Performance of large truss-tube dobs in a breeze?

[lacad01 (Adam)]

At this point of time I'm only dreamin' , but have caught the bug to one day own a moderately sized SDM or Obsession in the order of 15" to 18" mirror
Just wondering how these beasties handle in a breeze...does the light shroud act as a sail? I guess some form of wind-break could circumvent this but interested in experience from large light-bucket owners,
cheers

[wavelandscott (Scott)]

While I would not call my 12.5 inch truss scope large, I think I can give an answer to your question.

I have not had a problem with wind and my scope, in a breeze I was willing to observe in. I mean to say, if it was blowing enough to negatively impact my scope, I did not find it enjoyable to observe.

As a point on this issue, a "smooth" dobsonian mounted telescope has a balance between ease of movement with some pressure and friction. It does not spin freely like a lazy susan. It shouldn't move by itself but should use a gentle hand pressure to move. A light breeze should not be sufficient to make it move.

[lacad01 (Adam)]

Thanks for the reply Scott. What sort of prompted my question was after seeing a promotional video of the Obsession dobs, the large 18" and up scopes look really, really smooth in their movement and part of their promotion was that they can be moved around in alt/az with one finger
Granted, I don't think I'd be out in a strong wind but thought even a slight breeze would be enough to catch on the shroud and make observing frustrating.
cheers

[AstroJunk (Jonathan)]

No telescope likes a breeze, but you shouldn't leave a big dob unattended if it is gusty. Any manual dob should have a hook on the front to hang counterweights from it when using light eyepieces (Dobs are normally balanced a little tail heavy). I used to tether my beast down when not using it in poor conditions.

When in use, it isn't such a big issue - but it is an issue.

When you go to a Servocat Driven dob, you can lock the alt clutch at about 30 degrees and leave the dec clutch free - that way it acts like a weather vane! Stand well back when the wind changes direction

[wavelandscott (Scott)]

Originally Posted by lacad01

Thanks for the reply Scott. What sort of prompted my question was after seeing a promotional video of the Obsession dobs, the large 18" and up scopes look really, really smooth in their movement and part of their promotion was that they can be moved around in alt/az with one finger
Granted, I don't think I'd be out in a strong wind but thought even a slight breeze would be enough to catch on the shroud and make observing frustrating.
cheers

I understand but I think the one finger push is bit misleading in this instance...one finger can deliver a lot of force if it is needed.

The Obsessions (and others) are smooth in their movement...I might say "consistant" instead of smooth. You want to avoid the "jerkiness" (my opinion) which is more of an issue to me then force to start motion. I hope that makes sense.

[lacad01 (Adam)]

Originally Posted by AstroJunk

When you go to a Servocat Driven dob, you can lock the alt clutch at about 30 degrees and leave the dec clutch free - that way it acts like a weather vane! Stand well back when the wind changes direction

Nice
Sigh...now all I need to do is save them pennies and get budgetary approval from the minister of domestic affairs

[Paddy (Patrick)]

Definitely go big - you will not regret it!

[Screwdriverone (Chris)]

Hi Adam,

Last time I went to my dark sky club site at Linden, there was a substantial gusting wind running at about 40-60 kmh which picked up folding chairs etc and a couple of times, when the scope (12 inch collapsible Skywatcher) wasnt being held by me, it was pushed around on the azimuth bearing by the wind.

Not much mind you as there is less "sail area" as opposed to a solid tube, but as it was on a purpose built stand (a 4 x 2 box to raise it higher) I made sure I held onto it when the big gusts came in case it blew over.

Its certainly true that any strong winds make observing a nightmare, too much vibration to make it worthwhile, not to mention a pain to keep the object in view.

Obviously the bigger the scope, the heavier it is and therefore the more stable in medium to high gust winds, but generally, windy conditions dont make things enjoyable.

Cheers

Chris

[jayscheuerle]

Of course shrouds act as sails. They ARE sails! And just like sails, if you want to reduce the effect of the wind on them, just lower them. - j

[tnott]

If it get's windy you can just remove the shroud. If you are at a dark site it won't make a difference to the view anyway. Secondary may dew up slightly quicker.