HUT Data Products

HUT 1 Files

There are three categories of HUT1 reduced data: five minute averaged data, data summed over one pointing and postscript files.

The FITS files include the following types:

     target_nnn_mmm_[a|d|n]_[ct|fc]
     target_nnn_[a|d|n]_sum_[ct|ph]
     target_nnn_[a|d|n]_imcscor_[ct|fc|ph]
     target_nnn_[a|d|n]_imcslya
     target_nnn_[a|d|n]_imcsraw

Examples (Not a complete package):

     u-gem_190_01_a_ct.fits
     u-gem_190_02_d_ct.fits
     u-gem_190_03_n_fc.fits
     u-gem_190_a_sum_ct.fits
     u-gem_190_a_sum_ph.fits
     u-gem_190_n_imcscor_ct.fits
     u-gem_190_n_imcslya.fits
     u-gem_190_a_imcsraw.fits

HUT 2 Files

There are two categories of HUT2 reduced data: data summed over one pointing and postscript files. Note that there are no five minute averaged files for HUT2 data.

The FITS files include the following types:

     target_nnn_mm_[a|d|n]_[ct|ph|fc]
     target_nnn_mm_[a|d|n]_imcscor_[ct|ph|fc]
     target_nnn_mm_[a|d|n]_imcsraw

Examples (not a complete package):

     u-gem_152_01_a_ct.fits
     u-gem_152_01_d_fc.fits
     u-gem_152_01_n_ph.fits
     u-gem_152_01_n_imcscor_ct.fits
     u-gem_152_01_d_imcscor_fc.fits
     u-gem_152_01_n_imcsraw.fits

MET	Mission Elapsed time: time from start of mission (near the launch time).
IMCS	image motion compenstation system (pointing stability provided by gyroscopes
target	Target name as defined by the ASTRO mission.
nnn	The "nnn" portion of the name is a three digit number giving the approximate mission elapsed time (MET) at the beginning of the data set. Sometimes when observations were split for processing, unique filenames were created by incrementing the MET, so the name may not reflect the true MET.
mm	For HUT1 this is a two digit number that increments at 5 minute intervals throughout an observation or for the whole data set. For HUT2 this number is initially set to 01. It is incremented to indicate that the observation was split because an instrument setting that affects the calibrations has changed during the observation, e.g., a new slit was moved in place, or the doors were reconfigured to a different state.
[a \| d \| n]	_a_ files are all data with the detector turned on with acceptable background levels below 912 Å (<10 cts/s). So far no data have violated this limit. _n_ files are "good night" portions of observations. Again the detector must be on, background must be acceptable, and the orbiter must be in the night portion of the orbit. _d_ files are the complement of _n_, "good day" with the orbiter in daylight. These times are based on terminator crossings as supplied by NASA.
[ct \| fc \| ph ]	The "ct" indicates unprocessed data that are simple accumulated raw count spectra with no calibrations applied. The ct file is first converted to "fc" (flux calibrated) by applying a series of corrections for (phosphor) pulse persistence, dead time, dark count, scattered light, second and third order light, and pixel-to-pixel variations (division by flatfield). These corrected counts are converted to absolute flux units by means of the HUT effective area curve ( HUT Handbook, Fig, 4-4). The "fc" file is next converted to the "ph" (photometrically corrected) file by application of a wavelength independent constant given by the keyword PHOTOCOR in the FITS file header. This is the reciprocal of the effective fraction of the exposure time the target spent within the observing aperture (see Handbook, Section 4.9). One should check the headers of the "ph" files for comments related to the photometric correction factor PHOTOCOR. If the source is weak (<~10 counts/s), or is diffuse, the correction should not be trusted. In this circumstance, divide the applied factor through the data to remove it, or use the "fc" files, where no photometric correction factor has been applied. View the count rates plotted in the SCI3 postscript files to verify such instances.
imcsraw	File containing raw, uncorrected count data used by the image motion correction processing. These will be nearly identical to the *nnnmm_ct files except for slight differences in total integration time due to different selection criteria used to accumulate the data. (The IMCS system must supply valid data for this portion to work, so in general the times will be slightly shorter.)
imcscor	This, or the coadded sum_imscor.ph file, is the recommended final data product for bright sources. This is the ph file for which wavelengths have been corrected for image-jitter and thermal variations (see Handbook, Section 4.1). These image jitter corrections are applied to the ct, fc,* and ph, files as a separated operation, so files imcscor.ct, imcscor.fc, and *imcscor.ph generally are provided.
imcslya	(Available for HUT1 data only.) This file is a computed geocoronal Lyman alpha emission profile observed through the given science aperture and smeared by the image-jitter motions in the particular observation.

The FITS files may be read into IRAF using the rfits task of the dataio package or using IDL with a fits reader program. The resulting files are two dimensional tables (IRAF images). The first line of the image contains fluxes, and the second line of the image contains the 1 sigma errors for each data point. CAUTION: in low count rate situations the 1 sigma errors are not reliable because of limited Poisson statistics.

Data reduction

Documentation

See the paper by Kruk et al. ApJ, 122, 299, 1999, esp. Section 16, for details of data reductions. Users should be aware that the calibration files referred to in the data reduction steps of this paper are not contained in the MAST archives. However, they can be downloaded along with the IRAF "hut" data reduction installation package from ST. (Note that one may work with these calibration files in IRAF without actually installing the layered 'hut' package.)

Data-Processing Stages ("data type"):

ct - accumulated instrumental counts per second for "new spectral pixels" (1/4 of the original pixel spacing) for raw data files. This takes the data up to Step 1 of the Kruk et al. (1999) description.

fc - A semiprocessed file containing corrections for dark signal, scattered light, pixel-to-pixel variations (flatfield division), and application of the HUT effective area curve. Fluxes are in absolute cgs units. This file is the result of Steps 1-9 described by Kruk et al.

ph - The final data product. This file corrects for image motion off the edge of the aperture during the exposure. This file is obtained by performing Step 10 described by Kruk et al.

HUT POSTSCRIPT FILES

In addition to the spectral data files proper, each dataset includes a group of postscript files, five for HUT1 and four for HUT2.

The filename formats for the postscript files are:

target_SCI1_dddhhmmss.ps       e.g. u-gem_SCI1_344051032
target_SCI2_dddhhmmss.ps       e.g. u-gem_SCI2_344051032
target_SCI3_dddhhmmss.ps       e.g. u-gem_SCI3_344051032
target_SPMA_dddhhmmss.ps       e.g. u-gem_SPMA_344051032
target_IMCS_dddhhmmss.ps       e.g. u-gem_IMCS_344051032

where the target is that defined by the project and dddhhmmss is the GMT day of year and time at the beginning of the file. No SPMA plots were made for HUT2 datasets.

The postscript files depict housekeeping and engineering data sometimes helpful for the calibration of the spectral data. Information about the housekeeping parameters can be obtained from MAST upon request.