Re: [guide-user] Guide and _star_ trails

Bill Gray Apr 18, 2014

Hi Clay, Ted,

> A star is NOT going to move in 5 minutes.

There's a bit of trickery going on here. You're right that, in
normal conditions, Guide only generates trails for solar system
objects. However, you can get Guide to show the "trails" stars
make in alt/az.

A while back, I had an inquiry from Scotty Degenhardt, who had
devised a rather nice scheme for observing occultations remotely.
He would go to a site and set up a telescope pointing at the piece
of the sky where the event would be some time later. To do that,
he needed to be able to make 'star trails', running along in RA
from the star being occulted.

Thus, if the star is at RA=03h14m, and is going to be occulted
in two hours and five minutes, you'd want to point the telescope at
RA=01h09m, leave the clock drive turned off, and wait for the earth's
rotation to bring the target star into view. Then you could drive
off, set up a second telescope half an hour later pointed at RA=01h39m,
and so forth.

With the "trail" running in RA, you can look along the trail for
bright stars and use them as reference points. That way, you can go
out in the field and say, "If I point my scope at star X, which is
bright and easy to find, and get it set up at such-and-such a time,
then I'll know that when the occultation occurs, the earth's rotation
will have put the occultation right in my field of view."

Scotty eventually got me to see what he was trying to do, and I made
the necessary modification to Guide (about a dozen lines of code). At
the time, I thought it unlikely that anybody but Scotty would be using
this feature, so I didn't document it, but I think he's passed the
word on to a few others in the occultation community; it's been more
useful than I'd envisioned.

If you hit Alt-J and enter 1 (as Ted described), you'll toggle Guide
to accept such objects as "trailable". Usually, if you right-click on
a star and try to add a trail, Guide will tell you to click on a moving
object (planet, satellite, asteroid, comet) first. But the Alt-J/1
step tells Guide that if you click on a star, a "trail" will be of the
sort I've just described.

Ted, I do think Clay makes a good point here :

> Try your animation feature on any planet to test it and you will see
> that it is working....or asteroid, comet, etc.

Having hit Alt-J/1 should still leave you able to generate trails for
solar system objects. If you do so, do you get trails? Is it only
stars that are currently un-trailed?

-- Bill