Just a question about eyepieces

[l3gendluk3 (Luke)]

This might be a silly question but here it goes. Assuming that everything is perfect (mirror surface, eyepiece lenses, collimation etc), if I have a 25 mm eyepiece in perfect focus and change it to a 9 mm eyepiece, will I have to move the focuser 16 mm to achieve a perfect focus?

Thanks,

-Luke

[Astro_Bot]

Originally Posted by l3gendluk3

This might be a silly question but here it goes. Assuming that everything is perfect (mirror surface, eyepiece lenses, collimation etc), if I have a 25 mm eyepiece in perfect focus and change it to a 9 mm eyepiece, will I have to move the focuser 16 mm to achieve a perfect focus?

No, not generally. The focal plane of the eyepiece (where the image is formed relative to the eyepiece components) and the eyepiece focal length are two different things.

Some manufacturers try to make their eyepieces parfocal - which in layman's terms means the focal plane falls in the same place for every eyepiece in the range - whereas others have widely varying focal points. It's a different focal plane that requires racking the focuser in/out, not the focal length.

Clear as mud?

[l3gendluk3 (Luke)]

Well I wouldn't say clear as mud but I do get what your saying. Thanks for your answer

-Luke

[barx1963 (Malcolm)]

Luke
You know that is a really good question!
Astro Bots answer captures the essence, but think about it this way. A telescope creates a focused image at it's own focal plane. If you held a piece of paper just above the focuser without an eyepiece in it and moved it back and forth, it would at a point produce a focused clear image. This can actually be done quite easily with the moon or a bright target. So a telescope can be thought of as analogous to a movie or slide projector
A eyepiece can be thought of as a fancy magifying glass. When you were a kid using a magnifying glass, you would have to hold it at a particular height above the ant or whatever you were examining to get it in focus.
The job of your telescope focuser is to get the the focal plane of the magnifier (eyepiece) in the same place at the focal plane of the projector (telescope) so you can examine the image created by the scope.
The focal plane of a particular EP design is determined by factors other than just it's focal length. If it was a single lense that may be the case, but no such EPs exist these days.

Malcolm

[Wavytone]

Luke,

What matters is the distance between the focal plane of the eyepiece, and the top of the sleeve on the eyepiece (the tube that fits into the eyepiece holder).

If this dimension is the same for your eyepieces, then no, you do not need to refocus when swapping them. This condition is also called "Parfocal", and some sets of eyepieces (generally expensive ones) are par-focal.

For example, if I insert a 5mm Vixen LVW eyepiece and focus my scope for this eyepiece, then yes, it will also be correctly focussed for the other Vixen LVW eyepieces (8, 13, 17, 22 and 42mm). But not for any other type of eyepiece.

Cheaper generic eyepieces are usually NOT par focal, even within the same set from the same manufacturer.

Sadly, eyepiece manufacturers have never agreed on standardising on this dimension (a great pity) and unfortunately this means that if you "mix & match" eyepieces from different sets and/or different manufacturers, you will be refocussing each time you swap from one to another.

[julianh72 (Julian)]

Originally Posted by barx1963

When you were a kid using a magnifying glass, you would have to hold it at a particular height above the ant or whatever you were examining to get it in focus.

When I was a kid, holding a magnifying glass a particular height above an ant was rarely for the purposes of examining it!

(Do kids these days even know what a magnifying glass is, let alone what you can do with one?)