Antlia Field - FORS Data Reduction

Reduced Data and Data Reduction Notes

Flatfielding

All the frames for this program were obtained in 4 port readout mode.Bias subtraction and flatfielding were performed using IDL routines.The masterbias has had the mean ``pedastal'' value removed, to retainthe infomation about the noise characteristics of the bias, but toallow the exact bias level to be reintroduced using the overscan regionsof the chip.
For more standard reduction methods and problems, look at the informationon the the FORS-ISAAC Cluster Deep Field page. The V data was particularydifficult to deal with because of the high sky background caused by thepresence of a full moon. Contrary to the FORS-ISAAC Cluster Deep Fieldit was not felt safe to flat field using on target flat field because ofthe crowding of the resolved stellar population in the centre of the field.The Cluster Field was unfortunately taken too long before the Antlia imagesand in different moon conditions and the flat fields are thus not useablehere.

Overall, it was possible to flatfield the data (with the exception of the lower left corner of the chip) with flats constructedfrom the twilight sky by median-combining avaiable exposures. Dome flatsturned out not to be very effective and therefore were not used and arenot included in the data release. The following calibration frames aremade available:
 


mbias_norm.fits Masterbias
Bskyn.fits                Twilight Skyflat B
Vskyn.fits                Twilight Skyflat V
Isky1an.fits              Twilight Skyflat I
Isky2an.fits              Twilight Skyflat I

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Registration and image coaddition

The flatfielded images were straight forwardlyregistered (to the integer pixel) and combined using IDL routines.


Photometric calibration 

It was not possible to acquire a full set of photometric standards duringthe commissioning/SV phase. As a result, it was not possible to deriveaccurate photometric zero points. Particularly, an airmass term couldNOT be determined for each filter. 

Observations used, are of the standard PG1047+003 taken on 30/1/99, including short Antlia observations


PG1047+003
filename filter expt. airmass

FORS.1999-01-30T05:38:41.734.fits B 5 1.164
FORS.1999-01-30T05:57:40.393.fits B 5 1.135
FORS.1999-01-30T06:19:30.091.fits B 5 1.113
FORS.1999-01-30T06:40:23.782.fits B 5 1.102
FORS.1999-01-30T06:55:49.729.fits B 5 1.100

FORS.1999-01-30T05:58:42.245.fits V 3 1.134
FORS.1999-01-30T05:39:43.318.fits V 3 1.162
FORS.1999-01-30T06:20:31.915.fits V 3 1.112
FORS.1999-01-30T06:41:25.065.fits V 3 1.102
FORS.1999-01-30T06:56:51.408.fits V 3 1.100

FORS.1999-01-30T05:41:43.150.fits I 3 1.159
FORS.1999-01-30T06:00:42.030.fits I 3 1.132
FORS.1999-01-30T06:22:31.121.fits I 3 1.111
FORS.1999-01-30T06:43:25.558.fits I 3 1.101
FORS.1999-01-30T06:58:51.300.fits I 3 1.100


 short Antlia exposures


FORS.1999-01-30T05:47:11.628.fits B 60 1.005
FORS.1999-01-30T05:49:08.860.fits V 60 1.004
FORS.1999-01-30T05:53:02.568.fits I 60 1.003

FORS.1999-01-30T06:29:09.917.fits B 60 1.004
FORS.1999-01-30T06:31:06.731.fits V 60 1.005
FORS.1999-01-30T06:35:00.773.fits I 60 1.007

To request these files, you have to register as a VLT/NTT archive user. If you are already a registered VLT/NTT archive user you don't need to register.

Photometric Solution

All the frames taken were averaged - to get better statistics on the final image in each filter. The short exposuressuffer, by themselves, from atmospheric scintillation and are lessaccurate. Three of the four primary standards were useful(PG1047,PG1047-A,PG1047-B), which include a small variationin colour (B-V=-0.290 to B-V=0.688).There was no airmass variation,so the airmass terms were fixed to those found in the photometric solutionfor the Cluster Deep Field. The colour terms were found independently. The photometric zero points in the table should be considerd accurate atthe 10% level, with the caveat for the airmass.
  Photometric Transfer Coefficients

Filter Z C A

B 27.320 -0.021 -0.18
V 27.713 +0.024 -0.12
I 26.720 -0.037 -0.03

If the instrumental fluxes are expressed electrons/second, the magnitudes of the objects are given as follows:
 B-B' = 27.32 - 0.021(B-V) -0.18*AM
V-V' = 27.71 + 0.024(B-V) -0.13*AM
I-I' = 26.72 - 0.037(B-V) -0.05*AM

where (B',V',I') = -2.5*log(flux(e-/sec))

Coadded Images

The final coadded and registered images are provided as listed below.
The B and V band image are the average of 4x600sec exposures, the I band image is the average of 18x300sec exposures.
Note that the pixel values in these images are in electrons.


Filename Exptime Seeing
Antlia_B.fits 600 0.58
Antlia_V.fits 600 0.54
Antlia_I.fits 300 0.50
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