Star colors/asteroid occn's/Ax.0 concatenation

Bill J Gray Nov 28, 2001

Hi folks,

Ross is right about the 'colored stars' problem: the difficulty here is
that Tycho has color data, but GSC (used for displaying stars at fainter
magnitudes) does not. So when you zoom in far enough, Guide has to swap
datasets, and the colors go away.

Sorry. When Guide 9 comes out on DVD, I can presumably fit a decent
chunk of the new GSC-2.2 on the disks, thereby providing more stars,
better positions and magnitudes, and halfway decent colors. (I can't
use A2.0 for that purpose, for legal reasons.)

As to the companion appearing blue... the Tycho data (both Tycho-1
and Tycho-2) seem to lean toward a whiter color, and the spectral
type from Hipparcos is B8. Sirius. With Tycho-2, it shows up as
slightly bluish, but not very. Perhaps it looks bluer than it
normally would by contrast with the K0 primary?

Jan, about the binary concatenation problem: one way around this is
to switch to the new "A2.0 via Internet" function provided in the most
recent update. This beats the old system of downloading files manually
by a long shot... its only drawback is that the (ASCII) downloads are
somewhat larger than the previous (binary) ones.

About the asteroid occultation path: Guide can produce _very_ wrong
paths for these, unfortunately. To get it right, you generally have
to get some high-precision astrometry for the object (Ron Stone at USNO
is now producing quite a bit of this), feed it through Find_Orb,
get a very exact orbit with an epoch close to that of the event, and
feed it to Guide. After all that, you will get about as good an
occultation prediction as is currently possible.

I was doing this pretty regularly for a while, before I got sidetracked
with other projects. See

http://www.projectpluto.com/ast_occu.htm

for a description of the technique and some examples of the results.

-- Bill