Astronomy Equipment Mini-Reviews
Here are some brief evaluations of astronomical accessories that I use with my telescope, a Discovery 10" f/.4.5 DHQ Dobsonian.  As time permits, I'll put up some more reviews and eventually get around to reviewing the scope and my binoculars as well. 

Current items include:
Orion UltraBlock Narrowband Nebula Filter.
Orion Variable Polarizing Filter Set.
Orion Ultrascopic 2x Barlow.
Guan Sheng 32mm 2" Wide Field.
Celestron/Vixen 26mm Plossl.
Antares 15mm and 10mm Ultra Plossls.
Orion 9mm 0.965" Orthoscopic.
University HD 6mm Orthoscopic.
Astronomik OIII Filter.
Tele Vue 7mm Nagler type 6.

Orion UltraBlock Narrowband Nebula Filter.  Grade: B+.  Price: $60 on sale; $80 normal price.
Orion Telescopes and Binoculars.

Pros: Enhances most diffuse emission nebulae, some large planetaries, and even made Comet Encke look better.
        Secure threads fit all my 1.25" eyepieces.

Cons: Not much improvement on compact planetaries and very faint nebulae.
         Doesn't "blink" well on small planetaries (my Astronomik OIII does much better).

This is a good general-purpose nebula filter at a low price.  The UltraBlock offers excellent enhancement of a few nebulae, like the Lagoon and Rosette.  On many other objects, it reveals details that are either invisible or difficult without the filter.  Often, the best "view" is a composite of the view with and without the filter.  Don't expect the UltraBlock to bring objects out of invisibility.  It seems to work well throughout my standard DSO range from 44x to 230x. 

Orion Variable Polarizing Filter Set.  Grade: B+.  Price: $20 on sale; $30 normal price.
Orion Telescopes and Binoculars.

Pros: Amazingly, works as "seeing filter" for Mars.
        Doesn't seem to degrade views significantly.
        Applications for daytime viewing of objects near quadrature (as single filter).

Cons: Does not seem to improve splits of double stars.

I bought this filter for Moon viewing during public star parties and for daytime planetary viewing.  One morning, I tried it on Mars when seeing was mediocre.  To my surprise, the image seemed steadier with the filter, allowing the use of higher powers.  I used the filter most of the time during the recent Mars apparition.  When seeing was really steady (Antoniadi 2.5 or better), I preferred the unfiltered view, but the filter made below-average seeing acceptable.  I'm interested in seeing what the filter does on Jupiter and Venus, as well as using it for some of that daytime viewing.

Orion Ultrascopic 2x Barlow.  Grade: A.  Price: $57 on sale; $80 normal price.
Orion Telescopes and Binoculars.

Pros: Essentially invisible magnification; retains brightness and sharpness of unbarlowed eyepiece.

Cons: Long barrel means that it is difficult to reach inside lens surface for cleaning.
         Not parfocal

This has certainly been my best accessory purchase to date.  I think a good Barlow should be at the top of the list for new scope purhasers with limited eyepiece collections and budgets.  The Ultrascopic fills the bill splendidly, and works well with all my 1.25" eyepieces.  There is slight vignetting on my 26mm Plossl, but I don't notice this during use (only when holding the combination up to a light source).  The Barlow is essentially invisible; it doesn't seem to dim the view or add scattered light.  Magnification factors range from 2.0x to 2.2x with my current eyepiece collection.

Guan Sheng 32mm 2" Wide Field.  Grade: C.  Price $69 (now usually less).
Various GSO Dealers.

Pros:  Wide AFOV (~66 degrees) for reasonable price.
         Impressive-looking, well-constructed eyepiece.

Cons:  Obvious and annoying astigmatism and coma in my 10" f/4.5.
          Significant lateral color.
When I saw this big eyepiece on sale for $69, I just had to try it out.  The FOV is huge (I measured it at 1.86 degrees in my scope), and a great benefit when starhopping in difficult fields, especially near the zenith. This field is 45' wider than my 26mm Plossl, which doesn't look all that great at the edges either. A lot of large objects that span the FOV of the 26mm are more pleasingly framed in the 32mm.  The Pleiades and Double Cluster fit with room to spare, and the 32mm gives a wide-field perspective on M31. 

So, why the C? Well, less than half the way out to the edge, stars become noticeably distorted by astigmatism and coma.  The edges can be pretty rough, although positioning my eye closer to the lens seems to improve things somewhat.  The importance of critical eye positioning may make this eyepiece less attractive as a star party eyepiece.  There is fairly obvious lateral color as well, although this is not much of a concern for low-power deep-sky use.  There's also the matter of misrepresentation; this eyepiece was offered as a wide field Plossl or Super Plossl, when in fact most are Kellners.  GSO now has a 5-element SuperView series; the 30mm in that series may do better in fast scopes. 

For my tastes, the field generally holds together well enough to give pleasing views of large open clusters.  Your mileage may vary, depending on your eyes, scope and viewing conditions. I've seen at least one review that rated this eyepiece as unacceptable even at f/6. When I put an off-axis mask on my scope and made it an f/12.7, the edges were still pretty ugly on this eyepiece (although better than at f/4.5).  Detailed examination of most objects requires higher power, so this piece doesn't spend much time in my focuser. 

Celestron/Vixen "Silver Top" 26mm Plossl.  Grade: B-.  Price: ? (discontinued).

Pros: Adequate eye relief for eyeglass wearers.
        When Barlowed, edge correction is better than 15mm Antares.

Cons: Marked astigmatism and field curvature at f/4.5.
        Edges seem to fade out and lack contrast.
The 25-26mm, 50deg AFOV Plossl is standard fare with almost any telescope.  This one is highly rated by some, but doesn't do all that well at the edges at f/4.5.  I sometimes use it for starhopping or quick-and-casual viewing, and a rare object looks better at 44x than at 76x, but I could almost get along without it and permanently mate it to my 60mm f/11.8 refractor for terrestrial viewing.  I keep it in my eyepiece because: 1) I don't have a 2" nebula filter for sprawling nebulae like the Rosette.  2) In a pinch, I could Barlow it for star party use for eyeglass wearers.

When Barlowed, the eyepiece shows a new face.  A true field of view of 33' just takes in the whole Moon.  The result is a very immersive experience at 88x.  The edge problems? They essentially disappear.  Even though the limb of the Moon is close to the edge of the field, it is sharper than in the slightly wider FOV of the 15mm Antares.  The eye relief is still very comfortable. 

15mm and 10mm Antares Premium (aka Ultra/Elite) 5-element "Plossls".  Grade B.  Price: $56 on sale; $65-$75 normal price.
Various Dealers.

Pros: Very comfortable for me to use.
        Good magnification options both solo and Barlowed.

Cons: Ghosts on bright objects.
         More scattered light/less contrast than I would like.
         Astigmatism near edge of the field.
         True field a bit narrower than 52deg AFOV would imply.
         10mm gets little solo use now that I have a 7mm Nagler.  Ditto for 15mm with Barlow.

The 15mm gives me 76x magnification with a 40' TFOV. This is wide enough for a lot of starhopping, and is a great viewing power for many large deep-sky objects.  Add the Barlow, and 160x with an 18' TFOV is great for resolving globulars and seeing detail in many of the smaller deep-sky objects.  In fact, I found myself using this power so much that I effectively replaced the 15mm + Barlow with a 7mm Nagler.  160x is also a decent power for planets at star parties or when seeing isn't very good.  The 10mm brings a lot to the table, too.  114x is pretty much the optimum magnification for most deep-sky objects.  I used to use this power a lot, but my 7mm Nagler gives a higher magnification with virtually the same FOV.  At 230x (Barlowed) the 10mm is my eyepiece of choice for many demanding deep-sky applications, although its planetary/double star performance falls short of the 2 Orthoscopics I own within this range.  The 15mm is an extremely comfortable eyepiece if you don't wear glasses.  The 10mm is less comfortable, but still nice.

I thought these Antares eyepieces would be the answer to all of my observing situations; unfortunately, they fall short on planets and double stars due to ghosting and scattered light problems.  I prefer my Orthoscopics for these applications.  The ghosts appear as a small, round bright speck near the center of the field and a partial ring of light near the edge of the field.  Only the brightest objects (magnitude 1 or brighter) are affected, and the ghosting is less noticeable if the sky background isn't very dark.  Scattered light appears as increased glow around magnitude 8 or brighter stars.  I also feel that the Orthos reveal more planetary detail, although I'll have to do more testing.

9mm Orion 0.965" Orthoscopic.  Grade: B.  Price: ? (discontinued).

Pros: Good contrast and color rendition.
        Least scattered light and ghosting of any of my eyepieces.
        Killer planetary power when Barlowed.

Cons: Narrow FOV.
         Field stop not in focus.
         Astigmatism at edge of field.
         Nonstandard barrel diameter needs adapter.
When I first got the 10", this was my workhorse eyepiece for everything except starhopping and the largest deep-sky objects.  Now, my Antares eyepieces reign supreme for DSOs, but this funky little Ortho teams with my Barlow (275x) to remain my most-used planetary eyepiece.  I almost never use the eyepiece solo anymore.  It doesn't look like much during the daytime, but at night it becomes a fairly comfortable, well-corrected planet killer.  The astigmatism is normal with Orthos, and is what restricts their field of view.  Fortunately, the field is flat, so when a planet drifts toward the very edge of the field it becomes slightly elongated but stays in focus.  The fuzzy field stop is a minor aesthetic issue that amounts to a vignetting at the extreme edge of the field.

6mm University HD Orthoscopic.  Grade: B+. Price: $80.
Helix Manufacturing.

Pros: Sharp and contrasty.
        Bolt case included from Helix Mfg.

Cons: Narrow FOV.
        Tiny eye lens (~2mm wide) reduces viewing comfort.
        Slight astigmatism at edge of field.
        Slight ghosting when bright object is just out of field.

I bought this eyepiece to combine with my Barlow (390x) because I just knew that sometimes I'd need more power on Mars. These occasions turned out to be infrequent, but the high power was a nice luxury and I can attribute seeing Deimos to this purchase.  Saturn is a bit more forgiving than Mars, and I've already had several good views of it.  The combination is also simply dynamite on detail-packed planetary nebulae like the Eskimo and Cat's Eye when conditions permit.  The HD seemed to show bright regions on Mars better than my other eyepieces, and it will be interesting to see if it finds a niche as a non-Barlowed eyepiece (190x) on Jupiter when interesting events occur during mediocre seeing.  A lot of double stars look nice at 190x as well, and the image is cleaner than in the Antares eyepieces.     

Astronomik OIII (Oxygen-III) 1.25" Nebula Filter. Grade: A-.  Price: $100.
Oceanside Photo and Telescope.

Pros: Enhances the objects that should be enhanced when viewed through an OIII filter.
        Excellent for "blinking" small planetary nebulae.
        Well-made and durable.

Cons: High price.
         On many objects, UltraBlock is as good or better.
         Threaded portion is a bit short; would like more thread area.

I got this filter to complement my Orion UltraBlock.  Almost all emission nebulae show some improvement when viewed through a narrowband filter, but some look much better when viewed through an OIII "line" filter.  A few examples of objects where the OIII is a knockout winner: Abell 21, the "Medusa Nebula" in Gemini; NGC 2359, the "Duck Nebula" in Canis Major, and of course the Veil Nebula (though the Veil is really no slouch in the UltraBlock).

I've never been satisfied with using the UltraBlock as a "blinking" tool.  This Astronomik OIII is much better for this purpose.

Compared to the UltraBlock, the OIII will show a darker sky background with fewer stars.  Generally, I prefer this view when sketching a nebula.  On many objects, the differences in detail are pretty subtle. 

Tele Vue 7mm Nagler type 6.  Grade: A-.  Price: $240 on sale ($280 list).

Pros: Incredible wide field of view.
        Excellent edge correction with essentially no astigmatism or field curvature visible at f/4.5.
        Excellent coatings for decent contrast despite wide-field multi-element design.
        Very comfortable to use.
        Barlows well.

Cons: High price.
         Noticeable chromatic aberration (lateral color) in outer half of field when viewing bright objects.
         Minor "kidney bean" blackout issues in some eye positions.
         Slightly more scattered light and internal reflections than Orthos.

OK, I bought a Nagler.  The 7mm gives 165x in my 10" f/4.5 Dob, and this is one of my most-used powers.  I'm working on the Galaxy Groups and Clusters observing program, and having nearly 30' of FOV to play with at this magnification is a lifesaver.  Globulars are also great with some breathing room around them, and even smaller isolated deep-sky objects and planets benefit from a wide field of view on an undriven Dob.

Compared to all of my other eyepieces, the Nagler has fantastic edge correction.  All of the others show significant astigmatism (least apparent in the Barlowed Orthos).  The Nagler shows none, only the insignificant native coma of the primary mirror is visible near the edge of the field. There is some lateral false color visible in the outer half of the field, but it isn't really intrusive until very near the edge of the field and isn't a big deal except on the brightest objects (mainly the Moon and Jupiter).

The Nagler seems to be the equal of the Orthos and Antares Plossls when it comes to showing faint stars and nebulosity.  It is as good as or better than the Antares Plossls at revealing planetary detail.  The Orthos show less scattered light and seem a little bit more contrasty on planets, but in practice the differences in magnification between my eyepieces are more significant than these minor differences.  The Nagler, with its large eye lens, is much more comfortable to use for extended periods in an undriven Dob. 

The Nagler Barlows just fine in my Ultrascopic Barlow.   

You certainly don't need a Nagler or other premium eyepiece to enjoy astronomy, and frankly the price is astronomically outrageous.  On the other hand, this 7mm type 6 is a very nice eyepiece...