Right ascension and declination identify an object's position in the sky, but if we are looking at something through a telescope, there is a third angle of interest, namely how our view is rotated relative to the point we are looking at.

We could could think of this angle as a sort of celestial roll that goes with RA (yaw), and D (pitch).

Is there a standard name for this "roll" angle as a third element with RA and D?

What is its reference direction? (Orthogonal to the direction given by RA and D, in the plane containing that direction and the north pole?)

And would increasing the angle turn our view clockwise, or counterclockwise?




1 Answer 1


This angle is dependent on the specific positioning of the eyepiece on your telescope. For models on which the tube can be rotated or custom-positioned, this means there are even more possibility—technically, an infinity.

However, no matter which telescope you use, the angle of rotation of the field of view will not change as objects move across the sky due to the Earth’s rotation if you are using an equatorial mount. Of course, using an altazimuth mount would make this angle change with time.

Usually, angles increase eastwards at the eyepiece. See for example https://en.wikipedia.org/wiki/Position_angle

Clear skies!

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    $\begingroup$ It will also depend on the optics of the telescope as well! A right-angle mirror or prism secondary in a Newtonian or at the eyepiece of a refractor will make "the angle" useless until the mirror imaging is dealt with. $\endgroup$
    – uhoh
    Commented Dec 9, 2020 at 4:40

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