Jump to content


Photo

Long Focus Newts?

  • Please log in to reply
48 replies to this topic

#1 cwilson

cwilson

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 01 Oct 2012
  • Loc: SW Oklahoma

Posted 25 April 2014 - 08:13 PM

Does anyone sell high quality long focus Newts these days? Say something like a 6" f/8?

#2 jzeiders

jzeiders

    Viking 1

  • -----
  • Posts: 520
  • Joined: 07 Nov 2012
  • Loc: SF Bay Area

Posted 25 April 2014 - 10:10 PM

You can check with a top mirror maker and see what the rice would be for a custom mirror, You could also make your own, An f/8 is fairly easy and once you make your own there is nothing quite like first light with a mirror you made. Don't worry about better than 1.4 or 1.8 wave, that is plenty if you have a smooth curve, a good edge (not turned) and are fully polished out.

Back in the early '70s I made an 8" f/8 and it was a wonderful scope. Planets and doubles were wonderful and deep sky was just fine. The long tube was a bit of a hassle but not really a big deal. Ray trace it and make a set of baffles for it, it makes a big difference in contrast. Also use a good quality secondary.

I think Orion sells or did sell a 6" f/8 dob. How good are they? I dunno.

Jack

#3 ylem

ylem

    Viking 1

  • -----
  • Posts: 630
  • Joined: 27 Jan 2009
  • Loc: Entropy, Monroe, NC

Posted 25 April 2014 - 10:14 PM

Other then the Orion xt6, I don't think so......
I hope I'm wrong and others will let us know where to look :grin:

#4 youngamateur42

youngamateur42

    Surveyor 1

  • *****
  • Posts: 1855
  • Joined: 21 Nov 2012
  • Loc: La Verne, CA

Posted 25 April 2014 - 10:18 PM

If want to go a different route, you can get an old Criterion RV-6 for a relatively good price. They are excellent scopes, and the tracking is a plus. I love mine.

#5 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10462
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 25 April 2014 - 10:55 PM

Orion's xt6 has its fans to be sure. Refurbishing an old classic like RV6 would be a fine project too - particularly with the dandy coatings out there today.

Long focus newts aren't the rage they were back in the 60's as most observers these days are deepsky fans preferring larger aperture that would make f/8 impractical. My own 8" f/9 was a custom order from PARKS.

Like refractor fans, long focus reflector fans love the views specific to it.

Pete

#6 TCW

TCW

    Vanguard

  • *****
  • Posts: 2156
  • Joined: 05 Nov 2013
  • Loc: Somewhere in a galaxy far, far away.

Posted 25 April 2014 - 11:24 PM

Check out RF Royce.

http://www.rfroyce.c...e newtonian.htm

#7 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 25 April 2014 - 11:55 PM

Check out RF Royce.

http://www.rfroyce.c...e newtonian.htm


Bob Royce makes an excellent mirror (I have owned four of them over the years), and he still has mirrors as small as 6" on his price list. I am pretty sure he would do the slightly longer f/8 on a custom basis.

#8 TCW

TCW

    Vanguard

  • *****
  • Posts: 2156
  • Joined: 05 Nov 2013
  • Loc: Somewhere in a galaxy far, far away.

Posted 26 April 2014 - 12:54 AM

Check out these threads.

http://www.cloudynig...6080213/page...


http://www.cloudynig...6266266/page...

#9 ylem

ylem

    Viking 1

  • -----
  • Posts: 630
  • Joined: 27 Jan 2009
  • Loc: Entropy, Monroe, NC

Posted 26 April 2014 - 07:39 AM

Ahhh, life in the slow lane :smirk:

My fastest scope is f8

#10 herrointment

herrointment

    Aurora

  • *****
  • Posts: 4923
  • Joined: 12 Mar 2011
  • Loc: East of Poskin

Posted 26 April 2014 - 01:59 PM

I purchased a 1980's homemade 8" f/8.5 Newtonian on a Meade Starfinder mount for $250.00 a few years back.

When conditions are good you can hardly tear yourself away from the eyepiece.

It IS a planet killer!

#11 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 27 April 2014 - 12:35 AM

My old 10" f/9 is still the only telescope to show me spokes in Saturn's rings. Chicago optician Dan Joyce and I both saw them at the Astrofest star party.

I do have high hopes for my Zambuto mirrors - one is a 16" f/7, the other in process is a 8" f/9.

Here is the old 10" at the Winter Star Party, circa 1997.

Attached Files



#12 precaud

precaud

    Surveyor 1

  • *****
  • Posts: 1664
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 27 April 2014 - 07:08 AM

I love that pic, Jeff. It makes me think of building something like that for use at home, I don't think I'd want to haul it around. Some kind of motorized observing chair would be nice too.

#13 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 27 April 2014 - 08:55 AM

Thanks John. Selling that scope was one of my biggest astronomy mistakes ever.

I decided to rectify that by building a 8" f/9 scope. The aperture, true field, and focal length were chosen as a bridge between my 16" f/7 and a 4.7" apo refractor. I should be able to use it from a sitting position using a CatsPerch chair.

#14 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10462
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 27 April 2014 - 09:56 AM

You'll like the f/9 and the tube at a mere 35lbs is long but light to carry. Its never a bother. By contrast the heft of a 10" f/5 or bigger is significant heavier. I do use a simple step ladder but its a step up only. Its never a climb. The seat I'm making -an aspen chair- ought to be all that.



Pete

#15 Sean Cunneen

Sean Cunneen

    Let Me Think

  • *****
  • Posts: 3268
  • Joined: 01 Aug 2007
  • Loc: Blue Island Illinois

Posted 27 April 2014 - 10:17 AM

They are out there. The good news is that long focus optics tend to be less expensive, but they are rare and don't come p for sale often. If you get your hands on some, they are potent performers!

Attached Files



#16 cwilson

cwilson

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 01 Oct 2012
  • Loc: SW Oklahoma

Posted 27 April 2014 - 02:06 PM

I live in an apartment so ATM is not practical for me. But I remember the great views I got many years ago from an 6" f/9 that my uncle built. He had access to an optical shop at a major university, so it was of very high quality. It was a great scope!

I was thinking having something along those lines would make a great planetary scope. Far less costly then a larger refractor.

#17 azure1961p

azure1961p

    Voyager 1

  • *****
  • Posts: 10462
  • Joined: 17 Jan 2009
  • Loc: USA

Posted 27 April 2014 - 03:49 PM

I disagree,

Being an apartment dweller would seem to make ATM design and fab a solve for those issues particular to your dwelling needs.

Pete

#18 cwilson

cwilson

    Mariner 2

  • -----
  • Posts: 218
  • Joined: 01 Oct 2012
  • Loc: SW Oklahoma

Posted 27 April 2014 - 04:53 PM

I disagree,

Being an apartment dweller would seem to make ATM design and fab a solve for those issues particular to your dwelling needs.

Pete


The problem is lack of workspace. It's pretty small. ;)

#19 precaud

precaud

    Surveyor 1

  • *****
  • Posts: 1664
  • Joined: 05 Dec 2012
  • Loc: north central New Mexico

Posted 27 April 2014 - 05:14 PM

Oh, don't let that stop you, living rooms make great workshops in the absence of other available spaces. Of course, it helps not having a partner to ask permission from :) This is the 14 foot long solar heater I built in the living room, it was the only indoor place I could build it AND maneuver the finished unit outside from. Thankfully, I thought of that before I started on it!

Attached Files



#20 figurate

figurate

    Ranger 4

  • -----
  • Posts: 315
  • Joined: 09 Feb 2013
  • Loc: Metro North Texas

Posted 27 April 2014 - 08:14 PM

Do you guys feel that any perceptible improvements, in going from F7 - F8, to F9 or 10, are worth the additional length and bulk? (Given equal optical quality) I still remember the views through a 10" F7 at a star party many years ago. I know the long focal length comes in handy for planets, but is there any other determining factor over a comparably smooth, say F7 telescope?

This is one of my "slow" telescopes; since I masked the TDE (my first and only mirror) it works at about F7. More to the point, it was almost 30 years in the making...

Attached Files



#21 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 27 April 2014 - 08:18 PM

They are out there. The good news is that long focus optics tend to be less expensive, but they are rare and don't come p for sale often. If you get your hands on some, they are potent performers!


And add to that:

- Collimation is hardly more difficult than getting the mirrors in the tube;
- Collimation stays put all night long;
- $100 eyepieces perform great; and
- 1-2 mm exit pupils without resorting to super short eyepieces;

#22 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 4139
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 27 April 2014 - 08:49 PM

Do you guys feel that any perceptible improvements, in going from F7 - F8, to F9 or 10, are worth the additional length and bulk? (Given equal optical quality) I still remember the views through a 10" F7 at a star party many years ago. I know the long focal length comes in handy for planets, but is there any other determining factor over a comparably smooth, say F7 telescope?


That's a good question, and I've read opinions both ways. Not saying I believe the doubters, there's always been something appealing about slow designs for planetary, for sure.

You couple together this:

Smaller secondary
Likely better baffling
Ease of getting closer to ideal figure

The last one though is somewhat of a tossup. I've made a few long mirrors and I ended up redoing most of them several times before I was satisfied.

It is however probably easier for most people to maintain smoothness while introducing a gentle parabola.

Some years back during the impressive Mars opposition, at OSP I brought a modified EP with an occulting bar and wanted to bag both moons with my 8" f/5.8. I got Phobos, but everytime I tried Deimos was behind the disk. Ed Stevens had just finished a 10" f/7 in a PVC tube (sort of a bad idea due to vibration) and we tried for them with that as well. I spotted Phobos with his as well, but he never did for sure. My 8" has never lacked for planetary performance, but it is pretty well optimized, and the planet itself was spectacular.

The final factor is probably how easy it is to hog out the curve. :lol:

Best,
Mark

#23 Jeff Morgan

Jeff Morgan

    Fly Me to the Moon

  • *****
  • Posts: 5788
  • Joined: 28 Sep 2003
  • Loc: Prescott, AZ

Posted 27 April 2014 - 11:09 PM

Do you guys feel that any perceptible improvements, in going from F7 - F8, to F9 or 10, are worth the additional length and bulk? (Given equal optical quality) I still remember the views through a 10" F7 at a star party many years ago. I know the long focal length comes in handy for planets, but is there any other determining factor over a comparably smooth, say F7 telescope?

This is one of my "slow" telescopes; since I masked the TDE (my first and only mirror) it works at about F7. More to the point, it was almost 30 years in the making...


There was a discussion of this a couple of years ago on the Zambuto Optical group (Yahoo). If memory serves, someone had posited that f/7 was the "break-even" point since the comatic blur size falls below the eyes resolution limit at that focal ratio.

On the other hand, Chris Lord offers a neat mathematical derivation of the mechanical tolerances in collimation for diffraction limited performance in double star observing here. I would think the analysis would apply to planetary observing as well. His conclusions are that for high resolution work you need an aplanatic telescope, so the benefits keep coming by going longer. But he points out that due to physical layout the Newtonian in fact scales poorly, limiting useful size to about 10".

#24 figurate

figurate

    Ranger 4

  • -----
  • Posts: 315
  • Joined: 09 Feb 2013
  • Loc: Metro North Texas

Posted 27 April 2014 - 11:43 PM

Mark and Jeff, thanks for two excellent answers. I've always felt, based on views through various apertures, that the sweet spot for newtonians is
8 to 11 inches, maybe 12, working at F6 or higher.

Fred

#25 Mark Harry

Mark Harry

    Vendor

  • *****
  • Vendors
  • Posts: 6201
  • Joined: 05 Sep 2005
  • Loc: Northeast USA

Posted 28 April 2014 - 04:53 AM

Nice link, Jeff. It reveals -why- I like 6" F/10s so much.
:bow:
M.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics