The Tycho Star Catalog at MDM
John Thorstensen, Dartmouth College
1) Copy your MDM-style pointing list over
2) On agung, type "tychoselectmdm" and answer
3) Note that the date format it wants is
YYYY MM DD, e.g.
2004 3 17
for March 17, 2004.
4) Copy the output file back over to the
top level of chichon's
visitor directory, where the xtcs window
can see it.
IFAQs (Infrequently Asked Questions):
0) Why do I want to use this?
It makes it possible to check very quickly
that the telescope is
pointing right, and it can be a real time-saver
for finding and
centering on faint things.
1) What does it do?
The program reads each object in your pointing
list, finds the nearest
three entries in the Tycho-2 star catalog,
and makes a new copy
of your pointing list with the three nearest
Tycho stars inserted
after each entry. If your object is named
"foo", the Tycho stars
will be named "foo.pt1", "foo.pt2", and
"foo.pt3", ranked from nearest
to farthest from your object.
2) What is this Tycho-2 catalog?
Tycho-2 is the biggest, most accurate catalog
of bright stars
(like 11th magnitude or brighter); it is
a combination of positions
derived from the Hipparcos satellite with
a large number of other
3) What's a pointing list?
A pointing file can be read by the telescope
control program so
you just specify your target by name, avoiding
errors in typing
and delay as you sit and peck out the coordinates
on the keyboard.
If you have more than one target a night,
you're a "foo" if
you don't use one. The format for the list
is simple -- one
entry per line, of the form (e.g.)
name_no_blanks 18 22 33.4 -1 02 03 2000
for an object at 18h 22m 33.4s, -1deg 02min
03sec. Note no blanks
in the name, and no colons in the coordinates.
themselves are free-format, it's not fussy
about which column,
leading zeros, etc.
4) OK, but how do I use these extra stars
in the list?
Although the 2.4m points "blind" pretty well,
it's far from
perfect. However, in a small part of the
sky it can offset to
within an arcsecond or so. If you have
any trouble finding your
target, you can just whip off to the nearest
Tycho star, center it,
reset the encoders, and you'll offset right
back to where you
want to be. Actually, it's safer to look
at *two* Tycho
stars to be sure you've found the right
one, and a third one
is provided to give still more confidence
about the ID. If
you want to avoid resetting the encoders
(which is a little
dangerous), you might just note the location
of the Tycho
star in the field and look for your object
there when you set
back to it.
Because the throws are so short, the telescope
usually gets to the
nearest Tycho star in only 10-15 seconds.
Well worth it.
The stars are named "foo.pt1" etc. because
then you can immediately
call up the nearest Tycho star by simply
appending ".pt1" in the
"Object Name" window, and when you're done
you can get your object
name back by erasing the ".pt1". (Note
you have to hit a carriage
return to actually read the coordinates
of the object from the
list; also, you need to delete characters
explicitly with the
"delete" key, backspace doesn't work, you
can't black in characters
and delete them all at once. Weird interface.
5) Why should I use Tycho instead of the
Tycho-2 contains around 2 million stars,
compared to a few hundred
thousand in the SAO, so Tycho is much denser
-- the stars will be
closer to your target. Also, it's much
more accurate, typical
errors being 30 mas (that's milliarcsec),
more than good enough
for any offsetting or pointing correction.
Tycho's positions are
more recent, also, and the proper motions
are more accurate than
the SAO. There's a paper by Hog et al in
the A&A describing it.
6) Why does the program ask for the date?
It's silly in most cases, but the coordinates
are updated for
proper motion from 2000 to the time of
your run. For most stars
this is a miniscule correction but you
never know when you'll hit
something that's flying along. This way
you *know* it's right.
7) Will the program select something too
It rejects stars brighter than V = 5, which
counts for something.
8) Where can I expect trouble?
At the pole, which is a crazy place to observe
stars are extracted from box-limits in
ra and dec, rather than
a circle around your object. The program
does wrap correctly
around 0h, which is a more likely trouble
9) Hey, I like this! Can I get some at home?
Sure! Have your computer guy contact me,
I'll need to burn you a CD-ROM with my
compressed Tycho-2 catalog on
it (I took out all the fat and left only
vitamins, compressing it
from 500 Mbyte down to about 70 Mbyte).
The code would require
minor modifications to reflect your local
directory structure, but
it's easy to adapt.
If you're a C guru and would like to write
other applications using
Tycho, please go ahead. At this time ,
all I have set to go is this
program for settting up pointing lists,
which may be all that most
people would want anyway.
John R. Thorstensen Professor of Physics
Department of Physics and Astronomy and
6127 Wilder Laboratory
Dartmouth College voice: 603-646-2869
Hanover NH 03755-3528 USA FAX: 1446