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Equipment Discussions >> Binoculars

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GlennLeDrew
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Celestron 15X70 (and home-made 12.8X51) new
      #3641848 - 02/23/10 12:14 AM

Last night I did a little stargazing from my 9th floor apartment balcony. Well, I was mainly giving my recently purchased Celestron 15X70 its first nocturnal work-up. And comparing it with my Mk II RA bino in 12.8X51 mode (50mm f/3.3 aspheric objectives mated with 13mm, 100 deg. AFoV Ethos eyepieces), mainly because of the similar exit pupil diameter.

But I'll save that for the end, after supplying a 'review' of sorts. Before venturing out the door, I did some measuring of the 15X70. This included removing one of the eyepieces. The details:

- Effective aperture via the flashlight-through-the-eyepiece test; 65mm, both barrels. The projected circles through the objectives were round enough, although the edges were indistinct, mainly due to the slight offset from true concentricity in the prism train resulting in a series of superimposed circles. Only the slightest of lateral movement of the light resulted in their truncation into ellipses, but this is hardly surprising in light of the already clipped aperture.

- Other than the subtle effect pointed out just now, and what follows in the next point, there's generally good opto-mechanical alignment, with prism apertures and eyepiece field stop appearing to be *reasonably* concentric (as compared to several horrific cases I've seen in too many others of this breed.)

- With only the slightest off-axis tilt of the flashlight during the aforementioned test of the right barrel, one side of the dreaded blue-grey 'square of incomplete internal prism reflection' floated into view. None of the other three sides of the pattern could be seen, nor were any at all seen in the left barrel. Strange! makes me strongly suspect an angle error on one of the 45-degree reflecting Porro faces (it doesn't take much to cross the threshold between total and partial internal reflection.)

- Measured AFoV: 65 deg. Interestingly, this almost perfectly reflects the value calculated from magnification (which I *did not* measure) and TFoV.

- Eyepiece field stop diameter: 21.3mm. From this, and the provisionally verified TFoV of ~4.35 deg., the objective focal length is ~280mm, or f/4.0. The eyepiece focal length is therefore ~18.7mm.

- Rear prism aperture estimated as ~20mm from a visual comparison with the measured field stop diameter.

- The eyepiece design comprises four elements; a doublet eye lens and two singlets ahead. The closest analogue I can come up with is an inter-war wide angle design from 1925, whose name escapes me at the moment.

- Field illumination is generally uniform in appearance over most of the field, falling off rapidly in the outer ~3-4 degrees apparent. Along the top of the field the distinct edge of the field stop is completely gone. The combination of clipped objective aperture and a prism aperture somewhat smaller than the field stop produces the sharp edge-of-field vignetting. The vertical offset of vignetting is due to the slight decentering of the relevant apertures at the rear of the instrument.

- Other than the rear surface of the objective, which has been given a multicoat, all other surfaces have at best a single coating exhibiting the typical pale blue cast in reflected light. Actually, I suspect a couple surfaces have no coating at all.

- There is evident undercorrected spherical aberration, although stars do come to a reasonably tight best focus. There were no asymmetric flarings.

- Chromatic aberration is of agreeably subtle intensity. I'd say its almost surprisingly good in this respect.

- Image sharpness is decent to about halfway to the field edge. The degree of blurring at the field edge itself is about what to expect for a relatively simple eyepiece coupled to a fast f/ratio objective.

- Ghost reflections from bright objects in the FOV are easily seen and several in number. But they are mostly large and sufficiently blurred to be objectionable for only the very brightest of objects, such as the half-Moon I aimed at.

- There seems to be a shiny plastic spacer in the eyepiece which causes a semi-specular reflection from bright objects just about 'touching' the field edge. These reflections appear as radially oriented 'spikes' or 'shafts' of light lying in the outermost field and pointing straight toward the field center. I might disassemble the eyepieces in order to 'scuff up' the spacers with emery cloth (as I recently did with another bino having a rather similar eyepiece design/fault.)


Now to the observations/comparisons.

The Celestron 15X70, when properly put together and aligned, is a surprisingly decent bino in its own right. As expected, it outdid my 12.8X50 in depth of penetration, due to the combination of larger aperture and higher magnification.

But not by *too* much! My bino's aspheric objectives exhibit about 1/3 the spherical aberration afflicting the Celestron, and so stars are tighter. I didn't try a resolution test per se, but the view of "Winter's Albireo" suggested similar resolution capability.

The half Moon provided a good test for chromatic aberration. The 15X70 exhibited so very little--when my eyes were properly centered so that lateral color wasn't the issue--I was quite taken by the view. And bluish Rigel exhibited only a trace of a purplish fringe. Not bad!

As I'm about to haul myself off to my little cliff dwelling, I'll end here for the moment.


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Erik D
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Reged: 04/28/03

Loc: Central New Jersey, USA
Re: Celestron 15X70 (and home-made 12.8X51) new [Re: GlennLeDrew]
      #3641986 - 02/23/10 03:06 AM

Glenn,

Thanks so much for that clinical report. It's going to take me at least a 2nd reading to digest it all.

A few things come to mind:

1. If you can get your hands on another pair of Chinese 15X70 LW from a well know US importer, priced between $130 to $150 USD, subject them to the same disassembly and scrutiny, will they hold up better in optical performance and mechanical construction?

2. How about a pair of "Classic Japanese quality" binocular such as the Orion Little Giant 15X70s available till a few years ago?

Is this class of Chinese 60/70 mm LWs of sound design, but the finished product does not live up to full potential because it's simply impossible to lavish attention on a pair of binocular priced to sell under $50 USD?

OTOH, when the end user pays double or triple the $49 price tag will they receive a pair that's 20% better? 30%? Or is it simply improving the odds?

I suspect there are no easy answers ... but we are certainly becoming much better informed today than we were 5 or 10 years ago.

I do recall EdZ's decision to keep a pair of $149 Chinese 15X70s but to return a pair of $400 Japanese 16X80s in his first CN review so many years ago.

ERik D


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KennyJ
The British Flash
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Reged: 04/27/03

Loc: Lancashire UK
Re: Celestron 15X70 (and home-made 12.8X51) new [Re: Erik D]
      #3642856 - 02/23/10 01:37 PM

Glen ,

Thanks for yet another very interesting report .

I know I'm in a minority on an astro bino forum , but personally , I much prefer seeing tighter stars , even if there are far less of them , than sheer numbers , many of which appear overly distorted .

Kenny


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Celestron 15X70 (and home-made 12.8X51) [Re: KennyJ]
      #3643724 - 02/23/10 08:25 PM

Erik,
What does "LW" mean? To me it denotes the Luftwaffe!

Quote:

1. If you can get your hands on another pair of Chinese 15X70 LW from a well know US importer, priced between $130 to $150 USD, subject them to the same disassembly and scrutiny, will they hold up better in optical performance and mechanical construction?




I couldn't say with any certainty. As experience has taught us that price is a reasonably good indicator of quality, my provisional answer would be, 'yes.' For a given model of instrument sold by a variety of vendors, there can be demonstrable differences between them. If vendor X's price is higher than Y's, it *likely* due to improvements, better features or higher quality control.

Quote:

2. How about a pair of "Classic Japanese quality" binocular such as the Orion Little Giant 15X70s available till a few years ago?




If we're restricting ourselves for the moment to what are/were considered 'lower end' binos price-wise, then my knee-jerk reaction would be to place higher confidence in the older glasses.

Quote:

Is this class of Chinese 60/70 mm LWs of sound design, but the finished product does not live up to full potential because it's simply impossible to lavish attention on a pair of binocular priced to sell under $50 USD?




In the main, yes. Other than the needless aperture restriction, the design is basically good. If the components are well executed, this instrument would work very well. Here are the pluses, as already exhibited:
- Virtually no 'fingernails' of extraneous prism reflections surrounding the exit pupil.
- An enjoyably large AFoV (by the standards set by other binos.). Certainly better than that of the 25X70.
- Chromatic aberration is not at all objectionable.
- The motion on the hinge is clean and crisp, and holds the set IPD quite nicely.

The minuses:
- Spherical aberration could be reduced. Although not that bad when compared to a great many other binos of various size and livery, it does indicate some degree of turned edge on the objectives and/or non-flat prism surfaces, which are likely defects in rapidly-finished optics. And this is even more surprising considering the now-larger f/ratio (f/4.3 vs. the 'native' f/4.0) resulting from the aperture restriction.
- The biggest contributor to aperture restriction stems from the thick 'shoulder' of the barrel-to-body adapter. This new 'baffle' intrudes into the light cone ahead of the front prism aperture. If not for this, the bino just *might* work at the advertized 70mm, or at least close to this. But then, in the existing instrument I'd have to say this 'baffle' is a good thing, else any edge roll on the objective (and perhaps the front-most prism aperture) would be only more manifest than it is now, with really deleterious impact on the image.
- The eyepieces can definitely receive better treatment for control of stray reflections.
- As evidenced in other specimens I've examined, large variation in angle errors and/or dimensions on prisms often introduces or requires radical tilting in order to achieve collimation. If only the manufacturer would toss the bad prisms, so many problems with 'cat eye' exit pupils, field tilt and non-uniform illumination would be history. This is possibly the single worst aspect in the hit 'n miss crapshoot suffered by those who don't try before they buy.

Quote:

OTOH, when the end user pays double or triple the $49 price tag will they receive a pair that's 20% better? 30%? Or is it simply improving the odds?




That depends on the vendor, I should say. If a reseller specifies a certain degree of QC on the manufacturer's part, or are willing to inspect and weed out the lemons they get, I'd have some confidence in getting a specimen that's up to scratch.

In my case, I did pay $120 CDN for mine, from a reseller who does inspect all binos they recieve. But I do know that I subject an instrument to vastly more stringent scrutiny than they, and so will only purchase one that meets my criteria.


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milt
professor emeritus
*****

Reged: 09/13/04

Loc: Arizona
Re: Celestron 15X70 (and home-made 12.8X51) new [Re: GlennLeDrew]
      #3647444 - 02/25/10 06:17 PM

Hi Glenn,

Excellent report.

It certainly showed how an accumulation of issues - underused objectives, uncoated surfaces & spherical aberration - can conspire to reduce the limiting magnitude of a 70mm bino to close to that of a good 50mm.

Just curious, how did you estimate SA?

Thanks,
Milt


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StarStuff1
Post Laureate
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Reged: 04/01/07

Loc: South of the Mason-Dixon Line
Re: Celestron 15X70 (and home-made 12.8X51) new [Re: milt]
      #3647495 - 02/25/10 06:51 PM

Methinks Eric's mention of "LW" is meant to be the abbreviation of Light Weight.

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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Celestron 15X70 (and home-made 12.8X51) new [Re: StarStuff1]
      #3647513 - 02/25/10 07:07 PM

milt,
My estimate of spherical aberration was just that; an estimation. I use a couple of factors:
- how narrow is the region of best focus (a good optic snaps into focus, a poor one mushes through a considerable range);
- the brightness variation across the de-focused disk of a point source (with a good optic a defocused star is an evenly illuminated disk).


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