Subject: A comment on Don Wells reply to T.Forveille and S.Guilloteau proposal

From: "GUILLOTE@FRGAG51" <[email-masked]>
Date: Fri, 20 Oct 89 13:16:23 EDT

Date sent:  20-OCT-1989 09:30:31.22
To:   DISHFITS@NRAO

Dear friends,
 
I have a comment on Don Wells answer to the our (T.Forveille and myself)
document. It concerns point 3) of his answer, which presumably was
corresponding to item (*5) (XCOORD for not regularly sampled data).
 
There has been here some confusion between "irregularly sampled"
and "randomly sampled" data, which originate from an incomplete
description in our document. In our point (*5), we were implicitely
refering to FREQUENCY axis, not to mapping coordinates. The frequency
axis may be irregularly sampled, (e.g. Acousto Optical Spectrometers
in radio telescopes) but this is usually in a systematic (not random!)
way. If one envisage to store part of a 3-D spectral mapping with such
a frequency axis, the use of a Table extension to indicate Frequency
Value versus Frequency Pixel is probably unavoidable (**). Using random
parameter instead would imply more or less doubling the data size
(more precisely adding (np-1)*nf values, where np is the number
of map points, and nf the number of frequency channels).
 
This is specially important not only for single-dish data, but also
for many telescopes with spectroscopic capabilities (HST, ISO, and
most optical telescopes (***).
 
                Stephane

(**) In fact this is similar (NOT identical) to the extra 'IF' axis, connected
to the 'AIPS CH' table, mentionned in Section C.3.2 of Don Wells proposal.
 
(***) And here comes again the question of Frequency versus Wavelength.
A regularly sampled axis in one domain is no longer regularly sampled
in the other.