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Finding a particular object or objects

A search for a particular object is best done by searching on its sky position. There are two reasons for doing so: First, the object may appear in the catalogue under several different and perhaps abbreviated names, and secondly, the postion is an indexed catalogue item, which makes a search on it very quick. When searching only on the item object(_name), usually a wildcard has to be used, and the whole catalogue is tested, which can be much more time consuming than an indexed search.

A search on sky position is best done by either specifying a range for the catalogue items RA and DEC, or by using one of the derived keywords POSITION or BOX. For example, in order to find observations of NGC4151, at RA=12:10:32, Dec=39:24:30 (J2000 coordinates) any of the following searches could be made to obtain a first selection:

SEL/INP=0/OUT=1 RA = 12:08:00 to 12:13:00 , DEC = 39:00:00 to 40:00:00
SEL/INP=0/OUT=1 RA = 12:10:32 UNC 0:02:30 , DEC = 39:24:30 UNC 0.25
SEL/INP=0/OUT=1 POS = (12:10:32.0, 39:24:30, 30 arcmin)
SEL/INP=0/OUT=1 BOX = (12:10:32.0, 39:24:30, 30 arcmin, 24 arcmin)

By explicitly specifying the (here generous) search radius (with POS) and search box (with BOX), the defaults are overwritten. Those default sizes may also be modified (temporarilly) by using the /RADIUS and /BOXSIZE qualifiers with the select command, or (permanently) by redefining the default values themselves (and permanently, for the remainder of the session) with the DEFAULT function (sections and ).

Note that a selection on a precise position with SEL RA=12:10:32, DEC=39:44:40 is unlikely to produce the desired result, or even any result, because in this case the specified values of RA and DEC must be exactly identical to those in the Observations Catalogue.

Searching on object name is generally an uncertain strategy. For example, Markarian galaxies could be indicated by object names starting with MK, MRK, or MARK, but many have also an NGC or other designation. One can try e.g. :

SEL RA=(6:: to 12::) & DEC=(0:: to 30::) & obje=(MK*,MRK*,MARK*)

to find the observations with a Markarian-like name in the given sky area, but this search will not necessarily yield all objects of your interest present in the catalogue.

Further, it is possible to use Ecliptic or Galactic coordinates as well as another Equinox. The default coordinate system (Equatorial) and equinox (J2000) can be replaced by your own favorite with the DEFAULT command, or be specified, either with SELECT command qualifiers, or by prefixing the POS and BOX keys with `EQ', `EC' or `GA':

SEL/EQ=B1950 POS = .... etc. .

SEL/COOR=GAL/EQ=1985 POS = .... etc. , or: SEL/EQ=1985 GAPOS= ....

In order to search observations in relatively large areas one can also select by sky-cell number of the position index (see section and table ). Each cell covers an area of approximately 7 square degrees.

The following recipe may be useful when you know the name of your object, but forgot its position, assuming that there are indeed any data of it in the archive:

• Do: SEL/INP=0/LIST OBJE=*name* ;use wildcards, and the LIST qualifier.
• As soon as a record is listed, which includes the position, interrupt with: CTRL/C

  Save the table with: KEEP, or lose it with: ABORT.

• Do: SEL/INP=0/OUT=1 POSI= .... , which will find your observations.
• Similarly, you can find e.g. the sky cell with: LIST .... ,GAPOS, CELL, .... , after the interrupt and KEEP.