[daip] Help

[Eric Greisen]

Julie-Ann Goodlet writes:

 > I want to be able to use a task on the tv that will tell me the distance 
 > between two points in arcseconds. I've had a look through the manual and 
 > can't seem to find anything that helps.

Actually this is a bit interesting when the two points are not in the
same image (up to 256 images can be on a TV screen at the same time)
or when the angles get a bit large.  I just added a verb (IMDIST) and
proc (TVDIST) to the 31DEC03 aips to deal with this.  Unfortunately,
you have to install 31DEC03 to get the new verb - or run the MNJ if
you already have it installed.  Some sites in the UK will probably do
this.

The correct formulae are, for the separation,

      cos(theta) = sin(dec1)sin(dec2) + cos(dec1)cos(dec2)cos(ra1-ra2)

and the position angle

      sin(pa) = sin(ra2-ra1) * cos(dec2) / sin(theta)

POPS only does single-precision which may not be adequate which is why
I wrote a verb to do IMDIST.  TVDIST gets the pixel positions and
calls IMDIST.

Eric Greisen