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Transform sensitivity to catalog magnitudes - LMi 27 and 28

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JimK's picture
Transform sensitivity to catalog magnitudes - LMi 27 and 28

First an apology for a long post.  I erred on the side of providing some background for this issue.

A number of single channel PEP observers are using a pair of closely spaced red/blue stars in LMi (27 LMi and 28 LMi) to determine their transformation coefficients for the B and V bands. Using a closely spaced pair minimizes errors due to first order extinction, since the star readings are taken close in time and the stars are observed through the same air mass.  The drawback of this method is that it requires very accurate values for the difference in V magnitude and B-V color index for the pair.

The AAVSO PEP observing pages list a number of these pairs.  The best pairs are those that can be observed near the zenith, are closely spaced and have a relative large B-V value, preferably greater than 1. The LMi pair fits these criteria for many mid-northern latitude observers.  However, differences in published catalog magnitudes for these pairs need to be resolved to ensure consistency in determining transform coefficients.

The AAVSO web page describing the procedure for determing transforms using red/blue pairs is located at

The web page gives a delta B-V of -1.03 and V magnitudes of 5.50 and 5.878 for this pair which are sufficient for determing the V band coefficient. B-V data is not given for the individual stars but is needed for the B band transformation, so we must use catalog values . Searching SIMBAD and looking at the references I obtained the following data for this pair:

SAO 62019       LMI 28   V=5.521  B=6.686  B-V=1.165
SAO 62010       LMI 27   V=5.900  B=6.040  B-V=0.140

With these values delta(B-V) becomes -1.025 which is close to the -1.030 value given in the AAVSO web page.
The value of delta(V) becomes 0.379 compared to the value of 0.378 given in the AAVSO web page, again reasonable agreement even though the individual V values are different.  Note that with the method used the values that matter for the epsilon V calculation are the delta(V) and delta(B-V) values so there is only a very small impact in using the values I quote above for the epsilon V calculation versus those on the AAVSO web page.

However, the V value of 5.521 for LMi 28 was from the summary page in SIMBAD. Clicking on the measurement link in the SIMBAD page gives 5.502.  The 5.502 if from a private communication in the 1970's.  The 5.521 should be an average of the various sources but I have found these to be of varied quality. If we use the 5.502 value the AAVSO delta(B-V) is different, if we use the 5.521 they agree. So in order to have valid  transformations we need to know the values accurately. 

Tom Calderwood has pointed out that In a 1986 letter by Howard Landis, former PEP chair, he says Dyer Observatory established good magnitudes for LMi.

So here is the question after this long and rambling post:

Can anyoone point us to the Dyer data, or can anyone provide definative photometry on these stars?  

Thanks much.




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