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Skywatcher 18" and 20" Truss Dobsonians

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Beulah

Beulah

Posted
Posted

The 400p is a bit of a veritable monster .....

A strapping six foot lad would not need steps but anyone vertically challenged would.

I would also echo the sentiment that they have to be as portable as possible. The 400p weighs around 8 1/2 stone!!

After seeing the CN link in the post above (thanks Mike :)), I am pleased they have gone with the complete truss tube design. Be still my beating heart.........

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crashtestdummy

crashtestdummy

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It's not the size and total weight of the 400p that is the problem as such as my 16" lightbridge is a similar weight and size,the problem is that the tube assembly is one piece so is almost impossible for one person to handle and the eyepiece isn't positioned very well.the 18" prototype is a full truss assembly so will pack down smaller and be more manageable for one person.

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Rustysplit

Rustysplit

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TBH, I would rather have to carry a few spare kg discs weights in the boot of the car and have that weight shaved off the mirror box assembly in a lightweight design. If you are needing to lift in and out of car boots, it can make all the difference. So long as the end movements are ok and the views are good.

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Dude_with_the_tube

Dude_with_the_tube

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interesting.as soon as this scope doesnt have the same issues what Hubble optics UL has with scope being affected by any wind burp  and constant collimation issues ,balancing and the price tag is reasonable,i can see this as being a good aperture upgrade scope.and so far it seems that one thing:balancing is taken care of by having the adjustable balance system.So far so good and looks promising.

Hope Skywatcher is taking everything in consideration,so good luck and lets see what happens.Keep in mind that Explore Scientific is also working on a similar project for big aperture dobsonian so competition isnt sleeping either :)

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Rustysplit

Rustysplit

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 l always think that the single ring upper cage with the secondary sticking up on the spider vanes looks like a recipe for disaster when packing away. But as you say, a very good way to lose weight at the top end. I would like to see a compromise between weight and security of secondary. These are built to be shipped about to dark sites after all, so robustness can't be ignored.

I think you can to a certain extent Mike, but there needs to be a degree of balance first. If the bearing diameter gets too large, storing them as part of the main package becomes a problem. Which is where the Obsession UL in the link is quite clever with the folding bearing.

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AlexB67

AlexB67

Posted
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this is very interesting i wouldn't mind forking out for a 18"

Me too .. Long way in the future :D . They do look nice and portable, but the whole package will be costly investment when considering eyepieces too. At marginally over f4 collimation will be critical, the scope needs to be very stable and not so sensitive to temperature changes and mechanical stresses I suppose. At these sort of dimensions collimation tolerance is tiny, so I assume SW thought about this in designing it, and the weight and thermal properties of the mirror as well I guess. 

May as well start buying televue everything now :0), since at that focal ratio no other eyepieces claim they can hold up well enough, most of them being sort of around the f/5 range for good edge the edge performance, and a CC will be a must I would think. Wonder what sort of stock eyepieces they'll ship with it, because the typical eyepieces they ship would not do it much justice, perhaps selling without eyepieces would be an option, since anyone investing at this level and size will already have something decent in many cases.

Darn, this hobby can get expensive at this level :0)

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John

John

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..... Wonder what sort of stock eyepieces they'll ship with it, because the typical eyepieces they ship would not do it much justice, perhaps selling without eyepieces would be an option, since anyone investing at this level and size will already have something decent in many cases.

I think not including eyepieces would be the best route and would maintain credibility. 

I agree with you on the need for precise and stable engineering to hold the optics at these fast focal ratios. I'm sure Skywatcher can do it if they put their mind to it though. It would be nice to think their reps peruse forums like this and hoover up the ideas and viewpoints that are posted - it's free and experienced opinion direct from their potential marketplace after all :smiley:

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astronymonkey

astronymonkey

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I think this is an interesting move and the CN pictures shed some light on the SW plans but Id like to see some consideration of the materials and design used to make this a real option in the big dob market. 

The use of counter weights for instance is practical but also a pretty lazy design feature as a well calculated asymetric bearing, allowing the mirror box assembly to move backward as the scope is moved to the horizon, would negate the need for a lot of the weights and is a simple and neat solution......didn't Moonshane have this on his OO16 ?

They also mention on the CN site that the cell is all metal, well my LB16 has a all metal cell but being steel its heavy and an aluminium cell would be lighter and although it would be dearer it would be a small percentage increase on the overall price.

Im not a big fan of fully open rocker boxes as they put the mirror at too much risk of damage, dew and stray light and such lightweight secondary/upper frame structures will have the same effect. 

I think Alex has a point about which eyepieces this will ship with, I think the point about shipping with none is a sensible one, shipping with a decent laser collimator and RACI finder would be better than a poor eyepiece. 

So an interesting idea but Im still wondering whether or not something like a Dave Leukhurst Dob, with a mirror by Nichol Optical would be a better proposition. These artn made to a specific price point and David has perfected his design over the years to something that he knows will work.

Heres David Leukhurst with his type 2 dob with a 22inch mirror, ok £7k isnt cheap but once you get to this size I doubt a mass produced scope will come close and a f3.6mirror like this one is going to be a devil to make on a production line !

22-inch%20f3.6.jpg

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mert

mert

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Me too mert that's why I went for F3.9 just perfect on a 18"

I ended up buying a good set of steps, my scope is f/4.8, once I've got the Argos Navis & ServoCat setup it's not too bad, I wouldn't swap the views for anything. Big dobs are excellent :D

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swamp thing

swamp thing

Posted
Posted

I understand others points on counterweighting. I see no harm in home builds requiring counterweights. The key words are home builds. This is not a home build. This is a scope designed by a team of design engineers for a large company that builds scopes, not an amateur working out of his shed. I'm not gonna go so easy on them.

My 16" LB required counterweights = bad design.

My 10" home built Dob needs counterweights = my own bad design.

My 20" doesn't. I got this right on my second scope build, Skywatcher have made a few more than me.

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Dude_with_the_tube

Dude_with_the_tube

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skywatcher got it right on the 14" flex tube. scope is perfectly balanced and does not require any balances.I tested my one by fully racking out focuser,adding 2" extension to it,a 2" barlow followed by a 20mm ES in 100 deg and it was still perfect .that keeping in mind that original focuser was replaced with moonlite who has aditional raiser plates mounted on the tube so it is heavier as the original.

My 10" did require counterweights.

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