Focusing

[Andrew]

I have a couple of questions about focusing different eyepieces....

1. For a particular scope and make of eyepiece, as ep focal length gets shorter do you need to rack the focuser further in or out to achieve focus? This is presumably fairly basic and if the sky was ever clear I could go out and check this for myself!

2. Do all eps of the same focal length come to focus at roughly the same point on any given scope or are particular makes known for requiring big differences in travel?

[dabsond]

Unless eyepieces are labeled as "Parfocal" they will focus at different lenghts. Normally only series eyepieces (IE meade 4000's) will be parfocal with eyepieces of the same series. Two different brands even if labeled as parfocal will require focusing.

[GordonCopestake]

1) Theres no way of telling, different eyepieces focus at different focal plane positions it's independent of the eyepiece focal length

2) no, although manufacturers of premium eyepieces attempt to make eyepieces in the same series parfocal although in practice even the some tweaking is necessary.

[FLO]

Dabsond and Gordon have it covered except to say that - in my experience - you have to rack out further to bring 30-40mm 2" eyepieces into focus. Occasionally, I am asked to supply extension tubes for this purpose.

[Andrew]

Thanks guys, although I must admit, I was hoping to win a prize for the post with the most views without a reply

[The Warthog]

YOu can get a beast called a 'parfocalising ring' which you put on the eyepiece tube that goes into the focuser. It tightens agains the ep by means of a setscrew, and prevents the ep from going further into the tube. If you have two eps (or more) that you use all the time, which don't come to a focus at the same point, you can focus with the ep that comes to focus closest in, then with the ep that focuses a little farther out, you move it out of the focuser tube until it comes to focus, and tighten the parfocalising ring at that point, so you can switch eps without excessive refocusing.

My three X-Cels are only approximately parfocal; there is a little adjust ment required when I switch, but not so much that it gets really annoying. The refocusing that really drives me crazy is when I put the Barlow in.

[radioactive]

Thanks guys, although I must admit, I was hoping to win a prize for the post with the most views without a reply

Andrew if it helps I will say nothing!!