when viewing the double cluster in Perseus, the Cygnus loop, the Tarantula nebula in Dorado, or the Great Andromeda galaxy, then you really haven't been behind a telescope's eyepiece in awhile. These are all magnificent sights, typical of what you'll come across while observing, and much like the Siren's song, they can tempt you into altering your course so that you never reach your original destination.

Your plan will help determine your needs for an evening of observation and it will help keep you focused on the specific tasks needed to reach a particular goal. If you need a suggestion as to where to begin, visit the Variable Star Network, the British Astronomical Association Variable Star Section, or the American Association of Variable Star Observer's Web sites1 to discover what is afoot. See what other variable star observers around the country, or around the world, are observing. These Web sites report the latest variable star interests.

Perhaps a new nova has been detected or a cataclysmic variable has exploded into outburst. Possibly a supernova has been discovered in a bright galaxy or a gamma-ray burst has been reported. Of course, in the end, you should develop your own plan based upon your personal desires. For example, tonight you may wish to time an eclipsing binary, complete 50 visual observations, search for supernovae in 20 galaxies, or make 10 inner sanctum estimates. The increased anticipation that you'll feel when preparing your plan can add to the excitement of the actual observations.

An invaluable aid to planning is a log or record book and your log is a great place to record your plan so that you can refer to it during the evening, if necessary. Remember, not everything will go according to plan. As you begin your investigation of the variable stars, you will have successes and you will have failures. A failure is really a success if you look at it as successfully determining that this isn't the way to do something, so we'll refer to every happening as a success. It will be your responsibility to determine if any particular success should be repeated or avoided.

Records and logs will allow you to record your successes (those you wish to repeat and those you wish to avoid), observations, unusual occurrences (you're going to be surprised!), memorable events (you're going to be delighted!) and other things too numerous to mention here. Your log can serve not only as a record of your most recent observations but also as a living, historical document that will assist you during the years

'The Web address for these organizations are provided later in this chapter.

2Inner sanctum estimates, a term used by the AAVSO, are those positive visual observations made on stars 13™8 or fainter, or a fainter-than (you could not see the variable) observation of 14?0.

to come. It may contain short-cuts and more efficient methods that you discover during observing, notes to yourself regarding problems that you encounter, notes from one season to the next that will allow you to continue observing after a seasonal break of several months, events that you want to remember, ideas that you want to follow-up on later, weather notes, equipment behavior, discoveries, or a multitude of other notable items. I find it enjoyable to review my logs from past years. I'm able to recall great nights, interesting events and I just enjoy reviewing my exploration of the Universe. Not all entries need to have scientific value. Early morning entries, made when I was most tired, are probably best described as entertainment.

Your log or record book is a personal item. There is no preferred format for a log book, so you must develop a format that best suits your personality, observing needs, as well as scientific and historical obligations, if any. The contents of your log can range in style from casual to meticulous; in structure, from direct to elegant; in method, from hand-written to computer generated.

So as to provide a starting point, let's develop a simple log book and we'll begin with an observing plan. An example of a simple observing plan may look something like the following:

VARIABLE STAR LOG - Oct 27 , 2001

Nightly observation plan: Checked AAVSO; possible nova detected in Eridanus and V1159 Orionis (SU UMa type CV) is in outburst. VSNET reports a brightening of omega CMa (GCAS type variable) . Along with observing these three objects, check the following dwarf novae for outbursts: HL CMa and SU UMa, and estimate the brightness of the following LPVs: R Lep and T Lep. Locate the variable star RU Peg for future observation.

You don't need to make an observing plan every night either. You can make a plan for the month, listing the stars that you want to observe or the galaxies you want to check for supernovae. Then, when you find some unexpected free time, just grab your plan and you're ready to go observing with little time wasted wondering what to view. I've prepared an observing plan for each month, based on the stars visible from my location. Most stars, except those very close to the horizon, are visible for more than a month so you'll have the same stars on several of your monthly plans. The circumpolar stars, those that never dip below the horizon, will be available all year and so will be on every monthly plan. As a suggestion, review your monthly plans each year. Remove those stars that you never observe and periodically add some new ones. Also, consider the Moon and Sun during planning. Observing a variable star near a full Moon or just as the Sun begins to rise can be difficult to do.

Your plan need not be overly complicated but preparing one will help you concentrate on those observations that, for whatever reason, you feel are important and must be made during the evening. Once you have accomplished the goals set forth in your plan each evening, you're free to roam around the sky and look at the sights. Don't make your plan too complex or restrictive when you first begin observing variable stars. I've mentioned it before but, in the excitement of the moment, the idea gets lost sometimes; remember, the reason that you're doing this is to observe and have fun. A half-dozen well made observations will leave you with a sense of satisfaction and accomplishment. Add more targets as your abilities improve and as you come to better understand the time commitment. Estimates made in a rush or when you're frustrated and tired may well be less than valueless. Certainly if you are frustrated or unhappy, it won't be fun. It's my opinion, so I could very well be wrong, but I believe that the Universe with all of its attendant stars will be there sufficiently long for you to take your time and enjoy the experience.

Prepare your log or record book ahead of time, perhaps in concert with your nightly observing plan. Label the data columns and sharpen a couple of pencils. I recommend that you use a pencil for your data entry so that an error can be easily corrected. For the first few observing sessions, keep your administrative burden to a minimum. Your primary reason for doing all of this is to observe variable stars. Don't let the paperwork get in your way when you first begin. You will have plenty of time to refine your recording methods.

A simple log contains all of the essential information that you will need to satisfactorily record your variable star observations. I would like to recommend a couple things here. First, whatever format you use for recording the date, stick with it and don't change it from night to night or season to season. Second, record

your observations using local time. Trying to convert to Julian Date (JD) or Universal Time (UT) while observing is an additional distraction that is unnecessary. You can convert your observation time to JD or UT later.

A log like this can be kept in a small paper notebook or loose leaf binder, similar to what is used by students for their school work. If you keep your log on a computer hard drive, be sure and consider a back-up copy, such as on a ZIP disk. You'll find that as your logs age, their value increases. Don't risk losing them.

A log containing your observations may look like this:

t[j,jM-ly observation plan : Checked AAVSO; possible nova detected in Eridanus and V1159 Orionis (SU UMa type CV) is in outburst. VSNET reports a brightening of omega CMa (GCAS type variable) . Along with observing these three objects, check che following dwarf novae for outbursts: HL CMa & SO UMa, and estimate the brightness of the following LPVs: R Lep and T Lep. Locate the variable star RU Peg For future observation.







comp star

Oct 27, 2001



RU Peg


Std RU Peg (d)

126, 135

Oct 27, 2001


:4 5PM

V1159 Ori


Pre V1159 Ori (d)

124, 136

Oct 27, 2001



R Lep


Std R Lep (b)

75, 78

Oct 27, 2001



T Lep


Std T Lep (b)

103, 106

Oct 27, 2001





Pre HL CMa (e)

104, 116


Oct 27, 2001


: 16PM

Omega CMa


Tycho stars

HD 57821

Observer' s note: RU Peg is tough to find. Two faint stars are confusing. I really need to check the charts closely. I couldn't find a chart for Omega CMa so I used two stars with Tycho magnitudes. Clouds formed in the sky before I could observe SU Uma.

Observer' s note: RU Peg is tough to find. Two faint stars are confusing. I really need to check the charts closely. I couldn't find a chart for Omega CMa so I used two stars with Tycho magnitudes. Clouds formed in the sky before I could observe SU Uma.

As you can see, your records should contain information that is considered vital, such as date, time, star observed, estimated magnitude and chart data, but you should also include notes to yourself that will serve as reminders. Personalize your log. Describe in detail, and plain language, what is important during each night of observing. Cryptic notes or unusual abbreviations, made when you're tired, will be difficult to interpret days, weeks or years later.

Notice in this example that we've used local time. When reporting these estimates, you would convert the local time to Julian Date or Universal Time (more on dates and time later in the book). Also, the chart data

VJ0 planning indicates standard (Std) and preliminary (Pre) charts (more on charts later in the book) and the comparison stars are indicated by just using their magnitudes. You'll notice on the charts that the comparison stars do not have a decimal point. Decimal points, when placed on a star chart, look too much like a star and so are never indicated on a chart.

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