Why aren't all reflectors 'off axis'?

[overlord (Charles)]

Hershcell improved on this Newtonian stuff with his off-axis scopes but we have gone back to the olden days of 400 years ago.


How many decades before someone makes off-axis dobs as the main scope?!

[DavidU (Dave)]

The mirror is the problem. To work well the primary of an off axis scope must be also an off set parabola.
To make 4 six inch offset scopes you would get a 13" quallity mirror and cut 4 circular mirrors from this one blank.
If you could see one of these mirrors side on it would be 1/2 of a parabola with the thinnest edge (toward the centre of the parent 13" mirror)would be the side where the secondary mirror would be.
This complexity and cost are prohibitive.
Add to this collimating a scope like this is also a difficult procedure.
You could imagine a 10" offset Dob having to use a 22" parent mirror.

[overlord (Charles)]

Originally Posted by DavidU

The mirror is the problem. To work well the primary of an off axis scope must be also an off set parabola.
To make 4 six inch offset scopes you would get a 13" quallity mirror and cut 4 circular mirrors from this one blank.
If you could see one of these mirrors side on it would be 1/2 of a parabola with the thinnest edge (toward the centre of the parent 13" mirror)would be the side where the secondary mirror would be.
This complexity and cost are prohibitive.
Add to this collimating a scope like this is also a difficult procedure.
You could imagine a 10" offset Dob having to use a 22" parent mirror.

Thank you! That sounds pretty right.

I would buy a 12 inch off axis f/10 with spherical mirror any time!!!!!!!

Get to work skywatcher!!!!!!!!!!

Why can't i find a nice big planetary dob, whyyyyyyyyyyyyyyyyy!!!!!!!!!!!!!

[renormalised (Carl)]

Originally Posted by overlord

Thank you! That sounds pretty right.

I would buy a 12 inch off axis f/10 with spherical mirror any time!!!!!!!

Get to work skywatcher!!!!!!!!!!

Why can't i find a nice big planetary dob, whyyyyyyyyyyyyyyyyy!!!!!!!!!!!!!

You can find them, but you're going to have to pay for one. SDM make large dobs (of various f ratios). Some of the members here have them and will swear by them. They're good scopes. However, if you really want a light bucket, that will take time to build. How big a scope do you want??

[Waxing_Gibbous (Peter)]

I'm pretty sure that if you pay for it, someone will make it.

[M54 (Molly)]

Excuse my ignorance, but is it impossible to grind a mirror off axis, even with some kind of grinding machinery?

[renormalised (Carl)]

Originally Posted by Waxing_Gibbous

I'm pretty sure that if you pay for it, someone will make it.

That's true, Peter, but it's a case of reinventing the wheel making an off axis mirror. Just complicates matters trying to align the optical path for the scope. Dave hit the nail on the head.

[renormalised (Carl)]

Originally Posted by M54

Excuse my ignorance, but is it impossible to grind a mirror off axis, even with some kind of grinding machinery?

Yes, you could do that, but it's much easier to grind a large primary "template" and cut the off axis mirrors out of it.

[Satchmo]

Originally Posted by overlord

I would buy a 12 inch off axis f/10 with spherical mirror any time!!!!!!!

Get to work skywatcher!!!!!!!!!!

Why can't i find a nice big planetary dob, whyyyyyyyyyyyyyyyyy!!!!!!!!!!!!!

Mass production optics companies like Skywatcher would have to learn to make smooth surfaces with no macro-ripple before anybody would take marketing of a dedicated planetary scope seriously. Mass production and smooth surfaces just don't go together I'm afraid.

All you need for a good planetary Newt is an obstruction of not more than 20% and a good primary from a custom maker ( there are plenty of them to choose from ) with consideration of thermal issues in your tube . Thats not so hard.

[mswhin63 (Malcolm)]

Mass production optics are put to the lowest bidder. Skywatcher is made for the general astronomer not the more serious astronomer. Middle of the road stuff. Skywatcher is great for people like Myself and others that try to push the boundaries of a lower class scope.
SDM and TAK and minority others are designed for more serious observers or AP'ers.
SEBEN (Sperical Mirrors) on the other hand are for newbies that most will buy and then give up, not fancy but cheap.