Telescope Reviews: Denk Binotron rollout

Remember, this problem is somewhat unique to telescopes with moving mirror focusing and baffles, though it could be encountered in some refractors if the in-focus allows the front of the focuser tube to cut into the light cone, which is a very real possibility in some cases.

To answer the question though, in the case of the MCT, I was loosing apeture in reduced mode (severe aperture loss) and staight through mode (minor loss).

This has to do with the fact that the Super-system I used was based on the use of a 2" diagonal.

The C14 and C8 both start to loose aperture once the back focus goes past about 200mm. The C9.25 I think starts loosing aperture once the back focus goes above about 170mm and the falloff is quick.

As an example, in my C5, as measured directly, I am using a back focus of 203mm, and the 127mm scope is being reduced to 122mm.

The Denkmeier System I used was based on a 2" diagonal, and the entire back focus required to use it was about 230mm or so.

With 230mm of back focus, the C14 is working at slighly less than 14" of apeture (Maybe 13.75") and the C8 would be working at about7.7". The plot that I have for the C9 suggests that the 230mm of back focus would give an aperture of 8.75".

Because the aperture is reduced, the central obstruction becomes bigger by percentage of aperture so the contrast is further lowered.

This is in straight through mode.

With the High Power arm in, the back focus requirment is reduced. I don't know how much it changes, but this would mean that the high power arm will give the system more apeture than straight though, because the mirror needs to be moved back slighly to re-position the focal plane.

Now I can only be fair about this and say that many people (perhaps most people) will not care about this very slight loss in performance.

So, it only matter if it matters, but for those that want to get the absolute best performance from their SCT or refractor, they want to make sure that they are not working with a reduced aperture.

If the new Denkmeier can be attached to a Baader Prism or other device that will shorten the light path, aperture loss can be reduced or prevented in SCTs, MCTs, and many refractors that were not designed to allow a full apeture when the focuser was fully racked in (and this is not at all unusual, though after the Stellarvue debacle 10 years ago, I think it doesn't happen so much anymore).

Again, the back focus is the issue, and the point where aperture loss is very model specific (The C11 has the loosest baffle of the Celstron SCTs).

But if the back focus is over 200mm, most SCTs will start loosing aperture, which means in straight through or reducer mode, many will work at reduced aperture. With the high power arm in, the amount of apeture reduction will either be reduced, or in some cases, perhaps even eliminated.

The key to doing this in these scopes is the Baader approach where the binoviewer attaches directly to the top of the diagonal and keeping the connection to the telescope as short as possible.

If someone wanted to use the new Denkmeier in an SCT and keep the system working at full apeture, they would need to go to a Baader type diagonal with a short connector (Like the Baader SCT connector which only adds 15mm of back focus) and the Prism, which only adds I think 38mm of back focus. The Mark V binos add about 120mm of back focus, so this system will have a total requirement of about 173mm.

But in my Denkmeier, the power-switch alone added 18mm of back focus, so right there, if you go with a standard 2" SCT adapter and a standard lenght 2" nose, even if you used a Baader type diagonal, you are well over 200mm, which is the point that many SCT models will start to loose aperture.

Here are the numbers....

The Baader SCT adapter requires 15mm of back focus.

The Baader Prism requires about 38mm of back focus.

the Power Switch requires about 18mm of back focus (unless they have made it thinner).

A large prism bino will require about 120mm of back focus (this is what the Mark v requries).

Add this up and you get 191mm. If someone wants to use a powerswitch and avoid aperture loss with a moving mirror scope even in straight though mode, this will be the best configuration to use.

A 2" diagonal alone will put you well over 200mm, even if it is the Baader 2" diagonal.