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Russell Optics 85mm Super-Plossl

Posted by Clayton Cramer   03/24/2005 12:00AM

Russell Optics 85mm Super-Plossl
[ARTICLEIMGR="1"]One of the common mistakes of beginners is to assume that they want the most powerful eyepiece available. Alas, there are few occasions when the amateur can use the "50x per inch of aperture rule" that 1960s Edmund's Scientific catalogs used to tout as the "realistic" limit. Flaws in the telescope and poor viewing conditions means that even if you could buy a 1mm focal length eyepiece, almost no amateur would ever have occasion to use it. Even if you had a telescope with optics good enough to take advantage of it, eye relief—-the space between the eyepiece and your eye required to bring an image to proper focus—-gets smaller as an eyepiece's focal length get shorter. The small field of view also means that it gets more and more difficult to keep an object in the field of view—-and if you have an undriven mount, forget about it.

What about the other end of the focal length spectrum? Just as very high magnifications are far less useful than they might at first appear, very low magnifications can also present some problems. Still, since most of my telescopes have been f/7 or f/9, I have always been intrigued by the idea of buying a long focal length eyepiece to get a wider field of view. I have never been able to get all of the Pleiades into one field of view with my 8” f/7.

I’ve bought a few relatively long focal length eyepieces before: a 40mm Kellner and a 35mm Orion Ultrascopic, both limited by my requirement for a 1.25" barrel. Having bought a Photon Instruments 5" f/9 refractor with a 2" focuser, I was finally free of this limitation. Yes, I looked at the Televue 2" wide angle eyepieces, and 2" wide angle eyepieces offered by other vendors, but my blood ran cold at the thought of dropping a several hundred dollar eyepiece in the dark. I have also looked through a number of these high-end eyepieces over the years at star parties; I have not been spectacularly impressed with image quality, off-axis.

Then I ran into this 85mm Super-Plossl eyepiece, made by Russell Optics in Arizona(http://www.citlink.net/~optics/). To my shock and amazement, they are not an importer, but a real genuine U.S. manufacturer of eyepieces. (I confess this was part of what provoked me to start making telescope accessories as well)

The eyepiece barrel is made of Delrin, a hard black plastic made of polymerized formaldehyde, with somewhat similar mechanical and corrosion resistance properties as aluminum. Delrin has several desirable qualities for this purpose: it is much lighter than an equivalent metal barrel, and seems to suck heat out of your fingers much more slowly than metal. On a very cold night, you do notice the difference! I’ve never had an eyepiece stick to my finger because of the cold, but I have great confidence that this won’t happen with Delrin, even if I were observing from Antarctica.

With a focal length this long, Russell recommends it for "refractors and long focal length SCT's and Cassegrains." If you try it in a short focal length Newtonian, you will see why—-the diagonal produces a very large black dot in the middle of your field of vision—-a problem that becomes more apparent in daylight, I suspect because your pupils are open wider. Still, it is pretty amazing to use this 85mm eyepiece in a 17.5" reflector that I have-—the Pleiades at 23x with that much light are pretty overwhelming.

Used in its proper setting, however, you see both the positive and negative of an eyepiece with such a long focal length. The positive is the field of view. In my 5" f/9 refractor, 85mm gives 13.4x—-and something close to a two-degree field of view. All but the outermost stars of the Pleiades fit into the eyepiece. It is surprisingly easy to find yourself looking at objects just after dusk, and see satellites pass through your field of view. The field of view is so large that the chances of a random satellite passing through are quite high. It is even tempting to skip using the finder—-but even two degrees is too small to substitute for a finder.

To its credit, I could see no chromatic aberration introduced by this eyepiece. I have a 80mm military surplus eyepiece with a somewhat similar field of view and unsurprisingly, the 80mm produces a very noticeable green fringe on the moon. Surprisingly, so did a rare 2" barrel 18mm University Optics orthoscopic. The Russell Optics 85mm produces no color fringe at all.

There are a couple of negatives to an eyepiece like this, although I don’t see them as problems of this eyepiece so much as problems of any eyepiece with this long of a focal length. One problem is eye relief—-but unlike the 4mm eyepiece that requires you to rub your eyelashes on the glass, this eyepiece may be the first example I have ever seen of too much eye relief. I have to position my eye 4" back to get the full field of view. Of course, this can also be an advantage at a star party, where many members of the public do not get close enough to the eyepiece.

Another irritation—-and again, it is a reflection not on this eyepiece, but on the class—-is that it is very unforgiving of dirt on your diagonal. Yes, I know, I should keep that diagonal squeaky clean. This eyepiece (and other long focal length eyepieces that I have used) show every speck of dirt, superimposed on the image. This problem becomes less noticeable as the sky darkens, perhaps for the same reason that the black spot of a reflector diagonal becomes less noticeable in full darkness.

There is a bit of off-axis coma with this eyepiece, but it is not dramatically worse than the fancy brands of wide angle eyepieces that I have used previously-—it is just a quirk of eyepieces this long, I suspect.

If this eyepiece were priced comparably to say, the 31mm Televue Nagler Type 5, there might be a good case for doing a side-by-side comparison. (You supply the Televue Nagler, please, and insurance to cover me dropping it.) But here's the good news: the current price is $75. Not only will you not need permission from your spouse to buy this, you won't be tempted to weld to a chain to it, to make sure that it doesn't grow feet and walk away at the next star party.

This is a novelty eyepiece. I don't use it very often, but every once in a while, it is nice to pull it out and use it for its intended purpose—-deep sky objects that are very diffuse. If it cost $200 or $300, I would have had a hard time justifying the purchase. At this price, it's a bargain. Russell Optics also sells a number of somewhat more normal focal length eyepieces that you might want to consider if the 85mm is just a little too far outside the optical envelope for your telescope.