Subject: comments on single-dish FITS proposals

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Date: Wed, 25 Oct 89 09:13:34 EDT

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Comments on single-dish FITS proposals
--------------------------------------
 
From:  P.T.Wallace
       Starlink Project
       Rutherford Appleton Lab
       Chilton, Didcot, Oxon OX11 0QX
 
       PTW @ STARLINK.RUTHERFORD.AC.UK
       19457::PTW

These are hurried comments based on one pass of the document.  Apologies
if I've misunderstood anything.

PHILOSOPHY
 
In our recent work on Starlink data formats, we have distinguished between
two different design philosophies.  The one we have adopted ourselves
attempts to describe the relationships between the different data
components sufficiently well for programs to process data from diverse
sources correctly.  The more common philosophy - in fact the only one
which astronomers believe in and think they understand - is what we call
the "thought of everything", or TOE, philosophy.  The Single dish format
seems to be an example of the latter.  The TOE idea is that once you have
given implementors enough pigeonholes into which to stuff the data
generated by their telescope and instrument, you have thereby defined
the "meaning" of everything in data processing terms.  I don't believe
it's that easy in practice.
 
This is not to say that it isn't possible to use such designs, which
at least mean that FITS readers can accept the contents of a tape, and
that simple utilities can do things like plotting pixel value against
x-coordinate.  But I don't believe an astronomer can do astrophysics
from a FITS tape without an accompanying letter describing how the
various components have been used.  To proceed rapidly to the point,
I would aim for greater rigor in some parts of the single-dish proposal,
and to move some of the present items out of the main format into
unashamedly site-specific extensions.

POINTING CORRECTIONS
 
I think it is a mistake to try and define a standard set of telescope
pointing corrections.  While it is true that any two-axis mount
will have just six numbers describing the purely geometrical terms
involved, and many people (myself included) like to base a pointing
model solidly on these six terms, it is of course possible to define
a pointing model in a various completely different ways - lookup tables,
polynomials or harmonics, etc.  Moreover, all telescopes will have
extra terms, and some may have auxiliary readings (from sensors mounted
on the structure) or quantities involved in attempts to deal with
hysteresis.  Also, I have on some telescopes found that there is no easy
way to get at the basic pointing information.  On the JCMT, for example,
the current pointing coefficients appear to be relative to whatever
values are embedded in the system, and, when pointing tests are done, no
attempt is made to log the raw encoder readings to allow a model to
be fitted ab initio.  (Not how I would do it.)  There are also the
fearful complications of subreflectors which move to correct for
deforming dishes, chopping secondaries, etc.
 
So I propose that all telescope model information be entirely site-
specific.
 
Also, I distrust the idea of user-supplied corrections, especially when
it isn't made clear whether the one in azimuth (etc) is dA or dA*cos(E).
I would rather leave this information in the (site-specific) telescope
model.

SIDEREAL TIME
 
It doesn't say whether this is mean or apparent.  I propose the
latter.

OMISSIONS
 
If one of the objectives is to be able to reconstruct the pointing,
consider including (i) station coordinates and (ii) UT-UTC.
 
The station coordinates should be longitude (east of Greenwich),
latitude (geodetic), and height above the current IAU reference
spheroid.  The longitude and latitude should, formally, be as
affected by polar motion.

ENVIRONMENT PARAMETERS
 
Does "index of refraction" (REFRAC) mean the refractive index,
or the tan Z refraction constant?  I'm not keen on the former, and
would supplement the latter with one more term (in tan**3 Z).
 
Is it necessary to have HUMIDITY and DEWPT and MMH20?  I would
go for HUMIDITY alone.
 
To calculate the radio/mm refraction properly (also important
at large Z in the optical case) you need the tropospheric lapse
rate, in deg K per metre.  You also need the height, which I have
included in my "station coordinates" suggestion.

SOURCE COORDINATES
 
There is some evidence of a confusion between the term "epoch"
(meaning a moment in time) and "equinox" (shorthand for "the
mean equator and equinox at a given epoch").  I suspect the
keyword SINGLDSH GENERAL EPOCH means equinox for example.
 
I recommend that the following coordinate systems are recognised:
 
a)  Pre-IAU-1976 mean RA,Dec for any given equinox and epoch.  The
equinox should be Besselian (and is usually 1950).  The epoch is
important (because the system is rotating relative to an inertial
frame).  E-terms are included (i.e. the quoted position embodies
the E-terms just like catalogue positions).  Recommended name "FK4".
 
b)  FK4 but without E-terms.  I suggest the name "BN", standing for
Bessel-Newcomb.
 
c)  Post-IAU-1976 mean RA,Dec for any given equinox and epoch.  The
equinox should be Julian (and is usually 2000).  The epoch is
not required unless you are going to bother with proper motions.
E-terms are not involved, of course.  Recommended name "FK5".
 
d)  Galactic coordinates.  The document could mention that you
should really calculate these from the "BN" B1950 position rather
than from "FK4".  Names: "L2B2", or "GAL" would be OK.
 
e)  Mean ecliptic and equinox of date, IAU 1976.  Recommended
name "ECL", or "MEAN_ECL" or something.
 
f)  Geocentric apparent RA,Dec, IAU 1976 + 1980.  Recommended
name "RADEC".  Note that "FK5" to "RADEC" consists of precession,
nutation, annual aberration, and the gravitational lens effect
of the Sun.
 
g)  Geocentric apparent HA,Dec.  Recommended name "HADEC".
Computed from "RADEC" by subtracting RA from LAST.
 
I am doubtful whether it is worth providing for topocentric
HA,Dec, which differs from "HADEC" by the diurnal aberration.
 
h)  Topocentric Az,El, obtained from topocentric HA,Dec by
rotating into Az,El using the long,lat as affected by polar
motion.  Recommended name "TOPO_AZEL".
 
i)  Observed Az,El, obtained from "TOPO_AZEL" by adding the
effects of atmospheric refraction.  Recommended name "OBS_AZEL".
 
j)  Raw Az,El, obtained from the telescope encoder systems.  This
is related to "OBS_AZEL" by the telescope pointing model.  Recommended
name "ENC_AZEL".
 
The proposal needs to specify for all Az/El items whether they
are raw, observed, or topocentric (and whether Az means Az or
dAz*cos(El)!).

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