HJST CALCULATOR

A visibility calculator for the 2.7 m Harlan J. Smith Telescope.

Purpose: Show the hour angle and declination of your object's Apparent Observed Place on HJST's sky chart, compute the Hour Angle and UTC visibility limits and other observing ephemera, and return a saftey verdict.

The current version is April 8 2026 (see LEGEND). You may need to clear your history or cache if you have used an older version. At the moment, you cannot copy/paste in the worklist because of browser and operating system security requirements. In Firefox, if you release the mouse button outside the application's boundary the program will freeze and you'll need to reload. See Compatibility below.

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Enter your targets into the worklist, one target per line. Input is case insensitive. Click anywhere on the line: if the calculation is successful the parsed target will be highlighted in red and the chart updated. The formats are:

1. RA and DEC

The basic format is sexagesimal right ascension (RA) and declination (DEC), optional J2000 or B1950, and optional proper motion px py in milliarcseconds per year; single-space separated (colons are allowed in the sexagesimal values); and optionally embedded in freeform commentary text:

[prefix text] HH MM SS[.S] [+|-]DD MM SS[.S] [J2000|B1950[ px py]] [suffix text]

with J2000 and zero proper motion the default. This format is always tried first.

Examples:

15 17 20 +55 1 20
15 17 20.3 55 1 20 J2000
Prefix comment. 15:17:20.3 +55:1:20 B1950 10.3 137 Suffix comment.

2. Sun, Moon, Planets

Enter the name of solar system object - Sun, Mercury, Venus, Moon, Mars ... Pluto, with optional suffix comment:

Examples:

Sun
MOON -- displays approximate phase!
Jupiter
Pluto Poor Pluto!

3. Internal historical catalogs

Catalogs are FK5, Messier, New General Catalog and Index Catalog (Steinicke), using the single letter F, M, N, or I followed by a space and a catalog number and optional suffix comment:.

Examples:

F 732  Albireo is a good test of seeing for the Visitor Center programs.
M 33
N 6365 Pair of spiral galaxies in Draco

4. External SIMBAD search

Query a SIMBAD identifer using the single letter S followed by a space and the identifier. The retrieved

 RA DEC J2000 px py

will be inserted after the S command, and subsequent clicks will use this cached position, treating the original S command as a prefix comment in the basic RA DEC format.

Examples:

S alf Ori
S IRAS 23077+6707
S Andromeda Galaxy
S Sirius

Changing the date and time

The default date and time is NOW, but you can also enter any reasonable date and time by clicking on the Set button. For convenience entering UTC, you can omit the : separator and use just a space, in which case you can also enter just the hour, hour and minute, or hour minute second.

Note that after changing the date or time, updates are not automatic: after you click on the worklist line, the results are correct for the date and time shown.

Verdict

Your object will be either ABOVE HORIZON or BELOW HORIZON, and if above, it can additionally be VISIBLE or BLOCKED. If it's blocked, the reason will be either the pier limits, which takes into account all the stuff around the north pier, or a curtain limit, which is dictated by the lower curtain's fully dropped position. Whichever, it will be apparent in the graphical depiction.

The Limits in Hour Angle reports the visible hour angle interval, i.e. the setting-circle hour angle where your object first becomes visible and where it sets, taking into account the horizon (90º) zenith distance, curtain limit, and pier. Similarly, the Limits in UTC reports the visibility interval in UTC for the dates indicated.

Mouse-over information

Mouse-over the blue target to see its Apparent Observed RA, HA, and DEC.

Mouse-over the compass rose to see the LST (RA + HA).

Mouse-over the moon to see the mean lunar day.

Switching Tube East/West

A target that is above the horizon but blocked may be visible in the opposite tube (east/west). When you click East or West, the chart is redrawn and the controls flip to the opposite side. The collision limits in tube west may not be correct at the f/33 cage, elevator, and Hi-Ranger, and I will probe those limits as I develop the system.

Compatibility

Modern versions of Edge (Microsoft) and Safari (Apple) work best. Avoid clicking in the application window until it's fully loaded - depending on the browser you may to reload if you jump the gun. In Firefox, don't click in the application and then release the mouse elsewhere: this may cause the program to freeze, and you'll need to reload it. In iOS, the proxy connection to Simbad is blocked, so the S command won't work. Public computers, kiosk, and family protection modes may disable this application.