0.9m Filter List and Exposure Time Estimates


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This is a list of common filters used in the CCD camera at the CTIO
0.9m since SMARTS began.  For additional filter options, see

    http://www.ctio.noao.edu/instruments/filters/index_34.html
    http://www.ctio.noao.edu/instruments/filters/index.html

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STANDARD FILTER SETS
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                     size    wave    wave    peak     notes
                            central  FWHM    trans%


U (Johnson)          3x3"    3570    660     80.59    from Tek #2 set
B (Johnson)          3x3"    4202    1050    66.16    from Tek #2 set
V (Johnson)          3x3"    5475    1000    87.71    from Tek #1 set *
R (Kron-Cousins)     3x3"    6400    1450    81.09    from Tek #2 set
I (Kron-Cousins)     3x3"    8118    1415    96.63    from Tek #2 set

   * Tek #2 V filter cracked --- can be used for quarter chip work

g (Gunn)             3x3"    5118    900     81.39    Gunn-T
r (Gunn)             3x3"    6728    1000    93.83    Gunn-T
i (Gunn)             3x3"    8100    1500    93.00    Gunn-T
z (Gunn)             3x3"    9900    n/a     95.50    Gunn-T


u (Stromgren)        4x4"    3530    280     37.14
v (Stromgren)        4x4"    4118    146     52.04
b (Stromgren)        4x4"    4697    196     71.27
y (Stromgren)        4x4"    5478    244     70.83

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SPECIAL FILTERS
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Halpha-1             3x3"    6559    64      89.37
Halpha-2             3x3"    6598    69      87.77 

ND1     neutral density, attenuation by = 2.5 MAGNITUDES
ND2     neutral density, attenuation by = 5.0 MAGNITUDES
ND3     neutral density, attenuation by = 7.5 MAGNITUDES
ND4     neutral density, attenuation by = 10.0 MAGNITUDES

It is also possible to combine two ND filters to achieve additional
attenuation, e.g. ND2 + ND4 = 15.0 MAGNITUDES.

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ESTIMATING EXPOSURE TIMES
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Tests of the 0.9m CCD camera response were carried out through UBVRI
filters on 24 FEB 2006 during engineering time.  Landolt standard
stars in field SA 104 were targeted to determine how the 0.9m is
actually counting photons after the TCS upgrade and mirror cleaning.

Observations of the field on started at 05:07:14 CTIO local time and
ended at 05:48:49, already beyond the twilight limit (around 5:00 AM).
The airmass varied from approximately 1.3 to 1.4 during this sequence.
For details of the observability of this particular field at the given
date check here.  

Seven Landolt standard stars were examined:

                   ---------------------
                    Star         V Mag 
                   ---------------------
                    SA 104-334   13.484
                    SA 104-336   14.404
                    SA 104-330   15.296
                    SA 104-339   15.459
                    SA 104-325   15.581
                    SA 104-244   16.011
                    SA 104-338   16.059
                   ---------------------

Figure 1: Finding chart for Landolt field SA 104.

Three exposure times were used --- 10, 100, and 200 seconds --- in each of the UBVRI filters in the Johnson-Kron-Cousins set listed at the top of this page. Each image was corrected using a bad pixel mask obtained using flats from the same night. The raw images (not flat-field or bias corrected, but having very low backgrounds of about 1 ADU/s in V, see Table 1 for details) were used to determine the count flux in ADU/s for each of the Landolt stars with known magnitudes. Table 1. Determined Background and FWHM. ------------------------------- Filter Background FWHM ADU/s (pixels) ------------------------------- U 0.0 4.03 B 0.5 4.46 V 1.2 4.03 R 5.4 3.82 I 21.2 4.50 ------------------------------- Note 1: The scale of the 0.9m CCD camera is 401 mas/pixel, so the seeing was ~1.6". Note 2: the steep increases in ADU/s for the background flux in the filters R and I are presumably because the observations through these filters were at the end of the night, as sunrise approached. The relationships between magnitudes and fluxes shown in Figure 2 were then constructed for each filter. The fitted relations, which can be used to estimate integration times (note that variable seeing better or worse than the 1.6" seeing experienced in this experiment must also be considered), were of the form log(flux)=A1+A2(mag). Table 2. Coefficients for CCD response. ---------------------- Filter Coefficients A1 A2 ---------------------- U 7.75 -0.39 B 8.70 -0.41 V 9.42 -0.45 R 8.93 -0.41 I 9.09 -0.45 ----------------------

Figure 2: Data values and the fits for each filter Figure 2 in postscript format ___________________________________________________________________________