University Lowbrow Astronomers

To Zoom or Not to Zoom - and Why.

by Charlie Nielsen
Printed in Reflections: November, 2003.

Over the years I have managed to collect a wide assortment of eyepieces.  I believe the total is about 18 now.  One for every occasion you might say.  Some amateur astronomers might say this is overkill and you really only need about 3, and a good Barlow lens.  But it was fun collecting them.  And there really is something to say about having the right eyepiece for the job on hand.  Sometimes I use two scopes at the same time, so I need an eyepiece in each.  Well, I could go on making reasonable sounding excuses for buying eyepieces, but still, why would I need a zoom eyepiece?  Beside the fact that I just plain wanted one, there was a practical reason.  Once I locate an object that I want to study for a while I switch to an eyepiece (assuming it is not the one I located it with) that gives me that “just right” balance of magnification, contrast, and background star field.  This optimum selection is also effected by the prevailing sky conditions.  So the beauty of a zoom eyepiece is the ability to easily make this decision.  You simply locate your object at the zoom’s lowest power, then zoom in for the best view.  My plan was then to find my fixed length eyepiece that best matched the setting I left the zoom on, and switch to it.

So in comes the Vixen Lanthanum 8-24 Zoom.  I had read mixed reviews about this eyepiece, but I believed it would most likely be better than some less expensive models and I would have to spend a lot more money to get a noticeable quality improvement.  Also, this model sports plenty of eye relief, which as a glasses wearer, I always appreciate.  I purchased mine via the Internet for around $150.  It arrived unharmed and showing no signs of defects.  The coatings showed the reflection colors of a well multi-coated glass, but maybe not quite “fully multi-coated.”  The zoom action was very smooth and had just the right resistance down to about 10mm, then got a little stiff.  This seemed to diminish after some usage and now the whole range is quite smooth.  At one end of its range there is a little movement of glass felt if one shakes the eyepiece vertically.  I have noticed this with two other ones that I have examined, so this seems to be normal and harmless.  However, I would not recommend using one as a tambourine.

I was anxious to try my new toy and only had to wait a couple of days for a clear night.  The instant that I brought it to focus at the 24mm setting, I was stunned.  This eyepiece is extremely sharp.  In fact it is one of the sharpest eyepieces that I own.  I did not expect this with so many glass elements involved.  But I think it is sharper than any of my plossls, and gives up very little to my orthos.  Contrast is good and star images are sharp to the edge of the field.  I have read some reports that the image gets a little soft at the 8mm setting, and maybe that is true, but barely.  Eye relief is very comfortable, with or without glasses.  At the 24mm setting, I guess just short of 20mm and about 15mm at the highest power setting.  At the 8mm setting I do need to roll down the rubber eyecup.  The field of view varies from a very narrow looking 40 degrees at 24mm, to 60 degrees at 8mm.  The unfortunate part is that the field does not really start to “open up” until you get down to about 10mm.  At that setting I compared it to an Orion 10mm Sirius plossl that I used to own.  The zoom is a bit sharper, has somewhat better edge correction, a wider apparent field of view, and much better eye relief than the plossl.  This is why I traded the plossl.

The first complaint that I have is that the eyepiece is not parfocal.  Going from one extreme to the other does require some minor refocusing.  This is a hassle with a helical focuser.  Then again, I notice the same thing with most eyepieces that claim to be parfocal.  The second complaint that I have is the apparent field of view at the lowest power.  At 40 degrees, it is considerably harder to find objects than I had anticipated, and sometimes I switch back to a low power, wider apparent field eyepiece.  So as your low power, object locator type of eyepiece, one could do better.  In the range of about 20mm down to around 12mm it is sharper than most plossls, but with an apparent field of view more like an ortho.  At 12mm on down, the field of view grows rapidly, and it maintains that sharpness.  I have not had the opportunity to do a side by side comparison to the fixed length Vixen Lanthanums, but from my memory of testing some of those a couple of years ago, I think they would be very close.

So did I accomplish that goal of having an eyepiece selection tool as I mentioned earlier?  Well, not really.  The biggest problem is that once I start using the zoom I do not usually pull it out.  Again, the quality of the image is so good that I would most likely degrade it by switching to a fixed length eyepiece of the selected focal length.  So the reason for switching really comes about because I want a wider apparent field of view.  There is a redeeming factor though.  They are called planetary nebulas.  For example, the Blue Snowball.  I had been frustrated with small planetaries many times.  When I believe I have the little devil in the field of view, the object is stellar due to using low magnification.  But with the zoom you can just dial it up until you see that disk appear.  If all the stars remain stellar then you did not locate it, but having this instant power range makes the search much easier.  I first located the Blue Snowball using this eyepiece.  For the same reason, I like this eyepiece for searching out Uranus.  I look for that slightly blue-green star, center it, and zoom in.  If I got the right one, Uranus becomes a planet right before your eye... a very nice effect.

So would I recommend the LV Zoom?  Absolutely!  I believe it serves a useful purpose, and is just plain fun to use.  It also Barlows very well, so then you have a really wide range of focal lengths.  It also is convenient for light travel.  Just this eyepiece and maybe a 30 to 35mm wider field model, and you are good to go.  Although I do not use the zoom every observing session, it does have its purpose and performs its job well.  I certainly do not regret my purchase, and I think most observers would be very pleased also.


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