All good advice above. To elaborate some...
OCA - optical corrector assembly - basically a barlow with perhaps an additional corrective feature.
It's main job is the barlow effect that moves the focal point out farther to help make-up for the light-path needed in the binoviewer. It screws into the binoviewer itself. Note that the farther the barlow is away from the eyepiece, the more the magnification. Sometimes people will screw it into the 1.25" diagonal before the binoviewer if the OCA that they happen to have is not quite high-power enough to get the job done if screwed into their binoviewer. OCAs come in various powers. For example 1.3x, 1.6x, 2.0x, 2.3x, and more.
Most of the BV designs (except the linear version) need an extra 100mm (or so) of light-path that need to somehow be made-up for. For example, if you are at infinity focus with a particular eyepiece without the binoviewer and then use that same eyepiece with the binoviewer, you will need to rack the focus in around 100mm. That won't be possible with many scopes. Using an OCA with an eyepiece without a binoviewer will require you to move your focus position out. For example, with my refractor, I need to add an extension tube when using a OCA without a binoviewer. When doing this, when I add the length of the extension tube to the focus travel left for my focuser, if the sum exceeds 100 mm I can then remove the extension tube and use a binoviewer (with the OCA) and get infinity focus.
There are also special diagonals in which some binoviewers can directly screw into to save a bit in light path which can let you get away with using a lower-power OCA to reach focus. Some people with DOBs also move their primary mirror up a bit for this same reason.
Scopes that focus by moving the mirror (like the C8) typically have a much greater focus range and you may not need any OCA to reach focus. The only down-side is that with moving the mirror, you may not be working at F10 anymore and you may be loosing a bit of aperture and the center obstruction becoming a bit more in percentage. Most people simply don't worry about that.
Some binoviewers also have "switching" options to easily move various different OCA into play for different magnifications.
The linear binoviewer is a special case. It's design has two transfer lens in play which makes the total light-path actually zero for the binoviewer - therefore it can be used with any scope that you can otherwise reach focus without a binoviewer. It does have a few drawbacks - otherwise everyone would simply be using them. They are a bit more complicated with more air-glass interfaces and are more prone to internal reflections. They split the pupil for each eyepiece which has some strange effects when used for terrestrial viewing with how out-of-focus things look. The design has an internal field-stop of 17.4mm which will make any eyepiece used also be limited to a 17.4 field-stop. The complexity of the design also means that you need to have your eyes just in the right place for the image complexity to work - so eye placement is less forgiving than for conventional binoviewers.
I personally have both the Williams Optics (WO) BV with the supplied 1.6x OCA and 20mm eyepieces as well as the Orion Linear BV for which I bought the Baader 18mm orthoscopic eyepieces due to the field-stops being a good match. I also bought the Celestron 10mm Ultima wide eyepieces for when I want to use either BV with more high-power. I typically use my WO BV for high power (since I have to use the OCA with it) and the Linear BV for lower power. I can even use a .66 reducer with the Linear BV for even lower power. The linear is more finicky with getting the eye-position just right but I enjoy using it. I also enjoy using the WO BV for higher power.
Viewing comfort is HUGE for any visual observation and also important when using BVs. Most visual astronomers find viewing more comfortable with Alt-Az mounts.
Some people do like using two zoom eyepieces with BVs. Some other people prefer using single-FL eyepieces with BVs. It is really up to individual preference.
Others are big fans of the BVs with OCA switching capability.
Since you are using 2 eyepieces, the width of the eyepieces matter for your inter-pupil distance as well as your face geometry. Some people have issues with really fat eyepieces and prefer to avoid them. You would have to find out if two Baader MKVI 8-24 used together causes you any issues in comfort.
With whatever BV you get, spend some time working with it (a few days) before drawing any final decisions about whether it is for you. I almost sent my Linear back after a day because I was not yet used to its peculiarities. But after working with it a few more days, I learned how to use it and now really enjoy it. I also was initially frustrated with the Williams Optics BV since I could not reach focus with my refractors even with the supplied 1.6x OCA. Then I learned that I could screw the 1.6x OCA into 1.25" diagonal to make it an effective 2.2x OCA and reach focus. I could alternatively have purchased the WO 2.0 OCA and that would have also worked with my refractor.
I personally greatly prefer using two eyes for all my visual observation!
Edited by jprideaux, 19 September 2021 - 07:53 AM.