Field Orientation


One of the most confusing aspects of visual astronomy for me, at the beginning, was the simple matter of field orientation.

First there was the simple matter that the telescope optics would reverse or invert the image in some way. With a mirror diagonal, the image is reversed right to left; the vertical orientation is unchanged. For me this had a drastically disorienting impact on views of the moon, and I understood why moon charts printed in the reversed orientation were available.

On top of that, there was the confusing effect of changing the diagonal orientation from "erect" (in a plane perpendicular to the ground when the telescope was pointed away from the zenith) to one side, and the added effect of the change in head orientation to the optical axis that the change in diagonal position required. This rotated the field in some way, but because they always occurred together, I could not immediately distinguish their separate effects.

Finally, there was the effect on the eyepiece image of using the telescope remote controls to move the telescope (on an altazimuth mounting) to the left or right, or up or down.

To clarify these problems, I purchased a large plastic "For Sale" sign, blank on one side, at the local hardware store. I drew four large squares on the blank side and within each square the orientation of celestial coordinates as they appear near the horizon at the cardinal compass points. I placed the sign at a distance of about 100 yards, and examined each field with the diagonal in three different orientations, and with the median plane of my head either parallel to the optical axis (which required me to lean forward and turn my head when the diagonal was turned to one side) or vertical to the ground (the comfortable viewing position when the diagonal was turned to one side).

The image below summarizes the combined effects of diagonal and head rotation as observed in the celestial coordinates projected into different parts of the sky (H and Z refer to horizon and zenith).

And these effects can be summarized as four rules:

1. The "baseline" orientation of the eyepiece field occurs when the sky object is at the meridian, the telescope mirror diagonal is oriented vertically (toward the meridian circle), and the observer straddles the north/south line, facing either due north or due south, and therefore facing into the optical axis.

In this orientation celestial north, celestial south, the zenith and the horizon point are all aligned on the vertical line through the eyepiece field: north and zenith are at the top of the field, south and horizon at the bottom, when the viewer faces south; facing north, the orientations of north and south are reversed, but zenith remains at the top and horizon at the bottom. (See "erect" diagonal position in the N and S orientation, above.)

2. If the mirror diagonal, eyepiece and observer remain in the same relative positions, but the telescope is oriented to an object off the meridian, then the field is rotated to match the celestial coordinates of the area of sky being viewed, but in the opposite direction due to the mirror reflection left/right. (See "erect" diagonal position in the E and W orientation, above.)

3. If the mirror diagonal is rotated either left or right by a given angle (such as 45˚ to one side), then the field is rotated by the same amount in the same direction — counterclockwise, if the diagonal is rotated counterclockwise (to the left), clockwise if the diagonal is rotated clockwise (to the right).

4. If the observer turns in relation to the line of the meridian circle, then the eyepiece field is rotated by the same amount in the opposite direction — clockwise, if the viewer steps to the right of the optical axis, counterclockwise, if the viewer steps to the left.

In the "erect" image orientation for views predominantly south, the preceding field is on the left and the following field is on the right; for predominantly northern views the preceding field is on the right and the following field on the left. Viewing east, the preceding field is toward the top left; viewing west, the preceding field is at the bottom left.

For binary star observing in particular, an altazimuth mount is most convenient because it allows objects at all altitudes to be observed the meridian, which orients the field in such a way that position angles can be visually estimated quite precisely.

Note that the measure of position angle starts at north and is counted counterclockwise (first across the following two quadrants, then across the preceeding two quadrants), which means it is measured in clockwise direction in a CAT with mirror diagonal, regardless of the rotation changes caused by diagonal or head orientation.

The hand controls were easy to anticipate, provided that I kept the orientation of zenith and horizon in mind (always the directional change in altitude); the azimuthal left and right was just perpendicular. The Meade "Utilities" function also allows me to reverse the key direction of altitude, azimuth, or both, so I can configure them in whatever combination I find most comfortable with a reversed field.

My viewing method now is routine. I first look at the location in the sky I am observing, and with the North Star and landmarks of north and south as anchors. I determine the orientation of the right ascension and declination lines for that part of the sky. I visualize those in relation to the zenith. Then I reverse that imaginary grid left to right: those are the coordinate orientations of the "erect" field.

If I need to turn the diagonal to left or right, I visualize the added rotation of the "erect" field using the rules above. This allows me to anticipate the field motion that will be caused by different hand control skewing keys, and to identify the relative preceeding or following location of any two objects. This skill is especially handy for estimating the approximate position angle of binary stars, which I can now do within a typical error of about 10° to 20°.

Further Reading

Left Is Right and Right Is Left - A leisurely and user friendly orientation to field orientiation.


Last revised 06/06/16 • ©2016 Bruce MacEvoy