Williams Optics binoviewer

[Mak the Night]

7 hours ago, Knighty2112 said:

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. 

That should work. I use the T-adaptor above if I use a diagonal with a compression ring.

[Stu]

23 hours ago, Knighty2112 said:

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.  

If you are using any form of GPC or Barlow then I don't think you will see much vignetting on the 32mm Plossls. The Barlow narrows the light path so it doesn't hit the aperture stop of the Binoviewer as much/at all. Pretty sure that's right anyway  

[Knighty2112]

22 minutes ago, Stu said:

If you are using any form of GPC or Barlow then I don't think you will see much vignetting on the 32mm Plossls. The Barlow narrows the light path so it doesn't hit the aperture stop of the Binoviewer as much/at all. Pretty sure that's right anyway  

Yeah, need to use the WO 2x nosepiece ordered separately to achieve focus, so in reality my 32mm EPs become 16mm. Would be nice to get a wider view though, so may still consider perhaps lopping some off the tube of the ST120 perhaps to do that. I'll wait and see how the new Altair 102mm f11 frac coming next week works with them first before I make any firm commitment one way or the other.  

[Mak the Night]

44 minutes ago, Knighty2112 said:

Yeah, need to use the WO 2x nosepiece ordered separately to achieve focus, so in reality my 32mm EPs become 16mm. Would be nice to get a wider view though, so may still consider perhaps lopping some off the tube of the ST120 perhaps to do that. I'll wait and see how the new Altair 102mm f11 frac coming next week works with them first before I make any firm commitment one way or the other.  

I don't use any Barlow when I use the 32mm BCO's or 25mm AH ortho's on my 102mm Mak and they give me 40.6x and 52x respectively. I don't see any vignetting on either. Probably a Maksutov thing.

Edited August 12, 2016 by Mak the Night

[Stu]

5 minutes ago, Mak the Night said:

I don't use any Barlow when I use the 32mm BCO's or 25mm AH ortho's on my 102mm Mak and they give me 40.6x and 52x respectively. I don't see any vignetting on either. Probably a Maksutov thing.

The TV 32mm Plossls have a 27mm field stop, I guess the BCO's are similar given their similar focal length and afov? My understanding is that the 127 Mak also has a 27mm aperture in terms of its baffle tube. I think the Binoviewers have a prism aperture of 23mm so presumably some vignetting must occur, in fact I've read multiple posts on CN which say they do, although it's not a significant problem. That's unless as you say the longer focal length of the mak reduces the beam of light passing through the BV's but if it has a 27mm baffle tube that shouldn't be the case. I'm sure someone else can advise on this.

[Stu]

33 minutes ago, Stu said:

The TV 32mm Plossls have a 27mm field stop, I guess the BCO's are similar given their similar focal length and afov? My understanding is that the 127 Mak also has a 27mm aperture in terms of its baffle tube. I think the Binoviewers have a prism aperture of 23mm so presumably some vignetting must occur, in fact I've read multiple posts on CN which say they do, although it's not a significant problem. That's unless as you say the longer focal length of the mak reduces the beam of light passing through the BV's but if it has a 27mm baffle tube that shouldn't be the case. I'm sure someone else can advise on this.

Having done some more reading on this, it may be because of the change in focal length of the Mak when used with a Binoviewer. The focal length (and therefore magnification) increases when you use a 2" diagonal for instance, and will increase more with binoviewers. This is probably enough to reduce the diameter of the light beam through the BV to avoid vignetting. Does that make any sense?

[John]

The AFoV of the Baader Classic 32mm (which is a plossl) is closer to 46-47 degrees I felt when I compared one with other ~30mm eyepieces. 

[Mak the Night]

2 hours ago, John said:

The AFoV of the Baader Classic 32mm (which is a plossl) is closer to 46-47 degrees I felt when I compared one with other ~30mm eyepieces. 

I think you're right there John. I had a PDF somewhere that showed the specifications of all the BCO's. From what I can recall the 32mm is more or less a standard Plossl design with an eye relief of 22.4mm and the AFOV is 45°. I don't get any vignetting in a 102mm Mak.

Edited August 12, 2016 by Mak the Night

[Stu]

26 minutes ago, Mak the Night said:

I think you're right there John. I had a PDF somewhere that showed the specifications of all the BCO's. From what I can recall the 32mm is more or less a standard Plossl design with an eye relief of 22.4mm and the AFOV is 45°. I don't get any vignetting in a 102mm Mak.

I was assuming a 127mm Mak, not the 102mm which has a 20.7mm baffle tube which I guess matches the BCO closely at 45 degree afov. 

[Louis D]

1 hour ago, Stu said:

I was assuming a 127mm Mak, not the 102mm which has a 20.7mm baffle tube which I guess matches the BCO closely at 45 degree afov. 

When I mentioned vignetting with my binoviewers, it was with respect to my AT72ED which has a true 2" focuser with no aperture restrictions and with 32mm GSO plossls which have true 27mm field stops.  So the 20mm to 22mm clear aperture of the binoviewers (not exactly sure of the number) is quite easy to see with this combination.

[YKSE]

Just find a correct light path schematic for mirror and prism diagonals. Peter and Stu's understanding was correct, that prism reflects too, the shorter light path in prism is achieved by bending the lights closer to each other than straight reflection in mirror diagonal.

 

[Stu]

37 minutes ago, YKSE said:

Just find a correct light path schematic for mirror and prism diagonals. Peter and Stu's understanding was correct, that prism reflects too, the shorter light path in prism is achieved by bending the lights closer to each other than straight reflection in mirror diagonal.

 

Thanks Yong, that is really useful.

[Knighty2112]

3 hours ago, YKSE said:

Just find a correct light path schematic for mirror and prism diagonals. Peter and Stu's understanding was correct, that prism reflects too, the shorter light path in prism is achieved by bending the lights closer to each other than straight reflection in mirror diagonal.

 

OK, I see. Make sense now. Thanks!  

[Louis D]

I'm no optical expert, but that diagram seems to show the exact opposite of what is common knowledge, that prism diagonals shorten the path length over a mirror diagonal.  I'm still working out what is going on.  The angles of refraction seem correct.  The converging beam is actually made to diverge somewhat within the prism, lengthening its path while in the glass.  I always thought just the opposite was happening and accounted for the shortened path length.

[Knighty2112]

28 minutes ago, Louis D said:

I'm no optical expert, but that diagram seems to show the exact opposite of what is common knowledge, that prism diagonals shorten the path length over a mirror diagonal.  I'm still working out what is going on.  The angles of refraction seem correct.  The converging beam is actually made to diverge somewhat within the prism, lengthening its path while in the glass.  I always thought just the opposite was happening and accounted for the shortened path length.

After looking at it again I see what you mean! Is there an optics expert in the house?  

[YKSE]

45 minutes ago, Louis D said:

I'm no optical expert, but that diagram seems to show the exact opposite of what is common knowledge, that prism diagonals shorten the path length over a mirror diagonal.  I'm still working out what is going on.  The angles of refraction seem correct.  The converging beam is actually made to diverge somewhat within the prism, lengthening its path while in the glass.  I always thought just the opposite was happening and accounted for the shortened path length.

I'm no optical expert, but I think your observation is correct.

My thought: shortening the light path with a prism is more pictural description, because with prism diagonal, we gain backfocus compard to a mirror. What prism does is actually push the focus position more rearwards, therefore then gain. My 2" prism diagonal and mirror diagonal is of the same physical size, focusing on short distance (some 20-25meter), focuser needs to rack out clearly more with prism than mirror.

It is the same with a barlow or GPC in binoviewer, barlow or GPC, with their diverged beam(therefore actually longer light travel to focus) push the focus plan much more rearwards, so we gain backfocus, therefore the descriptive word "shorten the light path".

[Louis D]

5 hours ago, YKSE said:

I'm no optical expert, but I think your observation is correct.

My thought: shortening the light path with a prism is more pictural description, because with prism diagonal, we gain backfocus compard to a mirror. What prism does is actually push the focus position more rearwards, therefore then gain. My 2" prism diagonal and mirror diagonal is of the same physical size, focusing on short distance (some 20-25meter), focuser needs to rack out clearly more with prism than mirror.

It is the same with a barlow or GPC in binoviewer, barlow or GPC, with their diverged beam(therefore actually longer light travel to focus) push the focus plan much more rearwards, so we gain backfocus, therefore the descriptive word "shorten the light path".

I think you're absolutely correct.  Everyone is always saying they're shortening the path with a prism to increase available backfocus, when in fact they're lengthening the light path in glass to increase backfocus.  You want to shorten the air part of the path as much as possible with stubbier connectors, but you want to increase the glass part as much as possible.

[Paz]

I've come across this old thread whilst researching binoviewers, I have to say this is full of really informative posts.

[AndresEsteban]

Hi folks!
IMHO, my understanding is that a prism diagonal has a shorter optical path OP than a the mirror diagonal.
Let's call the optical path of the prismatic diagonal as OPP ~65 mm for a 1.25" one.
Let's call the optical path of the mirror diagonal as OPM ~75 mm for a 1.25" one.

Now let's consider our refractor with focal length FL used without a diagonal, pointed to an object far away with any eyepiece you'd like. The drawtube will be almost all racked out. Pick up a CD marker and make a line at the joint between the drawtube and the focuser body.  Now measure with a ruler the distance between this line and the drawtube end (we will assume here lies the focal plane... OK?). Let's call this distance LDO = Length of Drawtube Out. This is how much the draw tube goes out to achieve focus.

1) Now put your prism diagonal with the same eyepiece and focus again. The drawtube needs to be racked in, in order to have the object focused again, right? 🤦‍♂️
Make a new mark on the drawtube and write "P" on it. Now measure the distance between the drawtube end and this line and let's call it DPD = Distance for Prismatic Diagonal. This is how much the drawtube goes out to achieve focus with a prismatic diagonal.😄
2) Now replace the prismatic diagonal and put the mirror one. Put again the same eyepiece on it, focus. We'll realize that now the drawtube needs to be RACKED IN a few millimeters more! Make the same line with the CD marker at the junction between the focuser body and the drawtube. Measure the distance between the drawtube end and this line. Let's call it DMD = Distance for Mirror Diagonal. This is how much the drawtube goes out to achiEve focus with a mirror diagonal.😄
3) Now we have three measures: LDO, DPD and DMD. And we've measured that DPD > DMD.👍

Ok, let's see now whats going on here 😎. Before using any diagonal, the focal plane was at LDO from the focuser body. Then, when we put the prism diagonal, the focal plane went near the objective, thus, we saw that the drawtube WENT IN and now is at a distance DPD from the focuser body. Therefore, the focal plane "travelled" LDO - DPD, this is the Optical Path for the prism: OPP = LDO - DPD😎

Again, but now with the mirror diagonal, we have the optical path for the mirror diagonal OPM = LPO - DMD.😎

Now let's crunch some numbers in the real world. I've used a vintage 60 mm FL=910 mm achro DAN BEAM (Circle K), of amazing qualty and optics, from the '60. Also 25 mm Vixen NPL plössl (36.3x).
Values measured:
LDO= 107 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter without star diagonal!!!)
DPD = 41 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter with prism diagonal)
DMD = 30 mm (distance that the drawtube was out from the focuser body to the drawtube end with a 1.25" ep. adapter with mirror diagonal)

So:
OPP = LDO - DPD = 107 mm - 41 = 66 mm
OPP = 66 mm

OPM = LDO - DMD = 107 mm - 30 mm = 77 mm
OPM = 77 mm

OPD = 66 mm
OPM = 77 mm

In other words: the mirror has a greater optical length than the prismatic one, therefore the focal plane needs to travel 77 mm towards the objective, whereas the prism "eats" only 66 mm of the FL, so the drawtube is not so much inside the focuser WHEN A PRISM IS USED, in fact, we have more drawtube out with the prism than with the mirror, thus, leaving some room to focus other types of eyepieces!

Just my 2 cents...
Regards and clear skies for us all!
Andy

[AndresEsteban]

On 13/09/2016 at 17:16, Louis D said:

I'm no optical expert, but that diagram seems to show the exact opposite of what is common knowledge, that prism diagonals shorten the path length over a mirror diagonal.  I'm still working out what is going on.  The angles of refraction seem correct.  The converging beam is actually made to diverge somewhat within the prism, lengthening its path while in the glass.  I always thought just the opposite was happening and accounted for the shortened path length.

Hi Louis!
The schematic is correct and it shows what really happens! You may use the virtual image instead of the real one for practical purposes. Both mirror and prism diagonals advance the focal plane toward the objective. Mirror diagonals move it more forward than prisms. As OPM -the optical path of a mirror- is greater than OPP -the one of a prism- the new focal plane with a diagonal mirror will be closer to the objective than the prism. The drawtube goes further inside with a mirror diagonal than with a prism one. That's why more drawtube length is available when a prismatic diagonal is used!
Remember that both diagonal types shorten the path length, but prisms shorten it less than mirrors. That's the reason to use prismatic diagonal when our drawtube is almost all racked in and has no room for further in-travel. 

Regards and clear skies for us all!
Andy

Edited February 23, 2023 by AndresEsteban
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[AndresEsteban]

On 13/09/2016 at 23:35, Louis D said:

I think you're absolutely correct.  Everyone is always saying they're shortening the path with a prism to increase available backfocus, when in fact they're lengthening the light path in glass to increase backfocus.  You want to shorten the air part of the path as much as possible with stubbier connectors, but you want to increase the glass part as much as possible.

Exactly! The prism "eats" less light path from the objective than a mirror, therefore the drawtube doesn't need to be racked in so much and there's still a portion of it outside the focuser. In other words, prismatic diagonals provide more backfocus than mirror diagonals. With a prism, the focal plane is farther from the objective than with a mirror, so more in-travel is available!   (Thanks to Don Pensack for the feedback!👍🤜🤛🔭)

Regards and clear skies for us all!
Andy

Edited February 23, 2023 by AndresEsteban
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[Don Pensack]

On 04/08/2016 at 13:52, Knighty2112 said:

Got a Altair 2" dielectric mirror diagonal.  Got 80mm focus on my dual sped focuser, so hoping that should be enough also to bring the binoviewer into focus OK.

You will probably need an optical corrector (GPC) to come to focus.

If you do, don't get the one from William Optics.  I found it added chromatic and spherical aberration Big Time.

There are several other brands that would throw the focal plane far enough back that don't add appreciable aberrations.

[Don Pensack]

17 hours ago, AndresEsteban said:

Exactly! The prism "eats" less light path from the objective than a mirror, therefore the drawtube doesn't need to be racked in so much and there's still a portion of it outside the focuser. In other words, prismatic diagonals require less backfocus than mirror diagonals.

Regards and clear skies for us all!
Andy

You are using the term 'backfocus' incorrectly.

It may not be intuitive, but the term 'backfocus' refers to the ability to get closer to the scope for accessories requiring more in travel of the focuser (like a camera) by moving the focal plane farther away from the objective.

The prism diagonal throws the focal plane farther back, which allows the diagonal to move farther into the light cone from the objective, allowing accessories that require more in-travel to come to focus.

In other words, the prism diagonal provides MORE 'backfocus'  than a mirror diagonal.

An accessory that needs more in-travel of the focuser is said to need more backfocus.

 

I find that understanding of the word very non-intuitive, so I think about back focus as meaning how far back away from the objective the focal plane is.

And a prism diagonal moves it back, so an accessory that needs more in-travel can accomplish focus.

[AndresEsteban]

6 hours ago, Don Pensack said:

You are using the term 'backfocus' incorrectly.

It may not be intuitive, but the term 'backfocus' refers to the ability to get closer to the scope for accessories requiring more in travel of the focuser (like a camera) by moving the focal plane farther away from the objective.

The prism diagonal throws the focal plane farther back, which allows the diagonal to move farther into the light cone from the objective, allowing accessories that require more in-travel to come to focus.

In other words, the prism diagonal provides MORE 'backfocus'  than a mirror diagonal.

An accessory that needs more in-travel of the focuser is said to need more backfocus.

 

I find that understanding of the word very non-intuitive, so I think about back focus as meaning how far back away from the objective the focal plane is.

And a prism diagonal moves it back, so an accessory that needs more in-travel can accomplish focus.

Thanks for the observation, Don! You're right! 👍👍 A prism diagonal provides more backfocus than a mirror diagonal. The focal plane is farther from the objective than in a mirror diagonal! Therefore, more in-travel distance is available at the drawtube! 🤜🤛🔭
Thanks again, Don! Text above corrected! 👍

Regards and clear skies for us all!
Andy

Edited February 23, 2023 by AndresEsteban
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[Louis D]

OMG, my last post on this thread was 6.5 years ago.  Apparently, people really do go back through the archives and read these musty old threads.