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The "Ax" Files
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The "AX" Files:
Review of the Celestron 23mm Axiom LX Eyepiece
Phillip J. Creed
As a lifelong resident of the Northeast Ohio cloud factory, dodging inclement weather, last-minute go/no calls for observing runs and waiting to pounce on what others would consider"sucker holes" are facts of life. Thus, having a simple, no-frills, quick-strike"guerilla astronomy" set-up is essential. My equipment has always been either binoculars or a dobsonian, and a recently-acquired 120ST refractor fits in beautifully.
A similar minimalist approach was used for my eyepiece collection. After reading Ron Ravneberg’s outstanding article in Amateur Astronomy #49 on"The Music of the Night", I felt a need to cull my ocular herd. I had 6 eyepieces at the time, but I found myself always reaching for the same three focal lengths. So I decided to both consolidate and upgrade my eyepiece line-up by swapping 6 plossls with 3 wide-angle eyepieces, and throwing in a barlow to chase it all down.
My oh-so-voluminous eyepiece collection, up until June 2008 consisted of the following:
24mm Meade 5000 SWA
13mm Nagler Type 6
9mm Nagler Type 6
Siebert 1.75X barlow…and that’s all, folks.
Since my Dobs have been either f/4.5 or f/5, this line-up does the trick. I like having more contrast vs. 6- or 7mm exit pupils, so a ~5mm exit pupil will work just fine for low-power viewing. The 13mm Nagler hits the 2-3mm exit pupil"sweet spot" in virtually all dobsonians, and having a 9mm eyepiece with a barlow rounds out all the powers I’ll typically need in one night. The jet stream's never too far from my usual dark-sky haunts in eastern Ohio or West Virginia, so anything above 300x is more cost-effective if achieved through a barlow lens than another premium eyepiece, anyway.
I had a yearning, though, for a 2"-eyepiece. I've owned Dobs with 2" focusers since 1995, but never got around to actually putting in a BIG eyepiece in there. It felt a bit like having a sports car and never topping 80. Finally, in June 2008 I decided to take the plunge into a low-power ultrawide eyepiece.
But which one for"The Mauler", my 16" f/4.5 Dobsonian? I never got around to getting a Paracorr, and without coma correction, an f/4.5 mirror will shred apart most widefield eyepieces with almost primal ferocity. For sheer shock value, I borrowed a 30mm 1-rpd once for my 16" f/4.5. In my old 12.5" f/5, it was okay out to beyond 50% from center, but in the 16" f/4.5 I found stars in the outermost 2/3-rds were so frightening and distorted, you'd have thought they came out of an H.P. Lovecraft novel.
Needless to say, I knew it had to be a well-corrected eyepiece. After balking at the weight of the 24mm Meade 5000 UWA, I eventually narrowed my selection down to a 22T4 and a 23mm Axiom LX to keep the exit pupil a reasonable 5.1mm. I figured if I went much larger than 23mm, I'd need another ~17 to 20mm EP to fill the gap. Better to concentrate on upgrading, rather than expanding, the eyepiece line-up, I thought, so these were my two finalists.
I've heard that the 22T4 is a great performer, but the price difference of $299 for the 23 Axiom LX vs. $480 for the 22T4 made the"Ax" look like it was worth trying. Cloudy Nights itself was instrumental in cementing my decision to go for the Ax. I saw a favorable evaluation on the Cloudy Nights Eyepiece forum of both the 23mm and 31mm Axiom LX from Eurvin Jackson (“jack45"). Later on, I read another CN post from Keith Hoffman of OPT, who had a favorable impression of the 23-Ax when comparing to other ultrawide brands.
I called Keith up, and after a lengthy conversation, I decided to pull the trigger on this eyepiece in June 2008. Heck, at that price, I figured I could get a Baader MPCC and the Ax for the same price as a 22T4, but best to find out how the eyepiece would work in the first place. I finally had ordered my first 2" eyepiece.
If there's one thing I found a bit frustrating about this eyepiece, it's the lack of reliable specs on the eyepiece from Celestron's website or distributors. For instance, when I bought the eyepiece (and this is true as of the end of October 2008), the weight is mysteriously listed as 16.9-oz.
I say"mysteriously" because the first thing that came to mind when feeling it in my hand was,"Man, this thing's heavy!" The eyepiece doesn't cause balance issues in my dob, but I think that's in large part due to the stocky build (and, hence, more lenient balancing) of a 16" f/4.5 dob. When I weighed the eyepiece on a food scale over at the home of fellow CNer Larry Sayre, I got a much more plausible reading of 31.0-oz., or 1.94-lbs.
As much as I wanted a 2"-ultrawide, I specifically avoided the 31mm Axiom LX. The 31mm Axiom LX has a listed weight of 50-oz., very close to the 3-lb. weight of the 30mm Meade 5000 UWA. I haven't seen the former in person, but I've held the latter, and I had visions of needing a shoebox full of neutron-star matter to counterbalance my Dob. With a spec of 16.9-oz., I thought I was safe. Don't get me wrong; we're not talking 31T5-weight here, but something that dob owners ignore at their own peril. Why the weight spec is that far off is beyond me.
While 17mm of eye relief is listed, it seems quite a bit tighter in terms of usable eye relief. I have glasses that are fairly thin and sit close enough to my corneas that I can (with some effort) juuuuust see the field stop on a T6 Nagler. The eye relief of the 23mm Axiom is better than the T6s, but not by leaps and bounds. The large, flat eye/face plate requires many observers to tilt their heads slightly to see the full FOV. If you've ever looked through the William Optics 28mm UWAN, this is pretty similar.
The build of the eyepiece is solid, with a large rubber grip on the outside that also serves to adjust the eye relief. Coatings are reassuringly dark, with a bit more of a purplish tone vs. the greener coatings of a T6 Nagler. All in all, a really imposing piece o'glass and metal.
Besides the large eye/face plate, the only physical complaint I'd have is the cap on the field lens, which is too soft. I'm not sure if it's rubber, a thermoplastic elastomer (i.e. Santoprene), or flexible PVC, but whatever it is, it has a very low durometer! There is a negative lens that bulges to within 5mm of the exit of the 2" barrel, and the field lens cap is soft enough to easily press in against it, causing a nice little on-axis smudge.
"The Truth is Out There"
The parallel to the 28mm UWAN isn't just superficial. If you've looked through a 28mm UWAN, if you can tilt your head the right way, the reward is a beautifully-sharp, ultrawide low-power view.
Ditto the 23 Ax. I popped it in The Mauler and found this to be a very well-corrected eyepiece. It's very sharp on-axis, with that intangible, je ne sais quoi"snap" into focus that you'd get from a Nagler or Pentax. The first target for observation was M13. At 79X, the cluster's"spider leg" star chains were popping out in all their glory.
Next on the testing menu were edge aberrations. Here's the short version—the edge correction easily beats out a Hyperion, and is better than the 24mm Meade 5000 SWA (which, if I may say so myself, is a very good eyepiece in its own right). Away from the central FOV, the stars had the tell-tale"comet tail" attached to pinpoints—this was coma from the mirror.
What I had a tough time seeing was any mushiness from off-axis astigmatism. There was a tiny bit, but it's swamped by the effects of the mirror's coma. Stars at the edge had little"comet tails" on their points, but the most important thing was they were still points. M13 was resolvable right near the edge of the FOV, for instance. For an f/4.5 mirror lacking coma correction, this was an impressive performance.
One thing that was immediately apparent—and confirmed with later tests—was that the AFOV looked bigger than the advertised 82-degrees. My 16" f/4.5 actually has a documented focal length of 1,807mm (15.85" f/4.49), so a 23mm 82-deg AFOV, without distortion, should yield a TFOV of 1.04-deg. However, based on measurements using M13 and the 8th-magnitude stars HIP81551 and HIP81590, the TFOV came out to 1.09-deg.
A similar test with a 120ST (120mm f/5 refractor, 600mm focal length) should have produced a TFOV of 3.14-deg. However, one could put Gamma Sagittae at one end of the field stop and (just barely) include the entire"bowtie" of the Dumbbell Nebula on the other end, a distance of about 3.28-deg. With both instruments, it seemed about 5% greater TFOV than when calculated with the eyepiece's listed AFOV and focal length.
The eyepiece has a little bit of lateral color on the brightest stars. I didn't notice it at first, but subsequent trials have shown what looks like a tiny violet"tadpole" or smear if a 1st-magnitude star is placed near the edge. The Ax also had a little bit of field curvature in the 120ST, but I found my eyes could easily adjust. I couldn't detect field curvature in the 16". By comparison, I did not notice lateral color or field curvature with the 13T6 in either instrument.
There's a little bit of pincushion near the edges. Placing Jupiter near the field stop noticeably stretches it out with the axis pointing towards the center of the FOV. This wasn't a surprise, as many of the high-end premium eyepieces, especially the T5 and T6 Naglers and the Panoptics, employ pincushion distortion to get sharp edges.
The Axiom got quite a workout in the following months in the 16". Viewing the Double Cluster with it was mesmerizing. With a 1.09-deg AFOV, it's just wide enough in the 16" to squeeze in M31's nucleus, main dust lanes, and both satellite galaxies. In the 120ST, the view of M31 is almost spell-binding, essentially gulping in the entire galaxy and revealing dust lanes. The coup-de-grace with the Ax+120ST combo adding an O-III filter and viewing the Veil Nebula. As in the entire thing, including Pickering's Triangle.
Edge sharpness isn't the only criteria for a premium eyepiece. Any eyepiece that requires one to part with a trio of Benjamins should have good contrast, so I threw it at one of the most difficult deep-sky objects I knew of--dwarf galaxy IC-1613. This object is difficult regardless of aperture due to its abysmally-low 15.2-mag surface brightness. I tried this under Bortle Class 2 skies at Calhoun County Park, WV, but I knew this was still a tough, tough object. The Ax was up to the task, showing the amorphous glow without too much difficulty when averted vision was used.
Delving deeper into insanity, I decided to go for the northern section of Barnard's Loop. The"elbow" in the nebulosity near NGC 2112 was easily picked up using a UHC filter, and wasn't too difficult without filtration once"tube-tapping" and averted vision were used.
I didn't feel the evaluation would be complete until I saw it through more telescopes and compared it to a 22T4 or 26T5. Finally, one weekend in October 2008 I was visiting my in-laws in Lexington, MI, in the"Thumb" region of the Wolverine State. By happy coincidence, it was within a reasonable drive of Tom Trusock's home, so I made the 45-mile trek up there.
Tom had his 18" f/4.5 Obsession ready at the helm, complete with a Paracorr that effectively made it an 18" f/5.2 Obsession, an 111mm f/7 Astro-Tech apo (suh-weet scope; see the CN review)…and had a 22T4 for direct comparison.
By taking coma out of the equation, any aberrations still visible would be attributable to the eyepiece. Nevertheless, both Tom and I agreed …this puppy's sharp when paired with a Paracorr in a fast Dob. No discernible difference really surfaced between the edge sharpness of the 22T4 and the Ax, although the 22T4 was distinctly lighter and easier to look through vs. the Axiom. One key difference, though, was the Axiom's noticeably-larger TFOV vs. the 22T4 and slightly brighter sky background.
The 111mm f/7 apo was a marvelous instrument for low-power sweeping with the Ax. At f/7, the resulting exit pupil of 3.2mm and the TFOV of just under 2-1/2 degrees made for captivating views of M31, the Double Cluster and NGC 457.
"Trust No One"
I was convinced that I had made a good purchase, but there was still the matter of the manufacturer's specs. The weight and eye relief didn't match what I could see with this 23mm ultrawide…hey, come to think of it…is the focal length really 23mm?
I then embarked on a little crusade to verify the focal length and the AFOV. For the focal length, the methodology was simple—focal length = (exit pupil) x (f-ratio), so all I had to do was measure the exit pupil. I tested the Ax in my 16" f/4.5 and 120mm f/5 refractor, and I ended up with 5.2mm and 4.7mm exit pupils, respectively. I calculated the focal length is about 23.5mm based on these measurements/
Next came the fun part—AFOV. I went over to the home of fellow CNer Larry Sayre to take direct measurements of the AFOV. The method involves keeping both eyes open, but looking through the eyepiece so that the field stop appears superimposed over a specific span. We used a set of dots on the wall and a tape measure on the outside of the house. It was immediately clear that the AFOV exceeded that of the 13T6; it was just a matter of determining how much. Unfortunately, the sheer size of this eyepiece made using this method a bit tricky, and holding one's position while someone else measured your pupil-to-wall distance allows for quite a bit of data scatter. I got values ranging from 80 to 91 degrees, though it eventually settled around 87-degrees.
Back home, I got a message from Larry suggesting a method from David Knisely in which the field stop is traced out on a surface by either a bright flashlight or a laser. Because the eyepiece and/or observer don't wiggle while the distance is measured, this method gave me results I'm far more confident in reporting. The result was an AFOV of 84.8-degrees.
I didn't directly measure the field stop on the eyepiece, as it was an internal field stop due to the negative field lens. However, based on the TFOV measurements I got, I estimate the field stop is about 34.4mm. This puts the field stop almost in the same league as a 26T5 Nagler, and almost 10% larger than that of the 22T4.
You may have noticed that I didn't test this eyepiece out in a long f-ratio scope, like a 6" f/8 refractor of f/10 SCT. While I must admit it would have made for a more complete evaluation, I figured the $299 asking price would be justified only if it did well in fast scopes. There's a lot of eyepieces that you could get for $50 that would work just fine at f/10, but it had better work well in a fast scope if $299 is what it takes to bring it home.
That being said, the 23mm Axiom LX packs quite a wallop. It's a very well-corrected eyepiece that performs well even in fast scopes lacking coma correction, and is really something else once coma is taken out of the equation. Its overall performance falls in line with what a lot of amateurs have said about the 28mm UWAN—just shy of a Nagler, but very close and certainly belonging the league of"premium" eyepieces. The Ax's only optical shortcomings vs. the 13T6 Nagler was some lateral color and field curvature in shorter instruments.
It's not exactly an impulse buy at $299, but it's priced well for its performance as an ultrawide low-power eyepiece. It has some ergonomic issues that may preclude some from using it, notably those that don't like the"face tilt" required of the 28mm UWAN, but for those whom this doesn't bother, it's a great buy.
Okay, now I know there's a few of you that skipped to the end in anticipation of a brief synopsis with bullet points. Well, here ‘tis:
Very sharp, both on and off-axis
Very little off-axis astigmatism
Larger field than advertised (!); tested at 85-deg AFOV
Great performance/price ratio
Stocky, secure build
Listed specs differ from actual eyepiece
Field lens cap too soft; replacement cap highly recommended.
Ergonomics, especially for eyeglass-wearers
Some lateral color
Some field curvature in shorter focal length instruments.
I'd like to give a heartfelt thanks to Keith Hoffman, Eurvin Jackson, Tom Trusock, Larry Sayre and David Knisely for their assistance in the purchase and evaluation of this eyepiece. Cloudy Nights has provided a lot for this Ohio stargazer. I figured I owed something back in the form of taking the time to submit an equipment review. Thanks for reading, and keep looking up.