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Five-Way Shootout of Zoom Eyepieces for Solar Viewing
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Zoom eyepiece horserace
I systematically ran solar and nocturnal comparative performance tests of three zoom eyepieces-- the high end Pentax SMC 8-24mm, the Nikon 9-21mm (one of Nikon's premium models), and the highly touted budget-priced Apogee 7.3-22mm. I had a brief chance to test the new Proxima 8-24mm zoom from Hands On Optics, and Televue's 8-24mm, which is a useful benchmark since it is similar to the Vixen and the Orion.
This is one observer's attempt to look at these eyepieces as objectively as possible and rate them "head to head". I've got good eyesight and see coma and other optical imperfections quite easily, so I think I gave the Pentax, Nikon, Apogee trio a fairly rigorous test.
Criteria for this review included:
Let's meet the three main contenders:
1. The Pentax SMC 8-24mm zoom. List price ~$439, and about the size of a Nagler 20, this is one serious eyepiece! Given that the Pentax SMC's and now their successors the XL's are preferred by some to Naglers, if not the "king of zooms" --that may be the Leica-- it is at least the "crown prince".
2. Nikon 9-21 zoom. Purchased from APM Telescopes for $250 including a screw on 1 1/4" adapter, these model 7466 MC (for "multicoated") eyepieces were Nikon's top end 0.965" spotting scope eyepieces until they were superceded by the "MC II" zooms which offer 7-21mm power. Diminutive in comparison to the hulking Pentax, these are smaller than a Meade 26mm Super Plossl in stature.
3. Apogee 7.3-22mm zoom. Imported by Apogee Inc. (and previously possibly by "Hands On Optics") these $49.95 eyepieces are reputed to be OEM'd out of the same factory in Japan that makes Nikon spotting scope eyepieces. I believe that these are actually clones of Nikon's "Earth and Sky 15-45X" eyepiece (model 7675) rather than the Nikon 9-21's tested above. Although they are comparable in size, they are NOT identical to the Nikon 9-21's in size, form factor, or optical coatings, and thus I'd dismiss as "urban legend" the on-line belief that they are unlabelled clones of the MC 1 or MC 2 eyepieces which Nikon sells for five times as much money.
Part I: Solar Viewing
"Sidewalk glare test"-- as used in a lengthy thread on the Astromart "equipment talk" forum in 2004, this is an attempt to look at the propensity of the upper coatings of an eyepiece to either bounce back or pass bright point source lighting such as direct sunlight. Some (such as the owner of AstroPhysics, Roland Christen) believe this closely mirrors the test results obtained through using sophisticated spectrophotometer equipment. Others argue that it is a specious test that doesn't correlate to real world observing conditions. Put me in Roland's camp here, based on the results of my through the eyepiece testing as compared to this simple glare test. On the glare test the Pentax excels, reflecting very little light despite its massive upper element, and the Nikon does nearly as well. The Apogee, on the other hand, makes me think of St. Patrick's day, as its greenish coating (the other two are bluish/purple) bounces large amounts of green and white light back.
Score: Pentax 1, Nikon 2, Apogee a distant 3rd.
light transmission: All three contenders appeared to have comparable light throughput, as measured by their ability to see the same faint solar prominences. (I have found looking at solar proms to be a good discriminator between varying eyepieces, given the range of brightness and size that typically prevail in proms on any day of reasonable solar activity. Those eyepieces with more internal light absorption fail to detect proms that can be observed with high transmission eyepieces.) However, the Pentax and the Nikon did a better job at showing the corona (aka solar wind) surrounding the sun. This is not necessarily a pleasing effect, since it makes the area surrounding the sun brighter and reduces overall contrast, but looking at it repeatedly I am convinced that the Nikon and Pentax were actually picking up faint Hydrogen Alpha coronal light, even though to first blush it would look like off-axis glare. The Apogee discerned none of this-- which paradoxically gave it higher image contrast, as measured by the disc of the sun against a black background.
Score: tie between Pentax and Nikon, with Apogee a close 3rd based on "cosmetic appeal" of its particular defect. NOTE: this is strong performance, since the Nikon had previously been tested against a field including Cemax Plossls and TMB monocentrics and came in tied for the best light throughput. (see http://www.novac.com/solar/ObservingReport.php or Eyepieces on this site) That means that all three of these zooms should rival fixed focal length eyepieces in optical performance in solar observing.
On-Axis clarity: Again, all three appeared to be similar in terms of sharpness of focus at comparable magnification. All three also had comparable "snap" to their focus, though the Apogee may have lagged a bit-- I'll need to test further to be sure.
Score: three way tie, for now.
Range of Magnification: All three appeared to meet their nominal range of power. Here the Pentax scored well for offering the widest range. The Apogee scored well for offering nearly as high a range, with the highest power of the three contenders (55X vs. 50X for the Pentax, in the PST). The Nikon has the narrowest range and the least powerful high magnification (44X).
Score: tie between Pentax and Apogee, with the Nikon in 3rd place.
Coatings: The Nikon and the Pentax did a good job of rendering relatively glare-free images when used with modest shielding such as eyepiece cups (see image above and right). Under these same circumstances, the Apogee's Field of View was "flooded" with serious green-tinged glare, enough that I would not judge the eyepiece suitable for high end solar observing --as opposed to casual viewing by the public, or perhaps an unadorned PST (more on this in my "conclusions" below). Only the most extensive ambient light reduction regime-- the "Mathew Brady" style head shroud (image at right) did an adequate job of reducing the glare at the eyepiece to what I judged to be an acceptable level. Obviously, this is only an issue for solar or terrestrial observing (although I can't image how this eyepiece works well in a spotter scope!), unless you do your nighttime observing with lots of streetlights around. However, since this is the solar test portion, that means the score is:
Score: Tie between Pentax and Nikon, Apogee a distant 3rd.
Optical ease of use: Due I suspect to its larger top lens, eye placement is the most critical on the Pentax, and results in "blackouts" if the pupil of the eye is not on-axis. Eye placement is the easiest for the Nikon, and nearly as easy for the Apogee.
Score: Nikon in 1st, Apogee in 2nd, and Pentax in 3rd.
Parfocal zoom: The Nikon is advertised as parfocal, and in fact required only a bit of focus adjustment when zoomed from minimum to maximum or the reverse. (Since my parfocal Nagler 3-6 zoom behaves similarly, I think this is an artifact of my 20/15 vision.) The Pentax and the Apogee, on the other hand, required considerable re-focusing after any significant change in magnification.
Score: Nikon in 1st, Apogee and Pentax tied for 2nd.
Ease of zoom: The Pentax is somewhat difficult to zoom with one hand, because the built in adjustable eyepiece cup typically is grabbed and turned instead of the portion of the eyepiece that controls magnification. The Nikon is a positive nightmare to zoom-- the zoom portion is a ring immediately below the top rim of the eyepiece, rather than down on the barrel, so most folks grab the wrong area entirely. Resistance is firm enough that, even when you are twisting the correct ring, you need your other hand to restrain the eyepiece, unless you take "heroic measures" as I did in affixing duct tape to the barrel to make it a genuine one hand zoom. The Apogee, by contrast, has a logically placed ribbed twist collar that can readily be worked with one hand.
Score: Apogee a clear winner, Pentax 2nd, Nikon a distant 3rd.
"Other" PST-specific mechanical issues: The Nikon cannot come to focus in the Coronado PST without unscrewing it from the 1 1/4" adapter and gingerly setting the 1 1/4" shoulder of the eyepiece into the PST's holder. You have to be careful tightening the PST's set screw, since excessive tightening prevents the zoom from working. One of the two Nikons I own had a shoulder that was a bit oversized, and wouldn't fit comfortably in the PST. The Pentax and the Apogee had none of these issues. The Apogee is more petite and arguably better proportioned for use on a PST.
Score: Apogee 1st, Pentax 2nd, Nikon a dismal and distant 3rd.
Magnification labeling: Granted, having a magnification scale is not particularly important in practice--you zoom until you've framed the viewing object appropriately-- it nonetheless is nice to be able to calculate what magnification you are using with reasonable accuracy. If using these eyepieces in binoviewers (as I do with the Nikons, and as would be quite feasible with the Apogees given their low cost), having accurate and granular mapping of each eyepiece's magnification setting becomes imperative so that you can dial in identical power settings on each eyepiece.
The Pentax has a label for 8, 12, and 24mm magnification settings-- not as much granularity as I'd like to see, but apparently reasonably accurate. The Nikon has a ring calibrated with magnification factors for both Nikon's 60mm or 78mm spotting scopes, which makes for a confusing welter of numbers on the barrel-- and none of these are millimeter focal lengths relevant to astronomical use. ( What I did in practice was to make a paper scale calibrated in individual millimeters, allowing for accurate setting of the magnification to ~1/4mm. See the middle eyepiece of the top image at the right.) Apogee has labels for 7.3, 11, and 22mm settings, but they are applied backwards-- "22" is actually the highest zoom and 7.3 the lowest power, and the "11" is also placed suspiciously close to the 22 even though it should be nearer to the high powered end. Is a bogus scale better than no scale at all? I don't think so, and downgraded the Apogee accordingly.
Score: Pentax 1st, Nikon and Apogee tied for 2nd.
Late Entrants: At our Club's annual "star gaze" I was able to borrow a Televue 8-24mm zoom and a Proxima 8-24mm zoom for a few minutes each during our public outreach solar observing. I shared the views with two other observers, so these represent our consensus views.
Even though the Televue supposedly is the "pick of the litter" and undergoes more Quality Control than its Vixen and Orion brethren, my most abiding impression of this zoom was that it was pretty badly plagued by internal reflections-- certainly the most of any of the five eyepieces I've tested. Eye placement was also more critical with the Televue than with any of the other eyepieces, and it was relatively easy to cause "black outs" in which the image disappeared.
The "Proxima" zoom from "Hands On Optics" is a bit larger than the Apogee or Nikon, and was mechanically very easy to use-- on a par with the Apogee. It has a nice magnification scale calibrated with commendable granularity, with apparently accurate 8, 12, 16, 20, and 24mm settings. It has an adjustable built-in eye relief ring which frankly doesn't take the place of an eyepiece cup for solar viewing, and admitted considerable glare, albeit not as bad as the Apogee. FOV appears to be a bit less at a given magnification than the Pentax or Nikon, leaving the sun's disc appearing somewhat "crowded" in the eyepiece at higher power. Overall optical performance was nice, but even the zoom's owner didn't think it was quite in the same league as the Pentax or the Nikon in terms of overall solar performance. Not bad at all for the ~$100 price, though!
Based on this "quick impression" data I would rank the Televue zoom-- and presumably its' Orion and Vixen stable mates-- at the bottom of the heap in terms of solar performance. The Proxima would rank slightly above the Apogee. This would make the solar performance rankings read:
First place tie: Nikon 9-21mm and Pentax 8-24mm
Third: Proxima 8-24mm (Hands On Optics)
Fourth: Apogee 7.3-22mm
Fifth: Televue/Orion/Vixen 8-24
Part II: Preliminary Nighttime testing
Coma: Preliminary testing indicates that all three eyepieces are very good performers in fast scopes such as my f/10 LX200. At f/4.5, the Nikon and the Apogee continue to render images which are crisp and sharp across the majority of the field of view. The Pentax, however, has unacceptable levels of coma in the outer half of the field. I have yet to test Field of View and other factors. ("Ask me for anything but time...")
Score: Nikon first by a nose (due to slightly superior light transmission), Apogee second, and the Pentax third because of its inability to keep up in fast focal ratio scopes.
There are certainly a fair number of test criteria here, and I suspect each observer will have their own list of factors that matter the most. For solar viewing, I personally find the Nikon and the Pentax to be the clear winners because of their optical performance. I would give the Nikon the overall edge because of its lower price and smaller form factor. (The Pentax looks hugely out of proportion on the Personal Solar Telescope!)
For nighttime use, the Apogee and Nikon are great performers in both slow and fast scopes, with a slight edge possibly accruing to the Nikon. The Pentax is a strong performer in a slow focal ratio scope, and given its superlative (Nagler-grade) optics could be considered a one eyepiece solution for medium and high power use in a "Go To" SCT. However, the Nikon and Apogee also work well in SCT's, though, and at a fraction of the cost, so I'm not sure what a compelling case for owning the Pentax would be.
By one metric, the Nikon could be judged to be overall the winner, since it is a good optical solution for both day and nighttime use. Cost effectiveness cannot be discounted for most users and applications, though. The fact that you can buy 10 Apogees for the retail price of one Pentax or 5 for the price of one Nikon certainly is a factor that overcomes some level of performance gap-- how much is a subjective judgment for each user. Spending $250 or $440 for an eyepiece to use on a $499 scope like the PST might well be overkill for many observers. The Apogee is probably "plenty good enough" for a user with 'middle aged eyes' and a standard PST or SolarMax filtered scope. If you've got 'eagle eyes' or are running "stacked" SolarMax units and achieving 0.5 or 0.6A resolution, I'd think about getting a higher grade zoom than the Apogee. The Proxima seemed to be an even better choice for budget observing, but I'd want to have more than the limited time I had to be able to make a definitive recommendation. It's certainly worth considering, though.
As for me, I am keeping my pair of Nikons. I do all of my serious solar observing with binoviewers, and these are also good performers in nighttime binoviewing. (Though in practice, my pair of 22mm Panoptics tends to beat them out as my nighttime favorites because of their larger FOV.) I will also end up keeping the Apogee because I do a fair amount of public outreach solar viewing, and mono-mode is easier to make work with novice observers. By retaining the Apogee, I can keep my Nikons mounted in the binoviewer, rather than pulling one and dis-assembling its adapter ring to make it useable for "cyclops mode" viewing. The Apogee's discernable glare in solar viewing is more than outweighed by the "wow" factor novice observers experience at seeing the sun in Hydrogen Alpha for the first time... plus, if they are going to drip sweat into an eyepiece in the summer heat, I'd rather have it happen to a $50 one! The Pentax will go to a new home with a friend who owns an f/10 SCT. While the Pentax saw a lot of use as my mono-mode solar eyepiece because of the serendipitous fact that it was virtually parfocal with my Nikon-equipped binoviewer, this is not enough justification to hold onto a $440 eyepiece, especially in light of the fact that it is a mediocre performer in my f/4.5 scopes at night.
Your mileage may vary, but I'm a happy "three zoom" owner and likely to stay one for the foreseeable future.
Clear skies, day and night!
The three contenders: (from left) Apogee, Nikon, and Pentax
Aftermarket eyepiece cups still let a bit of light leak in-- which is enough to cause objectionable glare in the Apogee
A full hood "Mathew Brady" old-fashioned photographer style is the only way to make a good enough light seal to make the Apogee a strong performer for solar viewing