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Relative advantage of 4.5 newt vs 80mm refractor vs 90mm mak

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#26 daquad

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Posted 22 March 2020 - 06:39 PM

The 4.25" Edmund f/10 reflector I used to own gave superb images, even on the moon and planets. This was with its original rectangular secondary mirror, which was not ideal.

Same is true for the Criterion 4" f/10 Newtonian I owned as a kid.  Gave high contrast views of the moon and planets and double stars were resolved to the limit of its aperture with perfect Airy discs.

 

Dom Q.

 

Edit.  It's unfortunate that small 4"-5" slow Newts are not available.  Even a spherical mirror at f/10 -12 will have a wave rating of better than 1/5 wave.  They are easy to produce and the tube assembly can be made with all the latest technology for a perfectly ergonomic, CA-free and yes, coma-free (or nearly so) optic. 

 

A 4-1/2" f/10 Newt would be no longer and no heavier than a 4" f/10 ED and should cost much less and be just as good for deep sky and planetary observations.  

 

All the lens technology that is required for that same performance is really overkill if one is interested only in refractor-like views for visual observations.  

 

I like refractors, but I would also like a 5" f/10 Newt built to the same standards as a modern ED, which, unfortunately is nowhere to be found.


Edited by daquad, 22 March 2020 - 06:59 PM.

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#27 Auburn80

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Posted 22 March 2020 - 07:59 PM

Same is true for the Criterion 4" f/10 Newtonian I owned as a kid. Gave high contrast views of the moon and planets and double stars were resolved to the limit of its aperture with perfect Airy discs.

Dom Q.

Edit. It's unfortunate that small 4"-5" slow Newts are not available. Even a spherical mirror at f/10 -12 will have a wave rating of better than 1/5 wave. They are easy to produce and the tube assembly can be made with all the latest technology for a perfectly ergonomic, CA-free and yes, coma-free (or nearly so) optic.

A 4-1/2" f/10 Newt would be no longer and no heavier than a 4" f/10 ED and should cost much less and be just as good for deep sky and planetary observations.

All the lens technology that is required for that same performance is really overkill if one is interested only in refractor-like views for visual observations.

I like refractors, but I would also like a 5" f/10 Newt built to the same standards as a modern ED, which, unfortunately is nowhere to be found.


Why f10? To stay spherical? I do like your thinking; mine would be a well made 5" f8. Use a 1-1.25" secondary, a good 1.25" focuser and put it on an alt-AZ mount.
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#28 boyd

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Posted 23 March 2020 - 08:31 AM

Jon, yes I was looking at the deltona suspect. 🙂

I would mount it onto my eq1 in and alt/ax configure for now. I still love getting the ST80 out as a g&g, but my skies are horrid most nights with 5/10 seeing or below until around 2 am.

The mak c90 was free, so I’ll keep it for now.

A 4.5 newt would work from home in place of the st80. Then again I may just wait. I fear going too big, as I have no room.

A 102mm refractor was my original thought, but the cheap ST80 drew me in. 😒

#29 Jon Isaacs

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Posted 23 March 2020 - 08:34 AM

Jon, yes I was looking at the deltona suspect.

I would mount it onto my eq1 in and alt/ax configure for now. I still love getting the ST80 out as a g&g, but my skies are horrid most nights with 5/10 seeing or below until around 2 am.

The mak c90 was free, so I’ll keep it for now.

A 4.5 newt would work from home in place of the st80. Then again I may just wait. I fear going too big, as I have no room.

A 102mm refractor was my original thought, but the cheap ST80 drew me in.

Mounting it would not be so easy, it's too large for an EQ-1 and optically, who knows.  

 

Save your money for something better.

 

Jon


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#30 daquad

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Posted 23 March 2020 - 08:46 AM

Why f10? To stay spherical? I do like your thinking; mine would be a well made 5" f8. Use a 1-1.25" secondary, a good 1.25" focuser and put it on an alt-AZ mount.

Not to stay spherical, necessarily.  I remember how much I enjoyed the views through that 4" f/10 Creterion Newt.  One night a buddy and I spent several hours observing lunar features and describing to each other in detail what we saw.   I also liked the perfect Airy stellar discs.  F/8 is good too if it is a well made parabola.

 

Dom Q.

 

Edit:  Another feature I meant to mention of a long focal length is a relatively coma-free, flat field.


Edited by daquad, 23 March 2020 - 10:08 AM.



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