In future all large dobs will use this design

[overlord]

http://upload.wikimedia.org/wikipedia/commons/c/cd/Nasmyth-Telescope.svg

http://en.wikipedia.org/wiki/Reflecting_telescope

This is to avoid the ladder. Instead another mirror can simply be attached. Obsession telescopes use mirrors of I think 98% reflectivity + so there is no need any longer to use the antiquated Newtonian design. :thumbsup:

[overlord]

This will increase the focal length by perhaps 50% if the eyepiece is placed at standing height on say a 25" f/5 dob. They should put ME in charge of this stuff and no-one need use a ladder in the dark again, LOL!

[Astroman]

JMI use this design in their large scopes.

Now wouldn't that draw a crowd...

http://www.jimsmobile.com/images/ntt30.jpg

[JethroB76]

SDM were going to offer a folded newt design weren't they?

[renormalised]

Folded focal length telescopes....they've been around ever since the Rolling Stones were playing at the Circus Maximus for halftime entertainment!!!:):P

They don't make so many of them because they're more expensive and time consuming to design and build, even though they're a great idea.

[mithrandir]

Planewave too - CDK700 (http://www.planewave.com/index.php?page=1&id0=6&id=0)

The design is called Nasmyth or Nasmyth-Cassegrain.

For a dob you'd still have to worry about where the focuser would be. If it was OK near the zenith it would be too close to the ground for targets near the horizon. You'd have to move the altitude bearing a long way up to get the beam exiting through the bearing, and that would need a lot of weight added to the top to balance the scope.

[koputai]

Now THAT is a cool telescope. If I win the lottery, I'm buying one of those!

Cheers,
Jason.

[renormalised]

You and me both!!!:):)

[Waxing_Gibbous]

A 42% C.O.? The view will be dim as dishwater! :)

[xstream]

I don't believe Peter offers one but he will build to order. :)

http://www.sdmtelescopes.com.au/SDM008f.html

[koputai]

Well, no they won't.

The thing is, people with big Dobs want fast f ratio's and relatively
rich fields, which you can't get with the Nasmyth design.

Cheers,
Jason.

[Allan_L]

Its just like a big SCT isn't it ?
Without the corrector plate.

[Satchmo]

Market forces and fast dissemination of knowledge via the internet will push portable Dobs like 24" down to F3- the coma corrector is already there- so in the future ladders will be a thing of the past without resorting to long focus Cassegrain designs for eyepiece accessibility.

[renormalised]

The CDK700 is f6.6, which is fairly fast and wide...but not a f4 or f3.5 admittedly. That would be pushing things with a Nasmyth unless you made the primary optical path ridiculously short.

[overlord]

F/3 is very hard to make. Will always be pricy cos no machine can do it. it must be done by hand and will take years to get it perfect. :eyepop:

[rmcconachy]

I think Mark (Satchmo) has a pretty good handle on how difficult it is to make large, fast mirrors. The best really fast 20" mirror I've ever viewed with was one of his (f/3.7 from memory). The folded design is fine for imaging where secondary obstruction is not as critical but otherwise you have to pick your poison, a large obstruction or a very long focal length yielding a tiny field of view.

[Satchmo]

A 24" F3 with low profile focuser for visual use could use a 6" secondary giving 25% obstruction which is quite 'benign' compared to the common SCT's which have an obstruction at the secondary light baffle of around 40%.

While F3's are not really a viable commercial item at present I believe they will be in the future :)

[SkyViking]

Apart from being a lot more complex and expensive to manufacture, Nasmyth's are also much more difficult to collimate. Not exactly the thing you want with a portable scope.
However, most large observatory scopes (VLT etc.) are Nasmyth's. It works great in such a setting.

For a dob, what would you gain apart from a more conveniently located eyepiece?

[torana68]

via ion figuring?

[Satchmo]

When more people order them and in time, through repetition they will be produced more quickly and efficiently as new techniques are perfected and the price will come down. Normal pitch polishing techniques are perfectly adequate for this size of mirror.