Meade 16" mirror on Ebay

[avandonk]

Came across this by accident
http://cgi.ebay.com/MEADE-16-Spherical-Mirror-f1-2-w-secondary-mount_W0QQitemZ7546152906QQcategory Z28181QQssPageNameZWDVWQQrdZ1QQcmdZ ViewItem
looks very interesting?!

[asimov]

You'd need that massive central obstruction like you'd need a hole in the head..

[davidpretorius]

looks like something my dear old mum used to cook fondues with!

[asimov]

That's right Dave!? I thought it looked familiar! Your family dutch!??

[davidpretorius]

dad's from south africa, so sort off

[rmcpb]

With that focal length imagine the eyepieces you would need!!

[cometcatcher]

"Its diameter is 16" which as you probably know is a 35" radius." Um... :confuse3:

[Exfso]

I always thought diameter was 2xRadius. IE radius is 8". Neverthelesss it is still one ugly looking setup

:shrug:

[cometcatcher]

Hehheh, me too.

Maybe some bright spark can make a cassegrain of some sort out of it.

[avandonk]

I think he is refering to radius of curvature of the mirror ??

[cometcatcher]

That's what I thought at first, but the radius of curvature of a 16 inch f1.2 mirror is 38.4 inches, not 35. But it doesn't matter, I'm not buying it. :)

[westsky]

This would make an awsome Wright/Schmidt camera.

[avandonk]

That was my initial thought.

If you put a ccd or similar at prime focus.say even a toucam.The field of view would be 31'x23' with a focal length of 444mm at F 1.2!
As you increased sensor size the field curvature of the prime image could be a problem.A meniscus lens would flatten the field.

The other problem would be getting an accurate focus and centering of the sensor.But the real time image would make it straightforward.

[westsky]

You can get around the field curvature problem ( or at least lessen it) by using an aperture mask, if you masked it down to say 14" or maybe 13" bring the F ratio up to say f2 or f3 ,much less curvature.
I am currently building a 8" Wright/Schmidt using a F3.75 mirror, match this with a large format camera and you get one hell of a lens :-)) super fast with a large field of view.
You shouldn't have a problem getting focus, just allow the for the height of the focuser in your calculations.
I have no problem getting focus with a 8" F3 Wright/Schmidt, although I haven't tried with any form of CCD on this scope, Hmm! something to try next time the scope is out maybe
David.

[avandonk]

With a mirror 16" in diameter I had a mental picture of my 20D at prime focus.Hows that for a central obstruction?Maybe a 300D is better,it is smaller.Then as you said you could introduce an aperture at 2F to limit spherical abberation and field curvature.It was purely a gedanken experiment,but one worth trying if you could obtain a mirror this big relatively cheaply.
I only threw this in to see if anybody had similar evil thoughts.

[westsky]

I think if you can get this mirror cheap enough it would be well worth the effort to construct a superfast large lens, I have a friend who has a 18' F2.8 scope, he uses the modified 350d the FOV is huge and his images are nothing short of spectacular.
I have seen a few of these mirrors on Ebay over the last 12mths and they all seem to sell for around $800.00 US, abit too high a price for me, I will stick to finishing the 8"
it only cost me about $75.00 AU for the mirror.

David.

[avandonk]

I just did a quick calculation and found the central obstruction of a 20D
on a 16" mirror is about 15%!Assume the camera is behind a 160mm dia circular disc.It would be even less behind an ellipse.US $800 mmmm....

[westsky]

I think we have our telescope design's crossed :-(
The wright/schmidt use's a normal secondary the same as any newt, the focuser is in the same place as a newt at the focal point the only difference is the tube is twice the length,so the central obstruction is the same as a normal newt around the 15 to 17% with the 16" mirror you would still need a secondary around the 3" mark.
using a mask the effective focal length changes from F1.2 to F2 or more depending on the size of mask.
I think this would be a much simpler design to build.
Your'e thinking of this type the C18 (which happens to be my friends business.)
http://www.astroworks.com/

cheers
David.

[avandonk]

You are correct.I was only thinking of a quick and dirty experiment.I have a Canon 300mm F2.8L and this keeps me busy enough.But we are all after more aperture and lower focal ratios.that could explain the (dare I say it) dobs.

That is a superb design at a correspondingly superb price.Unfortunately that is the first rule of the universe.You dont get anything for nothing.

Bert

[westsky]

If we all had the money we all could own a C18 but alas it's not to be, so I will make do with my cheaper scope.
For those that don't know the Wright/Schmidt design heres a piccy, all self explanatery I think.
mine will be slighty longer the mirror is exactly F3.75

David.

[avandonk]

The fact that we are actually doing something, rather than being spectators says it all.Just because the Hubble takes better pictures than we could ever hope to,doesn't mean we should stop.Our hobby is more about self improvement and education than 'winning'.I look forward to your images.
Thanks for the info.

bert
As the mirror gets bigger its more efficient to place the sensor at the prime focus as the secondary mirror would be larger than the camera.

[westsky]

Totally agree with you Bert, if we all could take hubble quality images then the hobby would get boring very quickly. I found that part of the enjoyment is the apprentiship involved,it's taken me a long time to start getting respectible images on film,
these days it has become easier with the digital revolution and I guess someday I will have to bite the bullet and join in , in the meantime whilst I save for a decent digital camera I will still enjoy doing things the old fashioned way :-))

David.

[cometcatcher]

If stopping down an 8 inch f3 is the same as using a 5 inch f4.8, why not just use a 5 inch f4.8? What's the advantage to using a stopped down 8 inch f3? Won't there still be abberations at effective f4.8 using a spherical mirror?

[westsky]

Hi Kevin , thats what I thought at first then it was pointed out to me that although the effective F ratio changes the mirror doesn't it's still a 8" F3 and this is what makes the difference, it would not be the same as a 5" F4.8 the FOV would be smaller in the 5".
I will see if I can find an artical I have on the Lenless schmidt design it explains it so much better than I can.
David.

[westsky]

For those that are interested here is a PDF of how to make a Lensless Schmidt or Wright/Schmidt telescope.
it does tell you a bit about why you use a 8"F3 mirror and why it needs to be masked down.
http://members.iinet.net.au/~westsky/schmidt.pdf
It is about a 1mb download

David

[cometcatcher]

David, since both the 8" f3 and 5" f4.8 have the same focal length of 24 inches I think you will find the field of view to be identical in both scopes.

Thanks for the article, I had a good read. The advantage of the lensless scope is primarily reduced coma, and perhaps ease of finishing the spherical mirror since no parabolising needs to be done.

The main disadvantage to my thinking would be the rather large physical size for a scope of effectively only 5 inches. And you still have to hog out a fair bit of glass with an 8 inch mirror to go as deep as f3. But as long as your having fun that's the main thing.

[westsky]

Hi Kevin, you had me straching my head there for a few moments , you are correct the FOV is the same in both scopes in the artical, where I went wrong was with the calculation of the scope I am building, mine is only masked down to 6" and I used these figures comparing them to the artical's figures, I was never any good at math anyway :-))
Where this design really shines other than what you have already mentioned about ease of building, is the fast lens you get for imaging.
The artical mentions imaging in 15mins this was 10 years ago, with the better films today the time is cut to about 6 or 7 mins even much less with CCD or digital cameras.
This makes for easy guiding especialy if you are handguiding, I know whats it's like to sit there for an hour and manually guide, no fun at all :-))
The long tube is a pain to use but the final result is definatly worth it.
The production model Celestrons of the 80's ( not exactly the same scope but similar) still fetch high price's in the US, around $1500.00 or more and are in high demand this was the Schmidt Camera with a corrective lens.
I think the main problem with them at the time was that people didn't want to mess around with single pieces of film the Wright /Schmidt design did away with this and there are still lots of photographers using them.

cheers
David.

[cometcatcher]

There is another advantage. The long tube will give excellent shielding from stray light, something many modern reflectors ignore when they put the spider/secondary right at the end of the tube. I always use to build my tubes for conventional Newtonians longer than necessary to help in that repsect. And of course that aperture stop at the end helps prevent light hitting the inside walls as well.

I'd like to see your pictures through your scope when it's finished!

[westsky]

Hi Kevin , could be a while till the scope is ready, I have all the parts just have to make the tube.Which could take a while as I have other projects to finish first these are not astro things just the unimportant stuff the wife wants doing around the house :-))))
I have used a friends 8" schmidt and will take a look for some of those shots we did.

cheers
David.

[cometcatcher]

No worries. Whenever time and wife permits. :)