Tuesday, August 3, 2010

The William Optics 28mm UWAN

I tend to get a little stir crazy during the summer monsoon as far as astronomy goes- I am spoiled living in the desert southwest with many clear dry nights.  So when a few weeks go by with no opportunity for night time observing I turn to other aspects of my hobby (addiction) for entertainment.  Having already cleaned the observatory a few times (you could eat off the floor) and having already installed a nice new shelf from Target to hold some of my books and stuff, I made an impulse buy and acquired a used William Optics 28mm UWAN eyepiece.  I had heard good things about it from other TEC 140 users so when I saw one for sale on Astromart I asked myself  "Do I need this eyepiece?"  I quickly answered myself saying "Self....No.  No.  A thousand times, no.  You already have a very nice 31mm Baader eyepiece"  A few hours later I had made the deal and it would soon be shipped.

After a week of cloudy nights, the weather gods took pity on me, and the past two nights proved to be fantastically clear with very good seeing and no moonlight until after midnight.  I used the opportunity to test this new eyepiece extensively in my TEC 140 and what follows are my subjective observations and experiences using this eyepiece.

I decided to compare this eyepiece to my Baader 31mm Hyperion Ashperic which is obviously similar in terms of the power it delivers in the scope.  The picture to the left gives you an idea of the relative size of the monster in comparison to the Baader 31mm, and a can of Pepsi...keep in mind that these eyepieces both have 2 inch barrels, so you can see how physically massive the 28mm UWAN is.  Before I provide you with my subjective impressions, I'll give you some of the objective information as taken from the respective companies, as well as some of the exit pupil, power, and field of view calculations for the scopes I own.

Eyepiece                      William Optics 28mm UWAN                  Baader Planetarium 31mm Hyperion Aspheric

Manufacturer info          William Optics                                          Baader Planetarium
Price Retail/Used          $339/$250                                               $189/$150
Eye Relief                     18mm stated                                            18mm stated
Weight                         1000 grams (2lbs. 3 oz.)                           390 grams (13.8 oz.)
Apparent F.O.V.           82 degrees                                              72 degrees

Using TEC 140 (f/7)
     Exit pupil                  4mm                                                         4.5mm
     Magnification           35X                                                           31X
     True field of view     2.3 degrees                                              2.3 degrees

So now that we have the objective data out of the way, how did the eyepiece perform for me? My conclusion first...Overall, I am quite satisfied with the 28mm UWAN, and will be selling the Baader 31mm.  The Baader is a very, very good eyepiece for the money, however, the 28mm UWAN does most everything just a little better.  You can see from the data that these eyepieces are very close in both magnification delivered, exit pupil and field of view- so the differences that I am about to describe are my subjective experiences using the eyepiece.

Prior to explaining why I preferred the views in the UWAN, there are some ergonomic issues worth noting in the event you are considering one or the other of these eyepieces.  First, is the aforementioned weight.  The Baader weighs less than a pound, and is remarkably light for an eyepiece with a 72 degree apparent field of view.  The 28mm UWAN is over 2 pounds and will impact your telescopes balance.  Second, while the stated eye relief is 18mm for both eyepieces, the experience using them is quite different.  To the right is an image of the eye lenses and you can see that the UWAN eye lens is slightly smaller and you may be able to tell that it is recessed into the eyepiece top.  When using the 28mm UWAN, it does not feel like one has 18mm of eye relief.  In fact, to see the field stop, I have to place my eye almost all the way to the housing.  This is not an issue for me as the lens itself is recessed and my eyelashes do not touch.  I also find myself tilting my head slightly to get the best position to see the entire field.  The UWAN has a very smooth twist up eye-cup, however, I can not use this as if it is up at all I can not see the entire field of view.  The Baader 31mm is very comfortable to use and the eye relief measurement seems accurate.  For public outreach the Baader 31mm would be a better choice on its ergonomic characteristics alone.

On to the views, and the reasons why I prefer the 28mm UWAN.  I observed several types of objects with both eyepieces- nebula including M8, M17 and NGC 7000; planetary nebula including M57, M27, and the young, small NGC 7027 in Cygnus; Globular clusters including M22, M13, M20, M5, and NGC 7006 in Delphinus; The Veil supernova remnant NGC 6992; and the galaxy NGC 7331 in Pegasus. Observations of faint nebula were made both unfiltered and with my Baader planetarium UHC-S filter.  

I'll present my findings in descending order of the perceived difference.  In other words, I'll address sharpness first as I felt this was the greatest difference in the views...sky background will be second as I felt this was the second greatest difference.  Last will be color as I felt there was no difference in eyepieces.

First, the Baader 31mm is very sharp on axis, however, stars in the outer 25% of the field of view increasingly look like little flares as they approach the edge.  The 28mm UWAN on the other hand was sharp all they way past 90% of the field of view.  Even the outer 10% of the field was not nearly as distorted as in the Baader.  In terms of sharpness, the 28mm UWAN clearly had the edge (pun intended).

The second thing I noticed when using these eyepieces was that the 28mm UWAN revealed a darker sky background.  I was surprised by this given that the eyepieces are so close in focal length.  This was an obvious and marked difference between the eyepieces.  Had I not perhaps been unconsciously biased to look at sharpness first, the sky background may have struck me immediately.  

Third, and I hate to use such a subjective construct, was faint detail.  When observing star clusters, there were not any stars that I noticed in one eyepiece that I could not see in the other.  That being said, there were faint stars that were easier to see in the 28mm UWAN...easier than the magnification difference alone would dictate.  I do not know if it is related to sky background, coating differences, or anything for that matter.  Contrast is a touchy subject when discussing eyepieces and I am trying to avoid using that term...but faint detail was easier to see in the 28mm UWAN.  Nebula in particular were brighter and faint extensions were more readily seen in the 28mm UWAN.  This was most noticeable when observing NGC 6992, the eastern portion of the veil.  Also, the lagoon nebula (M8) revealed more structure in the UWAN.  To the left is a sketch I made of M8 using the UWAN 28mm and the Baader UHC-S filter.

Finally, I looked for differences in how the eyepieces rendered star colors.  With fainter stars within clusters I could not tell a difference between eyepieces.  I did point at the bright and colorful double Albireo and could not detect a color difference between eyepieces on this bright pair either. 

In conclusion, the William Optics 28mm UWAN is an excellent eyepiece for use in an f/7 refractor.  It is significantly more money than the Baader 31mm, but in my opinion, the views are worth the difference.  The Baader is more comfortable to use for a beginner or a member of the public at an outreach event, however, for the experienced amateur, the UWAN does manage to quickly get out of the way and let one observe.  Unless something is radically different in my f/10 SCT, look for the Baader Hyperion on the used market soon. 

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