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Equipment Discussions >> ATM, Optics and DIY Forum

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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: Optimized Newts vs APOs new [Re: bremms]
      #5581076 - 12/20/12 05:09 PM

Over on the eyepiece forum, they said it doesn't matter what kind of scope you use. All views will be superior if you use a Brandon eyepiece. Design and optical quality really have no bearing. Well, that's what they said.

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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Optimized Newts vs APOs new [Re: bremms]
      #5581085 - 12/20/12 05:12 PM

If we factor in the issue of cost, then nothing else comes even close to a newtonian. The price difference between a custom 6" newtonian with the finest mirrors, tube, focuser, etc; and even a low-priced 6" assembly-line apo is ridiculously large.

So why are so many choosing apos and even achromats over newtonians? Clearly the answer lies somewhere else than in theoretical performance. I think there are many answers, but one is that many newtonians made over the years have incredibly poor thermal behaviour, severely limiting their performance under real world conditions. One 6" that I have on loan take three hours to cool down... Before it has cooled, it is little better on the planets than my 80mm refractor. In contrast, my 5" f/14 achromat was almost immediately much more powerful than my 80mm and was utterly fantastic once fully cooled.

But when it had finished cooling, the 6" newtonian was scaringly good. Mars exploded in details. It elongated 0.4" double stars at 600x and showed perfect airy disks. Truly stunning performance.

So why don't I use the 6" all the time? Back then, I knew nothing about thermal design of newtonians, so I quickly gave up on it, due to its long cooldown time and crazy poor ergonomics (I had it on a german equatorial, which is a horror, if you don't have rotating rings. I didn't have rotating rings...). In comparison, the 5" refractor was easy to use, despite being very heavy. Now that I know more, I am thinking about rebuilding the 6". I also need to rebuild the 5" refractor. It is too heavy.

Newtonians can be crazy good, like apos, but they have their own subset of design weaknesses that need to be adressed in a succesful product. In a lot of newtonians of the past, these issues weren't given any attention at all and the result has been that newtonians have been dragging around with an unfair reputation for centuries.

I have been guilty of it myself. It took a 12" dob to really open my eyes to the qualities of the newtonian and that scope has LOTS of issues. The sad thing is that there seems to be no other way to remedy them other than build it oneself.


Clear skies!
Thomas, Denmark


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Dick Parker
scholastic sledgehammer
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Reged: 08/17/07

Loc: Tolland, CT and Chiefland, FL
Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5581195 - 12/20/12 06:02 PM Attachment (35 downloads)

Ed et al -

I have a 6 inch f/12 with a spherical mirror that gives incredible images. Saturn's moons are the smallest cleanest dots with this telescope. Last time I used it for a Messier Marathon I was blown away with it. I don't know why I don't use it more. But I actually plan to. On my list to Santa is a 1 inch secondary. I built it with .75 inch and that is a bit too small. Yup! I'd give it a go with a 6 inch APO. But it is a cheezy no cost thing. No one would give it credit against the nice looking expensive APOs.

My advice would be to not underestimate those long focus Newts

Dick Parker


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5581243 - 12/20/12 06:38 PM

@Astrojensen: in my experience no 6-inch would take that long (3 hours!) to cool down. A 6-inch mirror is only 1 inch thick. If you point the tube close to the Zenith, all the heat will quickly escape.

Mladen


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Astrojensen
Post Laureate
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Reged: 10/05/08

Loc: Bornholm, Denmark
Re: Optimized Newts vs APOs new [Re: MKV]
      #5581837 - 12/21/12 03:18 AM

That's what I thought, too, but alas. It really is a thermal pig, right up there with SCTs. I've found a big fan and have rebuild an identical mirror cell I had lying about. Now I just need a new base for the 2" GSO crayford and I'm almost there.


Clear skies!
Thomas, Denmark


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Astrojensen]
      #5581933 - 12/21/12 06:32 AM

Thomas, I've been an ATM since the 1970's. My first mirror was a 6-inch f/6. I've also had a Cave 6-inch f/8. All mirror blanks in those days were full thickness, no matter what diameter, we didn't use (nor have!) computer fans, and no one ever mentioned a 6-inch "thermal pig".

I have heard the same story about the Houghtons - they never "cool down". Rubbish! I made a 6-inch Houghton in the early 1980's and never had that experience. Maybe it's a generational thing. I don't know, but perhaps in three hours the atmosphere becomes less turbulent too, rather than your 6-inch mirror needing three hours to reach thermal equlibrium. Maybe you are observing on cement rather than grassy area, or over the rooftops.

You are comparing apples and oranges. I never had to cool down my 3-inch Unitron, or my Edmund's 3-inch f/10 Newtonian. A 3-inch aperture is not affected by the atmosphere because of the size of the thermal air cells. Thermal air-cell size is about 4-inches and any aperture above that will be prejudicially affected by the thermal heterogeneity of air to a much greater extent than apertures smaller than thermal air cells.

Any scope over 20 inches in aperture will see this no matter how long it has been "cooling" down unless it's in space.

Mladen


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GlennLeDrew
Postmaster
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Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Optimized Newts vs APOs new [Re: MKV]
      #5581970 - 12/21/12 07:35 AM

Metal scope tubes are awful things from a thermal standpoint, as they always 'want' to cool well below ambient under a clear sky. This differential maintains air currents for some time.

Mladen,
It's rather simplistic to state that thermal air cells are no smaller than about 4", and so apertures below this limit are not affected. One's sight line through the atmosphere must necessarily contain many cells overlapping, which will present a continuous variation on all scales. I've had 60mm scopes be very badly affected by seeing.


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: GlennLeDrew]
      #5582005 - 12/21/12 08:02 AM

Glenn, Cave Astrola used to sell fiberglass tubes. Parks Optical still does - with end rings. For a 6-inch f/10 it costs $139. I think that might be cheaper than aluminum!

I never used aluminum tubing. My scopes were always fiberglass, wood, or spiral paper. ATMs of my time knew better.

As for apertures smaller than 4" not being affected seriously by air turbulence, that has been well documented. You will notice that I also explained what I meant when I added that larger apertures are affected to "a much greater extent than apertures smaller than thermal air cells."

Of course, anything below 60 degrees will present thicker air mass and greater scintillation. Close to the astronomical horizon you never have air steady enough for observing with anything but the lowest power. Your best bet is always to observe at or near the Zenith.

Mladen


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Jon Isaacs
Postmaster
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Optimized Newts vs APOs new [Re: MKV]
      #5582961 - 12/21/12 06:09 PM

Quote:

Glenn, Cave Astrola used to sell fiberglass tubes. Parks Optical still does - with end rings. For a 6-inch f/10 it costs $139. I think that might be cheaper than aluminum!

I never used aluminum tubing. My scopes were always fiberglass, wood, or spiral paper. ATMs of my time knew better.

As for apertures smaller than 4" not being affected seriously by air turbulence, that has been well documented. You will notice that I also explained what I meant when I added that larger apertures are affected to "a much greater extent than apertures smaller than thermal air cells."

Of course, anything below 60 degrees will present thicker air mass and greater scintillation. Close to the astronomical horizon you never have air steady enough for observing with anything but the lowest power. Your best bet is always to observe at or near the Zenith.

Mladen




Unfortunately both Parks and Scope City have closed their doors, only Lumicon remains.

Regarding seeing stability in smaller scopes... If it's unstable it in a large scope, typically I can see it in 4 inch.

Jon


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5583013 - 12/21/12 06:48 PM

John Isaacs: The Parks site is still active.

If fiberglass is not available, phenolic is a good substitute, though it may be pricier. Paper tubes also make better tubes than aluminum (aluminum is one of the most heat conductive materials!). Tubes don't have to be round. I thin Texerau's method makes for excellent, lightweight, tubes made out of wood that anyone can afford and make with basic homeowner's tools. Anything but aluminum.

Mladen


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Mark Harry
Vendor
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Reged: 09/05/05

Loc: Northeast USA
Re: Optimized Newts vs APOs new [Re: MKV]
      #5583049 - 12/21/12 07:10 PM

I've used AL tubes for years, and -NEVER- had an issue with 6" aperture, period.
M.


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5583097 - 12/21/12 07:34 PM

I have built several Newts and always had good results with aluminum. A layer of 1/8" thick cork, spray painted black and cemented to the ID, removes essentially all thermal boundary layer effects.
Mike


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bremms
Carpal Tunnel
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Reged: 08/31/12

Loc: SC
Re: Optimized Newts vs APOs new [Re: Mike I. Jones]
      #5583132 - 12/21/12 08:00 PM

My 6" F8 has an 8" 1/8 wall tube. It's massive and is being replaced with a Protostar tube. 4 lbs vs 16 and better thermal properties. Needs end rings an a stiffener for the spider mounting. Mine takes about 2 hrs to really cool down properly. Turning my scope into a lightweight so it works on one of my Vixen GP mounts.

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Datapanic
Carpal Tunnel
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Reged: 10/17/09

Loc: Tucson, Arizona
Re: Optimized Newts vs APOs new [Re: bremms]
      #5583182 - 12/21/12 08:48 PM

The Horsetrail Cave has an aluminum tube (original paper/fiberglass tube was destroyed) and I use a nice fan that can be reversed if needed. It performs well with a flock-board interior. Aluminum is excellent for radiating and loosing heat faster than other materials such as painted fiberglass, wood or paper that act as insulators.

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StarStuff1
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Reged: 04/01/07

Loc: South of the Mason-Dixon Line
Re: Optimized Newts vs APOs new [Re: Datapanic]
      #5583194 - 12/21/12 08:56 PM

--

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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Mike I. Jones]
      #5583228 - 12/21/12 09:27 PM

Quote:

I have built several Newts and always had good results with aluminum. A layer of 1/8" thick cork, spray painted black and cemented to the ID, removes essentially all thermal boundary layer effects.
Mike



Of course! That's like insulating a house, Mike. Parks tubes needed neither paint nor cork, and their price was very competitive. Why buy aluminum?

Mladen


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: Optimized Newts vs APOs new [Re: MKV]
      #5583260 - 12/21/12 09:55 PM

One reason is, I've ordered a few Parks tubes over the decades, and every one I received had a few to several non-trivial cracks in the outer finish. It's not the fault of the tubing, but of shipping and packaging methods, and of the gorillas (mis)handling them. If I lived in the LA area again and could just drive over to Parks and buy one off the shelf, that'd be great. But I live in Texas, and smooth, pristine, dent-free irrigation pipe is available by the mile, and within easy driving distance.

Mike


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tim53
Postmaster
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Reged: 12/17/04

Loc: Highland Park, CA
Re: Optimized Newts vs APOs new [Re: MKV]
      #5583277 - 12/21/12 10:06 PM

I think the biggest problem with fiberglass is its flexibility.

People like Jeff Beish and Don Parker have bee outspoken proponents of aluminum tubes, unpainted on the outside, and of course at least painted black on the inside (paint is an insulator).

I make my tubes out of wood because I like it.

-Tim.


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plyscope
Pooh-Bah


Reged: 11/23/06

Loc: Perth, West Australia
Re: Optimized Newts vs APOs new [Re: tim53]
      #5583323 - 12/21/12 10:41 PM

I like wood too.

Here is a technical article on cooling for newtonians by Anthony Wesley.

web page


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5583542 - 12/22/12 02:48 AM

An optimized Newt would overwhelm any refractor. 12.5" to 14.5" is the sweet spot.

Edited by Daniel Mounsey (12/22/12 10:17 AM)


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