Newtonian Secondary Offset?

Jack Kramer

If you own a Newtonian telescope, you may have heard about secondary mirror offset and wondered about its purpose. The process of offsetting means that instead of installing the secondary centered geometrically in the tube beneath the focuser, it is positioned slightly away from the focuser and toward the primary. This is done because the optical and mechanical axes of Newtonians are not the same - due to the 45o angle of the secondary mirror, the geometric center of the mirror is not the optical center. Offsetting the secondary is meant to ensure a fully illuminated field of view without vignetting - a loss of light at the edge of the field of view. In the illustration below, the red lines show what happens in a mirror that is not offset.

The red dot from a laser collimator helps to identify the optical axis. In the photo of the offset secondary mirror here, the "return" position of the laser dot is seen slightly above the center of the secondary mirror, indicating the location of the optical center.

Here's the formula for determining the amount of offset: Offset = Diameter of Primary x Diameter of secondary divided by 4 x FL of primary As an example, in an 8" f/7 scope you would generally have a 1.52" secondary; plugging those figures into the formula, the amount of both offsets would be only a little over .05". That's certainly not very much. You can easily check this out by visiting telescope-making author Mel Bartels' web site, which calculates this for any telescope:

In a properly collimated system, the secondary appears positioned in the center of the focuser drawtube. This assumes the mirrors are perfectly aligned, which requires a center dot on the primary mirror plus some basic collimation tools. An offset in the direction away from the focuser (the "transverse" offset) requires that the spider vanes be slightly shorter on one side. An offset toward the primary involves only a simple adjustment to the secondary holder, which you actually do when lining up the secondary directly under the focuser.

After reading discussions on both sides of the issue, I'm persuaded that a full offset (toward the primary and away from the focuser) is unnecessary in most cases. Bryan Greer of Protostar (manufacturer of secondary mirrors and spiders) explains it this way:

When you get to the final collimation step of tilt adjusting the primary, you are pointing the primary back at the optical center of the secondary. Once again, the light cone will be intercepted perfectly symmetrically. The only consequence of not offsetting is that the primary mirror is no longer pointed exactly down the middle of the tube. It will end up oh-so-slightly tilted up towards the focuser. But this causes no optical problems at all, as there is nothing sacred about having a perfect 90-degree reflection at the secondary. Collimation, vignetting symmetry, and concentricity of the optical axis within the focuser drawtube are all still perfect.

Then why would anyone choose to fully offset the secondary? Many experts feel it isn't necessary, even in large fast scopes, since the size of most secondary mirrors is ample enough to provide full field illumination. One exception is when the size of the secondary is at the absolute minimum, in which case a non-offset secondary would not intercept the entire cone of light coming from the primary mirror. Typically this situation occurs in Newtonian telescopes intended primarily for planetary observation where the secondary mirror size is kept as small as possible to get the best image contrast. Offset also is an issue for long exposure astrophotography because tracking will be a bit more accurate if the optical and mechanical axes are coincident. Similarly, there's a body of opinion that says a full offset will improve the accuracy of digital setting circles on scopes with Dobsonian mounts.

If you've done a good job of collimation, you'll probably find that in the process of centering the secondary mirror beneath the focuser drawtube you've induced an offset toward the primary by pure serendipity!

Published in the April 2008 issue of the NightTimes