New Help for DAS 33
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New Help for DAS 33 |
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Introduction:
This
document contains help about the UVCS instrument and its software (DAS).
It contains a description of the modifications to the DAS
and detailed information regarding the changes to the calibration file.
This document is not directed toward the first time user of the DAS.
In order to make running the DAS trouble-free
and helpful to the first time user, the DAS33
is equipped with an ASCII tutorial file. The tutorial file will serve as
a user's guide, which takes the user step by step through running the
DAS33.
Due to its length the tutorial file will not be given here.
For detailed description of the DAS
functions, consult the help button inside the DAS
or see the web version of the tutorial file at http://cfa-www.harvard.edu/uvcs/
(click on Tutorial).
New from DAS32 to DAS33:
- Radiometric calibration
- Small slit calibration
- Added pointing display code
- Extended header support
- Updated mirror pointing calibration
- Added help text in menus
In addition to the changes to the DAS,
further help is provided, such as description of keywords
& configuration parameters
Intercalibration of Ly-alpha and OVI Channels:
The Ly-alpha channel and the redundant path of the OVI channel provide two independent measurements of Ly-alpha intensity. DAS33 has revised radiometric efficiencies for the wavelengths common to these two optical paths. The revised efficiencies are based on laboratory measurements, in flight comparisons to the spectral intensities of stars, and intercomparisons of the two Ly-alpha paths from co-registered coronal observations. The intercomparison ensures that observations with either path result in the same measured intensity within about ±10 % plus the statistical uncertainty.
These changes are included in the plots referenced below comparing overall
efficiencies between the previous and revised calibration.
Variations
of Efficiency as a Function of Height:
Previous versions of the radiometric calibration assumed that the efficiency of the UV channels for a particular wavelength did not vary with the amount of exposed mirror, yielding a linear "vignetting curve" of effective mirror area versus exposed mirror.
However, different portions of the UV gratings are illuminated for observations at different heights, and there are new inflight measurements and laboratory measurements of replica gratings that provides the variation in efficiency across the gratings. The radiometric calibration now accounts for this variation, resulting in the vignetting curves as displayed in these graphs (Ly-alpha, OVI, RED Ly-alpha).
Based on laboratory data the variations of efficiency as a function
of height are not a significant function of wavelength for both the Ly-alpha
and OVI channels.
The new radiometric calibration also accounts for a separate source of "vignetting" behavior in the UVCS "redundant path". The redundant path has a more complicated behavior because of its secondary mirror; for some telescope mirror apertures light from the primary mirror may overfill the secondary mirror, resulting in a flattening of the "effective area" beyond about 36 mm aperture. In addition, light from the secondary mirror only illuminates a portion of the OVI detector fully, with a varying "partial vignetting" away from this region.
It is not meaningful to perform a redundant
path radiometric calibration in a portion of the detector receiving no
redundant light, and the calibration becomes more uncertain in the partially
vignetted zone.
The DAS 33 radiometric calibration
correctly calculates the effective area for each channel. In the case of
the redundant path it performs the computation for each spectral bin, and
informs the user of detector areas in which calibration could not be performed
or had high uncertainty. Plots of the unvignetted area for each pixel of
the OVI channel at various unocculted apertures can be found here.
Modifications
of the wavelength efficiencies:
For OVI: The 1048 A and 1025 A were measured in the laboratory and found
to be the same for both lines. Component calibration implies that it should
be constant between 930 - 1125 A within the uncertainties. Thus DAS
33 continues to assume no wavelength dependence for the
OVI channel both primary and redundant.
For Ly-alpha: The wavelength dependence is based on component calibration
and inflight calibration using stars and line ratios. A comparison of the
old das (DAS 32) and the new das (DAS
33) is given here.
The
DAS was incapable of providing
accurate slit widths below 25 micron. DAS 33
provides slit widths as small as 2 micron.
Pointing display code provides a window where the roll angle and the
heights of the observation are plotted. An example can be found here.
New science and other descriptions retrieved from the appropriate day's
mission log, have been added in the form of an extended header.
The purpose of the extended header (see an example of
the extended header here) is to
present UVCS data to the SOHO archive in a way that most fully supports
the catalog's searching methods. The user can view the extended header
from within DAS 33.
Updated
Mirror Pointing Calibration:
Flight calibration of the mirror-pointing mechanism using known star positions has revealed a difference from the pre-flight mirror calibration used in previous versions of the DAS. DAS 33 provides the new, star based, calibration in the MIRANGL variable, which refers to the height above the sun center of a point intermediate between the two UVCS slits. See this diagram for UVCS FOV geometry.
The flight calibration is valid down to 1.74 Rsun. For observations below this height, DAS 33 shows '******' for MIRANGL, as well as for INS_YR and other display values that rely on MIRANGL.
Because of the current lack of an accurate calibration below 1.74 Rsun, and because individual observations since the start of the mission have been labeled with a consistent "commanded" value for the UVCS mirror angle, DAS 33 retains the old mirror calibration in the new variable "CMDMIR". This variable is useful for relating to the commanded value, for comparisons to earlier analyses with the old calibration and for a very approximate value for heights below 1.74 Rsun. The user should recognize that the "CMDMIR" value is not the most accurate calibrated value for the mirror angle.
In addition, the DAS is not as yet able to perform a correction for the "crosstalk" between UVCS mirror and grating mechanisms. Users needing precise UVCS pointing calculations need to correct the DAS pointing information for this effect.
Additional help buttons were added to the calibration pull-down menu. A detailed list of these help buttons is provided below:
Stray Light Help:
The DAS
33 makes no corrections for stray light. Currently the user
must subtract stray light using a postscript version of the stray light
plot available here together with the C III
97.7nm and/or Si III 120.6nm intensities from the user's observations, if available. The stray light is based
on laboratory and flight data.
Redundant Path Vignetting Help:
Please refer to the text
provided above.
Radiometric 
Second
Order Help:
There is a second order calibration
option within the radiometric calibration dialog box which is intended
for future use.
Flat Field Help:
Flatfield measurements have been made both pre-flight and on-orbit.
We estimate:
±10% pixel-to-pixel variation for the Ly-alpha detector (one sigma)
and ±5% pixel-to-pixel variation
for the OVI detector.
Given the magnitude of other uncertainties involved in UVCS radiometry,
we do not recommand flat field correction of the data because it would not significantly improve the uncertainties.
Each detector has one or two obvious defects where the detection efficiency
approaches zero. We recommend simply ignoring the data in these regions.
In general, the detector response is "dose dependent", i.e., the efficiency goes down slowly with accumulated photon detection. However, this effect is operationally kept to about 5% or less by periodically increasing the detectors' high voltage. This effect is especially present in the portion of the OVI detector where H I Ly-alpha is usually detected (the so-called Redundant Path Portion).
The OVI detector has a positively charged wire grid in front of the photocathode that prevents stray ions from reaching the detector. The wires that run along the 360 direction (i.e., those that cross and affect the spectral direction) are located at columns 30, 126, 222, 316, 410, 506, 600, 696, 793, 877 and 973. The uncertainty in the positions of the wires is ± 2 columns. The wires show a slight shift in location for spectral lines in the Redundant Ly-alpha path but it falls within the uncertainties. The wires reduce the observed intensity at the above locations. The amount varies with internal occulter setting and thus with heliocentric radius.The wires that run along the 1024 direction (i.e., those that cross and affect the spatial direction) have a constant reduction in intensity of about 10%. The wires start at row 14, appear approximately every 26 rows, and appear to be slightly tilted ( 1 to 2 rows shift over the full length of the detector) and bent due to distortion. Users with science results that are affected by these wires are encouraged to consult with members of UVCS team.
In the case of the Ly-alpha detector, it is sealed with a MgF2 window and thus the ions are not a problem.
Observations with the Ly-alpha detector after 97.09 are compromised in regions where the detector is worn. Flat fields are very uncertain and strongly time dependent. Use caution when trying to interpret these observations.
Flat fields produced on-orbit generally do not cover the entire detector
and are made with binning of the order of 3 (spatial) x 1 (spectral) pixels.
Pre-flight flat fields of the detectors (the so-called UCB or "Berkeley" flat fields) do
cover the entire detector on a single pixel scale but are not believed
to be fully representative of the flight condition. All flat flelds are
available for downloading from the UVCS
web site.
Description
of Keywords & Configuration Parameters:
In addition to the help file, additional file with listing of
all the keywords and configuration parameters is provided.
Primary
Header Keywords
| Keyword | Comment |
| DATE | Creation date of FITS file (UT): format DD/MM/YY |
| FILENAME | Name of this Spectral & Visible Light (SPVL) data file |
| TLM FILE | Name of original telemetry file |
| EXT | Name of Binary Table Extension |
| ORIGIN | Organization creating FITS file |
| TELESCOPE | Spacecraft identifier |
| INSTRUMET | Instrument |
| DETECTOR | Detector Identifier |
| EXCON | Experiment computer |
| EXPCOUNT | Total number of exposures on file |
| TAI_OBS | Observation start time in TAI seconds(*) |
| TAI_END | Observation end time in TAI seconds(*) |
| DATE_OBS | Observation start date/time in UTC:"dd-mon-year hh:mm:ss.ss" (IDL format) |
| DATE_END | Observation end date/time |
| STATUS | Calibration status |
| SC_STAT | S/C Pointing Calibration Status |
| FLT_FILE | Name of Flat Field Data file (or 'None') |
| DRK_FILE | Name of Dark Count Data file (or 'None') |
| CAL_FILE | Name of Calib. Parameter file (or 'None') |
| ORB_FILE | Name of SOHO Orbit Data file (or 'None') |
| ATT_FILE | Name of SOHO Attitude Data File (or 'None') |
| OBS_NUM | Parameter identifying the Table ID and Fixed Parameter Set of the top-level observation sequence |
| OBS_TYPE | Observation Type parameter (OTYPE):'SunCenter', 'Offset', 'Disk' |
| NPANEL1 | Original number of DATA panels (1-5) |
| ROWNGRP 1 | Number of row groups in original data (1-360) |
| ROWFRST 1 | First active row in original data (0-359) |
| ROWACT 1 | Number of active rows in original data (1-360) |
| ROWSPC 1 | Number of space rows in original data (0-359) |
| COLNGRPn 1 | Number of column groups for panel n in original data (1-1024) |
| COLFRSTn 1 | First active column for panel n in original data (0-1023): n=0, ..., NPANEL - 1 |
| COLACTn 1 | Number of active columns per group for panel n in original data(1-1024) |
| COLSPCn 1 | Number of spacer columns between groups for panel n in original data (0-1023) |
| UVOFST 1 | Image RAM offset address |
| UVBCNT 1 | Image data byte count |
| UVTABL 1 | Detector mask table ID |
| INS_ZR 2 | z-coordinate of the line-of sight to a reference point on the entrance slit (UV channels) or center of entrance aperture (VL channel) in the instrument reference frame |
| NDATA 1 | Current number of DATA panels (1-5) |
| Txxxx3 | Other keywords |
1 Present only for Spectral Data files.
2 The INS_ZR parameter is present only after the Calibrate Instrument Configuration Function has been performed: STATUS = 'sp_raw_conf', 'sp_flat_conf', 'sp_wave', 'sp_rad', 'vl_raw_conf' or 'vl_rad'.
3 Other keywords such as: TIDMm, TDESCm, TRVAL, TDELTm, TSIZEm, TRCALm.
(*) The parameters TAI_OBS and TAI_END give the observation start and end times in International Atomic Time (TAI).This is the time used on the SOHO spacecraft, and is defined as the number of standard seconds since Oh on 1 January 1958.
UVCS uses the IDL format for date/time strings (e.g. "18-JAN-!996 17:20:34.12").
The two
tables below show the configuration parameters for the UV channel and the
VL channel. Note that the configuration parameters in the shaded area are
also same for the VL channel data.
Basic
ConfigurationUV Data
| Keyword | Comment |
| DAT_XORB | Exposure start date/time in UTC |
| DAT_XEND | Exposure end date/time in UTC |
| OBLK | Observation science block counter |
| BUFFER | REU Image Buffer used |
| FLTPOS | Filter position (O=ENGAGE,1=DISENG,2=CLEAR) |
| ROLL | Raw roll mechanism position |
| PTGX1 | Raw X1 pointing mechanism position |
| PTGX2 | Raw X2 pointing mechanism position |
| SUNVT | Raw sun sensor TOP diode signal |
| SUNVB | Raw sun sensor BOTTOM diode signal |
| SUNVR | Raw sun sensor RIGHT diode signal |
| SUNVL | Raw sun sensor LEFT diode signal |
| MIRPOS | Raw mirror mechanism position |
| OCCPOS | Raw occulter mechanism position |
| SC_Y
SC_Z |
(SC-Y, SC-Z) is the angular offset of the spacecraft optical axis relative to the solar reference frame |
| SC_ROLL | Roll angle of the spacecraft reference frame relative to the solar reference frame |
| ROLANGL1 | Roll angle |
| POSX1 1 | X1 Pointing Mech. linear position |
| POSX2 1 | X2 Pointing Mech. linear position |
| MIRANGL1 | Intermediate point between Ly-alpha and OVI slits |
| OCCWDTH1 | Width of unocculted portion of telescope mirror |
| INS-ROLL 1 | Roll angle of the instrument reference frame y-axis relative to the spacecraft reference frame z-axis |
| INS-YR 1 | Distance from Sun center to the nearest point on the slit. (Differs from MIRANGL by +10 arcsec for VLD and Lya-alpha channel and by -27 arcsec for the OVI channel). A postscript file illustration of the FOV can be found here. |
| INS-YO
1
INS-ZO 1 |
(INS-YO, INS-Z0) is the angular offset of the UVCS roll axis (origin of ?rotating? instrument reference frame) relative to the spacecraft reference frame |
| SC-YR
1
SC-ZR 1 |
(SC_YR, SC_ZR) is the angular offset of the spacecraft optical axis relative to the rotating instrument reference frame |
| SUN-YR
1
SUN-ZR 1 |
(SUN_YR, SUN-ZR) is the angular offset of solar disk center relative to the instrument reference frame (from sun sensor) |
| TAI-XORB | Exposure start time (in TAI seconds) |
| GRTPOS | Raw grating mechanism position |
| EXPTIME | Exposure duration |
| SLTPOS | Raw slit mechanism position |
| SLTWDTH 1 | Entrance slit width |
| MCPV | Raw MCP voltage |
| MCPVOLT 1 | UV Detector MCP Voltage |
| WRVAL(2) | Wavelength corresponding to first column on detector (WRVAL(0) is for primary and WRVAL(1) for redundant path). |
| WDELT(2) | Wavelength interval per column pixel (after distortion correction). For OVI, WDELT(0) is the dispersion in primary in units of Angstrom per pixel. The Ly-alpha has just WDELT(0) which is not equal to zero. |
1 These parameters are present only for STATUS
='sp_raw_conf', 'sp_drk_conf', 'sp_flt_conf', 'sp_wav', 'sp_rad', 'vl_raw_conf'
or 'vl_rad'.
VL
Channel Data
| TTYPE | Comment |
| VLHV | Raw VLD high Voltage |
| VLHVOLT | Calibrated VLD high Voltage |
| pB | Fraction of polarized VL as determined relative to the center of the disk |
| pB_ERR | Error in the fraction of polarized VL as determined relative to the center of the disk |
| ANGLE | Angle of polarization |
| ANGLE_ERR | Error in angle of polarization |
| STOKES_I2
STOKES_Q2 STOKES_U2 |
Calibrated Stokes parameters I, Q and U as derived from 3-8 VLCNT measurementswith different polarizer positions |
| VLENUM | Number of sub-exposures (=r) in range 1 - 8 |
| VLCNT | Raw VLD photon count for each sub-exposure |
| VLEXP | Sub-exposure duration |
| POLPOS | Raw polarizer position for each sub-exposure |
| POLANGL1 | Polarizer angle for each sub-exposure |
| VLMODE | VLD readout mode (0=count, 1=diode) |
| VLRAD1 | Raw VLD analog radiance 1 for each sub-exposure |
| VLRAD2 | Raw VLD analog radiance 2 for each sub-exposure |
| VLSTRT | Sub-exposure start time (in TAI seconds) |
1 Present only for STATUS = 'vl_raw_conf' or 'vl_radf'
2 Present only for STATUS = 'vl_radf'
