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Long slow Newtonians

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#51 CollinofAlabama

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Posted 17 September 2018 - 03:03 PM

I agree with both Starcanoe AND Jon Isaacs.  If one has the patience, time, and mechanical ability to set up a 12" F/5 scope, it's definitely going to be the better instrument in terms of performance at the eyepiece.  However, it will never have the grab 'n go feel of a 6" F/10 instrument -- which is more likely to be 6" F/8 these days, but that would shift Jon's position down to a 10" F/5, and essentially the same argument.

 

A 6" F/8 or higher is a wonderful instrument.  In a dobsonian mount, they are truely grab 'n go as anyone's 110mm refractor, and much more wind resistant than just about any refractor, period.  The coma, tho there, is very, very small, and, I find, genuinely tolerable, unlike F/6, and especially at F/5, where, if you're using a Newtonian and care about a flat field, you've got to introduce a coma corrector, with its inherent weight on the focuser, and unique configuration issue-per-eyepiece, to say nothing of the extra stress on exacting collimation one concurrently moves up to.

 

For a more refractor-like viewing experience with less fuss, faster cooling, often better performance, the 6" F/8 newt is an unsung hero in the telescope world.  Not the stunning galaxy viewer a 10" F/5 is, for sure, but more likely to easily split tight doubles than most 10" F/5 owners can muster.  The 10" F/5 could produce every bit of star splitting capacity a 6" F/8 could, theoretically, only saying that the average 10" F/5 owner does not possess the patience, time, or mechanical prowess to make it happen, to say nothing of the extra thermal issues involved with a 10" mirror compared to a 6".  And the weight of a 6" F/8 dobsonian is about the easiest "large-sized" telescope to set up a person can find, being amazingly wind resistant, but throwing up consistently good images.


Edited by CollinofAlabama, 17 September 2018 - 07:53 PM.

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#52 Jeff Morgan

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Posted 17 September 2018 - 04:22 PM

Frequently the longer slower newt will have a smaller secondary.

Planetary contrast will be improved.

The long slow newt will have significantly less coma. More improvement.

It will also focus sharp much easier. More improvement. 

Although a fine adjusting focuser will help this in a fast newt. 

 

You can get to the same magnification in both scopes. 

With a barlow(s) or ep's with built-in barlow, you will reach equal magnification.

 

Plus the barlow or ep's with built-in barrows will clean up off axis astigmatism.

Correcting maybe half of the fast newts inherent aberrations in the ep.

 

If you use a coma corrector, and multi-element astigmatism correcting ep's, in a fast newt, then you can get really, really close to a long slow newt, on planets.

 

But the fast newts larger secondary robs some contrast.

Many folks maintain that the multi-element ep's that work best in fast newts, those rob some contrast too.

 

The long slow newt, with modest design ep's, delivers a great planet. 

A fast newt of the same size, at the same magnification, needs better ep's and coma correction to even come close to equaling it. IME

 

On deep sky targets, at equal aperture and magnification... they're qual. IME

 

End ramble.

 

Forgot one - collimation tolerance. Not a big deal on the Faint Fuzzies, but for planetary detail and close double stars - critical.

 

Longer focal ratios have a much larger "tolerance envelope" to work with than shorter focal ratios do.

 

Unless the scope has very well-engineered and beefy construction, the collimation will (not may, will) shift as the scope is moved. There are many mechanical connections where positional shift flexure can manifest themselves, particularly in a truss scope. It takes a lot of attention to detail to get the sources of play under control. And then there is flexure to consider, not just tubes, but focuser boards loaded with three or four pounds of equipment.

 

Of course, this can be done. My Takahashi Epsilon e-180 is f/2.8 and stays collimated for half a dozen sessions or more. And the 24"long tube weighs 28 pounds without the tube rings.


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#53 RobertMaples

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Posted 17 September 2018 - 04:37 PM

...

The long slow newt, with modest design ep's, delivers a great planet.

A fast newt of the same size, at the same magnification, needs better ep's and coma correction to even come close to equaling it...

 

 

My own thinking/experience is that a long, fast Newtonian will provide the better planetary views. One has to step back from the aperture paradigm and do some outside the box thinking.

 

Consider two scopes,  both with a 1500 mm focal length: a 150 mm F/10 and a 300 mm F/5. I think there's little doubt the 300 mm F/5 will provide the better planetary views. 

 

Sure , I'm comparing a 6 inch with a 12 inch, ergonomically that's the right comparison.  

 

Jon

I believe izar187 is referring to to telescopes of the same aperture size, not focal length size, in answering my question comparing a longer focal length newt to a shorter one with a barlow.


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#54 Jon Isaacs

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Posted 17 September 2018 - 05:26 PM

And what is a good 6 inch mirror going to cost vs a good 12 inch? And the weight of the mirrors and scope. Mirror cell complexity. Alignment tolerances. Coma corrector vs none. Eyepiece cost differences. Cool down times. Grab and go vs grab and grunt.

 

Looking at just focal length is silly.

 

Standing on a ladder for a 10 inch is even sillier.  

 

Thinking outside the box,  that means considering more than just aperture.  

 

Jon


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#55 ShaulaB

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Posted 17 September 2018 - 05:35 PM

4 inch f12 Dobsonian. I made the mirror and the rest of the build. Excellent on Jupiter, Saturn, the Moon, and bright open clusters. The child in the picture is,now over thirty years old.

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#56 Jim Romanski

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Posted 17 September 2018 - 06:02 PM

I inherited a 12.5" F/8 mirror and cell from my Dad.  It's an old Upton mirror so I think it's got a pretty good figure.  After many years of thinking about how I'd make a tube that broke down for transport I decided it just wasn't worth it.

 

But I'm thinking that it might be a good mirror to build a Chiefspeigler out of.  I think I could make one with a very comfortable eyepiece height.  It wouldn't be a huge project as long as I can get help with the corrector lenses.

 

But It'll have to wait for at least 2 other telescope projects to be finished (not to mention household projects). 


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#57 Jon Isaacs

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Posted 17 September 2018 - 06:06 PM

Not the stunning galaxy viewer a 10" F/5 is, for sure, but more likely to easily split tight doubles than most 10" F/5 owners can muster.  The 10" F/5 could produce every bit of star splitting capacity a 6" F/8 could, theoretically, only saying that the average 10" F/5 owner does not possess the patience, time, or mechanical prowess to make it happen, to say nothing of the extra thermal issues involved with a 10" mirror compared to a 6".

 

 

I don't know about other owners of 10 inch F/5s but I regularly split doubles not possible with a perfect 6 inch . It's not that much effort . Collimation, a good fan and stable seeing.

 

As far as collimation shift: I will just say,  it is possible to build a fast Dob that does not shift collimation.  It might take some time running down the various gremlins... 

 

I often think of Jeff's 16 inch F/7 with its 112 inch focal length amd his various attempts at making it more user friendly.  I'm more than happy with a ladderless 16 inch F/4.4. Ease of use equals more frequent use.. 

 

Jon


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#58 CHASLX200

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Posted 17 September 2018 - 06:49 PM

Chicken or the Egg?  The Chicken!  smile.gif

 

Jon Isaacs:  I worked in telescope retail in the late 1970's - early 1980's. At that time there were no commercially-produced F/4 Newtonians other than Edmund's Astroscan and Coulter's little collapsible 4-1/4" CT-100. (You might come across the odd Cave Astrola or Telescopics Newt/Cass convertible but those were not common.) One exception: Edmund's big red 8" F/5 on the fork mount that showed up about 1982.)

 

At that time, the best eyepieces available for these shorties were orthoscopics and the odd surplus Erfle. Meade supplied a 2" Erfle too. There were also the Clave Plossl's from France but they cost a lot of money to import and the best choices were the Meade R.G. Erfles and Orthos and the Brandon Orthoscopics. Even Edmund only supplied their 28mm RKE with the A-scan.

 

Then along came Coulter with their big blue 13.1" plywood push-along. I believe that was the first large-ish F/short produced in any numbers. It was about that time that Al Nagler came on the scene with his 1-1/4" Plossls which were the first of that design affordable by the average telescope user. Then he followed up with his 13mm Nagler design. We called it the "coffee can". Few could afford one and the kidney bean effect and its weight made it a very hard sell.

 

At least that's how my memory has it. Almost all Newtonians commercially produced at that time were F/6 - F/10. F8 was typical. Plossl's were new and all the rage. (I still use mine, but my Meade R.G's get the most use.  wink.gif

F/4.5 seemed to be the fastest Newts back then. Maybe the Volk scope was F/4?  Every fast Newt i owned back in them days gave coma filled mushy views. This was pre Paracorr days and before we had good collimaton tools. And it seemed not many were good at making fast mirrors back then.  Eyepiece tech was dead back then until Tele Vue came out around 1981, then later on with the Nags. The red 8" F/5 Edmund came out around 1978,.  The Coulter 10.1" Compact i got in 1982 was a sorry scope for planets.  The Naglers were a world changer as was the much better built Tectrons, around 1988 and then Obsession Dobs around 1990.


Edited by CHASLX200, 17 September 2018 - 06:54 PM.

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#59 CHASLX200

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Posted 17 September 2018 - 06:59 PM

You know what? Now that my memory is jogged a bit, there were some other sawed-off Newtonians available in ~1980: Meade sold a 6"F/5 on an equatorial mount (their model 645) and there was also an outfit (Star Instruments?) in California that produced a 6" F/4 tube assembly with Meade accoutrements. Both of these were considered to be "wide field" telescopes but, naturally, coma was terrible and there were no parracor's around then.

 

I think it was 1980 that the TV Plossls  hit the market. (Coma still sucked.)

 

These fancy new hand grenade eyepieces truly do make all the difference. They DO allow F/short (under f/8) telescopes to work and big ones to be portable. Al Nagler revolutionized the telescope industry. You're right. The egg enabled the chicken!  smile.gif

I had a Star instruments 6" RFT that had the mirror held in place by a center bolt i bought new in later 1978.  It came with the 2ndary and holder loose that chipped the main mirror when i got it.  Another mushy fast scope.


Edited by CHASLX200, 17 September 2018 - 06:59 PM.


#60 daquad

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Posted 17 September 2018 - 07:04 PM

I don't know about other owners of 10 inch F/5s but I regularly split doubles not possible with a perfect 6 inch . It's not that much effort . Collimation, a good fan and stable seeing.

 

As far as collimation shift: I will just say,  it is possible to build a fast Dob that does not shift collimation.  It might take some time running down the various gremlins... 

 

I often think of Jeff's 16 inch F/7 with its 112 inch focal length amd his various attempts at making it more user friendly.  I'm more than happy with a ladderless 16 inch F/4.4. Ease of use equals more frequent use.. 

 

Jon

Agree.  From my location. Antares skims along the tree tops when at the meridian.  Yet I was able to split it with an Orion 10" f/4.7 in mediocre (5-6) seeing.  I couldn't do it with my 6" f/9 Starfire which was set up at the same time until the 10" Newt showed me where to look.  IOW I saw it easily with the 10" f/5 Newt and with difficulty with the 6" f/9 APO.  I'm sure that if the APO were a 6" f/8 Newt the story would be the same. 

 

Dom Q.



#61 25585

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Posted 17 September 2018 - 07:27 PM

I think it worked the other way.. Because the good eyepieces and coma correctors were available F/4 Newtonians came into their own.  The market for TeleVue eyepieces was and is for CATs, refractors and reflectors.

Large aperture scopes were impractical before the Dobsonian.. This photo tells the story:

 

 

 

Two 12.5 inch scopes. Old style and one more modern.

One of these two scopes never left the driveway. One of these two scopes has been all over the southwest and in it's current truss form just got back from a week under the dark skies of the Navajo reservation.  And yes, the F/4.06 has been the better planetary scope.  Something about being stuck in the garage limits the image sharpness.

It was a nice old F/6, I had it out a few times a year. Not often for someone who gets out about 180 nights a year. Impressive but impractical in today's world where I can setup my 22 inch by myself but the 12.5 inch F/6 was a two person scope.

Jon

 

Yes, practical one-man F6 stops at 10 inches aperture. Practical F5 is 12 inches.

 

The difference in OTA diameter, coupled with extra weight - primary and cel - are the 2 factors determining handling ease. 


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#62 Galicapernistein

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Posted 18 September 2018 - 09:37 AM

The point of a slow scope is to funnel light into the center cones of the eye the best way possible. Cones are responsible for visual acuity, and rods are for low light conditions. The area of maximum resolution is tiny. Think of the time you have spent trying to tease out details in areas far smaller than even the Great Red Spot. Cones are very sensitive to stress, not only biochemically, but from muscles in the eye. The rods and cones are packed tightly together, and even slight stresses are enough to distort them. The big advantage of a slow scope is the large depth of focus. From experience I can say that slow scopes are simply more comfortable to use, but why that is I can’t say. But relaxed eye muscles allow the eye’s cones to perform their best, which is critical for resolution. Even small levels of muscular stress can affect what you see.  Eyeglass wearers know what kind of effort is needed to focus on objects with glasses off. Expensive eyepieces and coma correctors are compromises that we need because big, slow  scopes are hard to deal with. But big, fast scopes do maximize the performance of the eyes rods, which for dim objects is a big plus. Unfortunately, their shallow depths of focus cause problems when it comes to how the eye reacts to minute changes in focus of brighter objects. They will outperform smaller scopes, but only because of their sheer size, not to mention weight and expense. But even a 4” F9 can show an amazing amount of detail. It would be nice if big scopes could be used at F8 and higher, but few of us can enjoy such a luxury, and most of us will never even get to look through a big, slow scope to enjoy what a maximized monster dob can show. 


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#63 25585

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Posted 18 September 2018 - 12:47 PM

I would have a 12 inch F6 or 14 or 16 inch F5 if I could transport a solid tube version that was light enough. But they are not made, and a 12 inch F5 is enough though for carrying vertically and getting into and out of my 4x4 (short wheel base), I have considered a trailer but a caravan might be better.


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#64 A Y

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Posted 18 September 2018 - 12:52 PM

So are there any commercially-available slow Newts that aren't too tiny? Orion has a 4.5-inch f/8, but that's it. Is it basically a DIY project?

 

I love the idea of what Gary Seronik described in his slow reflector article: https://garyseronik....h-f9-reflector/


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#65 Jon Isaacs

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Posted 18 September 2018 - 01:15 PM

So are there any commercially-available slow Newts that aren't too tiny? Orion has a 4.5-inch f/8, but that's it. Is it basically a DIY project?

 

I love the idea of what Gary Seronik described in his slow reflector article: https://garyseronik....h-f9-reflector/

 

Pretty much I think.  There are 6 inch F/8 Dobs which can be mounted in rings on a Gem. 

 

Jon


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#66 25585

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Posted 19 September 2018 - 02:07 PM

If an 8 F6 is desirable, there are plenty on the market. Same FL as a 6 F8.  



#67 daquad

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Posted 19 September 2018 - 02:17 PM

If an 8 F6 is desirable, there are plenty on the market. Same FL as a 6 F8.  

And Orion Optics UK makes both with cooling fan, 8X 50 finder, rings and two speed focuser.


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#68 Jon Isaacs

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Posted 19 September 2018 - 02:27 PM

 

If I wanted a 6 inch F/8, I would wait for an RV-6.   The tubes where phenolic which is better thermally than metal and round..  The mirror cells were simple with a good clean flow.  The secondary was about 22%.  Without the rings, the complete OTA weighed 8 pounds.  The rings were beautiful,  sand cast aluminum and robust.  The mount, workable.  

 

If the optics are not to your standards, have Mike Spooner make a mirror for you.  

 

5623907-Meade plus RV-6 2.jpg

 

My limited experience with OOUK was not good.  A friend had a 6 inch F/5 built for him with the 1/10 wave mirror.  The secondary spider was mounted in the wrong position so the secondary could not be properly positioned.  I don't know if he ever got that resolved but they were not responsive. 

 

Jon


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#69 25585

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Posted 23 September 2018 - 01:08 PM

 

 

If I wanted a 6 inch F/8, I would wait for an RV-6.   The tubes where phenolic which is better thermally than metal and round..  The mirror cells were simple with a good clean flow.  The secondary was about 22%.  Without the rings, the complete OTA weighed 8 pounds.  The rings were beautiful,  sand cast aluminum and robust.  The mount, workable.  

 

If the optics are not to your standards, have Mike Spooner make a mirror for you.  

 

 

 

My limited experience with OOUK was not good.  A friend had a 6 inch F/5 built for him with the 1/10 wave mirror.  The secondary spider was mounted in the wrong position so the secondary could not be properly positioned.  I don't know if he ever got that resolved but they were not responsive. 

 

Jon

 

How long ago please?  



#70 Jon Isaacs

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Posted 23 September 2018 - 01:17 PM

How long ago please?  

 

Within the last 2 years or so.  

 

Jon



#71 gwlee

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Posted 23 September 2018 - 01:58 PM

For my purposes, a 6”f8 Dob offers the best balance of portability and optical performance. Wish a manufacturer offered a premium version. 


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#72 Astrojensen

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Posted 23 September 2018 - 02:23 PM

For my purposes, a 6”f8 Dob offers the best balance of portability and optical performance. Wish a manufacturer offered a premium version. 

The problem is that almost no one will buy it. A premium 6" f/8 would cost upwards of $1k and very few would spend that on a 6" f/8, when they can get a "good enough" 10" f/5 or even a 12" f/5 for that kind of money.

 

Fortunately, it's easy to build one yourself - or maybe contract the work out to a skilled ATM'er. 

 

 

Clear skies!
Thomas, Denmark 


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#73 gwlee

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Posted 23 September 2018 - 03:24 PM

The problem is that almost no one will buy it. A premium 6" f/8 would cost upwards of $1k and very few would spend that on a 6" f/8, when they can get a "good enough" 10" f/5 or even a 12" f/5 for that kind of money.

 

Fortunately, it's easy to build one yourself - or maybe contract the work out to a skilled ATM'er. 

 

 

Clear skies!
Thomas, Denmark 

I understand why there’s no premium 6”f8 Dob on the market, but that makes it no less desirable to me. I believe that I could build one, but don’t believe it would meet my definition of  “premium.” :-) 

 

Cobbling together a smaller (3-4 inch), but equally portable refractor rig with high quality, off-the-shelf components is a much easier task, so that’s the direction I am going for now, but I would prefer a premium 6”f8 newt for this observing site. 


Edited by gwlee, 23 September 2018 - 03:27 PM.


#74 Astrojensen

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Posted 23 September 2018 - 03:47 PM

I understand why there’s no premium 6”f8 Dob on the market, but that makes it no less desirable to me. I believe that I could build one, but don’t believe it would meet my definition of  “premium.” :-) 

Have you considered letting someone else do it for you? There are both skilled ATMs that could do it, but there are also some professional telescope makers that are willing to build one-offs. New Moon Telescopes comes to mind, as an example. 

 

I have a 6" f/8 newtonian myself (a Sky-Watcher, not half bad, actually) and I can most certainly understand your desire to get a premium sample. 

 

 

Cobbling together a smaller (3-4 inch), but equally portable refractor rig with high quality, off-the-shelf components is a much easier task, so that’s the direction I am going for now, but I would prefer a premium 6”f8 newt for this observing site.

I tend to swing back and forth between refractor and reflector phases. Right now, I'm in a refractor phase and two projects I'm currently entertaining myself with at the moment are designing tubes for a Zeiss C50/540mm and a Lichtenknecker 125/1750mm achromat and collecting parts for the builds. I have nearly all parts for a lightweight tube for the Lichtenknecker, I just need to order a phenolic tube made to my specifications.  

 

 

Clear skies!
Thomas, Denmark 


Edited by Astrojensen, 23 September 2018 - 03:48 PM.

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#75 WoodyEnd

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Posted 23 September 2018 - 07:05 PM

I have a Meade 826 which certainly is not long focus but the images are superb.  If Meade made a f8 or longer version I would sure like to have one.




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