LDAC - Special LDAC Table Names

Note

The concrete examples on this page use the LDAC catalogue D1_r_sex.cat.

A tar archive with the complete set of LDAC Tools example files is here

The LDAC format treats several table names specially. Two pairs are very important for each user of the LDAC tools.

The LDAC_IMHEAD - LDAC_OBJECTS pair

The SExtractor program produces with (-CATALOGUE_TYPE FITS_LDAC) an LDAC objects catalogue. The objects data are stored in the LDAC_OBJECTS table. The LDAC_IMHEAD table is a large ASCII table with metadata such as the used SExtractor parameters and the image header. It can be displayed, but its individual entries cannot be accessed easily.

user$ ldacdesc -i D1_r_sex.cat -t LDAC_IMHEAD

****** Key #1
       Key name:...............Field Header Card
       Key comment:............
       Key type:...............String
       Key dimension:..........2 (80 122)


user$ ldactoasc -s -i D1_r_sex.cat -t LDAC_IMHEAD

#   1 Field Header Card
SIMPLE  =                    T / written by IMCAT
BITPIX  =                  -32 /
NAXIS   =                    2 /

.
.

SEXFBUFS=                 2048 / Image-buffer height (scanlines)
SEXMWSCL=      4.345960617e-01 / Measurement-weight re-scaling factor
SEXDWSCL=      1.078257799e+00 / Detection-weight re-scaling factor

user$ ldactoasc -s -i D1_r_sex.cat -t LDAC_IMHEAD | fold | grep SEXMINAR

SEXMINAR=                    4 / Minimum area used for detection (pixels)

Note the necessity of the fold command in the last ldactoasc call if you want to isolate a certain entry!

Within THELI the LDAC_OBJECTS - LDAC_IMHEAD format is only used to perform astrometric calibration with Emmanuel Bertins scamp program. Otherwise, the ldacconv program is used to transform the catalogue to the FIELDS - OBJECTS pair.

The FIELDS - OBJECTS pair

The FIELDS - OBJECTS pair of LDAC tables contains similar information as their LDAC_IMHEAD - LDAC_OBJECTS cousins. However, the FIELDS table is a regular binary table whose entries can be accessed and used in the standard way:

user$ ldacconv
> SYNTAX: ldacconv
              -b camera_number
              -c camera_name
              -f filter_name
              -i Catalog1 Catalog2 ... [-o Output_catalog] [options]

user$ ldacconv -i D1_r_sex.cat -o D1_r_ldac.cat -b 1 -c MEGAPRIME -f r

user$ ldacdesc -i D1_r_ldac.cat
------------------Catalog information----------------
Filename:..............D1_r_ldac.cat
Number of segments:....3

.
.

****** Table #2
       Extension type:.........BINTABLE
       Extension name:.........OBJECTS
.
.

****** Table #3
       Extension type:.........BINTABLE
       Extension name:.........FIELDS
.
.

user$ ldacdesc -i D1_r_ldac.cat -t FIELDS
****** Key #1
       Key name:...............OBJECT_POS
       Key comment:............Position of the first object from that field
       Key type:...............Long Int
       Key dimension:..........0
.
.
****** Key #98
       Key name:...............SEXMWSCL
       Key comment:............Measurement-weight re-scaling factor
       Key type:...............Double
       Key dimension:..........0
****** Key #99
       Key name:...............SEXDWSCL
       Key comment:............Detection-weight re-scaling factor
       Key type:...............Double
       Key dimension:..........0

user$ ldactoasc -i D1_r_ldac.cat -t FIELDS -k SEXSFWHM SEXSATLV
#   1 SEXSFWHM            Source FWHM used for measurements (arcsec)
#   2 SEXSATLV            Saturation level used for flagging (ADU)
0.7300000000 50000.00000

Note

1. Traditionally the arguments ‘-b’, ‘-v’ and ‘-f’ must be given to ldacconv. The provided values are transferred to the FIELDS tablebut, as far as we know, their contents can be chosen arbitrarily.

2. Also for traditional reasons, the ldacconv program translates several important keys when converting LDAC_OBJECTS to OBJECTS. Those are:

   SExtractor / LDAC_OBJECTS	OBJECTS
   NUMBER	SeqNr
   X_IMAGE	Xpos
   Y_IMAGE	Ypos
   A_IMAGE	A
   B_IMAGE	B
   THETA_IMAGE	Theta
   ISOAREA_IMAGE	NPIX
   FLAGS	Flag

Warning

1. Do not manipulate the FIELDS table manually. It is closely linked to the OBJECTS table and it is used for important, internal bookkeeping! Programs that manipulate the OBJECTS table in a significant way such as ldacpaste or ldacsplit ensure the integrity of FIELDS!
2. Do not use the ldacrentab command to rename an LDAC table to one with the special names given above, especially NOT to an OBJECTS table! The non-presence of a well formatted FIELDS table often leads to difficult to spot errors in subsequent processing! It is similarily bad to rename special LDAC tables to something else.