Zoom Eyepieces

[stinky]

Have noticed some less expensive zoom eyepieces being advertised - eg. SkyWatcher 7 - 21 mm.

Presently I have 2 eyepieces that came with my refractor - a 10 & 20. Went out and bought a 6mm Omni, but find I'm missing ut having to switch between the 10 and 20. Would a zoom lens deliver comparable performence to the same brand fixed eyepiece? Or are these low end - to be avoided?
Regards

[Miaplacidus (Brian)]

Originally Posted by stinky

Would a zoom lens deliver comparable performence to the same brand fixed eyepiece? Or are these low end - to be avoided?

In general, no, and yes (in that order).

Zooms can be okay for terrestial use, but even then you need an expensive one (e.g. Zeiss, at +A$700). Just too many compromises are made in the design, so that for astronomical use the image is going to be very degraded. Also, in cheaper zooms, the AFOV is narrow to begin with and often shrinks even further as you zoom in.

The only advantage of a (good quality) zoom that I've heard of (like the TV 3-6mm model) is that it allows you to "dial in" the magnification to match the seeing when you're doing high powered lunar and planetary observing.

[ausastronomer (John Bambury)]

I agree, I try to avoid zoom anything in optics, although zoom camera lenses and variable power rifle scopes have come an enormous distance in the last 20 years.

A zoom eyepiece will never be as good as a high quality native eyepiece optically. Another issue with zoom eyepieces is that as the power is reduced so is the AFOV which is the opposite to what happens with fixed focal length eyepieces. Some high quality variable power eyepieces (Televue) have addressed the reducing AFOV issue in recent years but I seriously doubt that a budget variable will adress it.

CS-John B

[ving (David)]

theres a really good write-up on zooms at cloudy nights you should read

[cahullian]

I have a Seben 8-24mm zoom the optics aren't great, but you do get the wow factor when looking at star clusters. It looks like your flying through them. I find mine is also ok on the moon.

Gazz

[casstony]

I have a Vixen 8-24 and Proxima 8-24 and I think the images are at least as good as generic plossls, though I haven't had the opportunity to use the zooms at high power or make critical comparisons to high quality eyepieces. I'll probably sell one of these eventually but I'll always have one zoom in the accessory box - they are just too easy to use.

-Tony