OMEGA-PRIME

Flat Field

Flat fields frames for all filters will allow to correct the multiplicative effects of the pixel-to-pixel sensitivity variation of the detector. We can create flats by taking images of the dome illuminated by a lamp, of a screen in the dome illuminated by a lamp or/and sky flats fields from the twilight sky.
Find below a statistics and comparison of flat field frames taken at the screen and at the dome.



a) Dome Flat Field (after normalisation)

date quadrant int. time
(seconds)
filter mean
(counts)
median
(counts)
stddev
(counts)
deviation
(%)
11th Feb.
2001
down-left 1.677 J 0.989 0.990 0.05718 5.7
down-right 1.033 1.034 0.05754 5.8
up-left 0.975 0.974 0.0556 5.6
up-right 0.970 0.969 0.05952 6.0
full 0.995 0.994 0.06258 6.3
down-left 1.677 Km 0.997 0.999 0.05294 5.3
down-right 1.013 1.014 0.05154 5.2
up-left 0.975 0.977 0.05222 5.2
up-right 0.967 0.968 0.05544 5.5
full 0.991 0.993 0.05673 5.7
12th Feb.
2001
down-left 2 J 0.999 1.000 0.05519 5.5
down-right 1.026 1.027 0.05872 5.9
up-left 0.980 0.981 0.05592 5.6
up-right 0.960 0.961 0.05792 5.8
full 0.992 0.993 0.06148 6.1
down-left 1.677 Km 1.010 1.012 0.05245 5.2
down-right 1.009 1.011 0.05232 5.2
up-left 0.982 0.983 0.05211 5.2
up-right 0.961 0.960 0.05417 5.4
full 0.992 0.993 0.0567 5.7
13th Feb.
2001
down-left 3 J 0.995 0.996 0.05989 6.0
down-right 1.025 1.024 0.05850 5.9
up-left 0.978 0.978 0.05578 5.6
up-right 0.960 0.961 0.05764 5.8
full 0.990 0.989 0.06105 6.1
down-left 1.677 Km 1.009 1.010 0.05174 5.2
down-right 1.012 1.012 0.05172 5.2
up-left 0.980 0.980 0.05134 5.1
up-right 0.960 0.961 0.05353 5.4
full 0.992 0.994 0.05619 5.6


The deviation is around 5.5%, which shows that the variation of sensitivity pixel to pixel of the detector is very small.

b) Screen Flat Field (after normalisation)

date quadrant int. time
(seconds)
filter mean
(counts)
median
(counts)
stddev
(counts)
deviation
(%)
14th Feb.
2001
down-left 3 J 1.007 1.007 0.05988 6.0
down-right 1.037 1.034 0.06897 6.9
up-left 0.993 0.993 0.06330 6.3
up-right 0.979 0.975 0.06535 6.5
full 1.004 1.000 0.06795 6.8
down-left 3 H 1.012 1.009 0.06001 6.0
down-right 1.038 1.034 0.06690 6.7
up-left 0.990 0.989 0.06232 6.2
up-right 0.972 0.970 0.06500 6.5
full 1.003 1.000 0.06821 6.8
down-left 2 Km 1.020 1.016 0.06045 6.0
down-right 1.020 1.016 0.06383 6.4
up-left 0.998 0.994 0.06145 6.1
up-right 0.976 0.975 0.06295 6.3
full 1.003 1.000 0.06480 6.5


We can see that the results for dome and screen flats makes no difference, but if we divide a dome by a screen flat images with the same count level we obtain the next figure, where it is possible to see a small variation in the thermal emissivity in the Km filter. In the J filter there is a vertical gradient (up-left graph) of 0.5% and a horizontal gradient (down-left graph) of 0.2%. In the Km filter we can observe a vertical gradient (up-right graph) of 0.75% and a horizontal gradient (down-right graph) of 1%, which is nearly to the deviation of the flat (5.5%), so in this case is much better to work with the screen to avoid this small structure.



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Comments and suggestions :   Ana Guijarro