Too much cloud, too much thinking time,

[Ptarmigan]

It is cloudy again, and I have been thinking, again ! :-

The trouble with 'big' newtonians is that the eyepiece is too far off the ground unless you use fast F/ratios, which are hard on eypiece design (money) and on the grinding/figuring of the primary mirror (more money!).
The trouble with cassegrains ( that bring the eyepiece to a reasonable position ) is that they have long focal lengths and very slow f/ratios

So what is my error in thinking of achieving a reasonable eyepiece height in a semi-newtonian by employing an intermediate "strength" cassegrain type secondary which is not of extreme length, ie only brings the prime focus partway down the tube not all the way down to and through the prime mirror.
Then intercept that with a newtonian type elliptical secondary (I mean terceiry, 'cept I cant speel it !) to direct the focus out of the side of the tube about 1/3 or 1/2 way down ?

I originally thought of two flats, one circular at the usual secondary position but pointing down the tube followed by the newtonian elliptical flat to send it all out sideways. But I think that is a no-go because the first flat would need to be too large with too much obscuration of the primary, hence thinking of a "mild" cassegrain type circular secondary mirror (money over a newtonian but not as much as or maybe the same as a cassegrain secondary ).

It is not done, so I must be going wrong somewhere ??
 

[Ptarmigan]

Ps.

Maybe not as much as 1/3 or 1/2 of the tube length,

just enough to bring the eypiece down to a reasonable height.

[Moonshane]

can you not just decide on the focal length (i.e. height of the eyepiece) you want and then create a scope of the aperture you want/can afford? focal ratio then 'happens' naturally.

in my experience a newt with a focal length between 1200 and 1600 is a good compromise as the eyepiece (in a dob set-up) is at the right height for a standard chair (1200)  or for standing (1600). personally I made an observing stool that adjusts from about 8" off the ground to about 38" off the ground which covers pretty much all scopes. furthermore, cheap eyepieces such as Televue plossls provide

perfectly adequate views in fast scopes even without a coma corrector - better with but still decent without.

I do agree that too much cloud makes you think too much

[Ptarmigan]

can you not just decide on the focal length (i.e. height of the eyepiece) you want and then create a scope of the aperture you want/can afford? focal ratio then 'happens' naturally.

,,

I do agree that too much cloud makes you think too much

Thanks for your thoughts, I see what you mean but I was kinda thinking from the other direction , like :

 The bigest aperture, not the max focal length so,

I can afford a big aperture at f4 but I can afford a bigger aperture at f5 and even bigger at f6

but I am just a "low down bum" in the words of the wild west, at 5'6 and a bit of following wind, so a f6 big newt needs to be brought down a tad.

I can also afford a catadioptric but (see previous)

so why not use '1/2 a cat' to bring the newt eyepiece down to a managable altitude like I outlined, there must be a reason for not doing it ? not because I want to choose a focal length to suit a chair iyswim ?

and too much thinking leads to too much drinking hic. 3rd rum&coke, which leads to too much cloudy thought lol!

[Moonshane]

bear in mind that once you get beyond a certain focal length, you run out of eyepiece choices and your available field of view reduces.

in answer to your query about bringing eyepieces down there are some designs that do this e.g. http://www.cloudynights.com/ubbarchive/showflat.php/Cat/0/Number/319907/page/4/view/expanded/sb/9/o/all/fpart/

or http://www.reinervogel.net/index_e.html?/lowrider/lowrider_e.html

anything is possible with enough effort but there are always compromises.

personally I stick with f4 for bigger scopes (I have a 16" and a 12").

[Ptarmigan]

 schiefspiegler  ! ! gosh bad enough trying to speel it without trying to figure it ! I can guess what thingy of this parish would say if I wanted a set made, need to see my contract with the devil first ! But an interesting link, thankyou.

Your second link is interesting as well, I had thought of that (see, I said too much thinking time !) but I see two problems, using a flat the 'throw' is limited and a shallow incline (for distance) means the head needs to be narrow and one-eyed ! Oh, and thirdly the diameter of the interception (oscuration) gets bigger the longer the desired 'throw', whereas with the cass. the throw is adjusted by its figure. (bit like like inserting a barlow )
Oh and 4thly, it was easier to describe (without a diagram) my orig. thought ;-)

Still cant see why it cant be done,
think of it as a bit like a (mild, or intermediate) standard C11 or LX200 but with a diagonal throwing the focus out sideways instead of letting things go down to the primary with the diagonal down there , simples ? ?
 

[Ptarmigan]

and it is still cloudy,

thanks for the conversation,

nitenight !

[DaveS]

What you're after is a Naysmyth (Sp?) design, very common among the huge 'scopes on mountain tops. Big enough to have a manned platform on the mount at the Naysmyth focus.

The main problem I see for visual is the orientation of the field. An auto field rotator might be an idea.

[Beulah]

 schiefspiegler  ! !

Bless you.

[Ptarmigan]

Bless you.

Chortle !

Chuckle !

Clutches sides,

nice one

DaveS > thanks, nice find, almost what I was on about,

I'll be back, cant stay just now ,,,,

[swamp thing]

I'm no expert but I should think multi mirror systems could be a bit of a headache to collimate.

[rowan46]

Is this not what the original bird jones tried to tackle keeping the scope length down and the apparture up by putting a barlow in the optical train. I have never seen a good one but they must be theoretically possible

[Laurie61]

There is a design called the coudé focus which uses three mirrors to put the focus position on the side of the tube. As Steve says it would be more difficult to collimate. It looks like a modified sct, as the third mirror would need a spider support  arrangement you would also get diffraction spikes.        

[Ptarmigan]

Thanks everyone, very interesting cloudy-googling-reading

Looks like the Nasmyth, and the Coudé variation, are mainly intended to bring the focus out of one or both axis so usually lengthen the Fratio to F12ish or much longer.
My idea is that the Ptarmigan focus (hohoho, du'like the modesty ! ) should be not too far from the Newtonian focus such that it only needs a small stretch of the Fratio to bring the viewing position (of a modest Fratio primary) down to a reasonable height

There is actually one less component compared to a Cas. that would need to be collimated - no corrector plate! Plus1point !!
The diagonal is already used in a Cas. but of course it is in a mechanically stable/robust place being one solid lump integral with the inlet and eyepiece tubes and their axis, so not as much of a problem as the extra spider in my dream minus1point
Shouldnt be unsurmountable to the keen amateur tho'

If I am understanding the "bird jones" correctly that involves a double pass  through the corrector group of lenses with resulting chromatic considerations.

Now that bigbig parabolic mirrors are available/affordable it seems that the the difficulty of going below about f3.5 is a bit limiting to further size increase above 24"

[AndyWB]

Couldn't the third mirror be supported from underneath, rather than another spider? I'm pretty sure this is what I saw in a youtube vid about UT4 at the European Southern Observatory. 

My understanding is that it sits in the shadow of the secondary, kinda like a Cassegrain has a hole in the primary. 'Course now you'd have to engineer a hole in the primary. 

Just a thought - you could have multiple focussers and eyepieces, and swap between them by rotating the tertiary mirror... but that's getting silly.

[Ptarmigan]

Thanks for the interesting vid.
Yes, everything is within the shadow of the secondary and its spider.

So, either from underneath as in the vid., or the extra spider could be in the shadow of the first spider,
or on legs off the first spider.

hole > Or superglue the tube to the primary !

Actually not a bad idea rotating the tertiary, I sortof did that with the little 6" Newt I built years ago. I mounted the secondary with its adjusters on a (axial) rod. The rod passed through a hole in the central boss of the spider (that boss was smaller in dia than the secondary) The whole secondary gubbins was held in the spider by a set-screw. This meant that the rod with secondary attached could be slid up/down and twiddled round the axis of the scope so it could bring the newtonian focus out to a photographic port or to 2 visual ports 180deg apart on each side of the tube.
It was a proof of concept thing, in operation it was a bit fiddly and risked droping bits on the primary !