[vsnet-chat 810] Re: vsnet charts

["Fraser Farrell" <fraserf@dove.net.au>]

Lasse Teist Jensen wrote:

> I like the vsnet charts. I use these charts a lot.
> But lately many of the charts have been based on USNO data.
> This means that you get a lot of very faint stars (mag.18 - 20) on the
> charts. The charts become very crowded and I think that in most cases these
> faint stars are not needed to identify and estimate the variable.

The ability to display VSNET charts at different scales would go a long
way to reducing the crowding problem.  Not just with <20th mag USNO
stars, but with (for example) clusters and rich starfields generally.
I see real advantages in having two (or more) VSNET charts - CHTs - at
different scales for some objects:

- a CHT at the usual image scale of about 10' arc square showing the
  star field to 16th-17th mag (or the brightest 30(?) stars).  Good
  enough for most of us.

- a CHT of 30"(?) arc square centered on the object, showing stars
  down to the limits of the catalogue (or detector).  For those of you
  who have time booked on the Keck telescopes :)

All that is needed is a small amendment to the "CHT standard" so that
the display software knows the chart scale.  By default, the CHT scale
is 0.02 arcseconds per "unit".  One way to include scale information is
to append the word SCALE (followed by a number) to the end of the second
header line. An example of this in a CHT might be:

  ----------------------------------------------------
  frame ABC1234   500  Nova Imaginary 1996           V
  date 96 12 24.3485  (Tycho mag)           SCALE 0.08
  1122 7764 1322 0
  678  8992 1107 0
  1884 26561 1048 0 NSV 654321
  8930 14419  148 1 Nova
  12309 8638 1283 0
  11658 4651  821 1 (823)
  13519 22004 1020 0
  ----------------------------------------------------

This method has several advantages:

(1)  Older CHT display software that is not "scale-aware" simply treats
     the SCALE xx.xx as part of the header.  The chart will still
     display as usual.

(2)  Display software that _is_ "scale-aware" can easily search for the
     text string SCALE, read the number that follows; and use it to
     construct scale bars, exclude stars outside the plottable area,
     etc.  In the above example, the scale is 0.08 (possibly from a
     smaller telescope and/or low resolution CCD).

(3)  Authors of CHTs are not forced to use (or convert to) 0.02
     arcsec/unit scale.

(4)  It provides a consistent standard for indicating chart scale - even
     if the display software ignores scale.


Having a scale bar on the CHT display also avoids the confusion of image
sizes changing during printout, photocopying or publication.

"Scale-aware" display software should default to 0.02, if no SCALE xx.xx
is found.  This will make most older CHTs display correctly.


As for the other problem of showing legible labels on VSNET charts; I
say this is a problem for the display software - not the CHT author.
The CHT author cannot know the capabilities of the system which may be
used to show or print the CHT.  This includes such things as screen
fonts, colours, resolution and other issues that affect legibility.

Speaking as someone who has written CHT display software (one of
VSSPlot's "Toolbox" functions), I also say it is not difficult for the
software to make CHTs legible.  Here's what VSSPlot does to display
CHTs:

(1) First, stars are displayed.  Stars marked as "saturated" are drawn
as unfilled circles; so that half the display is not obliterated by a
few giant dots, and it's obvious which images are saturated.

(2) Next, the CHT is read again to search for extra information (such as
labels, comments); which is displayed as a label joined to the star
image by a short line.

(3) Finally, the star magnitudes are displayed in a similar manner
if there is enough space.

(4) The latest VSSPlot (still on my computer) also allows a limiting
magnitude to be defined.  Anything fainter is completely ignored.


I wrote a rudimentary intelligence into the label display, which greatly
improves legibility. VSSPlot searches several locations around the star,
looking for the "emptiest space" to put the label.  The definition of
"emptiest" being "minimum number of non-black pixels".  This checking
does slow down CHT displaying, but the result is a lot better than a
simple always-put-the-label-beside-the-star.

Extra information is always shown; usually this is important stuff
like the object identities.  Star magnitudes are displayed only if there
is space. Showing every magnitude in a crowded field seems both
unnecessary and silly to me.  Especially when the final product is a sea
of overlapping digits!

My $0.02 (or $0.03) worth...


cheers,
Fraser Farrell

http://vsnet.dove.net.au/~fraserf/  email: fraserf@dove.net.au
traditional: PO Box 332, Christies Beach, SA 5165, Australia

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