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Frankenbino Exposed II - Explained
If you reading this for the first time, you've probably got curious by the two things. First - is what's up with this Name and why is it marked as sequel and Second - what it's purpose . Please let me start with the first.
I gave to it that name, simply because there is Frankenbino Exposed I . You can read about it HERE on the forums of Cloudy Nights. The guy probably got his inspiration by the idea of being able to use one of the best Binoviewers out there made by the Baader - Mark V, with 30mm clear aperture prisms and superior optics and very neat Powerswitch made by Denkmeier, which allows you to switch 3 powers instantly without switching eyepieces.
You've got the two best things from the two best companies working together, but the problem is, that there is no ready made adapter, or fitting, to make this connection possible. So you can't just buy it, you have to make it. So that how the Frankenbino probably got Exposed in the first place, strangely, the guy's name is also Sergio. Now, two Sergio's making two Frankenbinos....Coincidence? Go figure...
What's about my case then? Well, if you into Binoviewers thing, you might already know, that the Baader Mark V known as shortest possible combination for Binoviewers out there. It comes with the prism/mirror diagonal that have very short light path and very easy removing/changing mechanism. But I got the Denkmeier II Binoviewer setup with it's magical 3 power Powerswitch and I wanted also to make it the shortest possible setup, by mating it with the same short Baader prism and made the switching/removing easy no less.
So once again, Baader product and Denkmeier product meet again, therefor the name - Frankenbino Exposed II.
So, now we go to the second thing - why to bother at all, to have this shortest possible setup you may ask. And I will say this, while shortest possible setup can bring some advantage when used with refractors, due to the shorter infocus travel, this is a must when used with Schmidt-Cassegrain Telescopes or similar designs with moving primary mirror.
Let's see for example my initial setup on the image to the right. It consist of Crayford Focuser, 2" Diagonal, Filter Switch, so you won't need switching filters in the dark, Powerswitch and Denkmeier II Binoviewer. All seems to be a reasonable things that can turn the observation night into a real joy. But, focuser is 40mm light path, 2" diagonal around 110mm, Filter Switch 15mm , Powerswitch 20mm and Binoviewer additional 110mm. Which totals to a sum of 295mm additional light path.
Meaning that now to come to a focus, I have to move the focal plane further back, to compensate for those additional 295mm of a light path and that would increase tremendously the operational focal length of the telescope. As a result, this will produce different than assumed magnifications by the eyepiece being used.
But the increased focal length is not what is really got me worried. To compensate that amount of a light path and move focal plane way back, I have to turn the focuser knob Counter-Clockwise and bring the Primary Mirror much closer to the Secondary Mirror. At some point, it got close enough, that the Baffle Tube, on which Primary Mirror slide, start to introduce it self into the field and will actually reduce the working aperture of the scope.
To demonstrate that this is exactly what happens, one can perform what is called a Green Laser Test. This test have been discussed many times on the pages of Cloudy Nights forums.
On the image to the right you can see my own experiment conducted with my equipment. The principle of the test, is to take the green laser and shine it through the Binoviewers eyepiece to try to illuminate the objective aperture backwards and then measure it.
But depending on the actual exit pupil of a given setup, we probably can light up only portion of the objective, as laser beam is about 1mm wide. My calculated exit pupil was 1.8mm in diameter, so almost twice as large.
So how do we fill all of the exit pupil with collimated beam of laser light then? Well, we need to expand that beam of light using the pair of Binoculars. Before we can use the Binoculars we have to make sure they are focused to infinity and we will get the nice collimated beam of light that would be expanded times the magnification of the binoculars being used.
After we got our expanded beam of light, we can shine with it through the equipment being tested and measure the working aperture. My setup, that shown on the first picture, were questioned, so I conducted that test with this setup. One more note, the Telescope also have to be focused to infinity.
After I got my whole setup aligned and telescope aperture being pointed to a wall, I lit up the laser pointer and nice, sharp-edged green disc of light emerged from the scope's aperture. Then I simply printed out exact paper ruler and stick it to the wall to measure diameter of the disc.
The results came out as expected, my working aperture were reduced to a size of being around 9". Wasting high-paid aperture already gave me a reason enough to be concerned, but there is more to it. While aperture got reduced, the secondary central obstruction stayed the same size, automatically giving higher percentage ratio to a working aperture. In simple words it will hurt contrast even more.
Of course I could give up on the focuser and the filter switch but still, at that long of a focal length, two of three magnification being produced in the binoviewer with PowerSwitch were almost not usable for DSO. And, using 2" Diagonal with 1.25" Binoviewer are no brainer, so the project for making shortest ever possible setup were born.
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