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Equipment Discussions >> Reflectors

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Bob S.
Carpal Tunnel
*****

Reged: 07/14/05

Re: just when we thought we were safe new [Re: derangedhermit]
      #5990785 - 07/26/13 06:38 AM

Quote:

Quote:

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)




I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee




Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else of the same aperture size as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.

Edited by Bob S. (07/26/13 05:41 PM)


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GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: just when we thought we were safe new [Re: Jon Isaacs]
      #5991442 - 07/26/13 02:56 PM

If you have a ladder scope...getting a ladder like that one Jon is very important...especially for when someone not experienced wants to take a look. I would love to join you out there.

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derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: just when we thought we were safe new [Re: Bob S.]
      #5991742 - 07/26/13 06:04 PM

Quote:

Quote:

Quote:

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)




I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee




Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.



Lockwood is one of the "few people" I mentioned above. It is a remarkable accomplishment that an f/2.75 primary can give better views than an equally well made f/3.5.

I retract the statement about doubting anyone wants to make an f/2 paraboloid, since Lockwood says on his web site that he will make them, although I don't know if he would recommend one for visual use in a Newtonian.

I would very much like to be able to afford a large excellent f/3 such as yours. That's why I have been looking into very fast large mirrors, and attempted to answer the question.

Do you know why the SIPS solution works better for coma correction over a range of eyepiece focal lengths than the Paracorr II? It is the same optical design, is it not? Are the eyepieces parfocal, or close to?

Can you give ballpark relative price differences between a 20" f/4, a 20" f/3.5" and a 20" f/3? I would guess going from f/4 to f/3.5 costs at least 50% more, and going from f/3.5 to f/3 perhaps double.

Lee


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Bob S.
Carpal Tunnel
*****

Reged: 07/14/05

Re: just when we thought we were safe new [Re: derangedhermit]
      #5991844 - 07/26/13 07:09 PM

Quote:

Quote:

Quote:

Quote:

Could a more exotic optical design handle F/2 for visual use? (No, SCTs don't count - any full aperture corrector will fail as it scales to larger diameters.)




I've been asking about this. You're going to have a large secondary in any practical solution. I doubt you'll find anyone wanting to make an f/2 paraboloid, hyperboloid, or ellipse, so you're left with the SCT-type spherical mirror. Cheap to make, but very very difficult to correct the tons of spherical aberration for large diameter primaries by using subaperture correctors. At least that's my understanding. CDKs and Cassegrains use higher primary focal ratios, above f/3. Oh, at f/2 you probably need a 3" focuser with 2" adapter to avoid drawtube vignetting.

About the best you can do is f/3 and a Paracorr, and you're still pushing it - there are a few people that know how to make such a mirror and build such a thing, but a lot less than can build at f/5, or even f/4.

Lee




Lee, Having done a serious "Pepsi Challenge" between my former 28" f/3.5 Lockwood/Starmaster and a 28" f/2.75 Lockwood/Webster, I can tell you for sure that the f/2.75 is not only doable but with a just noticeable difference was beating my f/3.5.

I would feel very comfortable putting my 20" f/3 Lockwood/JP Astrocraft up against anything else as long as you let me use the SIPS system from Starlight Instruments/TeleVue and Ethos ep's. The SIPS (Starlight Integrated Paracorr System) is able to more adequately correct for coma with different focal length eyepieces than the Paracorr II that I have.



Lockwood is one of the "few people" I mentioned above. It is a remarkable accomplishment that an f/2.75 primary can give better views than an equally well made f/3.5.

I retract the statement about doubting anyone wants to make an f/2 paraboloid, since Lockwood says on his web site that he will make them, although I don't know if he would recommend one for visual use in a Newtonian.

I would very much like to be able to afford a large excellent f/3 such as yours. That's why I have been looking into very fast large mirrors, and attempted to answer the question.

Do you know why the SIPS solution works better for coma correction over a range of eyepiece focal lengths than the Paracorr II? It is the same optical design, is it not? Are the eyepieces parfocal, or close to?

Can you give ballpark relative price differences between a 20" f/4, a 20" f/3.5" and a 20" f/3? I would guess going from f/4 to f/3.5 costs at least 50% more, and going from f/3.5 to f/3 perhaps double.

Lee




Lee, I cannot speak to the costs with any kind of authority. The Paracorr II has recommended settings that sometimes are kind of ballpark wheras the SIPS allows all of the ep's to be spaced at the correct distance for sharpest/tightest views. The SIPS has a larger range of movements I think than the regular Paracorr but it also only uses a 1.5" focus travel for the focuser itself. In the Pepsi challenge that we did, the one variable that was different between the two scopes was that the f/2.75 was using the SIPS and I was using a Paracorr II. On my recent f/3, I chose the SIPS but sometimes still revert to the Paracorr II for cerain applications. I think Mike Lockwood has suggested that going faster than f/2.75 is not recommended by him. He has a test scope with an f ratio of f/2.55 and so I think he has had enough experience given that he also has a 20" f/3 just like mine that is 1.25" thick and he has come up with those recommendations.

It is just my gut feeling but I would not feel comfortable using anybody but Lockwood for a mirror in the f/2.75-f/3.0 range at this time. I think he has amply demonstrated his ability to nail these ultra-fast mirrors. I would suspect that some other very skilled opticians are honing their skills in this range and it is likely that Lockwood will not occupy this rare space for long? Only time will answer this question. And, as Mel Bartel opined at a lecture I attended a couple of years ago, the speed of the mirrors will be constrained by the correctors and eyepieces built to handle ever faster optics. Bob


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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: just when we thought we were safe new [Re: Mike B]
      #5992254 - 07/27/13 12:19 AM Attachment (6 downloads)

Quote:

Considering the things which go bump in the night, and not wishing to numbered amongst them, i've always been inclined to keep both feet on the ground. So i stopped the fever at 15" F4.5.... however, given the opportunity to keep a larger rig permanently stationed in the boondocks, i think i could manage with a ladder like Jon's.




Actually these types of ladders move quite nicely. Even on rocky ground they are quite stable. You can even lean sharply over the rail and feel safe.

While I suppose a full-sized van or pick-up truck would work, the key is to have some type of trailer at your disposal. My kayak micro-trailer does double duty.

Of course the whole premise of this thread is flawed. Assuming more than a few dozen of the sub f/3's ever get built, even those owners will soon want more aperture (you just can't beat human nature). And then it's back to ladders.

Spend a little more for a ladder you feel secure on and a lot of problems get bypassed from the start.

Edited by Jeff Morgan (07/27/13 12:20 AM)


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jgraham
Postmaster
*****

Reged: 12/02/04

Loc: Miami Valley Astronomical Soci...
Re: just when we thought we were safe new [Re: Jeff Morgan]
      #5992602 - 07/27/13 08:56 AM

C'mon now, ladders have their benefits. Objects are brighter and clearer 'cause you are higher in the atmosphere and you are closer to the object.

I used a homebuilt 16.5" f/6.5 for many years with a nice lightweight fiberglass ladder and I have a 16" Lightbridge as a more compact and portable companion. The Lightbridge is uber comfortable to use, but there is a bit of magic climbing the ladder to look through my big scope.

Enjoy.


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: just when we thought we were safe new [Re: jgraham]
      #5993048 - 07/27/13 01:35 PM

Quote:

Objects are brighter and clearer 'cause you are higher in the atmosphere and you are closer to the object.



Plus, it's possible that, instead of flowing down into the earth for standing viewers, the starlight puddles in your toes for ladder'd viewing.




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Jeff Morgan
Postmaster
*****

Reged: 09/28/03

Loc: Prescott, AZ
Re: just when we thought we were safe [Re: jgraham]
      #5993934 - 07/28/13 12:03 AM

Quote:

C'mon now, ladders have their benefits. Objects are brighter and clearer 'cause you are higher in the atmosphere and you are closer to the object.






Now if we could just figure out a way to get the primary mirrors away from the ground ...


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