Subject: Re: TELESCOP-keyword.

From: Lucio Chiappetti <[email-masked]>
Date: Fri, 20 May 94 09:55 EDT

I do not see any reason the change the usual practice by which the
TELESCOP keyword denotes the satellite mission. Since this is hardly
relevant for software, the simplest thing is not to touch the existing
standard (TELESCOP is one of the early "reserved" keyword).

Actually in my s/w (which uses a local non-FITS FITS-like format) I use
a SATELLIT keyword for this purpose. But I convert it to TELESCOP when
I write out my files in FITS. That seemed to me the simplest solution.

There are already three keywords to determine the instrument data
are taken from, TELESCOP, INSTRUME and DETNAM (the first two are part
of the "reserved" keywords; I do not particularly like DETNAM from an 
aesthetic point of view, I'd preferred an 8-char-English-word like
DETECTOR, but since it is in widespread use I abide to it).

I would not be concerned by the fact that some satellites may host a
variety of actual "telescopes" (i.e. optics systems), some of which
may have a variety of instrument in the focus. There also cases in 
which there is no optics at all, i.e. no "telescope". It is just a matter of 
names, this is how I'll see it :

    TELESCOP (SATELLIT)     indicates the spacecraft
    INSTRUME                indicates the complex of optics (when
                            present) and instrument (if the association
                            is fixed)
    DETNAM                  indicates the sub-unit of a multi-unit
                            detector, or the detector in focus when
                            this can be interchanged

I'll make some examples :

    TELESCOP  EXOSAT
    INSTRUME  LE1 or LE2    one of two optics systems
    DETNAM    CMA or PSD    each of the two telescopes had the
                            possibility of having in focus one
                            of two detectors (they were on a moveable
                            mechanism)

    TELESCOP  EXOSAT
    INSTRUME  ME            a collimated proportional counter array
    DETNAM    ggn           (consisting of 8 Argon detectors Ar1 to
                             Ar8 and 8 Xenon detectors Xe1 to Xe8;
                             in addition one could also use combinations
                             in quadrants and halves)

    TELESCOP  EXOSAT
    INSTRUME GSPC          a collimated GSPC (no need of DETNAM)

    TELESCOP  SAX
    INSTRUME  LECS         one optics system + one GSPC in focus

    TELESCOP  SAX
    INSTRUME  MECS         one electronics controller ... with
    DETNAM    Mn           three separate GSPCs (M1 M2 M3) in focus
                           of three separate optics systems

                           this is tricky, may be I would use instead

    TELESCOP  SAX
    INSTRUME  Mn           and forget DETNAM; Mn indicates the combina-
                           tion of optics and detector, and neglects
                           the fact a single processor handles all 3

    TELESCOP  SAX
    INSTRUME  HPGSPC       one collimated GSPC (no need of DETNAM)

    TELESCOP  SAX
    INSTRUME  PDS          one collimated phoswich, consisting of
    DETNAM    Pn           4 sub-units (may also be used in couples)

    TELESCOP  SAX
    INSTRUME  WFCn         one of two coded-mask wide field cameras
                           (looking in opposite directions)

    TELESCOP  XMM
    INSTRUME  EPIC1        (or MOS1) a CCD array in focus of telescope 1
    DETNAM    CHIPn        consisting of 7 chips

    TELESCOP  XMM
    INSTRUME  EPIC3        (or MAXI, or PN1), a CCD array in focus of
                           telescope 3, uses a different technology
    DETNAM   CHIPn         consisting of 12 chips

    TELESCOP  XMM
    INSTRUME  EPIC2         either MOS2 or PN2, a CCD array equal to
                            one of the previous, in focus of telescope 2  
    DETNAM    CHIPn   
     
    TELESCOP  XMM
    INSTRUME  RGS1         a CCD detector placed after a reflection
                           grating sharing the beam of telescope 1

    INSTRUME  RGS2         idem, in the beam of telescope 2
                           (telescope 3 is unobstructed)

     In this latter example one sees that the same "telescope" has
     two permanently mounted instruments. It is hardly relevant to
     have a separate keyword to denote the optics systems.

Disclaimer:
The examples are made on real cases I work or had worked with. The names
used are indicative and in the case of Exosat may not correspond to what
is used in current archives, while in the case of SAX and XMM are just 
representative (although in a few cases correspond to conventions I am
privately using in simulation software).

If it could be of any use, I would accept to have a SATELLIT keyword 
duplicating the content of the TELESCOP keyword, and denoting the spacecraft.

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