digits and press the RA+ key to make your selection, Gemini displays the full designation
of the object in the catalog that is the closest match to the digits you entered. For
example, in the WDS catalog, if you entered the digits 00040 and pressed the RA+
button, Gemini would display the catalog item closest to your entry: WDS
00042+6217. You can then scroll through the catalog with the DEC buttons and must
press the RA+ button a second time to select the object and display the hour angle and
elevation. This same selection method is used for the SAO catalog, which is only a
subset of the full SAO catalog (i.e. some entry numbers are missing).
For convenience, pressing the RA+ button again causes Gemini to jump to the "GoTo
or DEC buttons to scroll to other menu items, or press the MENU button to escape the
Menu Mode.
Once the position of the selected object is calculated, additional data related to the object
can be displayed using the "Show Information" menu item. The information shown there
depends upon the catalog and object type selected. Mutual cross references,
constellation, common name, type, magnitude and size are shown for the Messier, NGC
and IC catalogs. Type and size are provided for the Sharpless 2 catalog, and
constellation, sometimes Greek letter designation, type, magnitude and spectral class are
provided for the Bright Stars. Only magnitude information is stored for SAO stars, and
double stars are marked with a "D."
The catalog numbers for objects in the Messier, NGC and IC catalogs can be entered
without the leading zeros. Simply select the digits you want and then use the
character in the last digit to indicate that you are done. For example, to select NCG 145,
you can enter NGC 145
and press the RA+ button.
5.3.2.2 Accuracy and Epochs
With the exception of the Solar System objects, all coordinates are stored rounded to 20
arcsec, giving 10 arcsec accuracy for the standard epoch J2000.0. The coordinates are
precessed to the equinox of the date when the object is selected; nutation is neglected.
The apparent position of objects is calculated (for standard air pressure and temperature),
taking refraction into account. For the Moon, topocentric coordinates are calculated.
By default, Gemini assumes that any input coordinates are for the epoch of the current
date. This default can be changed to Epoch J2000.0 by executing "Setup
Coordinate
Epoch
Epoch J2000.0." If this change is made and Epoch J2000.0 coordinates are
entered, Gemini will precess the entered coordinates to the epoch of the date so that
GoTo slews will be accurate. The coordinates of the resident catalogs will not, however,
be affected regardless of which epoch is selected.
5.3.2.3 Credits
In general, the whole astronomical community has to be credited with contributing to
Gemini
s databases by their collecting, checking, sorting, and providing this great amount
of data. Special credits are due to Wolfgang Steinicke for long years of work checking
Gemini Users Manual
4
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