SECTION 9 - IMAGE LABEL CONTANTS

9.1 IMAGE LABEL OVERVIEW

The discussion of Section 8.2.1 focussed on the physical and logical record format of image labels on tape, without regard to their actual contents. Similarly, in other sections of this document, the kinds of data added to the processing history portion of the label were discussed generally, without regard to specific formats within the label records. In this section, the contents of image labels are presented and explained so that users may more usefully interpret the documentary information in the labels.

Throughout this section, the illustrative displays of labels and portions thereof are in the form in which they would appear on the labelprint listings described in Section 8.1.4. In particular, the character decoding is EBCDIC, so that binary-format label entries are not decoded properly. Since the binary portions of the label are, with the exception of the "camera snapshot" entries pertaining to camera temperature data, not used by the processing system, little documentation of the binary data is presented herein. Users are referred to Thomason and Cheng (1978) for the original format specifications for the science header.

9.2 RAW IMAGE LABELS

file. Fields 1-4 collectively comprise the "size parameters" for the data file.

Table 9-1. Key to Figure 9-1

Field	Contents
1	Starting line (record no.) of data file	(bytes 25 - 28)
2	Starting sample (byte no.) of data file	(bytes 29 - 32)
3	Number of lines (records) in data file	(bytes 33 - 36)
4	Number of samples (bytes per record) in data	(bytes 37 - 40)
5	Camera scan step size (1-4)	(byte 44)
6	EDS file no. (1 or 2)	(byte 46)
7	Station flag (0 = HANDOVER, 1 = GSFC, 2 = VILSPA)	(byte 49)
8	Camera no. (1 = LWP, 2 = LWR, 3 = SWP, 4 = SWR, 8 = FES1, 9 = FES2)	(byte 50)
9	Dispersion flag (0 = high, 1 = low)	(byte 51)
10	Image sequence no. (1-99999)	(bytes 52 - 56)
11	Running number of label line	(bytes 67 - 69)
12	Continuation character (C = more lines follow, L = this is last line of label)	(byte 72)
13	SOC tape (raw image archive tape) no.
14	File no. of raw image on SOC tape
15	Total time camera was turned on (seconds), taken from FIN entry (see field no. 24). Sum of all exposures if more than one is taken. This is not a true exposure time for trailed spectra.
16	Guest Observer comments section, entered by telescope operator
17	Event "round-robin" section describing time- tagged sequence of procedures. Entries all begin with GMT time in hhmmss format. Oldest entries appear below the double blank lines. Note: SWLA = short wavelength large aperture, LWSA = long wavelength small aperture, etc.
18	Year of read
19	GMT day of read
20	Approximate time of read in hours, minutes, and seconds GMT
21	Exposure start tag. GMT time given is near start of exposure. Format is: EXPOBC cam. no. Tmintsec gain lamps where Tmintsec is the commanded duration of "camera-on" time (seconds are rounded). Actual duration may be modified by a subsequent MODTIME command.
22	Exposure end tag. GMT time given is usually near end of exposure but can be much later. Format is: FIN cam. no. T t S sec voltage U uvc voltage, where t is the total cumulative duration of "camera-on" time achieved in seconds (truncated) since last read of camera in question. Due to truncation, duration may not agree with that in EXPOBC tag.
23	Exposure start tag, in this case for the second aperture.
24	Exposure end tag, in this case following exposure in second aperture and showing truncated cumulative time for both apertures. Time in seconds here is passed to field no. 15.
25	Readprep tag. GMT time given is near start of image read process. Format is: READPREP cam. no. IMAGE image sequence no.
26	Program ID
27	Episode no. (1, 2, 3,. . . etc.)
28	Observer sign on name
29	Target list sequence no.
30	Catalog source (H, B, D,. . . etc.)
31	Object name
32	Object classification
33	Right ascension of object (hhmmsst where t is tenths of seconds of time)
34	Declination of object (± ddmmss of arc)
35	Spectral type
36	Luminosity class (1-9)
37	V magnitude or flux
38	Color excess E(B-V) or color B-V
39	Information from Preplanned Observation Tape (POT)
40	Binary section of label
41	Binary section of label
42	Orbital elements, periodically updated, for epoch specified by fields 43 and 44
43	Julian Date
44	Seconds since midnight of JD in field 43
45	a, semimajor axis (km)
46	e, eccentricity
47	i, inclination (deg)
48	, longitude of ascending node (deg)
49	, argument of pericenter (deg)
50	M, mean anomaly (deg)
51	Spacecraft attitude commands sent to spacecraft (most recent set of four)
52	Day:hour:min:sec at which new attitude commanded
53	Right ascension commanded (hhmmsst where t is tenths of seconds of time)
54	Declination commanded (± ddmmss of arc)
55	Spacecraft roll angle (dddmmss of arc)
56	Binary section of label
57	Image processing history section of label
58	File type identifier and image processing scheme name (see Figure 9-2 and Table 9-2)
59	Identifier for image processing program name and time (GMT) of image processing scheme initiation

Note that lines 3-9 are entered by the telescope operator at the console and may occasionally contain errors. Lines 36-37, normally input from the POT, may be modified by the telescope operator and hence are also subject to errors. The automatic entries on the other lines are more accurate but can be affected, for instance, by ground computer problems (see also Section 9.5.1).

As is apparent from Figure 9-1, the binary-format portion of the raw-image label in logical records 38-82 and 86-100 is not usefully decoded when interpreted in EBCDIC characters. The camera snapshot entries in logical records 86-100 are scanned by the processing system as described in Turnrose, Harvel, and Mallama (1982) in order to extract information on the camera head- amplifier temperature (THDA) which is in turn used to correct geometric distortion and dispersion-constant files for thermal effects (Sections 4 and 6). Users are referred to Turnrose, Harvel, and Mallama (1982) for details.

9.3 PROCESSING-HISTORY PORTION OF LABELS

9.3.1 LOW DISPERSION

Information identifying lines in the raw image affected by microphonic noise (see Section 3.1 ) is currently provided for LWR images only; an example is given in Section 9.3.2.

Figure 9-2: Labelprint Listing for Merged Low Dispersion Extracted Spectral File (MELO)

	

		(1)   (2)   (3)   (4)   (5)   (6)
		 T     n     H     L     A     C
			     L     S     M     S
				   B
				   E
				   T
				   X
				   Y
where
      (1)       T = new software (F formerly used for old software)
      (2)       n = camera number (1-4)
      (3)       H = high dispersion
		L = low dispersion
      (4)       L = large aperture point source
		S = small aperture point source
		B = both apertures point source
		E = large aperture extended source
		T = large aperture trailed source
		X = both apertures extended source
		Y = both apertures trailed source
      (5)       A = automatic registration of spectral order(s)
		M = manual registration of spectral order(s)
      (6)       C = current production processing calibration
		S = special calibration

Note: not all combinations are possible.

The information pertaining to the reseau displacements and thermal corrections thereof is documented in two separate lines of the label (fields 18-20). In this example, the label data state the following: the mean reseau displacements used were calculated from images acquired between GMT day 085 of 1978 and day 334 of 1979 (note that the entries in the label are not in decimal-year notation), 18 flat-field images from this time period were used to calculate the mean, and the average 1 scatter values among these images in the sample and line directions are 0.134 and 0.138 pixels, respectively.

The camera head amplifier temperature (THDA) at the time of image read obtained from the camera snapshot data (see Turnrose, Harvel and Mallama, 1982) is documented in field 19. This temperature is used as described in Section 4 to calculate thermally-corrected displacements, as appropriate. Should a specific THDA value ever be manually entered into the processing scheme to override camera snapshot data, the words "MANUAL OVERRIDE" would be added to this label record. Furthermore, when mean reseau displacements rather than temperature-corrected displacements are used, the message "MEAN RESEAU USED" would appear in field 20 (see Figure 9-3 for such a message). Such a situation will currently always occur for LWR or LWP data and will also occur for SWP data for which a valid THDA value is not available from either the camera snapshots or manual input.

The information pertaining to the dispersion relations and thermal/temporal corrections is documented in a number of lines of the label (fields 21-26). In this example, the label data indicate that the mean dispersion relations used were calculated from images acquired between GMT day 001 of 1980 and day 222 of 1982 (again this is not a decimal-year notation); that 44 wavelength calibration images from this time period were used to calculate the mean; and that the average 1s residual scatter values after correction for time and temperature effects in the sample and line directions are 0.287 and 0.252 pixels, respectively.

The camera head amplifier temperature (THDA) at the time of the end of the exposure, as obtained from the camera snapshot data, is documented in field 22 and used as described in Section 6 to calculate the thermal shift of the dispersion relation. If the THDA at the end of exposure is not available from the camera snapshots, the THDA at the time of image read is used and the words "AT TIME OF READ" would be added to the label record. In the event that a specific THDA value is manually entered into the processing scheme to override camera snapshot data, the words "MANUAL OVERRIDE" would be added to this label record. If the THDA is not available from the header label or manual input but the time of read is determined (either specified manually or extracted from the label) then a correction for time only is applied and the message "MEAN THDA" will appear in field 22 signifying a default to the mean THDA value. If neither the THDA nor the time of read is available from either the header label or manual input, the message "MEAN DC USED" is added to indicate that the processing defaulted to mean dispersion constants. Note that even in such an instance, the sigma values given in the label are still the average residuals described above.

The information labeled "THERMAL SHIFTS" in field 23 documents the net time/temperature shifts applied to the mean dispersion constants (see Section 6). The shifts labeled "REGISTRATION SHIFTS" in field 24 represent the residual shifts needed after the "THERMAL SHIFTS" listed above are applied to register the extraction slit with the order (see Section 6). For registration shifts determined manually (i.e., by operator intervention) the word "MANUAL" appears after the shift values; for registration shifts determined automatically by the software system, the word "AUTO" appears instead, as in this example.

The final dispersion constants (after all shifts have been applied) actually used in the spectral extraction step are recorded in the B (line direction) and A (sample direction) arrays in fields 25 -26.

9.3.2 HIGH DISPERSION

Table 9-3. Key to Figure 9-3

Field	Contents
1	Identifier for image processing program name and time
2	Microphonics presence indicator; listed are lines of raw image affected by detected microphonics.
3	Ripple correction K value, evaluated by the expression given, where M = echelle order number
4	Ripple correction value
5	Satellite velocity components
6	Earth velocity components
7	Net radial velocity correction which was applied to extracted wavelengths (km s-1)

The fields which are unique to high dispersion processing are those relating to the echelle ripple correction and to the velocity corrections made to reduce the wavelengths to a heliocentric frame of reference. The ripple correction parameters, applied as discussed in Section 6 , are given in fields 3 and 4.

The velocity components of the IUE satellite and the earth in a right-handed equatorial coordinate system, calculated as described in Section 6 , are given in fields 5 - 6. The resulting net radial velocity correction, whose wavelength equivalent is added to the extracted wavelengths in order to obtain the heliocentric wavelengths, is given in field 7; see Section 6 for computational details.

9.4 NONSTANDARD LABELS

9.4.1 TAPE HEADER FILE

9.4.2 RESEAU-POSITION FILE

9.4.2 LABEL ERRORS AND MODIFICATIONS

9.5.1 LABEL LINES 1-100

9.5.1.1 Errors

• History Playback Images. When a problem occurs with the archiving of IUE images during real-time operations, the spectral data can generally be recovered from the history tapes which record the spacecraft telemetry stream. Unfortunately, since the science header is generated by the ground system and is not contained in the telemetry stream, the history tapes cannot be used to recreate image header labels. Therefore when a history playback request is made, the header label generated contains only data input with the request. Such labels will not contain any of the binary data, (including all of the temperature information used as described in Sections 4 and 6), will lack the event "round-robin" portion of the label, and may lack the observer's comments and additional data as well.
• Corrupted Labels. An error was discovered in an IUESIPS utility program called ULFLBM which could corrupt the binary-data portions of the header label. Apparently the routine deleted from the label bytes which were not recognized as EBCDIC characters. The error was not discovered until June 13, 1983, so that images processed before that date could be affected by this problem, although the program was in fact rarely used. A more detailed description of this problem is contained in Turnrose, Thompson and Gass (1984).
• Incorrect Label Information. Occasionally the information stored in the label is either misinterpreted by the IUESIPS software (e.g., when the time in line 2 is used as the exposure time for a multiple-exposure image) or the label data are simply incorrect. As explained below, procedures now exist for modifying labels with incorrect data; however, such modification usually requires that the error be discovered before the image is processed and that the correct information be known.
• Truncated Labels. During the period from April 25, 1978 to February 6, 1979, some high dispersion IUE images were partially processed on an IBM 360 computer and, as a result, the header labels of these files were truncated (i.e., lines 11-35, 38-45, 47-82, and 84-85 were removed from the label). See Turnrose and Harvel (1982) for details.

9.5.1.2 Modifications

It was subsequently decided that a better policy would be to leave the original header intact and deal with corrections in an alternative way. The project therefore agreed that the IUESIPS software should eventually be modified to search for an appendage to the header label which would contain any corrections to data stored in lines 2 - 100. Line 1 would still be allowed to be modified directly, because of the importance of entries in this line as identifiers and keys used by numerous software systems. Because such an alternative procedure would require changes to almost every major IUESIPS applications program, an interim policy was agreed to in which although the label entries mentioned above would continue to be corrected in situ, the original (uncorrected) entries would also be stored in an appendage to the label. The applications program LABCOR which creates such an appendage was first used in February 1983.

9.5.2 IMAGE-PROCESSING HISTORY PORTION OF THE HEADER LABEL