Last modified on August 24, 1997 by A. Richichi

The FIRPO photometric system

Jump at the end if you just want the conclusions!

In the nights of August 11-16, extensive photometry was carried out both at TIRGO and with FIRPO at the 1.23m on CA. A total of 17 sources happened to be observed at both sites in one or more filters. This gives a nice opportunity to check the quality of the FIRPO photometry against that of TIRGO.
A first analysis of the mag differences at the two sites gives the following result:
diffj.dat: av =    -4.00000E-03 -- sig =    0.194227
diffh.dat: av =     2.10000E-02 -- sig =    0.104924
diffk.dat: av =     0.12412E+00 -- sig =    0.193666
However, it is noteworthy to look at a plot of these differences against the spectral type. Some editing has been done: a) a cluster of stars near K4.5-K5 in K seems to be discrepant; it turns out that 3 of them were observed at TIRGO at high airmass (3.5) and it is likely that the extinction correction was not very good up to this limit. The fourth one was centered in the IR without TV camera (dayight), Sam reports it is a binary. So these 4 stars were discarded. b) the A5 star seems uninteresting (although more or less in agreement with G5), also discarded.
There result can be seen in this other plot, where the spectral types have been converted to temperatures. It appears that although a constant fit is also not too bad, some slope gives a better result. The values are
 Program POLYFIT V 10/Jul/90 (A.Richichi) - 24-Aug-1997 13:38:25
 File: diff_temp_jc.dat
 Pol. degree:  1- N =   10- [ 2786.00- 4095.00] - [    0.00- 1194.00] 
 1  -.10304E+01  0.26412E+00
 2  0.29958E-03  0.74406E-04
 sdev: 0.16063E+00  chi2:0.24486E+01

 Program POLYFIT V 10/Jul/90 (A.Richichi) - 24-Aug-1997 13:42:59
 File: diff_temp_hc.dat
 Pol. degree:  1- N =   10- [ 2786.00- 4095.00] - [    0.00- 1194.00] 
 1  -.43327E+00  0.16240E+00
 2  0.13077E-03  0.45077E-04
 sdev: 0.10084E+00  chi2:0.34024E+01

 Program POLYFIT V 10/Jul/90 (A.Richichi) - 24-Aug-1997 13:44:31
 File: diff_temp_kc.dat
 Pol. degree:  1- N =   10- [ 2786.00- 4095.00] - [    0.00-  774.00] 
 1  -.46648E+00  0.23764E+00
 2  0.19942E-03  0.68655E-04
 sdev: 0.14413E+00  chi2:0.29095E+01

 Program POLYFIT V 10/Jul/90 (A.Richichi) - 24-Aug-1997 13:48:26
 File: diff_temp_kd.dat
 Pol. degree:  0- N =    3- [ 4095.00- 5085.00] - [  295.00-  990.00] 
 1  0.21865E+00  0.40645E-01
 sdev: 0.11161E+00  chi2:0.28824E+01
At K, the fit for stars hotter than K5 (4100K) was done with a constant.

How to interpret this values? A definitive conclusion can be reached only for the K filter, since we can compare the transmission curves for FIRPO and TIRGO. The K3 filter inside FIRPO is much wider than the one at TIRGO. As a result, when we are in the Rayleigh-Jeans regime, it tends to give an effective wavelength shorter than at TIRGO. At CA, our standard stars are generally bluer than the target stars, and we can expect that the K magnitudes with FIRPO are fainter than the true ones. This is shown well in the plot. The slope is due to the fact that this effect is a function of the redness of the target star. As soon as the target stars become less red, we can expect the slope to become zero.
For the other filters, the situation could be the same although we do not have a plot of the transmission curves of the FIRPO J and H filters. In any case, the color difference between standards and targets still remains.


In conclusion, the FIRPO JHK magnitudes will need to be corrected, and at Arcetri we are in the process of rebuilding the FIRPO photometric archive. It is my impression that without an extensive program of standard stars, it will be hard to reach the intrinsic accuracy, which is generally of 0.02-0.05 mag in the 3 filters. More likely, we will have to deal with uncertainties of about 0.10 mag unless the observer takes care to use a standard star of the same color as the target star.
For the future, we should plan to carry out at least another night of common observations from TIRGO and FIRPO (Jan 98?). As a final recommendation, pls always fill the "official" logsheets, and try to conform to a standard pattern for the file names.

				Andrea (with help from Sam)

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