I expect a number of new PEP observers to become active in the coming months and they will likely have common questions to ask and experiences to report. We’ll use this thread as a container for those communications.
Tom
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First thing: new observers will need an AAVSO online account and an Observer Code. It is also greatly encouraged and appreciated to become an AAVSO member. See the link below. Tom
Where can one purchase a PEP?
Peter
BPEC
Peter:
Optec no longer sells these photometers. I scrounge used ones to loan out.
Tom
I’m signed up now, observer code CMIM. The PEP is somewhere over the Pacific Ocean.
By way of introduction I am an amateur astronomer in Auckland, New Zealand. My main interest is Asteroid Occultations. I responded to Tom’s call for southern hemisphere observers and am well positioned for Antares observations.
Michael
First Steps
New PEP observers will need to get the photometer mounted on the telescope. This may require balance adjustments, for the SSP is heavy. If you have a cassegrain scope on a fork mount you will need an equatorial wedge. In alt-azimuth mode the optical tube can only be brought up to about 65 degrees altitude before the photometer case hits the base of the fork.
With the mechanical installation done, you will need to work out the best way to run the 12V power cord so that it reaches the photometer in any orientation. When operating the scope be careful not to start wrapping the cord around the mount.
With that done you are ready to start experimenting with the photometer. Try centering it on a star and see how well your mount keeps it in place. This may differ in different parts of the sky.
Next, get a handle on how far you must move the scope off-target to take sky readings. An easy way to do this is to set the integration time to one second, and with a slow slew speed move away from the star until you see the counts stop dropping. Flip the mirror and look in the eyepiece to see how far you have moved. The brighter the star, the further you must go.
At this point you should look at chapter one of the PEP observers guide (Photoelectric Photometry (PEP) Observer's Guide | aavso)
Try a single-filter sequence on one of the PEP program stars. At this time of year (May-June) you could try one of the “constant” targets, SY UMa or NSV 6687 (UMi), with V magnitudes given below. The complete target list is found in the “starparm” file (Photoelectric Targets and Data Reduction | aavso).
SY UMa 5.28, comp HD 82328 3.175, check HD 82621
NSV 6687 4.275, comp HD 136726 5.013, check HD 142105
Until you have established color calibration you will not be able to submit results - right now you are just trying to become comfortable with the steps of the sequence.
As you proceed with the sequence watch the counts. Those numbers are your best diagnostic of trouble. The counts should be fairly consistent during the sequence. It’s a particularly good idea to fix in your mind the approximate count for the first sample (comparison star) and see that subsequent comparison counts are comparable.
And I forgot to add: give your photometer time to acclimate to the outdoor temperature - at least half an hour (I try to allow 60 minutes), and let the electronics run for at least ten minutes. As the ambient temperature changes, your counts will change and this will be most apparent in the sky readings or when you have the mirror down for eyepiece viewing.
If you habve a Generation I photometer, there is a mechanically adjustable “offset” that governs how many counts take place with the mirror flipped down. A few hundred counts in ten seconds is appropriate. [The Generation II offset is harder to adjust.]
Tom
The Optec photometer arrived today. After weeks of storms and bad weather I should get a few clear nights over the weekend to start learning how to operate it.
Antares is in a good position from late evening. What comparison star should I use?
I can see Leo Minor at 18 deg elevation so could use it for practice but I don’t think I will get high quality results at that low an elevation.
Michael
Calibration and Target Stars
Information on PEP program target/comp/check stars can be found in the “starparm” file on page Photoelectric Targets and Data Reduction | aavso
The somewhat cryptic contents have, on each line, names, coordinates, V magnitudes, and B-V colors for target and comparison stars, plus names, coordinates and magnitudes for the check stars. Look for “ALF SCO” to get the Antares parameters. If you use the on-line PEPObs reduction tool you must identify your target with the starparm name and not any AAVSO synonym.
At this time of year, southern hemisphere observers will calibrate on the Centaurus red/blue pair (see Obtaining Transformation Coefficients | aavso). They transit at about 00:30 local standard time.
Tom
I had first light with the photometer tonight. Not great conditions with very high humidity and some cloud. Managed to get it all set up and had a go at the colour calibration using c1 and c2 Cen. Had to stop due to very high humidity and condensation, not that the results would have been viable anyway. But at least it all seems to work and I have figured out a basic workflow finding focus and the target stars and my tracking seems to be good enough with or without guiding. Tomorrow night should be better conditions and might be good enough for some real measurements.
Ambient Environment
Before using your photometer each night let it acclimate to the outside temperature. I like to set mine out an hour in advance. As the temperature changes the offset counts will change. As I understand it, the counts for a given photometer could go up or down as the temperature falls. Make sure your offset does not go too low. I aim for an offset count of about 40 per second during data collection. This improves linearity at the low end (though I only tested this on one unit).
Unless the temperture forecast is extreme (over 90F or under 20F) I leave the photometer attached to the telescope. However, if rain is expected I bring the photometer inside (moisture is not good for it).
Also, let the photometer run for at least ten minutes before taking data. This gives the electronics time to stabilize. I actually leave my SSP3 turned on all the time.
Tom
More on calibration
Each combination of photometer+filter+optics will have different color response. We accomodate this variation with a correction called transformation. Transformed data are, in principle, all on the same photometric “system” and can be directly compared.
Transformation calibration is established for each filter passband and yields a transform “coefficient” for the correction. Historically, these coefficients have been known as “epsilons”. All PEP observers will establish an epsilon for V band. If you work in B band you will need a second epsilon. We establish these epsilons with specialized observations of a pair of stars (one blue, one red) that are very close in the sky. Information about these observations can be found at Obtaining Transformation Coefficients | aavso and chapter 5 of the PEP Observers Guide. The way that transformation corrections are applied is covered in chapter 3 of the guide.
When establishing epsilons you will ideally perform the calibration observations on at least three different nights. Each observation will yield a standard deviation and they are then combined using a weighted average (I can do that averaging for you). The raw observation data can be reduced with a spreadsheet available on the web page noted above.
B band presents a special problem known as Second Order Extinction, an effect that must be allowed for in establishing the epslion for B band. More about that in a later post.
AAVSO observers usually establish epsilons once per year (they are presumed to be fairly stable). In the northern hemisphere the Leo Minor red/blue stars are considered the most reliable but they are a little dim. The Hercules pair also seems to work well. A new pair has been found in Cygnus that will hopefully extend the “prime” calibration season into the early fall. A pair must be observed when it is high in the sky, so certain pairs are useful only at certain times of the year.
Tom
Extinction
Before I address this additional kind of calibration let me first say that when it comes to processing your ordinary photometry the reduction software will apply the corrections for you - you needn’t be concerned with exactly how they work. You must only establish certain parameters that are fed into the software. The epsilon is one such parameter.
Another parameter is first-order extinction, sometimes knpown as primary extinction. Extinction correction comes into play because the variable star and the comparison star are almost never at the same altitude in the sky. The lower a star is in the sky, the more its light is extinguished by the atmosphere. Left unaccounted for, this effect messes up the brightness difference we measure between the variable and the comparison.
Extinction is expressed in units of magnitudes per airmass. Air mass is approximately 1/sin(altitude). Straight up is airmass 1. At 30 degrees altitude, the airmass is 2 and it climbs rapidly as you go lower. At sea level, a common extinction is 0.25 magnitudes per airmass. That means that a star at airmass 2 will lose 0.25 magnitudes compared to its brightness at airmass 1. The value of 0.25 here is known as the extinction coefficient, called k’ (k-prime) for short. Different passbands will have different coefficients, with B band having the highest and I band the lowest (let us not speak of U band).
Many observers use a fixed value of k’ as an estimate, but you can measure k’ with a little work and I try to do so every night. We will go into that in a later post. I observe from about 1000 meters elevation in a fairly arid environment. My median k’ for V band is 0.19 but I have seen it much higher and much lower.
Your data reduction software uses the k’ value to account for the airmass difference between the variable and comparison stars. The software will “brighten” the variable magnitude if that star is lower than the comparison, or “fainten” the magnitude if the comparison is lower.
Measuring first-order extinction
The simple way to measure extinction is to follow a star as it rises or falls in altitude during the night and record the change in brightness. Plotted as a graph with brightness versus airmass you get something like this…
The slope of the line is the extinction coefficient, k’.
This method is simple but not easy. First of all you must take data points over the course of 3-4 hours to get an adequate range of airmass. Second, many observers live in locations where extinction is not steady over that time span. There is a quicker approach to measuring extinction known as the Hardie Method.
The first method is not dependent on knowing the magnitude of your extinction star. But wIth Hardie, you choose a set of stars (I use 5) at a variety of airmasses, stars whose magnitudes are reliably known. They are observed in quick succession and a graph very similar to the one above can be created. We have a spreadsheet to assist with this for B and V bands (on page Photoelectric Targets and Data Reduction | aavso). The stars are selected from a list in Astronomical Photometry by Henden and Kaitchuck.
Keep in mind that the higher in the sky your target is located, the less the extinction matters. What we care about is the difference in extinction between the target and the comparison. At low airmass, this differential extinction is quite small and a good guess at the extinction coefficient is almost as good as a measurement.
For more information see Section 5.1 of the PEP Guide.
Second order extinction
Generally I like to see new observers work for a year just in V band. I think of it as an apprenticeship to become proficient in the basics. Once comfortable with single-band observing the newcomer can advance to two-band, which will usually be the BV combination.
Besides the direct benefit that accrues from collecting data in a second passband, BV photometry allows for more reliable transformation. Applying transformation requires knowing the color contrast between the variable and the comparison. The most common astronomical color is expressed as B-V (B magnitude minus V magnitude). For V-only photometry, we must adopt a fixed value of B-V, which works well for many stars but not all. With calibrated B and V filters it is possible to measure the color contrast before establishing the individual (transformed) B and V magnitudes.
Calibrating in B band requires establishing a coefficient for second-order extinction. Extinction increases as wavelength decreases. R band has less extinction than V, and B has more extinction than V. Within the B passband there is more extinction at the “blue end” than the “red end.”
What this means is that a star with lots of blue light will suffer more extinction (in B band) than a star with lots of red light. This extinction is accounted for in addition to first order extinction. Second-order calibration involves deteriming another coefficient, know affectionately as k" (k double-prime). The process involves following a red-blue star pair from low to high airmass (or high to low). At high airmass the blue star will lose more light than the red star, so the color contrast between the stars shrinks compared to that at low airmass. Sampling the pair over an adequate range of airmass takes 3-4 hours and your sky transparency must be steady in the interim.
In order to compute an epsilon for B band, you must know k". Because successful k" measurement is so dependent on good skies (hard to come by) that we settle for a single measurement rather than an average of three. Like transformation it is assumed to be fairly stable, so we only calibrate k" once per year.
Second-order extinction measurement is described in section 5.4 of the guide.
Without measuring a lot of stars on many nights, folks may find that determining the second-order extinction for B or B-V reductions difficult. Unless you are observing from a very good site, the night-to-night scatter will be large. One has only to look, for instance, at Arlo Landolt’s 1992 standards paper, and Table 1 there, where he shows mean extinction and ranges in his data from Cerro Tololo, arguably one of the best places in the world to do photometry. For k’’ in B-V he shows a mean value of -0.023, but a range -0.046 to +0.013. Many observers simply adopt a reasonable value like -0.025 or -0.03 and don’t try to measure it. I would suggest PEP observers do exactly this.
In his 2019 “48 globulars” paper, Peter Stetson, arguably the most careful CCD photometrist around (he wrote DAOPhot), says “the effect is too subtle to be measured with any precision.” He adopts a fixed coefficient of -0.016 by convolving stellar energy distributions with Landolt’s B bandpass and a standard model terrestrial atmosphere for 2km altitude (i.e. typical of Arizona or Chilean sites).
Why no second-order effect in V? It is because the atmospheric extinction trend is nearly flat across the V band due to the Chappuis bands of ozone flattening the broad decline from Rayleigh extinction of ordinary gaseous air.
\Brian
the papers cited above:
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This will be yet another post added to the folder on my computer labelled “Brian Skiff Gems”.
Roy
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Is there a document which describes the Hardie method in detail for determining epsilon? I see vague references in the PEP Guide, and other places but I’d like a detailed how-to/step-by-step to establish my transformation coefficients. Tom has mentioned he has a list of stars he uses throughout the year, and that he performs the calculation on each observing session. I’ve got my SSP-3A on my mount, tracking well, finding targets accurately using a user-defined database for the Losmandy G11 and 10" Meade ACF OTA. I need to get my extinction values so that I can use the spreadsheet to reduce data I’ve recorded for SY UMa.
More detail please!
Steve - BSTC
Steve:
We generally do the Hardie method with five stars selected from a list of semi-standard extinction stars. There is a spreadsheet to help you select the stars and reduce the data on web page Photoelectric Targets and Data Reduction | aavso
There is a ReadMe section of the spreadsheet with instructions and Erwin van Ballegoij (who wrote the sheet) can help (@BVE). He is in The Netherlands.
Tom
Thanks Tom. Not sure how I managed to miss that!! Weather looks to be decent here starting tomorrow evening and for the next 3 nights, so I’ll work to get my epsilons dialed in and start producing some useful data.
Steve - BSTC
Avoid a common mistake
When newcomers try to establish both epsilonV and epsilonB they sometimes try to perform the V and B observations separately in two successive sequences. No! To properly reduce the B samples they must be taken in a single sequence of interleaved B and V. See page 12 of the version 3 PEP Guide for how to acquire interleaved data.