Re: [guide-user] Epoch shown and scope's epoch

Bill Gray Sep 27, 2013

Hi Albert,

There are really two different things going on here.

Most telescopes expect coordinates to be sent to them in mean coordinates of
date, and that's what Guide will normally send and receive. So if you tell
Guide to find, say, Neptune, it'll compute the coordinates for that object,
precess them to the current mean coordinates as of "right now", and send those
to the telescope.

A few people with home-brew systems have asked that Guide instead use
J2000; for that purpose, you can hit Alt-J and enter


But that sounds like the opposite of what you want. In your case, if you
do nothing, Guide will send/receive mean coordinates of date, and you'll
be all set.

On-screen, using the Settings... Format dialog, you can tell Guide that
you want to enter and see RA/decs in J2000 (the default), mean coordinates
of date (corrected to current precession, but not nutation and aberration),
apparent coordinates (nutation and aberration included as well), or some
other system such as B1950 or B1875.

Yet another strange thing to keep in mind, that has caused me to get an
e-mail or two: for the telescope control, Guide knows that a physical
object is getting moved "right now". So the mean coordinates of date are
those for the current time, as read from the computer's clock. For the
on-screen display, the mean coordinates of date are those for the time/date
set in Guide. So if you've used Settings... Formats to display mean
(or apparent) coordinates, and you change the time by a few decades,
you'll see the coordinates change as well. (Which you would _not_ see if
Guide is showing J2000 coordinates; those are "fixed" with respect to time.)

-- Bill