Zoom Eyepiece

[Stephan]

Some times I wonder... why doesn't every body have a zoom eyepiece?

Sure they are not cheap but buy 2-3 normal eyepieces and you easily topped the price of a zoom ep. and you have the equivalent of 12 ep's. Sure the field of view is small at the longer focal length but usually you use the 'long-end' only to find an object and then you zoom in and observe. And if you want to watch nebulas you can just use 2"-widefield ep. The only other thing you need is a Barlow lens and you are set up through the whole range of possible magnifications. With a little bit shopping around, you can have it all that for under $300 (~$150 of that would be the 2"ep). That would be the ideal ep-set for beginners.

So why doesn't every body have one?

[janoskiss (Steve H)]

They usually have much narrower AFOV than fixed FL EPs (throughout the entire range of the zoom). The Nagler zoom only has 50 degree AFOV (IIRC) as opposed to 82 degrees for the fixed FL Naglers. (An exception is the Speers-Waler 5-8mm zoom.) Also, the optics for the zoom puts more glass in the way, inevitably decreasing transmittance.

[JohnH]

I am a relative newbie to this, the first ep I purchased was a zoom, 7-21 mm to go with an ETX90. Not sure this is the best way to go though as in my case I found the ep has a narrow useable viewing zone (that is your pupil must be in line with the centre of the ep to see anything at all at high magnification - not sure what the correct term is for this feature) this makes the ep hard to use for extended viewing vs fixed focal length eps.

[bytor666]

With today's coatings, the amount of glass wont hinder the views at all. I had a Speers Waler Zoom and it worked wonderfully. I only sold it because I wanted to get a Pentax 10mm XW , I was looking for a Pentax 10mm XW in the first place.
The field of view in the SW Zoom is huge!!!!....I saw incredible amounts of detail on Mars with this particular eyepiece!!!
I even put a GSO 2" barlow on it at the 5mm setting giving me 660x when I looked at the Eskimo Nebula in Gemini and the view was STILL crystal clear!!!!
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Mark
12" f/5 GSO Reflector
30mm 1rpd ,(soon)
14mm meade series 4000 UWA
10mm Pentax XW, (on the way)
2" GSO Barlow