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Binoviewer with Pentax 100ED Review (1 Viewer)

Can Popper

Well-known member
First off, this review is preliminary. I will add to this thread as things unfold.

Currently, I am working with Harry Siebert to get the right optics and components to allow my Williams Optics binoviewers to come to focus on my Pentax 100ED. After our second iteration, we were 33% successful.

With the new lens Harry sent me, I was able to bring objects within 30 yards to focus. The lens acts like a 3x barlow so the power of my eyepieces are tripled.

Within the 30 yards, the results are just glorious. When I view objects through the binoviewers using the included 20mm William Optics eyepieces, I really feel touched on a primal level. Looking at leaves and flowers through them feels like I am standing there with a giant magnifying glass and viewing them with both eyes. Others have said it feels almost 3D and I second that observation.

But outside the 30 yards, the binoviewers would not focus. I took some measurements of the scope and eyepieces and sent them to Harry. He will be custom machining an extension piece that will allow the OCA lens to sit deeper inside the scope. The same setup will also be able to screw onto my dielectric diagonal and use it with my Pentax. This will hopefully allow me to focus to infinity.

As for the eyepieces, I have tried my pair of 20mm 66 degree FOV William Optics eyepieces and my pair of 24mm 65 degrees FOV Siebert Ultras. The 20.5mm clear aperture of my binoviewer allows the 20mm eyepieces to work without vignetting. I do get some vignetting along the edges with the 24mm Ultras.

I will continue to extend this thread and add pictures as things progresses.
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Well, it has been a while since my first failed attempt at binoviewing on the Pentax 100.

Recently, Siebert Optical came out with the Universal 1.3x PST OCA which is stated to work for all binoviewers. My PST OCA finally came in today and I attached it to my binoviewers.

Presto! My binoviewer came to focus on my Pentax 100. I went through a number of eyepieces such as the Burgess Planetary 9mm's, Siebert Ultra 24mm's, SWA 15mm's and they all focused.

Now my observation binoviewing with this setup. The PST OCA seems to have two side effects. One, it lengthens the eye relief of the eyepieces used. Two, it seems to result in a noticeable degree of light loss. I guess when you are looking at the sun it is no big deal but for terrestrial viewing, the light loss is very noticable.

With this setup, I'm only comfortable binoviewing during the daytime.
Interesting info since binoviewing is one of the things I would like to test (see it at cr-telescopes on http://pt-ducks.naturlink.pt), and see if it would be useful for cr-birding.
Do you think that it increases resolution, even with the light decrease?
By the way, the 24mm eyepieces had vignetting with the 1.3x OCA?
Since my interest is to use it with zoom eyepieces, I would appreciate if you could test one Televue zoom or similar, even only with one eye… :)
The binoviewers I used were the Siebert Black Knights with 22mm of clear aperture. The 22mm clear aperture is just enough to ensure that the 24mm Siebert Ultras with 65 degree FOV do not vigenette.

I have sold my William Optics binoviewers in favor of the Black Knights because the WO's 20mm of clear aperture does vigenette my 24mm eyepieces.

I'll test the binoviewer setup with my zoom eyepieces (Scopetronix, Orion, and Pentax) though I'm not sure what you want me to test for.

I'll also run some resolution tests on a distant sign to see if the binoviewers improve resolution even with the significant drop in light throughput. Watch for pictures on this thread.
I performed futher testing of the PST corrector today. I compared it against the 3x OCA that was not able to bring the Pentax 100 spotter with binoviewer to focus at infinity.

Here are some observations:
1. The PST corrector actually results in brighter image than the 3x OCA.
2. Neither degrades the image much
3. The PST corrector can bring any eyepiece I tested to focus at infinity
4. Brightness is not an issue if I stick with medium and low power eyepieces
5. Bino mode does allow me to resolve objects better than mono mode. As noted elsewhere, our eyes were designed operate in bino mode. Viewing through the binoviewer results in a more natural feel.
6. The PST corrector results in images being flipped.

Below are some pictures of my setup and two pics I took through the Pentax/PST corrector/binoviewer setup. I will let the reader be the judge.


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Thanks for the additional info and the photos. The complete set results long but it's usable for me since I spend long hours cr-birding in hides.
Just a few questions:
- what you mean by "images being flipped"?
- Did the 3x OCA focused at close distances? I'm asking this since if it does, it would mean that the badder zoom will reach focus and, eventually, the 1.25 and the 1.6x OCAs could be also used instead of the PST corrector, shortening a bit the complete set and, eventually, increasing image quality.

Regarding the zooms, my question refers to their usability at high powers. With the XF zoom the PF100 results in 32.5-97.5x. With this set the 24-8mm zooms should result in 34-102x, plus the light is divided by 2 and have to pass through more lens. I use a XW 5mm eyepiece regularly in the PF100 (126x), with most light conditions, and can use also a TV Radian 4mm (158x) if light conditions are good. It seems that you don't have the XF zoom but you eventually have a 5mm eyepiece. Can you compare the light and definition on one of the 24-8mm zoom, on the binoviewer, with those of the 5mm eyepiece without the bino?
I think the image below will explain what I mean by the image being flipped. Also, I shot it using my Burgess Planetary 9mm (comparable to the Televue Radians but with shorter eye relief) through the PST corrector and binoviewer. Also this is shot at very high magnification (91x total) which is close to what you are looking for regarding cr ducks.

As for the 3x OCA, it only allows me to focus out to 50 yards max. The 1.6x OCA would focus out to 10 feet. Neither I view as being viable options for binoviewing on the Pentax spotter. The only thing that would allow the binoviewers to come to focus on the Pentax is the Siebert PST OCA.

If the below Planetary 9mm eyepiece photo is any indication, the zooms should be usable. The main adjustment you would need to make is to cover yourself with a black hood to block out ambient light. The contrast between the daylight around you and the lowered light levels at high power through the binoview and PST will make seeing difficult. I found high power to be more bearable on a darker day because the ambient light isn't causing my eye's pupils to shrink.

As for using the Baader Hyperion zooms specifically, I think coming to focus should not be a problem but their weight maybe a big issue. You can see from the above picture that the binoviewer hangs off the rather long tube of the PST corrector.

Putting two heavy Hyperion zooms into the binoviewer maybe result in damage to the PST corrector tube or the Pentax Spotter. The eyepieces I use are pretty light. In fact, even the Pentax zoom I have is pretty light though big looking.

However, there maybe a way to use the Hyperion zooms still. I'll test my setup by replacing the aluminum tube of the PST corrector with a 90 degree dielectric diagonal. That may lower the amount of force being placed on the collet ring of the Pentax.


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Ops, flipped images are not good for cr-birding...
With the 3x OCA it also result on flipped images?
Since I'm still interested in binoviewing the PST OCA maybe a solution for my old Celestron C5, that the problem in using it for cr-birding is the image degradation by the image erector, at more than 70x magnifications. With this solution, the image erector wouldn't be needed. In the other hand, the 24-8mm zooms would result in 68-203x, which is too much for terrestrial viewing...
Well, if you are considering switching to the Celestron C5 for binoviewing, I would advise you to give Siebert a call. He probably has a focal reducer that would shorten the focal length of the Celestron and flip the image right side up.

This way you would get lower magnification with your zooms and a right side up image at the same time.

He is very accessible for consultations on your needs.
Sorry for taking a while to get back to you. It has been raining cats and dogs down here.

If finally tested the 3x OCA and it does not result in flipped imaged.
Well, I finally got binoviewing on the Pentax working well. David emailed me and that got my mind jogging. I replaced the long PST erector tube with a 1.25" dielectric prism and the images got erected though they are still flipped left to right. No problem with near or far focus either.

I think using the 60 degree Televue should allow angled binoviewing with the Pentax spotters.


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