Astrophysics

Observation Conditions at Mount Mariquita

The sky at the Cananea site

Preliminary data available on sky brightness, extinction and seeing.

 

 Sky Brightness

 

Table 1. Typical Sky Brightness at OAGH (mag/sq. arcsec)
Date	B	V	R	I	Instrument	Observer	Ref.
4 Feb 2000	21.66	20.85	20.44	18.96	CamDir	D. Mayya
May 1998	21.90	20.94	20.49	18.77	LFOSC	A. Carramiñana+
Jan 1997	21.80	20.00	19.80	18.50	LFOSC	E. Carrasco et al.	[1]
May 1994	21.50	20.50	20.00	18.70	LFOSC	A. Buzzoni	[1,2]

 

We note that Buzzoni 1994 measures were taken towards the West, the direction opposite to the city of Cananea, while the 1997 measures were made in the direction of Cananea.Light pollution can account for the 0.7 and 0.5 mag/arcsec excess brightness observed in B and V respectively, with no significant excess in R and I.

 

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Atmospheric Extinction

 

Table 2. Typical Extinction coefficients at OAGH (mag/airmass)
Date	U	B	V	R	I	Instrument	Observer	Ref
23 Oct 1999	0.440	0.200	0.110	0.110	0.070	CCD at 16-inch	A. Carramiñana+
May 1998		0.347	0.217	0.132	0.128	LFOSC	A. Carramiñana+
30 Jan 1998		0.185	0.120	0.050	0.040	LFOSC	D. Mayya+I.Puerari
Jan 1997		0.243	0.139	0.090	0.064	LFOSC	E. Carrasco et al.	[1]
May 1994		0.215	0.094	0.069	0.099	LFOSC	A. Buzzoni	[1,2]
Average	0.44	0.20	0.11	0.07	0.05

 

• 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. 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

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Seeing

 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

 

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...

 

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References:

Carrasco, B.E., Carramiñana, A., Sánchez-Sesma, F.J. & Lermo, F.J., 1998,

1. in Preserving the Astronomical Windows,
   ASP Conf Ser 139, 141
2. Buzzoni, A., 1994,
   INAOE Technical Report 169.