Let us start off by touching on some reflecting telescope basics.
Collected starlight is reflected from our primary mirrors in the shape of a cone. This light cone starts off the same diameter as our primary mirrors. Then it tapers down to a single point once itís travels the full focal length of the primary mirror. Now we introduce another mirror called the secondary or diagonal mirror that intersects this light cone at a given point and deflects the cone 90 degrees outwards towards the focuser and ultimately to our eyes.
This article is not going to have anything to do with collimation because the action of offsetting your secondary optic has nothing to do with collimation. Whether your secondary is centered or offset, the procedure of collimation remains exactly the same.
Collimation by definition is: To make (as light rays) parallel. In a reflecting telescope, collimation refers to the incoming light rays reflected from the primary mirror to the eyepiece being parallel to the line of site from the eyepiece back to the primary mirror.
Secondary offset by definition is: The physical movement of the secondary housing in two directions - towards the primary mirror and away from the focuser.
Now that we understand that offsetting your secondary and collimating your telescope are two completely different things, we can ask the question, "To Offset or Not to Offset."
As a scope builder, I offer secondary offset as a free option. I have this as an option because secondary mirror offset is no doubt very misunderstood and always seems to be referenced as making a telescope more difficult to collimate. Offsetting your secondary should only be looked at from the point of view that it aligns both the mechanical and optical axis of a telescope.
With a centered secondary, the primary mirror will need to be tilted via the collimation bolts the amount of the offset towards the focuser side of the telescope to achieve proper collimation. This misalignment of the primary mirror from the telescopes mechanical axis will cause some vignetting off of the focuser side of the upper tube ID. Also, it will effect the telescopes goto and pointing abilities.
With an offset secondary and the alignment of the "entire optical system," vignetting is eliminated and pointing accuracy with all digital setting circles and goto systems is greatly improved especially above 75 degrees of altitude.
As mirror diameter gets smaller and/or focal length gets longer, secondary offset becomes less relevant. It is my opinion that an offset of .2" should be looked at as the mid point. An offset of less than .2" has very little effect on both vignetting and pointing accuracy. Offsets between .2" to about .25" should definitely be considered and offsets greater than .25" should be a must.
The formula used to calculate secondary mirror offset is:
(Secondary size) / (4x focal ratio) = Offset
Hypothetically, letís say we have a 22" telescope with a focal ratio of 3.7. This scope will use a 4.5Ē secondary.
To calculate offset for this size scope:
4.5 / (4 x 3.7) = offset
4.5 / 14.8 = .304 inches
Using our hypothetical telescope, we draw a line from the center of the primary mirror up through the center of the telescope tube (optical axis), then draw a line from the center of the focuser towards the optical axis. Where these two lines meet is called the intersect point. The calculation above is the amount we need to move the mechanical center of the secondary optic from the intersect point, both towards the primary mirror and away from the focuser. When using a site tube to align your secondary optic under the focuser axis, the offset towards the primary mirror is automatic. Only the offset away from the focuser needs to be completed for a fully offset secondary.
In conclusion, with todayís very "fast" focal lengths, it is highly recommended to go with an offset secondary. There aren't any negatives to an offset secondary unless your main goal is to star test optics. In this case, a centered secondary is desirable as it keeps the secondary shadow centered on your defocused star images. More importantly is not to confuse offsetting your secondary with collimation. Do not think that it will make your scope more difficult to collimate. If you choose to offset your secondary, all the math and positioning is absolute and youíll be collimating to a fixed secondary placement which isn't any different than collimating a centered secondary.
TO OFFSET OR NOT TO OFFSET YOUR SECONDARY