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the system works. You'll also see that some notes have already been supplied for NGC 253. Try extending that text to read something like: ~~253 The ^supernova^ 1940E was found in this galaxy on 1940 Nov 22 at RA 00 45.1, declination -25 34 (epoch B1950), magnitude 14.0. On 23 Aug 1997, I observed this galaxy with a 10" f/4 Newtonian and saw that... Of course, you can add remarks for new objects by inserting "tilde-number"s for them. There are similar files for a variety of other objects: IC.NOT, UGC.NOT, ASTEROID.NOT, and PLANETS.NOT. The system has to be modified in cases such as COMET.NOT and PK.NOT; you'll see that these files use a tilde- plus-name system, since tilde-plus-number would not be very helpful. 22: ABOUT GUIDE'S DATA As has been mentioned, Guide draws its data from a number of sources. It can be helpful to know something about these sources: their limits, purposes, and uses. This chapter will discuss these issues. For stars brighter than about mag 11, Guide uses the Hipparcos and Tycho-2 Catalogs. These catalogs were both generated by the European Space Agency (ESA) Hipparcos satellite, and are often spoken of as if they were essentially identical. They are not, and in some cases, the differences can be very significant! The Hipparcos catalog contains 118 218 stars that were primary targets for observation. Most are bright stars, but a few dimmer stars that were of special interest were included: mostly nearby stars and some variables. The satellite gathered positions to a precision of about .001 arcsecond, and highly precise proper motions, parallax (distance) data, and magnitudes in visual and "blue" systems, for these stars. (Some were more precise, some less so; the specific data for each star is listed when you ask for "More Info" about it.) The Tycho catalog contains a total of over 2.5 million stars. (Tycho has been released in two versions: Tycho-1 had about 1 million stars, but the data was reprocessed to detect fainter objects, and the current Tycho-2 catalog has about 2.5 million stars.) Tycho attempts to be complete down to the limit of what the instrument in the satellite could observe (about magnitude 12), and measured the same things as Hipparcos. But its precision is considerably less than that of Hipparcos. The magnitudes and positions are still better than those from any other catalog. The Third USNO CCD Astrograph Catalog (UCAC-3), is used for dimmer stars, down to about magnitude 17. It contains about 100 million stars, with good positional, magnitude, and proper motion data. It was generated by a small ground-based telescope
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