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However, some users claim that the UAG II causes aperture loss in this configuration, i.e., the light cone hitting the secondary would be too wide and thus cropped, due to the primary being pushed forward too much in order for the telescope to reach focus with the reducer.
My question is: is this true, and if yes, what would be the amount of the aperture loss? Also, do the UAG II and the original Alan Gee Mark II differ in this respect (the latter being inserted further up into the baffle tube)?
It is likewise NOT intended to work in front of a 2" star diagonal but only directly in front of the binocular viewer.
Absolutely any telecompressor causes "aperture loss". But not because it would crop the incoming beam, but because it does what the name says - it "compresses" the beam to become smaller in diameter. So, you get a smaller, brighter image.
At the distance of 121 mm the fully illuminatred field circle would be 18 mm and this is mercilessly visible in a CMOS-camera. The light fall off is so slow though that the human eye hardly notices the vignetting until the image circle has reached a diameter of ~23 mm. This is the reason why it is a joy to use with the binocular viewer and 1.25" eyepieces (which have field stop diameters of less that 26mm, and many binoviewers have even smaller prisms), but NOT with 2" eyepieces. Many eyepieces btw. work with this "vignetting trick"to -virtually- sharpen the outermost edge of the field. Check the web for TV-Plossel eyepieces of the first generations which used this "design feature" already 30 years ago.
Vignetting of UAG II and AG II are the same. The AG II was designed around the same time as the TV-Plossels. This was the time of emulsion films hundreds of times less sensitive than modern CMOS-cameras, and when 2" eyepieces did cost a fortune. Please do not try to pursue your project using the AG-lens principle for applications it was not intended for.
For a larger monocular true field of view, we'd recommend a 2" eyepiece like the Hyperion 36mm (the field stop diameter and thus the true field of view is comparable to thos of 2" eyepieces with even longer focal lengths, but the apparent field of view is much larger). If you aim for a brighter image with 1.25" eyepieces, then the UAG (in the standard version, which offers more options for configuration) is the better choice.
I also read about the ideal distance of "121mm", by using the bino + 2" TV diagonal, what is my distance?
Also can the UAG-II be used in conjunction with the GPC 1.7?
Thanks for your help, I am trying to learn!
Sorry - we do not think it makes much sense when trying to put a 2" diagonal into the beam - most likely you won´t reach focus.
The 112mm are equal to the light path of the bino-viewer, so that the UAG must be placed very close to the body to work. Also, the focal length of a Schmidt-Cassegrain changes, if you move the focus farther away from the telescope, so that the reducing factor changes - if you can reach focus at all.
You can not combine the Alan Gee with a glasspath corrector - this is one of the combinations where you do not reach focus at all with a Schmidt-Cassegrai - and you would combine an element to reduce the focal length with one to extend the focal length. You are better off if you use the Alan Gee for low magnifications and a glasspath corrector for higher magnifications.
Unfortunately, we have never measured the change in back-focus, as this is hard to do with an SC/EdgeHD with its moving main mirror.
The 7.5m T-2 extension #2 mentioned on the left side of the PDF is included in the scope of delivery of the UAG, so you can screw the UAG between any binoviewer with T-2-thread like the MaxBright II or the old MaxBright and any of our T-2 star diagonals.
If you have a standard 1.25" star diagonal without T-thread, this will not work to your satisfaction, as these star diagonals do have a longer optical length and no thread for attaching the binoviewer. We recommend a T-2 prism diagonal (they have the shortest optical length) plus the BTA-adapter for Schmidt-Cassegrains # 2408160, which can be screwed onto the back of your telescope, or a 2"/T-2 nose piece #2408150, if you have a 2" eyepiece holder on the back of your telescope.
I’d appreciate your input about field of view with the acf and if it can be used with my old binoviewer.
The focal length of the telescope will ve reduced approximately by factor 5.9.
Please note that the focal length of a Schmidt-Cassegrain changes, if you place accerories farther away, e.g. with a 2" star diagonal. That's why we recommend a smaller T-2-star-diagonal. We do not have data on how much the focal length of a Meade ACF changes with a 2" star diagonal exactly, but it shouldn't be too much.
Universal Alan Gee II - Telecompressor (UAG II)
The Universal Alan Gee II is the only telecompressor/reducer on the market which works fine with a binoviewer.
The standard Alan Gee Telecompressor Mark II was designed by Roland Christen (the producer of AstroPhysics apochromatic telescopes) as a reducer for Schmidt-Cassegrains which also delivers a flat field. The Universal Alan Gee II (UAG II) puts the optical elements of the Alan Gee in a regular 2" nose piece with an additional T-2-thread on the telescope side. The UAG II can be used for visual observations with Celestron EdgeHD telescopes. too.
The combination: Mark V Giant Bino / Baader 35mm ED-eyepieces / Universal Alan Gee II (UAG II) / Baader T2 Prism delivers a field of view of ca. 1,2° when used with a C8 – that's 72 minutes of arc, as much as a 2" 40mm eyepiece will show you! So, instead of waiting for an (unaffordable) 2" binoviewer, you can achieve the same effect on most SC-telescopes with the UAG II – be it monocular with a star diagonal or binocular with a binoviewer!
Please note that the reducing factor of the UAG II only depends on the distance between UAG II and eyepiece. It is designed for a working distance of 121mm then you get a reduction factor of 0.59. But if you focus to a focus plane farther away than the designed back focus of a Schmidt-Cassegrains or EdgeHD telescope, the focal length of the telescope itself becomes longer, and you get a different reducing factor than that which is calculated only from the distance between reducer ad eyepiece. The result depends on the individual adaptation and the telescope, we can't give a universal value for the reducing factor – but you will clearly notice the much larger field of view.
Use with a binoviewer
The UAG II can be attached directly to all Baader T-2 star diagonals with the included Baader T-2 / 7.5 mm Extension Tube (T-2 part #25C) (#1508155 , € 22,- ) . Although a binoviewer could mechanically also be attached to a 2" star diagonal, we recommend for a Schmidt-Cassegrain the short adaptation with a T-2-prism and the Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) (#2408160 , € 63,- ) , so that the ideal focus point (and the focal length of the telescope in front of the UAG II) don't change too much from the original design. A binoviewer with T-2 thread like the MaxBright® II or the Mark V Großfeld (Giant)-Binocular (#2456410 , € 1425,- ) can be attached directly to the UAG II. The distance to the eyepieces is then about 120 mm.
Monocular use with a star diagonal
The UAG II can also be installed in front of a T-2 star diagonal. In addition, you need a T-2 eyepiece clamp and – depending on the desired distance/reduction factor – a T-2-extension. The UAG should be as close to the telescope as possible, so we recommend the optional Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) (#2408160 , € 63,- ) to attach it to the telescope's SC-thread. If your telescope is equipped with a 2" ClickLock eyepiece clamp (#2956220 for C8/C925 or #2956233 for C11 and C14), you can also use this with the 2" nose piece of the UAG.
Baader 2" star diagonals with removable 2" nosepiece, you can also use the Baader Expansion Ring 2"a/T-2i with 1mm optical path length (T-2 part #28) (#2958242 , € 25,- ) to install the UAG in front of the star diagonal. It replaces the 2" nose piece. The distance between eyepiece and reducer is then equal to the optical length of the star diagonal. To place the UAG closer to the telescope, you can use the optional Baader T Adapter (BTA) for SC and MAK Telescopes (T-2 part #21) (#2408160 , € 63,- ) genutzt werden.
The UAG II may also be used with a camera (up to APS-C sensor size); the distance between UAG II and camera should be 121mm. The Alan Gee II was originally designed to be placed inside of the baffle of a Schmidt-Cassegrain, so for photography we still recommend the classic Alan Gee Telecompressor Mark II (T-2 part #20) (#2454400 , € 275,- ) in a configuration as described in its manual.
|Optical length (mm)||< 50|
|Usage||MaxBright II Binocular|
|Optical Design||Field Flattener, Telecompressor|