4 replies to this topic

[Mlad]

Hello everyone,
I am considering extending the truss in my Explore Scientific 12" Dob Gen. II telescope. I use the shroud along the entire length until the end of the secondary mirror cage.
I saw photos and entries on CN confirming that some of you use the extension and some do not.

What is the reason for the different approach to this topic?

 

Regards,
Przemek

[Phil Perry]

Could you clarify what you mean by "extend the truss"? I can't imagine that you're adding on to (lengthening) the trusses, as that would mess up the optic path (mirror is now too curved to hit the secondary). Do you mean you want to add an extension to the secondary mirror cage, to act as a dew shield and block stray light from nearby streetlamps, etc.? That sounds like a reasonable thing to do, so long as you don't make the assembly too top-heavy.

[Mlad]

Could you clarify what you mean by "extend the truss"? I can't imagine that you're adding on to (lengthening) the trusses, as that would mess up the optic path (mirror is now too curved to hit the secondary). Do you mean you want to add an extension to the secondary mirror cage, to act as a dew shield and block stray light from nearby streetlamps, etc.? That sounds like a reasonable thing to do, so long as you don't make the assembly too top-heavy.

This is about extending the tube (truss) to minimize reflections and reduce the risk of fogging the secondary mirror.

As you can see, I know why it is done, hence the question, because not everyone does it. I don't think I've seen such a solution in larger Newtonians on a Dobsonian yet.

 

For example, I don't remember his telescopes being equipped with such extensions in the photos published by Jon Isaacs.

 

Regards,
Przemek

[havasman]

In some locations stray light can be effectively blocked from the light path using various shields including extensions to the tube. Likewise mild dew conditions can sometimes be fought by those same sorts of tools. Whether they are worth implementing can depend on the observing conditions at your site and whether such passive measures can be effective against problems there. At my primary observing site stray light is not much of a problem and dew conditions are powerful and intrusive. Passive measures are not effective. So nobody uses tube extensions on their Dobs but many employ active, powered dew control systems.

 

Jon's desert observing site has the double advantages of being dark and free of dew on most nights. So for him neither use for a shield makes sense.

Edited by havasman, 11 November 2024 - 12:46 PM.

[Phil Perry]

This is about extending the tube (truss) to minimize reflections and reduce the risk of fogging the secondary mirror.

OK, technically you're extending the tube, not the truss. The truss structure ends at the start of the secondary mirror cage/focuser. Under the circumstances I listed, a tube extension for dew and stray light (if needed) makes sense. Just don't make it too top-heavy, or so long that it introduces vignetting.