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Findings with Celestron SkyMaster 15x70 binoculars

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#51 PowellAstro

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Posted 18 July 2013 - 12:55 PM

This second pic is one of the porro prisms after I have blackened the edges:

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#52 PowellAstro

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Posted 18 July 2013 - 12:57 PM

This third pic shows the bright reflective AL insert at the bottom of the barrel tube and the shinny plastic walls. These will be blackened with flat black as well:

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#53 GlennLeDrew

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Posted 18 July 2013 - 01:58 PM

Note that what appears as being highly reflective barrel surfaces from the front end might not be so awful as seen from the back end. I won't go into the factors involved, as they will require more verbiage than I'm prepared to go into at the moment.

#54 PowellAstro

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Posted 18 July 2013 - 03:31 PM

This is true but looking through the tube with no lenses at all and a bright light source toward the edge finds you with a bright glare and haze as though you were out on a foggy morning. So, at least in this case it is causing a lot of reflective light scatter. Also, with them apart this far, why not just flatten this reflective surface as well. ;)

#55 Binojunky

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Posted 18 July 2013 - 03:34 PM

The Oberwerk LW 15x70 is very similar to the Skymaster, close enough to say its from the same factory, however it costs more.
Internally its got a good bit more flat black paint on assorted surfaces and the epoxy putty is painted over, also the prisms have a secondary clip to help the collimation stay a bit more reliable,all optical surfaces are multi coated as against fully coated,maybe worth considering when thinking about the Skymaster yet not able to afford the very high end 15x70,s out there?,DA.

#56 PowellAstro

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Posted 18 July 2013 - 03:45 PM

I spent some time on the phone with Kevin the other day talking about the Oberwerk LW 15x70. They are the same housing and setup. All mounting, prism clips, etc are the same. The big difference in the Oberwerk LW 15x70 is that they have a higher select grade of optical components, prisms, lenses, etc. They also have way better coatings and lastly they are indeed assembled and aligned better than any of the other 15 x 70 binos in this class. The Oberwerk LW 15x70 are the way to go in this range unless you just can't spend the money. I already had these and I do optical work and have for many years. So, I just wanted to see how good they could perform setup like they should of been to begin with and the known issues corrected. The prisms in the Skymaster seems to be the weak link. I placed one of the objectives in a 340mm refractor tube I have an used a Nagler eyepiece at 20x per inch and the image was rather good!

#57 PowellAstro

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Posted 18 July 2013 - 04:35 PM

Even the eye lenses do not have blackened edges.

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#58 PowellAstro

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Posted 18 July 2013 - 04:36 PM

Another view of the eye lenses:

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#59 Binojunky

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Posted 19 July 2013 - 10:38 AM

I spent some time on the phone with Kevin the other day talking about the Oberwerk LW 15x70. They are the same housing and setup. All mounting, prism clips, etc are the same. The big difference in the Oberwerk LW 15x70 is that they have a higher select grade of optical components, prisms, lenses, etc. They also have way better coatings and lastly they are indeed assembled and aligned better than any of the other 15 x 70 binos in this class. The Oberwerk LW 15x70 are the way to go in this range unless you just can't spend the money. I already had these and I do optical work and have for many years. So, I just wanted to see how good they could perform setup like they should of been to begin with and the known issues corrected. The prisms in the Skymaster seems to be the weak link. I placed one of the objectives in a 340mm refractor tube I have an used a Nagler eyepiece at 20x per inch and the image was rather good!



Interesting, my Oberwerks have a distinct clip behind the prism on top of the putty used to fasten them in place, however my Skymasters do not, maybe the specs have changed over the years,I had my suspicion about the optics being a better grade in the Obies, thats why they cost me more,DA.

#60 PowellAstro

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Posted 19 July 2013 - 11:01 AM

The SkyMasters I have, have these same clips behind each prism to hold them in place. As I complete the pair, I will post a few more pics that will show the clips.

#61 PowellAstro

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Posted 19 July 2013 - 12:59 PM

Here is a shot after I blackened the inside of the body and the edges of the prisms. This shows the prism clips.

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#62 PowellAstro

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Posted 19 July 2013 - 01:03 PM

And a shot of the now Blackened Barrels:

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#63 PowellAstro

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Posted 19 July 2013 - 01:05 PM

This shows bad reflections in the non-blackened lens edges in the stock eyepiece and these major reflections now much cleaner in the eyepiece with the blackened edges:

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#64 PowellAstro

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Posted 19 July 2013 - 01:09 PM

The eyepiece field stops were also found to be a major source of reflected and scattered light. They are made of a very shinny plastic and produce a lot of glare. These were blacked out as well:

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#65 Rich V.

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Posted 19 July 2013 - 01:36 PM

Hey, Glen, have you determined just where in the system the 70mm objectives are stopped down to the effective 63mm that has been measured in all of this style of binos? I'd expect it's the front prism aperture but can't say for sure.

Since you're being so thorough, I'd like to hear what you have to say about this...

Enjoying your well documented breakdown! You'll have the best pair of Skymasters in the country when you're finished! ;)

Rich

#66 GlennLeDrew

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Posted 19 July 2013 - 02:29 PM

Rich,
Yes, it's the frontmost prism aperture which stops down the system.

PowellAstro,
Good looking work! Some questions:

- Did you also dull the shiny inside surfaces of the eyepiece spacer rings?

- How did you measure the eyepiece and objective focal lengths?

- You mention your suspicion that the prisms are likely the 'weak link.' Do you have a small optical flat to place against prism surfaces so as to assess the degree of flatness?

- Upon reassembling, how are you ensuring there is no prism 'lean' (which if present would introduce image rotation and eye strain)?

#67 PowellAstro

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Posted 19 July 2013 - 02:32 PM

It is the front prisms that stop them down. Good news, I now have the full 70mm use of the objective lenses! I moved the lenses back in each barrel just over 1/4 inch. The center of the lenses are now flush with the outside edge of the barrels. With this I now have fully illuminated exit pupils all the way around! I can also now focus as close as 32 feet and still have a 1/3 turn left on the focus knob. I have no idea other than their lock ring setup, as to why they had the lenses screwed so far into each barrel. Mine are back together now and they look like a 500 dollars pair of binos. Images are very sharp, contrasty and with way better color saturation!

I will document all that I have done to them as soon as I can pull it all together. I am glad to answer any questions.

#68 PowellAstro

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Posted 19 July 2013 - 02:40 PM

- Did you also dull the shiny inside surfaces of the eyepiece spacer rings?


Yes, I did all spacer rings in the eyepieces as they are of the same shinny material.

- How did you measure the eyepiece and objective focal lengths?


I use the sun and a slide rule jig made just to test FL.

- You mention your suspicion that the prisms are likely the 'weak link.' Do you have a small optical flat to place against prism surfaces so as to assess the degree of flatness?


Yes I do and most are 1/2 wave, some close to 1/4.

- Upon reassembling, how are you ensuring there is no prism 'lean' (which if present would introduce image rotation and eye strain)?


I have a bino laser collimator I use.

#69 Andresin150

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Posted 19 July 2013 - 03:02 PM

Excellent! for a moment I wanted a Skymaster to experiment too!

#70 GlennLeDrew

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Posted 19 July 2013 - 08:26 PM

Having moved the objective that 1/4" farther out, this must leave not much extra in-travel beyond infinity focus.

Are you certain you now have full aperture performance? For unmodified binos which have a 22mm aperture prism stopping the system down to 63mm, the on-axis beam footprint at the front prism aperture would be 70 / 63 * 22 = 24.4mm. For an f/4.7 objective, moving it out 6mm increases the light cone diameter at the prism by 4.7 / 6 = 0.78mm, which is rather less than the required 2.4mm.

Are you aware of the flashlight test for effective aperture? This will provide a direct measure. Remember to set the focus for infinity, and to locate the flashlight a good foot or so from the eyepiece.

#71 PowellAstro

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Posted 19 July 2013 - 08:45 PM

You are correct. If you look in the pic that shows the prism you will see the hole is now open to the very edge of the larger prism. I milled the front holes 3mm larger on the CNC as part of this mod. The large prism were right at 25mm wide. Due to better optical alignment, the edge correction looks better than it did before this work. I was worried the edge correction might look bad but it turnned out to be fine. As to the focus I have about a 1/3 turn left after focus at infinity.

#72 PowellAstro

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Posted 19 July 2013 - 09:25 PM

About your flashlight test. I have a 20" diffusion panel that I use. Before the mod, you could see vignetting around the exit pupil. Using a fully illuminated light source and focus set at infinity, the exit pupils are now fully illuminated with no detectable vignetting and the exit pupils measure 4.67mm. This is what should be seen with 70mm lenses of 330.2 FL at 15x.

#73 GlennLeDrew

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Posted 20 July 2013 - 01:33 AM

You say the first prism to intercept the light from the objective is the larger 25mm width, compared to the second, which is 22mm? If so, I'm a little surprised. And furthermore, that the first prism opening was not enlarged so as to take advantage of this and thereby gain back the full aperture is another puzzling aspect.

#74 nicknacknock

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Posted 20 July 2013 - 05:05 AM

Really enjoying this thread.I have a decently collimated pair of these and I always knew about some of the mechanical / optical issues of the SkyMasters but this is really an educational experience for me reading this, so thank you all for this excellent reading material!!!

#75 PowellAstro

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Posted 20 July 2013 - 09:33 AM

@ GlennLeDrew

I think they did this by design. As you may know, most lens mounting cells cover the extreme edge. So, if there is a slight edge problem, it is covered by the cell. In the Skymaster 70s, the plastic cell is glued to the outside of the lens. This leaves the whole lens in the optical path. If there was to be a turned up or down edge, and most times there are, it could have a large effect. This way it is vignetted and the effect all but canceled.






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