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HJST CALCULATOR is the premier visibility planning tool for the 2.7 m (107") Harlan J. Smith Telescope that takes into account the curtain and pier limits.
 
In the Firefox browser, avoid releasing the mouse outside the application, which causes a Javascript exception. See Compatibility below.
 
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Basic operation: optionally Set the desired date and time (the default is NOW); enter a target into the worklist; and click anywhere on the line. If the calculation is successful the parsed target will be highlighted in red and the chart updated with the object's position, observing limits in UTC and hour angle, airmass, zenith distance, azimuth, dome azimuth, and a visibility verdict.
 
The four worklist 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 hour, hour 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. Note that the Limits in Hour Angle are simply computed using the object's current declination, but the Limits in UTC are recursively calculated for high accuracy, and setting the date and time to this limit reveals the exact hour angle of the Apparent Observed Place.
 
Zenith Distance is reported even when the object is below the horizon, useful for civil (90º, 96º), nautical (96º, 102º), and astronomical twilight (102º, 108º). Note that while the object is above the horizon the ZD and AZ are for the Apparent Observed Place, which takes into account refraction. When the object is below the horizon, ZD and AZ are calculated using the just the Apparent Place to get the actual geometric position. Hence, crossing below the horizon you will see a jump in ZD as the refraction calculation is omitted.
 
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).
 
Cow-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. Currently, TS23 and DIAFI are the only instruments that can freely switch tube east/west.
 
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 need to reload the page 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 the page. 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.
 
Credits
The north pier limit functions are computed from data mapped by Judson Richards in 2025.
 
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