Call for a PEP campaign of Alpha Coma Berenices
1. Alf Com is an ideal PEP target: it is a bright star for which we have highly reliable comparisons. The eclipse provides the opportunity not only to contribute to scientific study, but for the PEP community to study its systematic errors. We will hopefully be able to compare our data directly with a professional observatory. Observations are requested beginning immediately and lasting through February.
2. The B-V color indexes of the comparisons are very close to that of the variable. Transformation values will be quite small, which will allow us to detect errors not due to transformation. A goal of reducing our systematic errors to less than 0.01 mag will enable us to create an accurate light curve.
3. In parallel with photometry reported directly to AAVSO, we propose to create another analysis pipeline, in which the data can be studied under uniform reduction. Participants would send their raw data to a volunteer, who would reduce them and lead a discussion of the results. Jim Kay (KJMB) has volunteered to do this reduction. In addition to your AAVSO submittal please send a text file of your raw data to Jim at stargem.jk@gmail.com. He will provide each participant with a summary of all data reductions as well as an analysis of our errors. Please include your location and the values of your transformation coefficients as well as the time and values of the raw deflections.
4. Because Alf Com will be low in the sky, we will need extinction data to make good reductions. Due to both position and time dependency of extinction particularly at low altitudes we plan to use the measures of the check and comp stars directly to calculate extinction rather than using a separate extinction star. Please also provide a brief qualitative comment of the weather conditions at the time of the observation, noting any haze, clouds etc.
5. We should be able to use fixed values of delta(B-V) for the comparisons, and so be able to work strictly in V band.
6. A slightly revised observing protocol for this campaign will allow us to quantify, and hopefully correct for our systematic errors. This modified protocol measures both the comparison and check stars for each measurement of the target star. This differs from our standard method of only measuring the check star once toward the end of our observations. The recommended observing sequence is listed below:
CS KS VS CS KS VS CS KS VS CS KS
where C is the comparison star deflection, S is the sky deflection, K is the check star deflection and V is the Alf Com deflection. Due to the position of Alf Com our observing window will be limited by sunrise so working in the V band only is recommended to maximize throughput. Multiple measures each night are encouraged. The comparison and check stars are SAO 82650 and SAO 82692 respectively. Magnitudes and positions for these stars are available from the alert notice 506 and also in the starparm_2015Jan05 file available on the AAVSO site by navigating to the PEP observing section page.
7. Please send a note indicating your intent to participate to Jim at stargem.jk@gmail.com or better yet through a posting on peptalk at peptalk@cantordust.net.
We are planning to go ahead with a PEP campaign on Alpha Coma Berenices as a "tune-up" for our northern hemisphere observers. All PEPers are encouraged to make observations to see how well we can agree. If the expanded observing sequence, CS KS VS CS KS VS CS KS VS CS KS, seems daunting, go ahead with the standard CVCVCVCKC sequence. The comp and check stars are SAO 82650 and SAO 82692, respectively, and are known to the PEPobs data entry system. Save your raw data, and send them to tee jay see at cantordust dot net (Jim Kay is currently on the Appalachian Trail).
There is a final window this year, from about April 8 to April 20, to calibrate your eV with the Leo Minor red/blue pair. Leo Minor is quite high in the sky, so you can probably allow the observation to go beyond 1 hour after transit.
Tom Calderwood