Meade 16" mirror on Ebay

[avandonk]

Originally Posted by 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.

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 (Kevin)]

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 (Kevin)]

Originally Posted by 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".

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 (Kevin)]

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 (Kevin)]

No worries. Whenever time and wife permits.