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Long slow Newtonians

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#1 25585

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Posted 07 September 2018 - 03:32 AM

Does anyone else own a long slow Newtonian?

 

I used to own a F8 8 inch but traded it in for a SCT of same aperture F10, as the GEM mount could not really handle the Newt's length and moving house, a SCT was more realistic. However the Newt was superb for double stars, getting good results from any eyepiece, and held its collimation very well. 

 

To try and recapture those days I have a 10 inch F6 on a Dob mount. But a 6 inch F11 has been offered to me to add to the shedful I already have. 

 

Does anyone here have longer than average make Newtonians? Complete, being made or just a mirror? 


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#2 Astrojensen

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Posted 07 September 2018 - 03:39 AM

I have a 6" f/10 mirror with a very good figure, that I should definitely put in a telescope. 

 

I'm thinking about building an optimized tube with cooling fans, baffles and all the bells and whistles around it. 

 

 

Clear skies!
Thomas, Denmark


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#3 happylimpet

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Posted 07 September 2018 - 05:02 AM

I havent, having only had f5 f6 and f4 Newtonians (and my first scope, an f7 4.5" Newt, but not sure that counts) but I can fully appreciate what superb scopes they are.

 

Do it.


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#4 db2005

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Posted 07 September 2018 - 05:12 AM

I have a 6" f/8 Skywatcher 150PL. Probably the best-performing Newtonian I've owned, but I find it's so large and cumbersome to mount and use that it unfortunately sees too little use. My C8 sees much more use.


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#5 TOMDEY

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Posted 07 September 2018 - 05:15 AM

I had a 6-inch F/12.5 Newtonian over 50 years ago that formed Spectacular planetary images. And was actually excellent on the other usual suspects. It's hard to explain, but a slow system feels somehow more comfortable on the eyes that these fast ones can't match. As if the eyes know the telescope isn't fighting itself to form a good image. That might be just psychological, but I suspect there is something to it. Kingslake and Sinclair called it "ray-bending." The less the optics have to bend the light, the easier is the design, fabrication, alignments and comfort of use.  Tom


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#6 CHASLX200

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Posted 07 September 2018 - 05:18 AM

I have owned around ten 8 f/8 Newts and all gave a great image.  Also had a 10" F/9.6 back in the early 80's.


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#7 db2005

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Posted 07 September 2018 - 07:14 AM

I had a 6-inch F/12.5 Newtonian over 50 years ago that formed Spectacular planetary images. And was actually excellent on the other usual suspects. It's hard to explain, but a slow system feels somehow more comfortable on the eyes that these fast ones can't match. As if the eyes know the telescope isn't fighting itself to form a good image. That might be just psychological, but I suspect there is something to it. Kingslake and Sinclair called it "ray-bending." The less the optics have to bend the light, the easier is the design, fabrication, alignments and comfort of use.  Tom

It's an interesting point. The same discussion pops up from time to time in the refractor forum, where short vs long telescopes are discussed, and long achromats still have a loyal following despite the advance of short APOs. Personally, I think f/7.5-8 and slower is a very nice place to be. Easy on eyepieces ... and I like you description "As if the eyes know the telescope isn't fighting itself to form a good image". I think I know what you mean from observing with my own f/11.3 achromats, even if it's only psychological...


Edited by db2005, 07 September 2018 - 07:23 AM.

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#8 macdonjh

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Posted 07 September 2018 - 07:37 AM

Does anyone else own a long slow Newtonian?

 

I used to own a F8 8 inch but traded it in for a SCT of same aperture F10, as the GEM mount could not really handle the Newt's length and moving house, a SCT was more realistic. However the Newt was superb for double stars, getting good results from any eyepiece, and held its collimation very well. 

 

To try and recapture those days I have a 10 inch F6 on a Dob mount. But a 6 inch F11 has been offered to me to add to the shedful I already have. 

 

Does anyone here have longer than average make Newtonians? Complete, being made or just a mirror? 

I haven't owned any slow Newtonians, but almost every scope I own is long focal length: 4" f/13 Mak, 6" f/12 achro, 8" f/20 classical Cassegrain, 14" f/15 classical Cassegrain.  The exception is my 10": it can be either f/15 or f/4 depending on whether I install the Cassegrain or Newtonian secondary mirror.

 

I think I gravitated to long focus when I had my second bout of aperture fever.  I was deciding between a 15" Dobsonian and a C11.  I chose the C11 because I could mount it on my G11 (a really good combination), but I would have had to buy a ServoCat for the Dobsonian.  So go-to ultimately lead me to long-focus scopes.  While I understand why fast scopes are popular (and there are a lot of advantages), I've grown pretty comfortable with my slow optics.


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#9 *skyguy*

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Posted 07 September 2018 - 08:38 AM

Does anyone here have longer than average make Newtonians? Complete, being made or just a mirror? 

I have a 6" f/10 Newt with an Edmund Scientific premium, spherical primary mirror with a small (20mm) secondary mirror on a curved vane spider. When I do a direct comparison with my 6" f/8 Criterion Dynascope  ... which is a nice scope in it's own right ... the 6" f10 has better contrast for planetary observing and a darker sky background when observing deep-sky objects at similar magnifications

 

The differences are very noticeable and somewhat surprising since the f/10 is a spherical mirror. However, the differences and capabilities of long focal ratio spherical mirrors versus the more traditional medium focal ratio parabolic mirrors is a well worn topic on CN.

 

6F8_6F10.jpg


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#10 Jeff Morgan

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Posted 07 September 2018 - 09:12 AM

Three of them over the years: 10" f/9, 8" f/9, and 16" f/7.

 

It's nice when the entire FOV is the sweet spot.

 

It's nice when any eyepiece that finds its way into the focuser gives a great view.

 

It's nice for minimum glass planetary work.

 

It's nice when collimation is easy (getting the optics mounted inside the tube is almost enough).

 

It's nice when collimation holds all night. No excuses performance.

 

Not so nice of a tube size and mount. But, I still have a soft spot in my heart for them.


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#11 Galicapernistein

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Posted 07 September 2018 - 11:15 AM

I had a 6-inch F/12.5 Newtonian over 50 years ago that formed Spectacular planetary images. And was actually excellent on the other usual suspects. It's hard to explain, but a slow system feels somehow more comfortable on the eyes that these fast ones can't match. As if the eyes know the telescope isn't fighting itself to form a good image. That might be just psychological, but I suspect there is something to it. Kingslake and Sinclair called it "ray-bending." The less the optics have to bend the light, the easier is the design, fabrication, alignments and comfort of use.  Tom

I've thought the same thing. I get eye strain with any F5 scope, but not with my 8" F9. This is undoubtedly due the greater depth of focus in the F9, but how that translates to less eye strain is not clear. I agree that the eye may compensate for minute changes in focus that the observer might not even be aware of, which may be the problem I have with F5's. 


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#12 Stephen Kennedy

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Posted 07 September 2018 - 02:37 PM

This is my Mikage Newtonian OTA on a Pentax MS-5 GEM.  Both were purchased in 1988 while I was stationed in Japan.

 

IMG_1168 (3).JPG

 

The aperture is 210 mm and the focal length 1,623 mm for a focal ratio of 7.7.  In recent years I have been using it mainly for Astrophotography with my Canon T3 DSLR  but also observe the Moon and planets with my Pentax XW 7 eyepiece from time to time.


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#13 barbie

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Posted 07 September 2018 - 05:33 PM

I have a 6" F8 Newtonian that is excellent on everything in a still manageable tube length

 


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#14 Starlease

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Posted 07 September 2018 - 07:30 PM

Gotta 6 inch f\11 Torus optics newt very good optically.

But it is mostly stand up only because of the length.

Used it a lot for 3 months after assembling only every

now and then comparing views with other scopes now.

 

An f\8 looks tiny next to it. Still easy to handle compared

to a 10 inch.


Edited by Starlease, 07 September 2018 - 07:31 PM.

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#15 Starlease

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Posted 08 September 2018 - 10:05 PM

6" f\11 and f/8, 5" f/7.

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#16 Starlease

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Posted 11 September 2018 - 11:30 AM

Just finished the 6" f/8 Edmund. Mirror came off eBay and it is fantastic like all Edmund mirrors. Stars are pinpoint sharp and sky background is pitch black. Hate to say it is so refractor like. People idolize lenses but quality mirrors are every bit as good.


Edited by Starlease, 11 September 2018 - 11:30 AM.

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#17 Binojunky

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Posted 11 September 2018 - 12:54 PM

I have a 6" F8 Newtonian that is excellent on everything in a still manageable tube length

Same here, love mine,(Orion basic 6" F8 dob), D.


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#18 Alterf

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Posted 11 September 2018 - 02:18 PM

I also have a 6" f/8, a Parks mirror in a Parallax tube.  It's pretty portable, probably the scope I use most.  I also enjoy longer focal lengths.  The sky background seems darker to me, and the stars more contrasty.  The other scopes I observe with are a 80mm f/7.5 refractor and a 290mm f/13.5 Classical Cassegrain.  All of these scopes keep coma, field curvature, and the like to a minimum and present very nice views.

 

Val


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#19 starcanoe

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Posted 11 September 2018 - 03:41 PM

Just finished the 6" f/8 Edmund. Mirror came off eBay and it is fantastic like all Edmund mirrors. Stars are pinpoint sharp and sky background is pitch black. Hate to say it is so refractor like. People idolize lenses but quality mirrors are every bit as good.

 

I ve got a 6 inch f 8 mirror I bought in the late 90s...is probably an Edmund  one as well (says ES and few other things on the back).

 

That thing REALLY performs. And this is only so so main mirror collimation, a slightly twitchy focuser made of plastic plumbing parts....the diagonal is just some run of the mill thing that I have no idea of the specs on but it was something cheap I also bought back then (or in other words not some fancy 1/30 wave thing). The spider is made of something like 1/8 RODS (or bigger!) rather than thin metal vanes. The secondary can't even be adjusted. My high power eyepieces are a oooolllld University Optics 6.8 ortho and a 10 mm 3 element Vite $10 eyepiece (with a plastic lens!).

 

I've been watching the Mars observation reports here. I'm seeing more detail than most people are reporting here (even when a fair number of them are seeing nothing more than the polar cap). Some photographs with signifcantly large scopes are about the only thing besting my observations. Even during the height of the dust storm I was getting large scale stuff (low contrast to be sure but definitely there).

 

Been doing public star gazes. Random John Q publics can often even see the large scale details with the $10 Vite !

 

 

Imagine if I did this thing up right !

 

I would prefer it to be more like f10 or a bit more....its too short most of the time.

 

I drool to think what a good 8 inch f9 ish could do.

 

Gawd I need to finish my 10 inch f8 1/50 wave rms scope !


Edited by starcanoe, 11 September 2018 - 03:45 PM.


#20 James Ball

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Posted 12 September 2018 - 11:36 AM

I was looking for long focal length Newts the other day and the f/8 6" was about the longest I could find at most places.  Does anyone sell complete scopes of longer focal lengths or are they ATM only now days?



#21 Starlease

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Posted 12 September 2018 - 11:44 AM

Discovery telescopes has longer 8 inchers.

But you mite die before delivery.

Optcorp mite have some in stock or their own versions.


Edited by Starlease, 12 September 2018 - 11:46 AM.


#22 Jeff Morgan

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Posted 12 September 2018 - 04:01 PM

I was looking for long focal length Newts the other day and the f/8 6" was about the longest I could find at most places.  Does anyone sell complete scopes of longer focal lengths or are they ATM only now days?

 

Probably not in the mass market scopes.

 

 

You could approach an optician about a custom aperture and length. If you want to avoid building the structure yourself, Joe Nastasi at Parallax will build you an excellent scope. Rob Teeter might also be able to accommodate a custom length in his STS series.

 

If you have a little space in the garage or basement, building a simple dob mount is really really easy. Or, a local wood shop or cabinet maker could do the cutting of the big pieces for you.



#23 Mike Spooner

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Posted 12 September 2018 - 08:58 PM

I've made several 6" f/9 and f/10 scopes (and even more mirrors). They can provide incredible images if the optics are well figured. I mostly use larger scopes but occasionally pull out 6.1" f/10 when I need a quality 'fix'. smile.gif

Mike Spooner
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#24 Bob S.

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Posted 12 September 2018 - 09:05 PM

I've made several 6" f/9 and f/10 scopes (and even more mirrors). They can provide incredible images if the optics are well figured. I mostly use larger scopes but occasionally pull out 6.1" f/10 when I need a quality 'fix'. smile.gif

Mike Spooner

Mike, I can fully attest to some of the most incredible planetary views and double stars with one of your 6" f/9 mirrors in a custom made telescope. IIRC, the secondary was only .75" and the views of Saturn were simply breathtaking. Thanks for having made some of the finest long focal length mirrors on the planet. Bob Schilling


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#25 Mike Spooner

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Posted 12 September 2018 - 11:00 PM

Mike, I can fully attest to some of the most incredible planetary views and double stars with one of your 6" f/9 mirrors in a custom made telescope. IIRC, the secondary was only .75" and the views of Saturn were simply breathtaking. Thanks for having made some of the finest long focal length mirrors on the planet. Bob Schilling


Hi Bob!

The 6" seems like a humble scope size but under the most stable skies it becomes apparent how limiting the atmosphere really is for high definition viewing. I'm more convinced than ever how important accurate figure is for the finest and detailed images. Those breathtaking nights are rare enough that most folks may never get to appreciate what can be achieved. Indeed, there are areas where seeing always limits even small scopes and I feel blessed to live where the veil is often lifted.

Best,
Mike
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