Collimating a Newtonian with an Offset Secondary Mirror

Jack Kramer

A few LCAS folks have inquired about a problem when trying to collimate their scopes. They get everything lined up as well as possible, but when they look down the empty focuser tube, the secondary mirror is not centered. If the secondary appears in the center of the focuser tube, then everything else is misaligned. The reason for this is usually that the telescope manufacturer has purposely offset the position of the secondary. An offset is of two types: a radial offset (moving the secondary slightly away from the focuser, putting it off-center in the tube) and an axial offset (moving the secondary slightly toward the primary). The radial offset requires the spider vanes to be slightly shorter on one side. The axial offset usually involves a simple adjustment to the secondary holder, and may be done to a small extent even when there is no radial offset.

The purpose of offsetting the secondary is to provide a fully illuminated field and avoid vignetting - a gradual fall-off in brightness toward the edge of the field in your eyepieces. But offsetting is unnecessary if the secondary mirror is large enough to intercept the entire cone of light from the primary mirror. It's also said to be unnecessary except in the larger-sized telescopes. According to Phillip Harrington, author of the book Star Ware, "Only when we start talking apertures in excess of 15" (maybe even 20") will there be any benefit to offsetting the secondary, and even then, the benefit is arguably so small that it might not be worth the effort."

Collimated and concentric optical components

Apparently, some manufacturers of moderate-sized telescopes have chosen to offset their secondary mirrors, despite the fact that there is no advantage to doing so for these particular telescopes. For example, as reported on one of the Internet chat groups, at least some of Orion's model XT8 and XT10 telescopes have offset secondaries. This has created frustration among owners because the diagrams in Orion's collimating instructions show all the optical components appearing concentric when you look down the focuser tube. I noted this same thing when collimating our club's 10-inch Dobsonian. I struggled with the alignment for awhile until I realized the secondary has a slight offset.

So if your secondary mirror doesn't appear centered when everything else is collimated properly, don't worry about it. The real test is the quality of the images when observing. If you have Internet access, there is information on this topic at:

http://www.fpi-protostar.com/collim.htm