Image of the day

Captured by
Mark Eby

M51 Whirlpool Galaxy

My Account

New to Astromart?

Register an account...

Need Help?

Orion's Collimating eyepiece

Posted by Robert Piekiel   01/27/2007 12:00AM

By Bob Piekiel

Orion’s collimating eyepiece, otherwise known as a Cheshire eyepiece, is a highly useful, often overlooked tool that can be used to quickly and easily tweak the alignment of virtually any type of telescope.

Laser collimators have become the “fad” in the last few years, but there are certain situations or telescope designs where a laser collimator is difficult or impractical to use. A Cheshire eyepiece like Orion’s does not have these limitations.

The Orion collimating eyepiece is basically a precision-centered peephole mounted in front of a crosshair sight, fitted inside a standard, 1-1/4 inch aluminum tube. An angled cutout on the side of the tube permits light to enter and be reflected off a polished ring, serving as a highly-visible reference of exactly where the sight is with respect to the optical axis.

What most telescope users probably don’t realize is that you can get any telescope into reasonably good collimation just by eye alone, IF you can perfectly center your eye in the eyepiece tube of the scope and IF you can SEE the reflection of your pupil in the mirrors and IF you know the exact nature of the reflections visible in the system (the edges of the mirrors, the baffles, etc.) Since it’s nearly impossible to do the first just by looking into your scope, and often difficult to effect the second, the procedure becomes difficult or frustrating for most. That’s where the Cheshire eyepiece comes into play.

Once you insert the collimating eyepiece into a scope, you then look into it and see the crosshair centered (hopefully) on the optics. That’s where the peephole and crosshair work together. They precisely locate your eye on the optical axis, which is where you need to be in order to “reference” proper alignment. That’s not all the eyepiece does, however. The polished, reflective ring inside the device projects an annulus of light into the telescope that becomes visible in the reflections of the mirror(s). In this way, you can see the exact centerline of the system (the crosshair), center it on the optics (through the scope’s collimation screws), and then center the reflected light ring squarely on that, again by adjusting the scope’s optics. Because you’re actually LOOKING into the scope, and not at a reflected beam hitting a target, you can see and assess the entire optical system as a whole, and adjust each component as necessary while viewing it in “real time.”

There are several situations where the Cheshire eyepiece will work where a laser won’t:

1) If you have a large Cassegrain-style scope where the secondary mirror is held in place by a bolt through its center. A laser can’t be used because the beam will simply hit the bolt, and there will be no reflection back to it for collimation.

2) If the physical layout of the scope is such that you can’t easily see the target on the housing of the laser where the reflected beam is supposed to hit. Some lasers have cutout windows for viewing this, but if they don’t they can be difficult or impossible to use.

3) If you’re trying to adjust the PRIMARY mirror of a Cassegrain-type scope. (Yes, some CATs have adjustable primaries, such as larger semi-professional models, and ALL Vintage, blue-white Celestron SCTs.) A laser by itself will simply return the reflection from the secondary mirror to itself. The beam never touches the primary, hence it cannot be used to collimate it.

The collimating eyepiece does not suffer any of these limitations, since it introduces a VISIBLE (viewable) light source into the scope that can be seen directly by the eye, on-axis, and will work with all types of telescopes.
There are a couple of tips and tricks for getting one of these to work in all conditions. First, you need some kind of light source to aim into the side of the collimator to be seen inside the scope. If outside in the daylight, ambient light is usually enough. Because I often work at night, I simply shine a small, LED flashlight into the device to REALLY illuminate the ring. Second, the scope’s focuser must be physically square and centered on the mechanical and optical axes of the scope. For most commercially-made telescopes, this is the case. It’s rare to see one come from the manufacturer with gross machining errors. If you have a home-built instrument, you need to accurately center and shim the focuser accordingly. If you don’t, the collimating eyepiece won’t work (and neither would a laser). Lastly, you need to identify the EDGES of your mirror holders so that you know when you have the reflection of the mirror(s) centered on the crosshair.

To use the collimating eyepiece, insert it into the eyepiece tube of the scope and illuminate the angled ring (as described above). Sight down the tube and note the crosshair target. Center the FIRST MIRROR (diagonal in a Newt, secondary in a CAT) on the crosshair, and adjust its tilt until the bright ring the collimator is producing is centered in the reflection. (The image of the ring doesn’t actually move, only the reflection does.) Next, adjust each successive mirror in the scope so that the image of the ring is concentric with the optic. Professional opticians refer to this as “working BACK THROUGH the system,” ie.: adjusting each mirror in succession from the eyepiece outwards. In other words, before you align the primary on a CAT system, you have to have the secondary aligned, because it’s the first thing you SEE while looking up the tube.
That’s basically it. The Cheshire eyepiece can be used day or night, on just about any reflecting telescope. It works well enough to get a scope aligned to about 95% of “perfect,” with the final 5% done by high-power star testing. At about $35, it’s a real bargain.

Once you get used to using one, and know the appearance of circular reflections inside your scope, you’ll probably use it far more than your laser, particularly if you have a scope with more than one adjustable mirror.

Click here for more about this subject. -Ed.