16" Lightbridge

[Andrew]

This may already have been discussed, but I couldn't find a recent post on this.........

Looks like the 16" lightbridge may soon appear. A thread on CN includes links to it in the Meade catalogue plus pictures:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/1147531/Main/1142742

Also, Telescope House are listing it (price TBC).

Anyone here know any more?

[GazOC]

The mirrors are GSO and the price will be around £1400-£1500.

[FLO]

Aunty FLO has them on backorder (I am not expecting them to arrive anytime soon).

There is also talk of a 14".....

The 12" Lightbridge has dropped in price by £100 to £599 (Oct Sky&Tel carries a good review).

[Andrew]

Thanks for the info.

I’ve seen the reviews on CN and in S&T, both of which are fairly positive (problems seem to be minor and involve things that can be modified). I’m certainly tempted by the 12”, but I think I’ll hang on until the 16” arrives and see if I can convince my wife that it’s too good a bargain to ignore!

One thing that does concern me little with the lightbridge is the six pole truss design. Several reviews (including Tom Trusock’s CN report) have noted the problem of keeping a light shroud out of the line of the mirror (thus reducing effective aperture) and the need for added stiffening hoops to do this because of the 6 pole design. Does anyone know of a UK supplier of light shrouds that account for this?

It isn’t clear to me from what I’ve read whether this is only a problem if the light shroud sags (as opposed to if is made from some stretchable material so that it can be kept taut). It occurs to me that the 6 pole design means that poles are not placed at the same point around the circumference of the upper and lower OTA components. Consequently, the pole itself must cut across the inside of the tube to some degree. A quick back-of-the-envelope calculation suggests that the pole must extend inside the outer edge of the tube by a fraction of the radius of the OTA that is approximately (1 – cos(pi/6)), plus half the thickness of the pole. Assuming that the poles are approx 20 mm thick and based on a figure of 356 mm that I’ve seen for the OTA diameter of the 12” model that would suggest that the poles might extend up to 8 mm inside the edge of the mirror! Of course, I’ve had to guess some of these figures. However, interestingly the S&T review gives a figure of 9.75” for the effective aperture of the 10” lightbridge. They don’t explain where this figure comes from, but maybe this represents 6+ mm of mirror that are blocked by the truss poles?! Its not much I suppose, but it may be worth keeping an eye out for on the 16” model.

[GazOC]

Very good point Andrew, the difference may not be too much but who wants to buy a 16" scope and stop it down to 15.5"?

I can't belive it's the "sag" in the shroud is responsible, my guess is that at least 8 poles rather than 6 would be needed to avoid the light being cut off.

[FLO]

A quick back-of-the-envelope calculation suggests that the pole must extend inside the outer edge of the tube by a fraction of the radius of the OTA that is approximately (1 – cos(pi/6)), plus half the thickness of the pole. Assuming that the poles are approx 20 mm thick and based on a figure of 356 mm that I’ve seen for the OTA diameter of the 12” model that would suggest that the poles might extend up to 8 mm inside the edge of the mirror! Of course, I’ve had to guess some of these figures.

Blimey Andrew! Not bad for a 'back-of-the-envelope' calculation.

I have used the 10" and 12" and not noticed a problem but at 8mm, I wouldn't really expect to. I will have a close look next time I have one setup.

As for the shroud. That is a good point and isn't something that I had considered. After saying that, it is normal for the poles to fix at an angle (its stronger that way) so I guess it must be the same with other Truss Dobs(?).

[Andrew]

In fact, S&T quote the effective aperture of the 12" as 11.9" which suggests this more of a problem for the 10" than 12". It does make me wonder how the 16" will turn out - it could be it will just be a couple of mm like the 12", or if they really mess up maybe you will lose half an inch

[FLO]

I would imagine the amount/percentage a Dob's truss tubes intrude would decrease as mirror size increases.

[FLO]

It occurs to me that the 6 pole design means that poles are not placed at the same point around the circumference of the upper and lower OTA components. Consequently, the pole itself must cut across the inside of the tube to some degree.

Been thinking about this: The tube is wider than the mirror so the poles probably don't intrude enough to effect the mirror...

Will have a look at the first oppurtunity.

[GazOC]

Thats how they usually do it, the mirror sits in a square box thats a few inches larger than the mirror.

[FLO]

The UK SRP price has been set at £1499.

I still don't expect delivery this side of Xmas