PDS_VERSION_ID        = PDS3
RECORD_TYPE           = STREAM

OBJECT                = TEXT
   INTERCHANGE_FORMAT = ASCII
   PUBLICATION_DATE   = 2019-06-01
   NOTE               = "Description for Venus Climate Orbiter (VCO, also
                         known as PLANET-C and AKATSUKI) UVI data
                         calibration."
END_OBJECT            = TEXT

END


UVI data calibration
====================

  The header keywords relevant to the flux calibration of UVI data are as
  follows:

    keyword    description
    --------   -------------------------------------------
    UV_FLAT    filename of flat-field image
    UV_C2F     method to convert pixel counts to radiances
    UV_C2FK1   Conversion factor K1 appeared in UV_C2F
    UV_C2FK0   Conversion factor K0 appeared in UV_C2F
    EXPOSURE   Exposure time

  The first keyword UV_FLAT is filename of flat-field data used to calibrate
  image. This file is stored in 'calib' directory. The keyword UV_C2F briefly
  describe the function using the following 3 parameters.

  The image is calibrated with a "L1 to L2" program in the following steps:

    A) The header keyword P_ID (e.g., "VCO_UVI_283") of the image is used to
       determine which flat-field image file should be used.

    B) The entire image is flattened by dividing by a reference flat-field
       image selected at step A. The flat-field filename used here is recorded
       in UV_FLAT.

    C) The counts are converted to radiances as described in the header
       keyword UV_C2F. K1 appeared in UV_C2F is extracted from the header
       keyword UV_C2FK1 in flat-field image FITS file. If the header keyword
       UV_C2FK0 exists in flat-field image FITS file, the value of UV_C2FK0
       is used for K0 appeared in UV_C2F. If not, K0 is set to zero. The
       values K0 and K1 are recorded in UV_C2FK0 and UV_C2FK1 in the header of
       calibrated data file in FITS format, respectively.

    D) The special values, missing value, saturated pixel flag value, and dead
       pixel value, are substituted into the data. For the details of the
       special values, see 'Data' section in 'catalog/dataset.cat'.


Smear correction for images with TOP_OB mode
============================================

  Most of the images are acquired with 'NORMAL' mode, and these images are
  applied smear correction onboard. The images acquired with 'TOP_OB' mode are
  not applied smear correction onboard, so the smear correction has been
  applied on the ground. The procedure of the smear correction for the images
  with 'TOP_OB' mode is as follows:

    1. Calculate S_topob(y), mean count value of the rightmost two pixels
       along x-axis in the TOP OB area (pixels at x=1023 and x=1024 in the
       full-size image coordinate) as a function of y. Using S(x, y), the raw
       count value as a function of x and y, S_topob(y) can be represented as
       follows:

         S_topob(y) = ( S(1023, y) + S(1024, y) ) / 2

       where x and y are the x- and y- positions in the full-size image
       coordinate without 3 pixels offset in x for 'TOP_OB' mode.

       The pixels at x=1022 are also located in TOP OB area, however, the mask
       nominally covering optical black area may not ideally block the
       incoming light, so the values of the pixels at x=1022 are not used for
       smear correction.

    2. Subtract the mean count values from raw counts and we get smear
       corrected count values S'(x, y):

         S'(x, y) = S(x, y) - S_topob(y)


Usage of UVI flat conversion factor file
========================================

  In this section, usage of flat conversion factor file is described.

    1. Check value of P_ID and choose appropriate conversion factor file. Note
       that the conversion factor files are already rotated, i.e., it has same
       direction with l1b and l2b image files.

    2. Read values of P_POSLLX, P_POSLLY, P_POSURX, and P_POSURY. These values
       indicates the region of the image stored in the data part of the HDU.
       The index of the lower-left corner is (P_POSLLX, P_POSLLY), and the
       index of the upper-right corner is (P_POSLLX, P_POSLLY).

       If ROI function is not used, these values will be 1, 1, 1024, and 1024
       for 'Normal OB' case and will be 4, 1, 1027, and 1024 for 'TOP_OB'
       case. If ROI function is used, these values will differ from the above.

    3. Read data in conversion factor file and crop it using P_POSLL[X,Y] and
       P_POSUR[X,Y].

    4. Multiply the cropped data obtained at step 3 and image data together.