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File meta-data (headers)

PGM

Earlier imagers, such as Wilbur and Poca, tended to store header information in fairly obscure ways. One advantage of using newer standard image file formats is that they were generally designed so that meta-data could easily be stored along with the image itself. Trond took advantage of this with the CANOPUS prototype imager (Polaris). He used the PGM file format, which allows for header information to be included after the file identifier (P5) and before the image size information (399 399). Any lines beginning with the pound sign (#) are assumed to be comments and are skipped by file reading routines. An example header is shown here. 

P5
# IMAGER Polaris . TYPE All-sky . CLASS General . PROJECT CANOPUS . MODE Norm ..
# LOCATION Gillam, Manitoba, Canada . GDLAT 56.38 N GDLON 265.36 E .............
# EXPOSURE_AT Fri Nov 10 11:41:05 UTC 2000 . DAYNO 315 .........................
# WAVELENGTH 5577 AA . INTEGRATION_TIME 005000 ms . IMAGER_GAIN 3 . FILTER_NO 3 
# DIGITIZER_AD_TOP 102 . DIGITIZER_AD_BOTTOM 031 . DIGITIZER_DC_OFFSET 000 .....
# DIGITIZER_INTEGR 150 . DIGITIZER_CLAMPCTRL 001 . DIGITIZER_CLAMPLEVL 061 .....
# SEQNO 52687 . SHTR OPN . INTENSIFIER_HV  ON ..................................
# .RESERVED.....................................................................
# *GPS_PALISADE(0) .GPS.            0 l   84  128  377    0.000    3.224   3.820
# T0/[n/a] T1/14.3C T2/17.5C FW_PV/27.0C FW_SV/27.0C @ Nov 10 2000 11:30:16 UT .
399 399
255

Information is grouped into name/value pairs, using an isolated period to separate fields.

PNG Version 1.0

Data from the next generation (BetaCam) imagers are stored as PNG (portable network graphics) files. This allows for meta-data to be stored along with the image as name/value pairs (tEXt). In the example given below, the "Creation Time" name (quotes are required because of the space) corresponds to a value of 13 Mar 2002 08:01:30 GMT. All the information stored in the PGM files mentioned previously is placed in a Description field, with additional fields for other useful stuff. 

Title           CANOPUS All-Sky Image
Author          Polaris H-Beta-Class Imager
Wavelength      558.1 nm, Narrowband, BW = 2.0 nm
Description     
                IMAGER Polaris . TYPE All-sky . CLASS H-Beta . PROJECT CANOPUS ...............
                LOCATION Gillam, Manitoba, Canada . GDLAT 56.38 N GDLON 265.36 E .............
                EXPOSURE_AT Wed Mar 13 08:01:30 UTC 2002 . DAYNO 072 . FILTER_NO 3 ...........
                WAVELENGTH 5581 AA . INTEGRATION_TIME 001000 ms . II_GAIN 0 . CCD_GAIN 0 .....
                SHTR OPN . SR01 02 . SR02 10 . MODE NORM .....................................
                SBIN 02 . RGN_S1 000 . RGN_S2 511 . PBIN 02 . RGN_P1 000 . RGN_P2 511 ........
                SEQNO 40025 .................................................................
                .RESERVED.....................................................................
                *GPS_PALISADE(0) .GPS.            0 l    5   32  377    0.000  129496. 981.341
                T0/ 21.3C T1/ 13.5C T2/ 12.1C FW_PV/27.0C FW_SV/27.0C Mar 13 2002 08:00:08 UTC
"Data Level"    level-0.0 (raw)
"Creation Time" 13 Mar 2002 08:01:30 GMT
Source          CAMERA_ID 12652.A01G2201.A01F9101.278849
Software        ASCD v0.95 for Linux, by T.S. Trondsen trondsen@phys.ucalgary.ca
Copyright       
                (c) 2002 Institute for Space Research (ISR).  All rights reserved. Any use of
                these data for other than on-screen viewing requires prior approval by the
                CANOPUS Science Team.
Disclaimer      
                Use of the data contained within this file is at your sole risk. You are
                responsible for ensuring by independent verification its accuracy or
                completeness

The PNG spec allows for compression of the value fields (zTXt). This is certainly useful for reducing file size, especially for lots of header information. However, it complicates the task of quickly extracting specific fields from the header. Currently, we are only compressing the description field. I'd prefer that we use uncompressed (tEXt) fields everywhere. Note that we should definitely not compress the Copyright or Disclaimer fields, so that they are always obvious.

PNG Version 1.1

Essentially all the information from the PGM header was placed in the PNG version 1.0 header under a single description field. It would be more convenient to arrange things so that each field contains only a single value, or a collection of closely related values. A suggested example is shown here:

Title
NORSTAR All-Sky Image Data
Author
PoCa Dual Bare CCD Imager
Camera Name
PoCa0
Exposure
Wed Mar 13 01:01:30 UTC 2002
Shutter State
Open
Integration Time
001000 ms
Filter Name
Green-line
Filter Wavelength
558.1 nm
Filter Bandwidth
557.0 - 559.3 nm
Filter ID
XXXX.YY.ZZZ
Filterwheel Slot
03
Intensifier Gain
1
CCD Gain
0
CCD Bin
2x2
CCD Region
S1 000 , S2 511 , P1 000 , P2 511
GPS Info
PALISADE(0) .GPS. 0 l 5 32 377 0.000 129496. 981.341
Temperature
T0 10.0C , T1 12.7C , T2 19.2C
Voltage
Intensifier 230V , Heater 12.0 V , etc.
Site Name
Gillam, Manitoba, Canada
Site Location
56.48 N 265.36 E
Image Number
40024
File Creation Time
13 Mar 2002 08:01:30 GMT
Data Level
0.0 (raw)
Hardware
CAMERA_ID 12652.A01G2201.A01F9101.278849
Software
ASCD v0.95 for Linux, by T.S. Trondsen trondsen@phys.ucalgary.ca
Copyright
(c) 2002 Institute for Space Research (ISR). All rights reserved. Any use of these data for other than on-screen viewing requires prior approval by the NORSTAR Science Team.
Disclaimer
Use of the data contained within this file is at your sole risk. You are responsible for ensuring by independent verification its accuracy or completeness.
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