Explanation of Declination

For those thinking about a Big Ugly Dish, here is the first post of a few that I will make so that anyone with a little patience can setup one. I can tell you first hand that after getting a big dish setup, the rewards are great. I myself run a 10 foot c/ku dish with a 4dtv and pansat 9200 with s-2 tuner. I am able to watch everything from uplink trucks to studio feeds as long as I am in the footprint. This first bit of reading is simply ment to help one understand one of the many things that is needed in order to set up a big dish. This is just a little about declination.......


Declination angle keeps satellite antenna from aiming into deep space and changes value with latitude in accordance to earth's curvature. Early astronomers developed the polar mount to track stars. For deep space tracking, i.e. star observation,the elevation setting of the polar mount is equal to the latitude of the site location. However, geostationary satellites are much closer to earth than are celestrial objects and their observation requires a slight tilting 'downwards' from deep space observation settings to see the geostationary satellite arc. This adjustment of the polar mount, the tilting downwards, is the declination adjustment of the polar mount and its amount is the declination offset angle (not to be confused with the declination angle which is the sum of the declination offset angle and the polar axis zenith elevation angle).
In summary, the declination angle lowers the satellite antenna from looking into deep space to look at the arc of geostationary satellites. However, upon practice, it has been discovered that using the latitude as the elevation angle, the satellites at the top of the arc, in a tracking mount system, were in perfect alignment with the satellite dish, i.e. were tracked perfectly by the polar mount, whereas the satellites at the lower ends of the arc where not tracking correctly. Conversely, if the satellites at the ends of the arc, on the horizon, were tracking correctly then the central satellites, at the top of the arc would be slightly off target. This effect is caused by the slight deviation from the true north/south line, caused by the act of the tilting of the dish, as the dish moves to view satellites lower on the arc, i.e. closer to the horizon. With this in mind, it was formulated the elevation/declination angles of the modified polar mount whose application has no effect on seeing satellites at the top of the arc but has the effect of better tracking on the sides and low end of the arc. This effect is accomplished by slightly increasing the elevation angle and accordingly slightly decreasing the declination angle by the same amount. Elevation and declination angles are always measured and applied when the dish is at its zenith, i.e. highest point in the arc, when the dish axis is aligned along the true north-south line.



Elevation Angle is the angle measured on (at) the pivot axis of the mount. To get the Total Declination Angle setting as measured on (at) the back of the dish ring (dish mount), add the Declination Offset Angle to the Elevation
Angle.

The difference between the Total Declination Angle and the
Elevation Angle is the Declination Offset Angle.

Elevation and Declination Angles are always measured and applied when the dish is at its zenith, i.e. highest point in the arc, when the dish axis is aligned along the true north-south line.

Now don't be intimidated by the above mumbo jumbo. It will simply help you understand what the dish is doing later on when you instal and adjust your big ugly dish. :) Have a great day!