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PEP transformation calibration

tcalderw's picture
tcalderw
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I have been making PEP observations since last July, and over the months I have attempted epsilonV calibrations on several of the red/blue star pairs on the AAVSO PEP calibration web page.  I find myself getting some large differences (eg: 40%) from pair to pair, and I wanted to ask which star pairs people are successfully using, and how much variation in epsilonV you are finding from one calibration to the next.

Tom

variable standard stars
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tcalderw
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A possible source of trouble is that most of the 12 red/blue pairs in the AAVSO eV calibration list contain, according to Simbad or VSX, at least one variable or suspected variable. Below is a summary:

 

Andromeda: blue star (HR 477)

Perseus: red star (HR 1052)

Serpens: blue star (HR 5859)

Hercules: red and blue stars (HR 6418, 6436)

Ophiuchus: blue star (HD 161261)

Aquarius: red star (HR 8453)

Pegasus: blue star (HR 8880)

Columba: blue star (HR 2265)

NSVs and low amplitude variables
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Matthew Templeton
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Hi Tom,

These are all low amplitude variables or suspects at best.  I would assume any variations (where they exist) would not impact your measurements at that level, but I could be mistaken.  Here's what I found from looking at the Simbad and VSX entries and from looking at the Hipparcos light curves (I have not checked for BSM photometry):


Andromeda: blue star (HR 477)

        NSV star
        All photometry within uncertainties in Hipparcos time series

Perseus: red star (HR 1052)

        NSV star
        All photometry within uncertainties in Hipparcos time series

Serpens: blue star (HR 5859)

        Not an NSV star
        Slight evidence of variability at the 0.02mag level in Hipparcos

Hercules: red and blue stars (HR 6418, 6436)

        NSV star, variability uncertain
        6418 slowly variable at the level of 0.01mag in Hipparcos

        NSV star, variability uncertain
        6436 slowly variable at the level of 0.01mag

Ophiuchus: blue star (HD 161261)

        V2315 Oph, definite variable (ellipsoidal?)
        Slight evidence of variability at the level of 0.03mag in Hipparcos

Aquarius: red star (HR 8453)

        NSV star, variability uncertain
        Slight evidence of variability at the level of 0.02mag in Hipparcos

Pegasus: blue star (HR 8880)

        tau Peg, definite variable (dSct, 0.02mag)
        Slight evidence of variability at the level of 0.02mag in Hipparcos

Columba: blue star (HR 2265)

        NSV star (possible low amplitude dSct?)
        Slight evidence of variability at the <0.02mag level in Hipparcos

Aquarius pair
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tcalderw
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Admittedly, I am new to this, but it seems like such small variations can be significant.  Consider the Aquarius pair. On the PEP eV web page, the magnitude of the red star is listed as 6.015.  But on the PEP eV archival chart, the magnitude is 6.005.  The 5 mmag difference is enough to change my computed eV by 10%.  << OOPS! TYPO:  1st magnitude should be 6.01 not 6.015 >>

It was my understanding that I should expect an eV for my instrument in the neighborhood of -0.050; is this correct?  The archival magnitudes of the Aquarius pair gave -0.0448, while the web page magnitudes gave -0.0397.  A later red/blue observation of the Andromeda pair gave -0.0449.

I'm not sure you how you're
Matthew Templeton's picture
Matthew Templeton
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I'm not sure you how you're doing the calculation -- would you please email me your observations and calculations offline?  I'd like to calculate your epsilon(V) using our copy of RBTRAN, which is what we do for most PEP observers. 

In the epsilon(V) calculation using the Aquarius pair, a difference in catalog V values of 5 millimag should result in a difference in epsilon(V) of .005 if delta(B-V) is assumed to be -0.985.  That will yield photometry with systematic offsets of less than 1 percent for nearly all stars in the PEP program.

And remember, your photometry
WI's picture
WI
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And remember, your photometry of a red-blue pair will include uncertainty at the +/-0.005 level (if you observer on a very good photometric night). Many measures of the pair will beat down the uncertainty. Observations on additional nights will also improve the average.

But best of all, remember that the comp stars in the AAVSO PEP program are selected (in almost all cases) to match the variable as nearly as possible in color.  A B-V color difference of 0.2 and an error in your epsilon-V of 0.01 will only add a transformation uncertainty of 0.002 magnitude to the variable observation.

 

I almost always used the red-blue pair in Leo, but it's not mentioned in your list.

Leo Minor
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tcalderw
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The Leo Minor, Orion, Centaurus, and Sculptor pairs do not appear to have any variables.  What eV values do you get with LMi?

Regarding color difference between variables and comps, the 5 recommended stars for new PEP observers have delta B-V values of 2.173 (CE Tau), 1.265 (RS Cnc), 0.38 (IN Hya), 0.608 (W Boo), and -0.517 (Rho Cas), most of which are significantly greater than 0.2 in absolute value.

The eps-V for my SSP-3 with
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The eps-V for my SSP-3 with the LMi pair was -0.36 (I believe it was negative; it's been more than a decade since I used my SSP-3, I've been using the SSP-5 in recent years). I tested it about every year and always got the same result, give or take a smidgen. I also tested it on different telescopes and got a very similar value - it would seem that the transformation coefficient is more dependent on the filter and detector than on the optical system.


Once or twice I tried other star pairs and got somewhat different results. I also did a all-sky determination and again got similar but somewhat different results. But all these determinations were the same within 0.01 magnitude. So I just decided to go with it. A B-V color difference between a variable and comp star and an error of 0.01 in eps-V is still going to produce a delta V within 0.02 of the ideal value. The AAVSO PEP program would welcome several thousand observations with that precision.

The eps-V for my SSP-3 with
tcalderw's picture
tcalderw
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I can't follow your next-to-last sentence.  How big a value for delta B-V?

Jim Fox's introduction to the AAVSO PEP program states:  "Integrity of our database requires that observations be accurate to within about 0.02 magnitudes."  It seems that the scenario you describe would put the observation at the limit of acceptable deviation - or am I misinterpreting? 

Also, an eV of -0.36 seems huge; do you mean -0.036?

eps-V for SSP-3
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JimK
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I purchased my SSP3 in 2014.  Its epsilon V is -0.054 using the Hercules pair (using my C14).  As a beginner photometrist I selected W Boo as one of my program stars.  If you want to coordinate some times of observation we could determine if there are any systematic errors between our setups using this star. May be a bit of an issue with the difference in airmass for our observations as we are about 3 hours apart in longitude.  I am also open to picking serveral non-variable stars to test for systematic errors between our setups.  Let me know if you are interested in coordinating any observation.  Maybe we can get some of the other single channel PEP observers intersested. A periodic comparison of our setups through observations would be useful.

The reason I chose the SSP3 was its accuracy, it would be great to know how my setup compares/transforms to others. I believe there are good bright program stars that could benefit from the precision of SSP3 setups.  Unfortunately, with few single channel PEP observers the density of the observations can be a bit sparse.

Jim

Oops. I meant to type a B-V
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WI
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Oops. I meant to type a B-V difference of 2.0 to represent some of the worst of the red variables.

Yes, eps-V for my SSP-3 = -0.036.

Test comparisons already exist in the data base. Look for a variable such as P Cyg that is getting currents observations, and see how well your own observation fits for light curve.


The desired precision of +/-0.02 mag applies, I believe, to the internal consistency of the observations. Means of three measurements with a standard deviation of +/-0.02 are accepted, bigger values are rejected. Of course, the program also wants data that is within a hundredth or two of the actual differential magnitude of the comp and variable.  But the transformation coefficient plays only a small part. In the extreme cases (comp-var B-V difference of more than 1.0 magnitude) a 10 percent error in the eps-V would change the final result by less than 0.01.

eps-V for SSP-3
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tcalderw
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Jim:

I am very interested in simultaneous observing sessions and would be glad to coordinate them.  Feel free to contact me offline and we could go into details.  I am planning a project this summer where three photometers would be used simultaneously from my location to see how well they can agree.  As part of that project, I am trying to sort out the eV calibration variations I am experiencing.

RE: Oops. I meant to type a B-V
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Thanks!  I made a typo myself in an earlier post, where I said the Aquarius Red star was 6.015 (should have been 6.01).

I maybe suffering from confusion regarding precision and accuracy, but I interpret an accuracy of 0.02 magnitude as meaning the differential magnitude measurement is no more than 0.02 from the "true" value.  If my eV compensation introduces a shift of 0.01, then I have used up half of the error budget before considering the measurement uncertainty.  Perhaps the web page really means precision instead of accuracy in this context:

"Integrity of our database requires that observations be accurate to within about 0.02 magnitudes. "

http://www.aavso.org/photoelectric-photometry-pep-observing-program-aavs...

 

 

epsilon(V) measurement
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Matthew Templeton
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Hi Tom,

There's a lot to respond to here.  To discuss the issues you raised, we think in general you're correct in what you talked about. Realize the methodology used for the PEP program was developed several decades ago, and has produced very consistent results since that time since all of the observers participating in the PEP-V program were using essentially the same observing method.  It is not perfect, but it produces truly good and consistent photometry.

Some specifics:

1) The error budget of 0.02 is an overall goal, but given uncertainties in calibration the *total* error budget of random + systematic is likely higher, but probably not as high as 0.05; we agree with David that for most stars in the PEP program, systematics are probably much lower (on order of 5 mmag).  We do stress that the random errors should be around 0.02.  Note that we also historically use an assumed mean extinction value regardless of location or conditions, and of course we assume a mean color for the variable star, even though many change color through their light curve.  These are not bad assumptions, but they will also increase the systematic uncertainties.

2) Catalog photometry: the errors you're seeing in your epsilon(V) calculation using different catalog values are for the most part unavoidable because there really is no "true" catalog value for these stars.  The data that were given on the page of red-blue pairs were *mean* values adapted from the General Catalogue of Photometric Data, and are derived from a number of photometric sources.  To our knowledge, none of the stars among the red-blue pairs are fundamental standards (e.g. Cousins or Landolt stars).  For example, if you look at the data for the Aquarius pair from GCPD, the mean photometry shows uncertainties in the V-band magnitude of 5 and 8 millimags for HR 8451 and 8453 respectively.  So from the V-band catalog uncertainties *alone* you have an uncertainty in epsilon(V) of nearly (10 millimag) / (delta(B-V)).  And that's before you make a single measurement.

3) Different results for different pairs: again, this probably goes to the issue of there not being a "true" catalog value for these stars. In addition, every star has a unique spectral energy distribution (SED), and so wide-band filters average the SED over their bandpass.  Each pair will then give a unique measurement that may be slightly different from the standard system from pair to pair.  We think you suggested this, but it would probably be more rigorous to determine epsilon(V) based on measurements of several pairs of stars, under the assumption that the uncertainties in catalog magnitudes and SED measurement will decline by the square root of the number of pairs that you use.  Again, the current procedure was developed early on in the PEP program to yield a scientifically useful balance between statistical rigor and ease of measurement.

What we would suggest is the following:

* Use the photometric data from the web page, rather than what is printed on the PEP finder charts.  That is what we intended the observers would do.  Those values may not be "true" to better than 0.01 mag, but at least all observers will be on that same system.

* If you want to get a more rigorous measure of your epsilon(V), measure as many red-blue pairs as is feasible given your location and the time of year.  If you're able to measure three or more, look at the standard deviation of the resulting epsilon(V) values.  Our suspicion is that you'll find they're all within 5-10 percent, which will again translate to an improved calibration at the level of a few percent of the color difference of the variable and comparison star (which, as David said, is small for most stars).

Matthew & Arne

Re: epsilon(V) measurement
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Matt:

Thanks for the detailed response!  I don't mean to cause trouble, but I have been very concerned about my eV scatter.  I have, in fact, made multiple eV observations.  Some have come out absurd, which may be inexperience on my part, but I offer, below, the results I trust most.  All reductions are based upon web page magnitudes.

Aqr    -0.040

And    -0.045

Ori      -0.061

Ori      -0.066

Per     -0.037

LMi     -0.068

LMi     -0.068

The distribution appears to be bi-modal, with peaks at roughly -0.041 and -0.066. 

Tom

 

transformation
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HQA
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Hi Tom,

Did you follow the procedure on the web page, measuring the red/blue pair(s) about 8 times each?  If so, what was the standard deviation of the measures for each star?  These were done on pristine nights in Bend?  Interesting distribution.  I'm a little concerned about the ones that you say came out absurd, as that gives me less confidence that these others, which are "cherry picked", are also correct.

With BSM (CCD, not PEP), I measure ~100 stars over the sky, determining extinction at the same time as the transformation coefficients.  Based on several nights, my results have +/- 0.006 in the slope.  Yours have +/- 0.01 or so, which doesn't appear to be that bad to me.  You may be worried about a seemingly systematic difference when in reality it is noise.

Arne

RE: transformation
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Arne:

My absurd result was for Pegasus: -0.1330.  I have a value for Ophiuchus of -0.0746 which was computed from only 5 measurements of the blue star due to a mistake on my part.  Those are the two values I do not trust.  Aside from Oph, all reductions were based on the standard eV series of deflections.

Below are the mean v(0) values and standard deviations of v(0)

Oph  -0.4181   0.0042

Aqr    0.2209   0.0031

Peg    0.1416   0.0089

And  -0.0419   0.0010

Per     1.4180  0.0050

Ori     1.2140  0.0067

Ori     1.2073  0.0051

LMi    0.3076   0.0036

LMi    0.3085   0.0110

On the Perseus and one Leo Minor observation I had to go beyond one hour from the meridian, but both these pairs were at high sky altitude, and my telescope is at 6300 feet elevation.  A few observations were made at less than pristine transparency (as forecast by clear sky clock); I assumed they would be usable because of the elevation.  

I have never made an extinction observation, can you explain the meaning of "slope" in that context?

Tom

 

BSM transformation
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Hi Tom,

By "slope" I mean the value of the coefficient, as this is usually determined, when using several stars, by the least-squares fit of (V-v) vs (B-V).  For a pair of stars, this is uniquely determined by Doug Hall's technique.

Your conditions sound good enough, and you were following the procedure correctly, so I'd say that you are dealing with small-number statistics (where it is not two peaks, but a continuum, with missing values), there was something occurring during your observations (such as some taken on one night, and some taken on another night with different extinction), or the catalog magnitude/color values have some systematic error included.  In any case, the results are "good enough" if you just average your calculated values.

Arne

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