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FourStar Calibration

FILTER

Gain

Read Noise Well Depth Typical Background Typical Background AB Mag Vega Mag
 

[e-/ADU]

[e-] [e-] [e-/s/pix] [mag/sq"] Zeropoint [e-/s] Depth1 Zeropoint [e-/s] Depth1
J1 (Y)
2.51
20
143,000
200
16.9
27.3
25.4
26.6
24.7
J
2.51
20
143,000
850
15.6
27.9
25.7
26.9
24.4
H
2.51
20
143,000
4100
13.8
28.1
24.8
26.8
23.5
Ks
2.51
20
143,000
3100
13.1
27.9
24.5
25.8
22.7

1. Depths are the 5.0-sigma background fluctuations within a 0.7" circular aperture in 1 hour (no correction for light outside aperture).
Total magnitudes for point sources at 0.5" seeing are approximately 0.7 mag brighter.

 

FourStar Standards

Spectrophotometric standards were convolved with the FourStar bandpasses to create a set of standard stars of appropriate brightness.
The stellar SED data are available from: http://www.stsci.edu/hst/observatory/cdbs/calspec.html

HST Standard J1(Y) J H Ks
RA DEC Name AB VEGA AB VEGA AB VEGA AB VEGA
03:32:32 -27:51:48 C26202 16.285 15.607 16.313 15.395 16.446 15.083 16.883 15.034
05:52:27 15:53:16 GD71 14.309 13.631 14.612 13.693 15.152 13.789 15.744 13.894
12:57:02 22:01:56 GD153 14.652 13.974 14.958 14.040 15.503 14.140 16.112 14.262
16:18:14 00:00:08 SF1615+001A 16.271 15.594 16.278 15.360 16.345 14.982 16.753 14.903
23:19:58 -05:09:55 FEIGE110 13.107 12.429 13.432 12.514 13.983 12.620 14.593 12.743
21:32:16 00:15:14 LDS749B 15.555 14.878 15.817 14.898 16.295 14.932 16.846 14.997

 

FourStar Linearity

Linearity correction maps are provided for each array for the FullWell mode and the LoNoise mode. The filename is divided into 2 parts: the gain setting and the chip number. The deviation from linearity is a single term of order 2.5. The data were taken using dome flats with the H filter at varying exposure times scaled by interspersed 10s exposures. The data were fit as:

exptime = A[0]*raw + A[1]*raw**2.5 + A[2]

The coeffiecient A[0] represents the (inverse) linear gain of the pixel in units of [s/ADU]. The true expected counts are determined by dividing by A[0]:

True counts = exptime/A[0] = raw + A[1]/A[0]*raw**2.5 + A[2]/A[0]

The coeffiecient A[1]/A[0] represents the non-linear correction and the term A[2]/A[0] is representative of the read noise (in ADU).

These coefficients are stored in an image cube for each array. The cubes contain three layers: The first is A[0] which provides an indication of the pixel-to-pixel variation in linear gain, the second layer is the coefficient of non-linearity of the 2.5th order and the third is zero-point determined at the time of the fit. Since the zero-point is presumambly random it is not included when applying the linearity correction.

Since it is possible for raw CDS data to be negative in cases where the background is low, one can either use the absolute value when finding the half root or one can simply set a lower threshold above which to apply the linearity correction.

In theory, the first frame (A[0]) can be normalized to unity and multiplied (since it represents inverse gain) with raw data frames to remove pixel-to-pixel variations in sensitivity. This would leave only flat-field variations in the telescope-plus-filter combination. Twilight flats contain all this information so it is not necessary to multiply by A[0], it is only mentioned here as a possibility.

DATE LoNoise FullWell NOTES
2011_10_17 lonoise.zip fullwell.zip In IRAF: cl> imexpr "a*(1+a*b*sqrt(abs(a)))" output input,lc_??_?[*,*,2]

 

FourStar Bad Pixel Masks

The bad pixel masks (bpm's) were created qualitatively using the linearity frames above and an IRAF cl script. The bps's for each chip are the same for either FullWell or LoNoise gain mode. The bpm's are shown here.

DATE   NOTES
2011_10_17 bp.zip good = 1, bad = 0

 

 

   

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These Documentation pages are Design in Progress and content may change. Last updated: March 13, 2010

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