• Excepto por la noche de mayo de 1998, de condiciones anormales, el resto de los datos son ajustados por una curva con dos términos:
  1. Rayleigh scattering: lambda^-4
  2. Aerosol scattering: lambda^-0.8
  La ecuación exactada con los coeficientes de ajuste aparece en la gráfica y los coeficientes de extinción correspondientes están tabulados en la Tabla 3.

• Except for the night in May 1998, of conditions anomalous, the rest are fitted by a curve involving 2 terms:
  1. Rayleigh scattering: lambda^-4
  2. Aerosol scattering: lambda^-0.8
  The exact equation with the fitted coefficients is written on the plot and the corresponding extinction values are tabulated in Table 3 below.

The resulting curve is plotted in red. The night of May 1998 fitted separately and the resulting curve is shown by light blue (this night was affected by smoke from serious forest fires all over Mexico and USA!)

Considering the proximity of KPNO (longitude = 111:36.0 deg, latitude = 31:57.8 deg, altitude = 2120 m) to OAGH (longitude = 110:23.0 deg, latitude = 31:03.2 deg, altitude = 2480 m), it is of interest to compare the extinction curves of the two observatories. Dashed line in the above figure represents the typical curve for KPNO (taken from IRAF database). It should be remembered that the extinction curve of KPNO is from narrow band photometry, where as our curve is based on UBVRI bands. Mean values of extinction at OAGH (excluding the smoky night) at the UBVRI bands are given in the last column of Table 2.

Table 3: Extinction Curve for OAGH (Preliminary!)
lambda (Å)	K(lam)
3000	0.689
3200	0.550
3400	0.447
3600	0.371
3800	0.312
4000	0.266
4200	0.230
4400	0.201
4600	0.178
4800	0.159
5000	0.143
5200	0.130
5400	0.119
5600	0.109
5800	0.101
6000	0.094
6200	0.088
6400	0.083
6600	0.079
6800	0.074
7000	0.071
7200	0.068
7400	0.065
7600	0.062
7800	0.060
8000	0.057
8200	0.055
8400	0.054
8600	0.052
8800	0.050
9000	0.049
9200	0.048
9400	0.046
9600	0.045
9800	0.044
10000	0.043

• Mediciones realizadas con el monitor de seeing DIMM desde abríl de 1998. Mostramos análisis preliminar de los dos períodos con mayor cantidad de datos.
• Measurements made with the DIMM seeing monitor starting in April 1998. Preliminary analysis of the periods with more data available gives:

  Table 4: Seeing measurements
  Statistic	February-March 1999	October-December 1999
  1st cuartile	0.851	0.708
  Median	0.993	0.828
  3rt cuartile	1.177	0.979

• Debido a diferentes configuraciones del seeing monitor los valores mostrados son en realidad límites superiores. La diferencia entre los dos períodos de 1999 puede deberse a diferentes configuraciones y no a una mayor calidad de la atmósfera.
  Seguimos trabajando....
• Because of the set-up used for the seeing monitor all values are in fact upper limits. The difference between the early and late 1999 data might be due to the different set-ups, rather than the atmosphere.
  Work in progress...

Referencias / references:

1. Carrasco, B.E., Carramiñana, A., Sánchez-Sesma, F.J. & Lermo, F.J., 1998,
   in Preserving the Astronomical Windows,
   ASP Conf Ser 139, 141

2. Buzzoni, A., 1994,
   INAOE Technical Report 169.