I see that Vixen is now selling a replica of Sir Isaac Newton's first reflecting telescope. The specs list the scope as being f/4. Wouldn't Newton have designed this first scope with a MUCH slower f ratio to combat coma inherent at this focal length OR did he actually polish the metal mirror into a parabola? - OR did it not even matter because of the limited field size of the eyepieces of his day. In any event, I'm still suprised he didn't make his mirror slower to make fabrication easier. Does anybody know who came up with the idea of the parabolic mirror - if NOT Newton? ... and for that matter, if anyone out there has one of these new replicas - how do you like it. (A LITTLE too pricey for me though!)- BUT I think it's very cool, Thanks, Dave Sewell
Newtons' First Reflector
Started by Dave Sewell, 08/25/2008 05:16AM
Posted 08/25/2008 05:16AM
Opening Post
Posted 08/25/2008 04:16PM
#1
Wouldn't Newton have designed this first scope with a MUCH slower f ratio to combat coma inherent at this focal length OR did he actually polish the metal mirror into a parabola? - OR did it not even matter because of the limited field size of the eyepieces of his day. In any event, I'm still suprised he didn't make his mirror slower to make fabrication easier.
Dave:
When one goes through engineering school one finds that Newton had his hand in just about everything, some of his contributions are quite surprising. For example most people are aware that he built the first "Newtonian" but few know that to go with that first "Newt" he also designed and patented not only the Paracorr but also the original Type 0 Naglers.
Of course those patents had long run out by the time Al Nagler came along so Al just altered the designs slightly in his Type 1's and of course the rest is history.
Now on to a bit about reality: Apparently Newton's mirrors were spherical so that coma was not the problem, spherical aberration was the issue.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/newton/index.php
Were the mirror parabolic, then coma probably still was not an issue. The coma free region at F/4 is about 1.1mm. With a 280mm Focal length, this is 0.225 degrees. Given the eyepiece designs of the era, I doubt coma would have mattered.
I happen to have a 3 inch Newtonian with a 280mm focal length and a parabolic mirror. Eyepiece astigmatism can be an issue but coma is really not a serious problem...
Jon
Dave:
When one goes through engineering school one finds that Newton had his hand in just about everything, some of his contributions are quite surprising. For example most people are aware that he built the first "Newtonian" but few know that to go with that first "Newt" he also designed and patented not only the Paracorr but also the original Type 0 Naglers.
Of course those patents had long run out by the time Al Nagler came along so Al just altered the designs slightly in his Type 1's and of course the rest is history.
Now on to a bit about reality: Apparently Newton's mirrors were spherical so that coma was not the problem, spherical aberration was the issue.
http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/scopes/newton/index.php
Were the mirror parabolic, then coma probably still was not an issue. The coma free region at F/4 is about 1.1mm. With a 280mm Focal length, this is 0.225 degrees. Given the eyepiece designs of the era, I doubt coma would have mattered.
I happen to have a 3 inch Newtonian with a 280mm focal length and a parabolic mirror. Eyepiece astigmatism can be an issue but coma is really not a serious problem...
Jon
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