Williams Optics binoviewer

[Louis D]

11 hours ago, Stu said:

The speed of light is constant in a vacuum but it does change (slow) in other media such as glass. I think astronomers only normally consider the speed in a vacuum because that is what it is for the vast majority of its journey to us from the stars.

The shorter light path on a prism however has nothing to do with the speed of light, it is simply due to the geometry of the prism vs the mirror i.e. The prism has a physically shorter light path. I'm not totally sure why, but putting my Baader Zeiss T2 prism next to the BBHS T2 mirror, the prism is clearly a smaller package.

Well, EdZ on CN stated the following back in 2012:

The optical path of a prism of same size is shortened (by physics) by the slowing of light traveling thru glass. The convergence of the rays is slowed and we can determine the optical path length by taking the physical length divided by the refractive index. So prisms (of same size) will ALWAYS have a much shorter optical path than mirrors.

So I'm going to go with EdZ because he always seems to have a pretty good grasp of optical physics.

[Stu]

1 hour ago, Louis D said:

Well, EdZ on CN stated the following back in 2012:

The optical path of a prism of same size is shortened (by physics) by the slowing of light traveling thru glass. The convergence of the rays is slowed and we can determine the optical path length by taking the physical length divided by the refractive index. So prisms (of same size) will ALWAYS have a much shorter optical path than mirrors.

So I'm going to go with EdZ because he always seems to have a pretty good grasp of optical physics.

Well that's a very different statement to the one you made, and EdZ's version makes  sense. The shortened length is due to the refractive index of the prism changing the convergence of the light beam and meaning that the optical path is shorter. It's  nothing to do with the light travelling more slowly through the prism, but is everything to do with the refraction of light as it enters and exits the prism, which ultimately is caused by the slowing and speeding up of light at the interfaces.

[YKSE]

I think a shemetic drawing of light path through these diagonals might partly explain the differences:

Mirror diagonal:                                                                                                                        Prism diagonal:

                                                          

For mirror diagonal, light path should be roughly diagonal size (2" or 1.25") plus the thickness of diagonal walls, while it's about 0,71x diagonal size for prism diagonal.

As an example, Baader's T2 mirror (2456100) has light path 53mm, while T2 mirror (2456095) has 41mm, that's about 0,77x, roughly right.

 

[Knighty2112]

1 hour ago, YKSE said:

I think a shemetic drawing of light path through these diagonals might partly explain the differences:

Mirror diagonal:                                                                                                                        Prism diagonal:

                                                          

For mirror diagonal, light path should be roughly diagonal size (2" or 1.25") plus the thickness of diagonal walls, while it's about 0,71x diagonal size for prism diagonal.

As an example, Baader's T2 mirror (2456100) has light path 53mm, while T2 mirror (2456095) has 41mm, that's about 0,77x, roughly right.

 

Thanks YKSE. That makes sense to me now, thanks!  

[Stu]

2 hours ago, YKSE said:

I think a shemetic drawing of light path through these diagonals might partly explain the differences:

Mirror diagonal:                                                                                                                        Prism diagonal:

                                                          

For mirror diagonal, light path should be roughly diagonal size (2" or 1.25") plus the thickness of diagonal walls, while it's about 0,71x diagonal size for prism diagonal.

As an example, Baader's T2 mirror (2456100) has light path 53mm, while T2 mirror (2456095) has 41mm, that's about 0,77x, roughly right.

 

Yong, I'm far from an expert on these things, but I thought the light path through the prism still reflected off the back internal surface by total internal reflection?

As I understood it from EdZ's post, it's the refraction of the converging light beam that actually shortens the light path.

As said, and this emphasises the beauty of SGL, I'm learning again today as I've never considered this stuff before, all very interesting.

[Knighty2112]

1 hour ago, Stu said:

Yong, I'm far from an expert on these things, but I thought the light path through the prism still reflected off the back internal surface by total internal reflection?

As I understood it from EdZ's post, it's the refraction of the converging light beam that actually shortens the light path.

As said, and this emphasises the beauty of SGL, I'm learning again today as I've never considered this stuff before, all very interesting.

Uh oh! now I'm getting confused again! Someone pass the ibuprofen please, I feel a headache coming on! Hehe!  

[Stu]

16 minutes ago, Knighty2112 said:

Uh oh! now I'm getting confused again! Someone pass the ibuprofen please, I feel a headache coming on! Hehe!  

Hang in there! We will get it resolved. As said, I'm no expert and may be wrong but Yong's post was not my understanding so I thought I would check!

[Highburymark]

I think I'll sit out this debate. My knowledge of light refraction reached a peak with grade C at Physics O level, 36 years ago. 

[Stu]

6 minutes ago, Highburymark said:

I think I'll sit out this debate. My knowledge of light refraction reached a peak with grade C at Physics O level, 36 years ago. 

Mark, I'm beginning to think I'm at the limits of my knowledge too

What I do know (or think I know) is that a light ray hitting an air to glass interface at 90 degrees will not be refracted at all. I think that a parallel beam of light passes into the prism without deviation, and is then reflected through 90 degrees by the internal surface which is at 45 degrees to the light, and it then exits the prism without further deviation as it passes the glass to air interface at 90 degrees.

The case for a scope is different as the light beam is not parallel but is converging towards the focal point. As I now understand it, the prism affects the angle of this convergence and so shortens the light path through the prism and increases the back focus available. This part I am still a bit unclear about and would like to see a ray trace through a prism showing this changes in convergence vs mirror to be clearer.

[Peter Drew]

Stu is correct re the light being refracted from the 45 degree angled face. 

[Knighty2112]

So if the light has to hit the 45 degree slope of the back of the prism to get deflected up 90 degrees then this is the same surely as the light been deflected up 90 degrees off the 45 degree surface of a dielectric mirror diagonal. So in reality the light path is the same, just that the focus point of the optics are shifted because the prism design is smaller than the design of mirror diagonals, hence the difference is in focul points and not light paths. That seems to make more sense to me. 

[Stu]

10 minutes ago, Knighty2112 said:

So if the light has to hit the 45 degree slope of the back of the prism to get deflected up 90 degrees then this is the same surely as the light been deflected up 90 degrees off the 45 degree surface of a dielectric mirror diagonal. So in reality the light path is the same, just that the focus point of the optics are shifted because the prism design is smaller than the design of mirror diagonals, hence the difference is in focul points and not light paths. That seems to make more sense to me. 

If I understand the post on CN by EdZ correctly then I don't think you've quite got it. There is an effect going on which changes the convergence of the light beam which shortens the light path. I am hoping someone will come along with a ray trace comparison between the two to help clarify it.

[Knighty2112]

Got the binoviewers this morning, and testing them out now. Have to use my Omni 2x Barlow with the binoviewers to get focus with my 2" diagonal. Get a good view with the 20mm EP's supplied with them (which with using the 2x Barlow give me 10mm EPs obviously). Tried them also with some stock plossl SW 25mm EPs OK, but when I use stock 10mm plossl  EPs I get I weird thing in the right hand view on the binoviewers. I though at first it was just stray light getting in somehow as I was testing during the day, but now realise it  its likely due to the prisms themselves in the bonoviewers themselves, however I did put my 10mm ES Maxvision 68 EP in the righ hand side and it went away. What I get with the plossl 10mm EPs is a bright flare at about the 4 o'clock position in the SE part of the EP view. Not sure how much this might be noticible when I use it at night, but wonder if anyone else has noticed this if you use 10mm plossl EP? As these 2 10mm EPs are been Barlow to 5mm EPs is this too much for the binoviewers to cope with? 

Edited August 8, 2016 by Knighty2112

[Knighty2112]

Managed to do a quick session earlier on to observe the moon, Saturn & Mars, and all I can say is Wow!

After earlier testing during the day I though there may be a problem with the binoviewers, but observing the crescent Moon first off was just brilliant. Had to use my 2x Barlow still to achieve focus, but the view with the 20mm EPs that came with the binoviewer were just wonderful. Then looked at Mars, which although no major detail could be seen, I could just make out the polar cap glinting away, and the gibbous shape very well. Saturn itself was just stupendous, even with all the LP from nearby houses and street lamps too as the grainy picture below shows. Mars is on the right next to the lamppost, and Saturn is to the top left. I also was able to use the 1.6 x nosepiece that came with the binoviewer too, so that I guess gave me 3.6x views, along with 2x views without the nosepiece fitted.

Hoping that it is clear for about 11pm when I hope to view M57 & M32. If they look half as good as the Moon, Saturn & Mars earlier then I that will be great.  

 

Edited August 8, 2016 by Knighty2112

[Knighty2112]

Received the WO 2x binoviewer nosepiece from Agena Astro from the US earlier than expected. Screwing in the 2x nosepiece I can achieve focus OK with my new Baader prism diagonal. Still awaiting a good clear night to test them further, but also got another 32mm Celestron Omni EP to make a pair with one I have already, so can use these too in the binoviewers, so hoping to get some nice wide'ish views with them.  

[F15Rules]

On 05/08/2016 at 11:31, Star Struck said:

A lot of getting Binoviewers to work is suck it and see as you will discover.

One thing not mentioned so far is Inter Pupillary Distance i.e. the distance between the centre of your eyepupils. I have found this to be critical for Binoviewers, a lot more so than for normal binoculars. I could not get the images to merge unless I got this spot on. It is easy to measure yourself, but if you wear glasses then your optician should know what it is exactly as they use it when making sure your glasses fit correctly.

This is the prism I use, it represents very good value for money if you cannot get your existing mirror diagonal to come to focus: http://www.365astronomy.com/Baader-Prism-Diagonal-T-2-90-degree-32mm-Set.html?gclid=COzow5qJqs4CFXYW0wodF4gKqg

Tony

Tony is right, this is cheapest way to buy the Baader set. Its a superb setup and because it uses a prism, not a mirror, it has a significantly shorter light path and so takes up less of your precious in-focus:-)

Dave

[F15Rules]

2 hours ago, Knighty2112 said:

Received the WO 2x binoviewer nosepiece from Agena Astro from the US earlier than expected. Screwing in the 2x nosepiece I can achieve focus OK with my new Baader prism diagonal. Still awaiting a good clear night to test them further, but also got another 32mm Celestron Omni EP to make a pair with one I have already, so can use these too in the binoviewers, so hoping to get some nice wide'ish views with them.  

Your 32mm plossls will likely vignette a little (show a darker edge to the field of view) but should still give a nice view..(the longest focal length EP you can use in a 1.25" barrel with the widest true field of view is around c25mm.

As your binoviewers have a maximum clear aperture (diameter of barrel on the binoviewer eyepiece side) , any eyepiece with a wider than 20mm field stop will vignette ( ie you lose some of the light). I think that's right:-)

Dave

[Louis D]

2 minutes ago, F15Rules said:

Your 32mm plossls will likely vignette a little (show a darker edge to the field of view) but should still give a nice view.

Oh, they'll vignette more than a little.  The outer 25% drops off to blackness before the fieldstop in my Arcturus binos.  I still like the views of nebulae in them because it really brightens them up by compressing all the light into a smaller image than with higher powered eyepieces.  That, and the eye relief with glasses is super comfortable.  Enjoy using them.  They're a lot of fun once you get a good setup.

[Knighty2112]

35 minutes ago, F15Rules said:

Tony is right, this is cheapest way to buy the Baader set. Its a superb setup and because it uses a prism, not a mirror, it has a significantly shorter light path and so takes up less of your precious in-focus:-)

Dave

Yeah, that's the one I got too.  

[Knighty2112]

30 minutes ago, F15Rules said:

Your 32mm plossls will likely vignette a little (show a darker edge to the field of view) but should still give a nice view..(the longest focal length EP you can use in a 1.25" barrel with the widest true field of view is around c25mm.

As your binoviewers have a maximum clear aperture (diameter of barrel on the binoviewer eyepiece side) , any eyepiece with a wider than 20mm field stop will vignette ( ie you lose some of the light). I think that's right:-)

Dave

Tested the, out in the daytime so far. Couldn't see any vignetting, at least not in the daytime. I'll check them out obviously better when the clouds roll away to view some nighttime targets.  

[Knighty2112]

25 minutes ago, Louis D said:

Oh, they'll vignette more than a little.  The outer 25% drops off to blackness before the fieldstop in my Arcturus binos.  I still like the views of nebulae in them because it really brightens them up by compressing all the light into a smaller image than with higher powered eyepieces.  That, and the eye relief with glasses is super comfortable.  Enjoy using them.  They're a lot of fun once you get a good setup.

Couldn't see an issue using them in the daytime, but like I said I need to check them on nighttime targets.  

[Mak the Night]

I have used a pair of 32mm BCO's in my WO bino's and not noticed any vignetting. That goes for the 25mm orthoscopic pair too.

 

[Knighty2112]

7 minutes ago, Mak the Night said:

I have used a pair of 32mm BCO's in my WO bino's and not noticed any vignetting. That goes for the 25mm orthoscopic pair too.

 

Good to know.

Do you need to use the 2x Barlow nosepiece to achieve focus to in your scope?

[Mak the Night]

2 hours ago, Knighty2112 said:

Good to know.

Do you need to use the 2x Barlow nosepiece to achieve focus to in your scope?

I only use the bino on scopes that utilise a diagonal. I've never had any problems achieving focus. I've yet to use it with my Newtonian though.

If I had any problems with focusing I might try placing the bino into this Celestron T-adaptor. You can get these almost anywhere. I am reluctant to put eyepieces with either undercuts or (even worse) flared draw tubes into the WO dioptres though. Why WO supply eyepieces with flared draw tubes for a bino with dioptres that have compression rings is beyond me. The same goes with the WO bino nosepiece itself. I wouldn't recommend placing it in a diagonal or adaptor with a compression ring for any length of time as it often deforms the ring itself, making insertion of other eyepieces difficult.

[Knighty2112]

When I put the binoviewer in to my diagonal, which has a compression ring, in only insert it up to the shoulder of where it slopes inwards, that way it is always on a a straight part of the neck of the binoviewers.