Error type 2 - the optical axis strikes the secondary mirror at a point away from the optical center.

The secondary mirror has an elliptical surface with a major to minor axis ratio equal to the square root of 2, for 90 degree deflection. Depending on its size, it lets some of the focal plane be fully illuminated, that is any point within the area of full illumination sees the whole main mirror reflected in the secondary. Outside of this, some light is lost.

Due to the 45 degree tilt, the elliptic surface appears circular when you see it with your eye on the optical axis near the focus. However, due to the perspective, the center of the circle you see is offset from the center of the ellipse, towards the edge nearest to the focuser. To be optically centered, the secondary mirror must be offset both in the direction away from the focuser and towards the main mirror.The offset in each direction can be calculated with very complex formulae, but the formula offset=minor axis/(4*focal ratio) is accurate enough for practical purposes (it is exact if just the center is fully illuminated - with a larger fully illuminated field, the error is insignificant anyway). The distance along the mirror face from the center of the ellipse to the optical center is the offset multiplied by 1.414 (the square root of 2).

Example: with a 33 mm diagonal mirror (the size refers to the minor axis) in a f/6 Newtonian, the offset is 33/24 mm = 1.3 mm.

An error of type 2 causes the fully illuminated field to be offset relative to focus, and will cause an uneven light loss near the edge of the low power field. The tolerance should not be larger than the radius of the fully illuminated field, to ensure that the focus is fully illuminated. But at least for short focus instruments, light loss is very gradual outside the fully illuminated field, and an offset of a few millimeters should have little effect visually. Sufficient accuracy is easily achieved with suitable tools.

For wide-field photo, the whole film frame should be fully illuminated. This may require a diagonal size of 30% of the main mirror diameter, or even more. For visual use, a diagonal size of no more than 20-25 % is commonly preferred, in order to minimize unwanted diffraction effects. Here, a smaller area of full illumination is sufficient, but at least the focus should always be fully illuminated. If you design your own telescope, use a "low-profile" focuser to let the focus, and the eyepiece, come close to the tube (but an extremely low focuser can cause problems with baffling against stray light).

To calculate the diagonal size or the size of the fully illuminated field: let D be the diameter of the main mirror, d the diameter (minor axis) of the secondary, F the focal length, b the distance from the optical center of the secondary to the focus, and x the diameter of the fully illuminated field: x = (Fd-Db)/(F-b)ord = x + b(D-x)/F

Error type 3 - the combined optical axis is not reflected at 90 degrees. Standard secondary mirrors and holders are designed for 90 degree reflection, and seen from the focus the elliptical mirror appears circular. An angle of more or less than 90 degrees will make it appear elliptical, and may also cause vignetting (i.e. blocking some of the light) by parts of its holder. If you have collimated, and look at the reflection of the secondary mirror, its holder should appear on-axis and you should not see its side. If you can, and it appears "tilted", you should consider shimming or otherwise "squaring" the focuser. An error of type 3 will have no other effects on the image (contrary to common belief).

Error type 4 - the optical axis is not centered in the tube. If it is grossly off center, the tube opening may cause some vignetting, and this should be avoided. Otherwise, it will have no optical effect. It may cause problems with some mounts, as an offset axis will not trace a great circle when the tube is moved in declination. This might introduce some error when using digital setting circles.

End of heavy theory - at least most of it.

So what steps do I take to collimate my telescope?

The lining up of the optical components should be done in a sequence that is as simple and ordered as possible. Ideally, you would start at one end of the optical chain and then proceed in steps to the other, without going back to readjust what once was adjusted. With real telescopes this is not possible, the adjustments affect each other in different ways depending on design details. For instance, with common secondary supports, it is not possible to adjust the tilt without moving the optical center significantly.

One practical way is to do it in the order described below (you could do it in the opposite order, but I believe it is much more complicated). Remember that this refers to a full collimation, like when you assemble the telescope from parts - you do nothave to go through all of this just to get your telescope ready for the night's observing! Step 5 (and maybe step 8) is usually quite sufficient for this.

The tools will be described in a later section, with details of their use. The error numbers are explained in the section on theory, that you may have skipped.

Square the focuser

If the focuser appears to be squarely mounted on the tube, it is not likely to be badly off. If you make your own telescope tube, you can make a small mark directly opposite the focuser hole. With the secondary mirror out of place, use a sighting device in the focuser. Shim the focuser to center the mark. A piece of tubing that fits your focuser, long enough to reach across the tube, will make it even easier, and so will a laser collimator or a crosshairs sight tube. (This minimizes error type 3)

Date: 2015-12-11; view: 755