VLBA Declination Limits
Because the VLBA stations are widely distributed with a large range of latitudes, there is no simple declination limit for the instrument. The table below presents several declination limits for each station.
Horizon Limit: A source with a declination below the given value will never rise above the horizon at the specified station.
10-degree Elevation Limit: A source with a declination below the given value will never have an elevation above 10 degrees at the specified station.
20-degree Elevation Limit: A source with a declination below the given value will never have an elevation above 20 degrees at the specified station.
| Station | Horizon Limit | 10-degree Elevation Limit | 20-degree Elevation Limit |
| Brewster (BR) | -39 degrees | -32 degrees | -22 degrees |
| Hancock (HN) | -43 degrees | -37 degrees | -27 degrees |
| North Liberty (NL) | -45 degrees | -38 degrees | -28 degrees |
| Owens Valley (OV) | -50 degrees | -43 degrees | -33 degrees |
| Los Alamos (LA) | -52 degrees | -44 degrees | -34 degrees |
| Pie Town (PT) | -52 degrees | -46 degrees | -36 degrees |
| Kitt Peak (KP) | -55 degrees | -48 degrees | -38 degrees |
| Fort Davis (FD) | -57 degrees | -49 degrees | -39 degrees |
| Mauna Kea (MK) | -57 degrees | -57 degrees* | -50 degrees |
| St. Croix (SC) | -62 degrees | -62 degrees** | -52 degrees |
*At MK, low-declination sources do not get above the horizon until they have an elevation of about 12 or 13 degrees.
**At SC, low-declination sources get above the horizon when they have an elevation of about 10 degrees.
In general, the NRAO recommends observing sources when they have an elevation above about 20 degrees at a participating station. Observing at lower elevations makes calibration more difficult. An elevation of 10 degrees is the practical limit for most stations. Below 10 degrees elevation, observers will need to worry about “spill-over” (signal reflected from or originating from the ground) at lower frequencies, and a very thick atmosphere at higher frequencies. Also, the amount of RFI tends to increase at lower elevations.
The large range in the station longitudes also has an impact on low declination observing. A source with low declination may set at some of the stations before rising at others, limiting the number of antennas that can be on-source. The table below gives the approximate amount of time a source with a given declination will be visible from at least the specified number of stations over a 24-hour period.
| Declination | 4+ stations | 5+ stations | 6+ stations | 7+ stations | 8+ stations | 9+ stations | 10 stations |
| -20 degrees | 9.8 hours | 9.5 hours | 9.4 hours | 9.2 hours | 7.2 hours | 5.1 hours | 3.9 hours |
| -25 degrees | 9.1 hours | 8.9 hours | 8.8 hours | 8.6 hours | 6.6 hours | 4.5 hours | 3.2 hours |
| -30 degrees | 8.3 hours | 8.2 hours | 8.1 hours | 7.5 hours | 5.6 hours | 3.6 hours | 2.3 hours |
| -35 degrees | 7.4 hours | 7.3 hours | 6.2 hours | 5.8 hours | 4.5 hours | 2.7 hours | 1.1 hours |
| -40 degrees | 6.4 hours | 6.4 hours | 6.2 hours | 3.8 hours | 3.2 hours | 0.4 hours | 0.0 hours |
| -45 degrees | 5.2 hours | 5.0 hours | 3.9 hours | 2.0 hours | 0.4 hours | 0.0 hours | 0.0 hours |
| -50 degrees | 3.4 hours | 2.8 hours | 1.0 hours | 0.0 hours | 0.0 hours | 0.0 hours | 0.0 hours |
NOTE: This table is only for the 10 VLBA stations and does not take into account adding Y1, the phased VLA, the GBT, Effelsberg, or any other stations.
For more details on planning observations of low-declination sources, see the Guide to Proposing for the VLBA (and HSA/Global VLBI): Scheduling Considerations.
