Note: Sunce this article was written, there have been a large number of comments that the newer high reflictivity diagonals are a considerable improvement over older diagonals. A number of companies, including TeleVue, have made these "super reflective" diagonals available. They should be considered for the highest quality results.
The following is a discussion of the design of the Meade verses the TeleVue 2" diagonals. Considering how simple this device is, it is hard to believe that the two diagonals could be designed so differently. The TeleVue is shown on the left and the Meade on the right. Superficially they are very similar and both do the same job. Interestingly, and amazingly to me, the TeleVue is actually made from a single piece of material. (not the mirror and mirror mounting plate of course) To do this, a large amount of machining has to be done. This makes the diagonal expensive. It is however beautifully machined. The Meade is made more conventionally. The housing is one piece and the two tubes screw into the housing. There is one slight advantage to this design. The 2" tube can be removed and a Schmidt adapter screwed in place. Thus one design can serve for two types of adapters. The Meade is available as shown with the 2" tube and alternately with at Schmidt tube and locking ring that fits directly on the back plate of most SCTs. With the TeleVue an additional adapter tube is required to fit it to the standard SCT Schmidt thread. Both diagonals have a full 2" internal opening and the mirrors are full size. I do not have information on the optical quality of the mirrors or details about the coatings that may be provided.
I do not use the 2" diagonal with a Schmidt threaded tube at all since I consider
the inside diameter of the Schmidt tube to be too small. I use only the
full aperture 2" tubing for the 2" diagonal and the 2" eyepieces. (I also
use the JMI or Optec rear plate on the SCT to insure a full 2" opening at the
back of the telescope. (This is only possible with the 10" and 12" optical
Below is shown the Meade diagonal with the 2" tube removed.
Note that I have modified the 2" tube by cutting a groove to secure it better
into the JMI focuser (or alternately a 2"size SCT rear plate adapter tube).
Additionally, I have inserted a 2" filter adapter ring so that standard 2" filters
(48 mm) can be used. The TeleVue diagonal has a thread for the 2" filters
machined as part of the tube. At the right is the Meade diagonal
with the weighted adapter and a 26 mm eyepiece. There has been no
hard evidence that I know of that the two diagonals function differently from
an optical viewpoint. The TeleVue is more finely made in my opinion
but is considerably more costly.
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