FORS1 - blue upgrade

A new CCD mosaic detector with blue optimised E2V detectors for FORS1 has been successfully commissioned the first week of April 2007. It is then offered for P79 (since April 7, 2007).
Users preparing their Phase I package for the P80 Call for Proposals, should use the information given below in order to best take advantage of the greatly increased blue sensitivity of the system. Please note that this includes the new high through-put filters which will replace the old Bessel filters from the beginning of P80.
We have prepared a phase 2 package for operations with the new E2V mosaic detector. Users are requested to confirm the validity of the modified package considering the informations which are collected in the following fact sheet.

Users may want to review:

  • Offset strategies considering the fringing.
  • Exposure times considering the lower readout noise which may allow to take more exposures with shorter exposure times.
  • Filter selection considering the new high throughput cutoff filters
  • Target positions considering the gap between the two detectors about 33" South (for rotator angle 0 degree) of the nominal pointing position.
  • Users which require high S/N ratios at >650nm should contact us (if they were not yet contacted)
  • The use of un-binned readout mode, if the reduced sampling (0.25"/pxl in default 2x2 binned modes) appears problematic, would require a waiver for authorisation.
  • Windowing will no longer be offered with the new CCD, as is currently the case in FORS2.

detector parameters

default readout imaging 200KHz,2x2,low
spectroscopy 100KHz,2x2,high
read out noise 200KHz,2x2,low (chip 1,2) 4.16, 4.03 e-
100KHz,2x2,high (chip 1,2) 3.18, 2.78 e-
gain factors high gain (chip 1,2) 0.54, 0.58 e-/adu
low gain (chip 1,2) 2.22, 2.37 e-/adu
image scale binning 2x2, COLL_SR 0.25"/pxl
binning 1x1, COLL_SR 0.125"/pxl
binning 2x2, COLL_HR 0.125"/pxl

Fringes at >650nm

The fringe amplitude of the new E2V detector at >650nm is largerthen for the Tek 2K CCD.

  • The fringes will remain in the nightsky background in imaging modes with filters of a central wavelength > 650nmeven after the flat fielding of the data:


    [IMG fringes]

    For the further data reduction it will be required to subtract thescaled night sky background from the frames.

  • In spectroscopic modes only a partial correction of the fringe can be obtained from the flat fielding:


    [IMG fringes]


    [1-D spectral fringes]


  • At wavelength >650nm, signal noise ratio of >15 may not be obtained due to the residual of the fringe corrections. Sky subtraction at these wavelengths will require that the target is observed atoffset positions (nodding on the slit). This will however not correct for the residual fringes in the extracted science spectra.

Instrument response

There is a great improvement in quantum efficiency at UV (0.8 mag) and blue (0.4 mag) wavelength range as seen in the following table:


filter zero point (e-/s) colour term gain
U_BESS 26.033+-0.045 0.091+-0.015 +0.83
B_BESS 28.112+-0.034 -0.107+-0.016 +0.41
V_BESS 28.214+-0.022 0.020+-0.010 +0.16
R_BESS 27.962+-0.022 negligible -0.04
I_BESS 26.993+-0.028 -0.060+-0.024 -0.

The increased response of the new mosaic in the blue part ofthe spectrum can be seen also in the spectroscopic efficiencies measuredfor grisms #600B and #300V:

[efficiency grism#600B]


[efficiency grism#300V]


New filters

There are in addition new filter available which can either be used in the ubvRIz sequence or the ugrIz sequence to cover the full wavelength rangewith broad band filters:



Some filters like u_, b_, v_, g_HIGH and B_BESS are only available with FORS1 and otherslike U_, B_, V_BESS and R_SPECIAL only with FORS2. With the new FORS1 filters we have obtained the following photometric zero points:

filter zero point (e-/s) colour term gain
u_HIGH 26.525+-0.051 0.106+-0.017 +1.33
b_HIGH 28.480+-0.030 0.069+-0.014 +0.78
v_HIGH 28.332+-0.022 0.049+-0.010 +0.28
g_HIGH 28.888+-0.026 0.031+-0.012