GROND calibration issues

This page gives updates on calibration issues of GROND. This overrides any previous information, e.g. that published in the instrument paper: PASP 120 (2008)

New CCD bias levels

• 30. Oct. 2011, 20:00 UT
  New bias levels have been set for both read-out modes (normal and fast). New bias frames should be used accordingly for the data analysis.

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  Broken K-band detector

• Jan 31, 2010
  The temporarily mounted engineering chip in the K band channel has been replaced by a new science grade HAWAII1-1 detector from Teledyne. Starting from today, the new calibration files should be used.
• July 28, 2009
  On 2009, Jul 11, During cool-down after a Closed-Cycle Cooler maintenance, the HAWAII-1 chip in the K-band broke into pieces ( http://www.mpe.mpg.de/~jcg/GROND/Bilder/Kkaputt.html). We are attempting to get a replacement from Rockwell/Teledyne as quickly as possible.
  In the meantime, an engineering chip of the same size has been temporarily inserted. This chip is missing the south-eastern quadrant. Also, it has much more cosmetic defects (as is typical for an engineering-grade device). During the next days, we will make any effort to establish proper calibration files.

  Users are warned to NOT use old calibration files together with observations taken after July 28.

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  Zero points, AB system, for airmass 1

• January 8, 2009
  New zero points have been determined from data taken Oct. 28, 2008. These are based on a night-long observation of a field covered by SDSS with full airmass coverage. Calibration against SDSS provides absolute photometry with an accuracy in the range of 0.02 to 0.04 mag.

  Filter Band	Zero point
  g'	24.96±0.02
  r'	25.17±0.01
  i'	24.11±0.02
  z'	23.92±0.02

• Previous zero points from the instrument paper: Tab. 2 in PASP 120 (2008)

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  Conversion between GROND g'r'i'z' and SDSS g'r'i'z'

• January 8, 2009
  The GROND filters g'r'z' are identical to the corresponding SDSS filters within the errors:
  g'_SDSS - g'_GROND = (-0.006±0.014) + (0.015±0.025) * (g'_SDSS - r'_SDSS)
  r' _SDSS - r' _GROND = (-0.004±0.004) + (0.012±0.015) * (r' _SDSS - i' _SDSS)
  z'_SDSS - z' _GROND = (-0.003±0.005) - (0.009±0.027) * (i'_SDSS - z'_SDSS)

  The GROND i'-band is substantially narrower than the SDSS i'-band, and thus there is a color term as follows:
  i' _SDSS - i' _GROND = (-0.031±0.005) + (0.113±0.014) * (r' _SDSS - i' _SDSS)
  or alternatively
  i' _SDSS - i' _GROND = (-0.023±0.010) + (0.216±0.054) * (i' _SDSS - z' _SDSS)

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  Conversion between GROND g'r'i'z' and Johnson BVRI

• January 8, 2009
  The conversion for GROND g'r'z' is straightforward, since the SDSS-Johnson conversion can be used (taken from Jordi, Grebel, Ammons, 2006, A&A 460; alternatives at http://www.sdss.org/dr4/algorithms/sdssUBVRITransform.html):
  g'_GROND = g'_SDSS = -0.124 + 0.630*(B - V) + V
  r' _GROND = r' _SDSS = +0.088 + 0.267*(V - R) + R
  z'_GROND = z'_SDSS = +0.298 - 1.584*(R - I) + 0.267*(V - R) + R

  For the GROND-i' band, we use
  i'_SDSS = +0.329 + 0.247*(R - I) + I, as well as the above transformation with with SDSS r' and i' and derive
  i'_GROND = 0.034 + 1.113*i'_SDSS - 0.113*r'_SDSS

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  Conversion between USNO B1/R1 and Johnson BVRI

• May 2, 2009
  The only useful conversion equation is
  B-V = 0.556 (B1-R1) ± 0.24
  Any other relation is pretty useless - details see here
  Johnson B = USNO B1.0 B1 ± 0.5
  Johnson R = USNO B1.0 R1 ± 0.5
  V = 0.444 B1 + 0.556 R1 ± 0.5
  In other words, V calculated from USNO B1.0 B1 and R1 are no better than using GSC1.x magnitudes directly, the latter itself being a normally decried practice.
  In summary: V magnitudes from B1 and R1 are really not suitable for any use, except possibly when there is no other alternative whatsoever.

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  Transformation between AB magnitudes and fluxes

• May 2, 2009
  F_ν [μJy] = 10^{(29-(m[AB-mag]+48.6)/2.5)}
  see e.g. Eq. 1 in http://adsabs.harvard.edu/abs/1996AJ....111.1748F