I have a very nice f/10 4.25" Newt from days of yore which I have only rarely used (Optical Craftsman). My son has a 4.5" Star Blast.
Compared to my f/6.5 Vixen ED doublet (102mm), the f/10 Newt does pretty well on planets. But it is longer and more difficult to move around.
You get into all kinds of weirdnesses with these small apertures in Newts. Let's say you want:
Excellent wide field performance with a 2" eyepiece
Excellent planetary performance at magnifications up to 35x per inch
You're going to have a problem with the Newt more than the refractor. One deficiency is that you can't illuminate a wide field eyepiece very well. If you have long focal length you're also going to sacrifice field of view. If you enhance the field of view with shorter fl, you get other aberrations and compromise on the planets.
The 4.5" Star Blast armed with a Pan 24 makes a really nice wide field instrument, and I suppose that one could be fashion which is better than my son's (sustaining magnification on the planets better) but I'm not quite sure how.
Six and eight inch scopes, by the way, are no longer really small instruments from the point of view of portability. My friend's 6 dob takes up a good deal of space, as does my FS128. But the Newts tend to be bulkier for any given aperture I suppose because they try to provide one inch around the mirror cells. Maybe a truss arrangement would take care of that.
Anyhow I'm not saying one can't have high end Newts in smaller apertures. What I am saying is that in an era where high end often means things like a Pentax XW40 or a Nagler 31 as well as time viewing Saturn or Jupiter throuogh a 3.5mm eyepiece, there are a host of issues from balance to field illumination on which the refractor has an edge. So why not bite the bullet and get a good refractor? (for the high end clientele)
On the other hand, while the Star Blast does not really come close to the Vixen 102's capabilities, it is such a good deal for the money, and does so well at low powers with the Pan 24, that it is in my personal estimation preferable to a set of binoculars. Your arms don't get tired, you have over three degrees, and there is much to done with a little scope like that. So I am sensitive to the principle. But having overhauled my old optical craftsman I came to appreciate that there are limitations on small mirrors.
Well I've meandered but I think what kills the small aperture Newts is the desire for crisp wide field low X views in two inch eyepieces, combined with the desire to be able to "bump it up" and still have good performance on a planet. Newts seem to have to choose one or the other.
regards Greg N
Compared to my f/6.5 Vixen ED doublet (102mm), the f/10 Newt does pretty well on planets. But it is longer and more difficult to move around.
You get into all kinds of weirdnesses with these small apertures in Newts. Let's say you want:
Excellent wide field performance with a 2" eyepiece
Excellent planetary performance at magnifications up to 35x per inch
You're going to have a problem with the Newt more than the refractor. One deficiency is that you can't illuminate a wide field eyepiece very well. If you have long focal length you're also going to sacrifice field of view. If you enhance the field of view with shorter fl, you get other aberrations and compromise on the planets.
The 4.5" Star Blast armed with a Pan 24 makes a really nice wide field instrument, and I suppose that one could be fashion which is better than my son's (sustaining magnification on the planets better) but I'm not quite sure how.
Six and eight inch scopes, by the way, are no longer really small instruments from the point of view of portability. My friend's 6 dob takes up a good deal of space, as does my FS128. But the Newts tend to be bulkier for any given aperture I suppose because they try to provide one inch around the mirror cells. Maybe a truss arrangement would take care of that.
Anyhow I'm not saying one can't have high end Newts in smaller apertures. What I am saying is that in an era where high end often means things like a Pentax XW40 or a Nagler 31 as well as time viewing Saturn or Jupiter throuogh a 3.5mm eyepiece, there are a host of issues from balance to field illumination on which the refractor has an edge. So why not bite the bullet and get a good refractor? (for the high end clientele)
On the other hand, while the Star Blast does not really come close to the Vixen 102's capabilities, it is such a good deal for the money, and does so well at low powers with the Pan 24, that it is in my personal estimation preferable to a set of binoculars. Your arms don't get tired, you have over three degrees, and there is much to done with a little scope like that. So I am sensitive to the principle. But having overhauled my old optical craftsman I came to appreciate that there are limitations on small mirrors.
Well I've meandered but I think what kills the small aperture Newts is the desire for crisp wide field low X views in two inch eyepieces, combined with the desire to be able to "bump it up" and still have good performance on a planet. Newts seem to have to choose one or the other.
regards Greg N
"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin
