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True aperture of Skymaster PRO 20x80?

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#1 wolfli

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Posted 12 December 2016 - 08:54 PM

It would be really interesting if anyone would like to share the actual true aperture of these binoculars. Thanks!

 

I am also very interested in the more general optical performance of these, as well as the comparison to similar 20x80 binoculars, for example the Oberwerk 20x80 Deluxe III.

 

Thanks!  :)



#2 Binojunky

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Posted 13 December 2016 - 11:44 AM

The Obies sell for a lot more money and are far superior , the Celestron 20 x 80 I bought was unusable,TD.

#3 wolfli

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Posted 13 December 2016 - 03:57 PM

The Obies sell for a lot more money and are far superior , the Celestron 20 x 80 I bought was unusable,TD.

Please note that this is for the new skymaster PRO that sells for $250. Not the standard skymaster. The new pro series seems to have improved a long way.



#4 GlennLeDrew

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Posted 13 December 2016 - 05:16 PM

If this new model does operate at full aperture, or *very* nearly so, perhaps the makers are noting and taking into account the recent discussions about working aperture here and elsewhere. Until less than a decade ago I don't recall much talk of aperture reduction in binos. If that were still the case I do suspect the makers would be content to continue doing this for other than the alpha glasses. After all, a stopped-down system does generally benefit from some reduction in aberrations.

 

Supporting a fuller aperture places a somewhat tighter constraint on objective quality, particularly as regards turned edge that exacerbates spherical aberration. Due to the large area occupied by even a narrow peripheral annulus, the narrow zone in which turned edge exists has an impact seemingly out of all proportion to that narrowness. Polishing action therefore cannot be conducted to the same degree of haste, so that the edge does not tend to fall away more rapidly. This is of not so great import for interior surfaces which will be cemented; a cemented interface negates the far larger refractive difference for an air-glass interface. But any and all non-cemented surfaces should be finished with due care.

 

The foregoing may or may not be so relevant for more recent practices, or for larger vs smaller lenses. But my experience with turned edge in the past inspires a continued wariness, or at least awareness.



#5 Allardk

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Posted 14 December 2016 - 02:12 AM

" .......tighter constraint on objective quality".

 

Quality comes with a price. 

Oberwerk seems to spend more time on quality control. Time equals money.

 

I find it already impressive that you can buy an Oberwerk 20x80 Deluxe for 309.95 USD. 



#6 Binojunky

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Posted 14 December 2016 - 10:35 AM

 

The Obies sell for a lot more money and are far superior , the Celestron 20 x 80 I bought was unusable,TD.

Please note that this is for the new skymaster PRO that sells for $250. Not the standard skymaster. The new pro series seems to have improved a long way.

 

That depends, fish around  and their are more negative reports than good ones, collimation still seems to be an issue, the Obies are still checked out before shipping, price wise the Obies are still more,TD.


Edited by Binojunky, 14 December 2016 - 10:36 AM.


#7 wolfli

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Posted 14 December 2016 - 11:23 PM

 

 

The Obies sell for a lot more money and are far superior , the Celestron 20 x 80 I bought was unusable,TD.

Please note that this is for the new skymaster PRO that sells for $250. Not the standard skymaster. The new pro series seems to have improved a long way.

 

That depends, fish around  and their are more negative reports than good ones, collimation still seems to be an issue, the Obies are still checked out before shipping, price wise the Obies are still more,TD.

 

 

Yes, from all available information it seems like an Obie would be a safer bet. But I am asking about the skymaster pros very specifically because I am curious to see whether they have improved on this very particular aspect: the true aperture.


Edited by wolfli, 14 December 2016 - 11:24 PM.


#8 Riccardo_italy

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Posted 15 December 2016 - 03:06 AM

I had the Skymaster PRO 15x70. True aperture is 70mm, measured with the torch-light method.

 

If you compare the prism housing, in the PRO prisms are significantly bigger. It's probably the only major modification on the PRO with respect to regular skymaster (plus better exterior, waterproof and XLT coating). I suggest you to go to the Celestron website and open, in two different tabs, pictures of the prism housing of both skymaster. The difference is obvious.

 

I sent them back because there was a lot of chromatic aberration in daytime using.


Edited by Riccardo_italy, 15 December 2016 - 03:09 AM.


#9 GlennLeDrew

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Posted 15 December 2016 - 07:08 AM

A larger prism aperture will not necessarily increase the working aperture. That's because a wider prism is attended by a concomitantly longer optical path length through glass. And so while the front opening of the prism system is wider, it's also located farther up the converging light cone, where the cone is also wider. If the proportions are maintained, the longer path length exactly counters the increased prism aperture.

 

What has to be done in order to increase the working aperture:

 

- Use an objective if slower f/ratio. This provides a narrower light cone width at the prism entrant aperture, thereby reducing or eliminating clipping of at least the axial light cone.

 

- Get the eyepiece's field stop closer to the rear prism aperture. This has the effect of moving the prism system farther back and closer to the focus, thereby providing a somewhat narrower light cone width at the prism system entrant aperture. Larger prisms could help here by providing a tad more space beside the side of the first prism where looms up beside the rear prism aperture.

 

- Related to the immediately previous aspect, one can use a smaller field stop so as to permit it to slip past the adjacent prism's side and hence get closer to the rear prism aperture.

 

-Use a tapered or stepped first prism. Such a prism has the half that first intercepts the light cone of larger dimensions than the other half. This better accommodates the light cone where widest upon entrance, with the 'normal' sized half being OK after the light cone has tapered down somewhat before getting there. Note that simply installing a larger prism of the usual form (no taper or step up in size on one side) will not by itself help, for the same reason that two such larger prisms will not.

 

Again, bear in mind the fact of 'conservation of proportionality' as prisms get larger. A scaled up prism comes with a matching increase in glass path length, thus nulling out the advantage ostensibly provided by the increased aperture.

 

The *primary* determinant for prism size is eyepiece field stop diameter. One does not want the prism aperture to be smaller than the field stop, or at worst get down to about 0.9-0.95 times the field stop diameter. Otherwise notable field edge darkening results. And using a prism of rather larger aperture than the field stop confers essentially no gain, again because of the longer optical path length through the prism system. Indeed, it would be inefficient, in that extra bulk and expense are serving no real purpose.



#10 wolfli

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Posted 15 December 2016 - 08:31 AM

I had the Skymaster PRO 15x70. True aperture is 70mm, measured with the torch-light method.

 

If you compare the prism housing, in the PRO prisms are significantly bigger. It's probably the only major modification on the PRO with respect to regular skymaster (plus better exterior, waterproof and XLT coating). I suggest you to go to the Celestron website and open, in two different tabs, pictures of the prism housing of both skymaster. The difference is obvious.

 

I sent them back because there was a lot of chromatic aberration in daytime using.

Hi Riccardo:

 

What you find seems to be consistent with a previous post:

http://www.cloudynig...ster-pro-15x70/

So it seems like 15x70 has full aperture. Not that there's reason to doubt it, but I have not seen any direct measurement for the 20x80.



#11 Riccardo_italy

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Posted 19 December 2016 - 05:30 AM

Wolfli, true they work at full aperture. But I sent them back. They really have a lot of chromatic aberration, more than what should be standard in an achro binocular.

I had them with me in a big public garden. There was a tree, with a white sky as background (it was cloudy). Each branch of the tree was surrounded by a violet halo. The tree was full of birds, but the vision was not good. The violet halo was very evident and far higher than with a 8x42 binocular.

 

The point is this one: chromatic aberration is fine in-focus. But as soon as you go out of focus (even one mm), chromatic aberration explodes. In a situation like the one above, you can't have the full picture in focus because objects in the frame are at a different distances, so some branches will be in focus and some won't be.

Moreover, the focuser is quite stiff, and this adds difficulty in finding the correct focusing position.

 

Instead, what's extremely good is the 3D effect. Very very lovely.

 

So, if used only for astronomical use and on a tripod, the binocular is good. But I do not suggest it for daytime.


Edited by Riccardo_italy, 19 December 2016 - 05:37 AM.


#12 carlcat

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Posted 19 December 2016 - 06:53 PM

I just returned the Celestron Pro 20x80 binoculars. I measured with a flashlight for prism clipping and they do indeed have a full 80mm. The fit and finish was pretty good but I returned them because of too much color and even though they were collimated, I couldn't get them to focus sharply. It seems my next option is Oberwerk 20x80mm Deluxe III but they weigh a whopping 7lbs. Way too much weight for hand held short looks on a zero gravity lounge chair. Not really sure where to go from here.


Edited by carlcat, 19 December 2016 - 06:55 PM.


#13 Riccardo_italy

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Posted 20 December 2016 - 03:34 AM

I wish a price a tag slightly higher (around 250$ instead of 200$) for a slightly better (=flatter) eyepiece and better colour correction. 50$ to be used all in the optics can make a lot of difference.

But, even if the fit and finish is very good, and the mechanical construction is OK (the focuser too stiff is not really an issue: generally with use it gets better and if not you can replace the grease), there are too much compromises on the optical performances.

 

It's a shame because the idea was exactly what I wanted: central focus, not too heavy, waterproof 15x70 to be used both handheld for daytime and on a tripod for night observations.




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