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Collapsable tube?

Started by brobinson82, 11/16/2010 07:44PM
Posted 11/16/2010 07:44PM Opening Post
So, I have 10" primary, f/4.5. I just purchased a Blacklite tube that's 12" in diameter and 48" long. Unfortunately, space is an issue (no pun intended!). It's looking like, once finished, this scope will be stored in a one bedroom apartment... I've seen collapsable scopes online with a sliding truss system between the tube surrounding the secondary mirror and that surrounding the primary. My question is–Does such an opening affect the image quality? Also, is the length to be cut out specifically measured. Obviously, the focal distance would be maintained with the trusses fully extended, but exactly how much each mirror needs to be contained, I have no idea! Any help would be greatly appreciated! Comments on the practicality of such a thing are also welcome!

Thanks,
Brian R.
Posted 11/16/2010 08:07PM #1
If I follow your question, there is no need for a tube at all around a telescope. My twelve and a half inch dob has a ring of half inch plywood at the top, from which hangs a focuser, a spider to hold the secondary, a little bracket to hold the Rigel Quickfinder, and four brackets to anchor the tubes coming up.

I have seen designs where the primary is virtually naked. THe bottom of the tubes attach to a metal frame that connects to the mirror cell and the bearings.

Now, I prefer having at least cloth around the trusses and mirrors to cut down on stray light. (I don't like bare mirrors). And you do need something to hold the top a given distance from the bottom. A tube does both these things well (but then take a lot of room when stored!).

So, as I said, one does not need a tube at all. (ANd tubeless scopes can be easy to store.)


Alex
Posted 11/19/2010 05:25PM | Edited 11/19/2010 05:25PM #2
If this is your first scope, I'd probably keep it simple and go with the solid tube you already have. For a 10" F/4.5 you might even be able to go somewhat shorter. Depending on your mirror cell, spider, and focusser it might be possible to shorten the tube by 5" or 6" (i.e. reduce the length to 42" or 43") if that makes some significant difference to your situation. Although some extra length in front can be useful to keep out stray light and dew.

A collapsable tube, or one with trusses in the middle, needs to be carefully executed to ensure a rigid structure that will hold collimation. For example, you would probably need to reinforce the bakelite tube with plates or rings where the trusses attach to the tube, etc. The size of the opening does not specifically affect the image quality, but there is still a requirement to keep the entire structure very rigid.

You will also need to think about mounting the scope. A round tube can be simply mounted with commercial rings or a saddle. But if you have to mount the scope by the trusses, you will need to make some further reinforcements (more complexity).