Comet 6x24 binoculars from the inside – what went wrong?

28 September 2017
Marcin Górko

5. Objective lenses

It is the last part of that Comet puzzle – the 24 mm objective lenses or rather, as it was measured by Arek during his test, the bigger, 25.4 mm lenses. The tubes of objectives are screwed into the prism bodies like the slides of the eyepieces. The difference in coatings’ colour are the first thing that sticks out – it’s so obvious that you might think the objectives actually come from two different sets of binoculars. I mean here “two sets of binoculars produced by two different manufacturers” of course…

Comet 6x24 binoculars from the inside – what went wrong? - Objective lenses
The Comet is equipped with two objective lenses which feature quite different antireflection coatings. Lack of due diligence? Slack quality control? Nah, that’s certainly the newest „Dual wavelength coatings” technology…

The edges of the elements are of course snow-white and they’ll remain so, it cannot be helped. The objective rings fixing the objectives inside their tubes had been screwed so tightly in the factory that I didn’t manage to unscrew them. As we are touching the topic of objective lenses I also decided to find out what is the reason of image fuzziness in the “objective + eyepiece” combo. I started by determining the focal length of the objective lens which amounts to about 100 mm. It fits beautifully the rule saying that achromatic objectives in binoculars are as fast aperture-wise as f/4. With 6.13x magnification, determined precisely in the test, and the 98 mm focal length of the objective lens, the focal length of the eyepiece must be more or less close to 16 mm – all the pieces of this jigsaw puzzle were finally fitting each other. I tested the quality of the objective lens by joining it with a top-of-the-range astronomical eyepiece Tele Vue Nagler 16 mm; the eyepiece of the binoculars, for a change, I attached to the Nikon Series E 100/2.8 photographic lens.

Comet 6x24 binoculars from the inside – what went wrong? - Objective lenses
The quality test of objective lenses and eyepieces of the Comet was done by joining them with a high quality astronomical eyepiece Tele Vue and a good photographic lens produced by Nikon.

As I expected the objective lens of the binoculars and the astro eyepiece provided very good image for the majority of the field of view (as much as 82°!) but the eyepiece of the binoculars with the photographic lens stopped down to f/4.0 showed some off-axis aberrations. As both objectives in their respective slides are identically positioned we can state with certainty that those 1.4 mm of difference in eyepieces protrusion is due to careless prism assembly; it causes a different length of optical paths.

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What can be said in the summary of this specific anatomy lesson? The Comet 6Ś24 should be treated like a piece of Ikea furniture – it’s more a DIY set, provisionally assembled in the factory. Even though particular parts of the binoculars are at least decent, sometimes even good, negligence in designing and assembling them thwarts the final result which could have been much better. However, if you own necessary tools, have some experience and you are not afraid of tinkering you should be able to remove at least some of the flaws. Returning to my culinary allegory from the introduction – the dish seemed to be great in the menu but eventually I found out the restaurant was stuffy inside, somebody fried the cutlet in old oil and burnt it, undercooked potatoes were too salty and the dessert way too sweet. It could have been such a nice experience…

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