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a neglected design: Pfund telescopes

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#1 catalogman

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Posted 04 January 2015 - 12:53 PM

All,

 

Attached are some schematics of a little-known design -- the Pfund telescope.
The first one may properly be called a Pfund-Newtonian; the second one may
properly be called a Pfund-Herschelian. The Porter Turret telescope is the
most famous example of a Pfund telescope, except that the flat pivots and
the mount is at the end of the tube, opposite the concave mirror; the
illustrations given here assume a stationary flat and a conventional
alt-azimuth or equatorial mount.

 

Pfund telescopes have some advantages over the more conventional designs:

 

- LESS DIFFRACTION. Using curved vanes is the most common way to reduce
  diffraction in conventional Newtonians, but this approach smears detail
  out of the final image. The Pfund design eliminates the vanes entirely
  so there are no diffraction crosses on star images and no loss of
  planetary detail.

 

- LESS OBSTRUCTION. The size of the perforation in the flat is determined
  by the eyepiece O.D., so fast mirrors and slow mirrors would have the
  same obstruction. This means that a 16" f/4 Pfund mirror with a 1-1/4"
  eyepiece would suffer a light loss from obstruction of only 0.6%. No
  secondary vanes also means no obstruction of binary stars.

 

However, Pfund telescopes also have some disadvantages:

 

- MORE COSTLY. Pfund telescopes require two large mirrors instead of one;
  in fact, the flat has a major axis 1.414 (the square root of 2) times
  larger than the concave mirror.

 

- MORE COOLING TIME. Both of the large mirrors require temperature adjustment
  (cooling fans, heating resistors, or some other method of temperature
  compensation).

 

- MORE DIFFICULT TO MOUNT. The perforated flat cannot be made as a conical
  mirror and mounted by a bolt through the middle, so the problem of a pinched
  flat has to be carefully addressed with this design.
 

Despite the flaws in the Pfund design, the possibility of improved images
suggests that construction of these telescopes should be attempted. Any
reports or comments would be welcomed.

 

                                                                                          -- catalogman

Attached Thumbnails

  • pfund_newt.JPG
  • pfund_hersch.JPG


#2 bremms

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Posted 04 January 2015 - 01:04 PM

It would take a lot Pfunds to build such a scope with the giant flat. But it might be Pfun.


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#3 catalogman

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Posted 04 January 2015 - 01:09 PM

very pfunny :p


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#4 m. allan noah

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Posted 04 January 2015 - 01:21 PM

The pfundamental pfunctional dilemma is the mounting of the pfunky, pfunnelpform OTA. The odd eyepiece location might even require a pfunicular!

 

allan


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#5 Jeff Morgan

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Posted 04 January 2015 - 01:49 PM

- MORE DIFFICULT TO MOUNT. The perforated flat cannot be made as a conical

  mirror and mounted by a bolt through the middle, so the problem of a pinched
  flat has to be carefully addressed with this design.

 

The cell could be a bit of a challenge.

 

But as far as the whole telescope, there could be some huge advantages. See the August 1992 issue of Sky & Telescope for an example of how to mount a Pfund reflector. That solution is ergonomic, stable, and both mirrors can be mounted high enough off the ground to avoid the worst thermal gradients.


Edited by Jeff Morgan, 04 January 2015 - 01:51 PM.


#6 Mike I. Jones

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Posted 04 January 2015 - 06:50 PM

Watch "Wren-Marcario Accessible Telescope" on YouTube
Wren-Marcario Accessible Telescope: http://youtu.be/DTSJ2qJ9Ta8
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#7 Chriske

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Posted 05 January 2015 - 04:23 AM

Neglected...?  Not completely..!

Years ago we made one for our handicapped friend.

It's basically a 10" f/8 Newt with a 16" flat in front of it to collect the light.

There's a thread about this scope on CN, but I can't find it.

 

 

 

TM-team06.jpg

 

TM-team07.jpg

 

TM-team08.jpg

 

The team who built this scope.

 

TM-team01.jpg


Edited by Chriske, 05 January 2015 - 07:53 AM.

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#8 Mike I. Jones

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Posted 05 January 2015 - 07:07 AM

All,

 

Attached are some schematics of a little-known design -- the Pfund telescope.
The first one may properly be called a Pfund-Newtonian; the second one may
properly be called a Pfund-Herschelian.

Correction: both scopes in the diagram are on-axis systems; neither are Herschelian.  The eyepiece is on the same optical axis as the primary mirror.  The steering flat has (or better have) zero optical power; it merely redirects starlight from any direction into any fixed optical system focused at infinity.  That system can be a paraboloid, a Cassegrain, a refractor or any telescope, centered or tilted.



#9 wh48gs

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Posted 05 January 2015 - 07:51 AM

Neglected...?  Not completely..!

Years ago we made one for our handicapped friend.

It's basically a 10" f/8 Newt with a 16" flat in front of it to collect the light.

 

How cool! Made with brains and love.

 

Vla



#10 AlphaGJohn

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Posted 05 January 2015 - 09:06 PM

Very impressive, Chris.

 

It appears that the capture mirror is motor-driven. Neat. Azimuth also?

 

Is the eyepiece position adjustable? It looks to be. Does the whole Newt OTA rotate or just the, uh, UTA?

 

What a slick design & beautiful execution!

 

John



#11 Chriske

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Posted 05 January 2015 - 10:40 PM

Very impressive, Chris.

 

It appears that the capture mirror is motor-driven. Neat. Azimuth also?

 

Is the eyepiece position adjustable? It looks to be. Does the whole Newt OTA rotate or just the, uh, UTA?

 

What a slick design & beautiful execution!

 

John

Nope John,

 

There's no motor at all.

 

TM-team06.jpg

 

From right to left you see :

1- Scope itself 10" f/8 flexed mirror - fixed scope. At the very right (not visible in the picture) there are two large fans.

2- Focuser part, also housing the secondary mirror / curved spider.  Depending on the height of the wheelchair (child or grownup) this part can be rotated up or down.

3- Part three is housing the large 45° angled / 18" elliptical shaped mirror. This flat was the most difficult part to make. After completion I had it waterjetted in a elliptical shape. You'll see the telescope during open door at our observatory. It was a sunny day so at the very top you'll see a Mylar solar filter(see other picture higher up in this thread)

4- A cannibalized 3" refractor transformed into a drum-scope, acting as finderscope. But because of it's (to long) focal length we added a green laser pointer(Black curled wire) for easy location of objects in the nightsky.  Focuser of that drumscope is at the very end on the left(not adjustable in height).

5- At the very left we added a small solar finder. (small gray tube)

6- Because that scope was finished only hours before opening of our open door we had some balancing problems(we quickly added these two solar filters) so we had to add a clamp holding a piece of lead(very lower left).

 

Part 2 , 3 and also azimuth are ball bearing supported and rotates very smoothly just using fingertips.

As matter of fact in the vertical wooden pillar we housed a axis out of an old car to allow azimuth movement of course.

 

It took us a few months to design and build this scope, having lots of fun of course.


Edited by Chriske, 05 January 2015 - 11:37 PM.

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#12 catalogman

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Posted 06 January 2015 - 03:06 PM

 

All,

 

Attached are some schematics of a little-known design -- the Pfund telescope.
The first one may properly be called a Pfund-Newtonian; the second one may
properly be called a Pfund-Herschelian.

Correction: both scopes in the diagram are on-axis systems; neither are Herschelian.  The eyepiece is on the same optical axis as the primary mirror.  The steering flat has (or better have) zero optical power; it merely redirects starlight from any direction into any fixed optical system focused at infinity.  That system can be a paraboloid, a Cassegrain, a refractor or any telescope, centered or tilted.

 

Your point is technically true, but that's because in a Pfund
telescope the roles of the primary and secondary mirror are
interchanged. So in a true Herschelian, the concave mirror is
tilted slightly out of the optical path; in a Pfund-Herschelian,
the flat mirror is tilted slightly out of the optical path.

 

This method of naming Pfund configurations is easy to

generalize to the other types of Pfund scopes that you

describe. If there's a better way of naming these various

types of Pfund telescopes, then we'd like to know.

 

                                                                 -- catalogman



#13 Mike I. Jones

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Posted 06 January 2015 - 03:55 PM

Glad to tell everyone. No, the mirror roles are not interchanged in a Pfund. The flat diagonal secondary mirror in a Newtonian sequentially follows the only mirror with optical focusing power in the system, the paraboloidal primary mirror. If you tilt the Newtonian diagonal in a different direction, it changes the required eyepiece location, but does not change where the telescope is pointed at in the sky. The Pfund steering flat has zero optical power, and precedes the paraboloidal primary. All it does it feed the stationary telescope starlight from any angle in the sky its clear aperture can reflect from. The paraboloidal primary and eyepiece or camera are still a centered optical system with no tilted components, and are not a Herschelian configuration.
Mike
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#14 Ed Jones

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Posted 06 January 2015 - 04:07 PM

This design is a lot like my window scope .  The only difference is that I tilted the primary to avoid perforating the flat making it a Heschelian.  It needed 2 correcting lenses to correct the tilt aberrations.


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#15 Mike I. Jones

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Posted 06 January 2015 - 07:44 PM

Maybe this picture will help clarify the nomenclature.  Pfund's contribution was to make any telescope stationary, in a comfortable orientation, by feeding it with a steerable flat mirror.  It is entirely possible to have a Pfund-Herschelian as you note, by leaving a tilted-primary Herschelian and eyepiece stationary and feeding it with a steerable flat.  The Pfund-Herschelian flat would not be perforated.  Cassegrains, dialytes, schiefspieglers, Schupmann's, and any other telescope can be mounted stationary and fed with a high-quality steering flat.  Ed Jones's window-mounted Chief design is simply a significant improvement on the Herschelian by adding decentered and tilted lens elements in the path to improve the image quality over the field.

 

You can even sightsee and birdwatch with a Pfund; again, a well-made flat has zero optical power, and its only effect is to redirect light into a fixed telescope rather than having to slew the telescope assembly to the desired view.  Objects not at infinity still appear at the same distance when steered by a flat mirror, and the telescope is focused on the nearby object as though it was looking directly at it.

 

A special case of the Pfund is the heliostat, in which the outer gimbal rotation axis of the flat mount is aligned to the north or south celestial pole.  In this case, the flat is driven in right ascension at the same rate as a convention telescope mount, and the declination axis is simply orthogonal to the polar axis.  

 

Mike

Attached Thumbnails

  • Pfund configurations.png


#16 Mike I. Jones

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Posted 06 January 2015 - 07:56 PM

Making a Pfund telescope track objects requires a simple 3D vector-sum algorithm in the control computer software.  You can't drive the Pfund flat with conventional Dobson alt-az software.  Oh you can try, but it won't track anything.  With this algorithm, and accounting for atmospheric refraction, the Pfund will track an object all night.  The image does rotate over time, however, and this field rotation would have to be added to the algorithm, along with a precision computer-controlled rotation stage to keep from getting arcs around the edge of the FOV rather than points.

Mike

 

Attached Thumbnails

  • Pfund drive algorithm.png


#17 Mike I. Jones

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Posted 06 January 2015 - 08:29 PM

This is a nice overview chart of the 18" Wren-Marcario Accessible Telescope (WMAT) at the McDonald Observatory Visitor Center.  I designed all the optical systems for it.  By using two matching primary mirrors, we were able to cover the entire sky except for small keepout zones around each primary.  Carl Zambuto made both 18" f/8 fused silica primaries (incredibly accurately, BTW), and the 24" perforated flat received final figuring at the Zygo Corporation to a flatness well below 1/10 wave P-V.  Las Cumbres Observatory made the tracking mount, and it points and tracks like a dream.  The staff at McDonald say this telescope consistently gives the sharpest images on the mountain.  We dedicated it in July of 2010, and even had several very excited wheelchair-bound visitors look through it on the inaugural night.  It was a wonderful project and team to work with.
Mike

Attached Thumbnails

  • WMAT overview.png

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#18 Chriske

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Posted 07 January 2015 - 12:52 AM

This is a nice overview chart of the 18" Wren-Marcario Accessible Telescope (WMAT) at the McDonald Observatory Visitor Center.  I designed all the optical systems for it.  By using two matching primary mirrors, we were able to cover the entire sky except for small keepout zones around each primary.  Carl Zambuto made both 18" f/8 fused silica primaries (incredibly accurately, BTW), and the 24" perforated flat received final figuring at the Zygo Corporation to a flatness well below 1/10 wave P-V.  Las Cumbres Observatory made the tracking mount, and it points and tracks like a dream.  The staff at McDonald say this telescope consistently gives the sharpest images on the mountain.  We dedicated it in July of 2010, and even had several very excited wheelchair-bound visitors look through it on the inaugural night.  It was a wonderful project and team to work with.
Mike

Nice one...!.... :waytogo: :bow:


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#19 GShaffer

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Posted 07 January 2015 - 01:10 AM

Wow....very cool design and implementation and for such a WORTHY reason!

Mike I.Jones, your words back when I started building my 8" F/16 Muffoletto Optics refractor just took on a whole new depth of meaning. It would appear you have forgotten more than most of us know lol. That project is finally back underway BTW :) Already built a mockup and did some testing. Looks like you were right about the Muffoletto lens. I believe it is going to be an awesome scope.....

Edited by GShaffer, 07 January 2015 - 01:10 AM.


#20 joemazda585

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Posted 07 January 2015 - 03:57 PM

forgive a newbie for this question- is there another name for this setup? im trying to search online and cant get more than 1 or 2 hits about "pfund" scope, and only one significant is McDonald Observatory.

 

also- Great thing you did designing an accessible scope.  :)


Edited by joemazda585, 07 January 2015 - 03:59 PM.


#21 Astrojensen

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Posted 07 January 2015 - 05:01 PM

Another amateur and I discussed Pfund telescopes many years ago over on AstroMart. We called them reverse newtonians. He had built a 10" f/6, which he said had absolutely amazing performance. I can't for the life of me remember his name, but I do believe he may be here on CN as well. 

 

I wanted to build one as well, for a 6" f/10 mirror I still have lying about somewhere, but I am no mirror maker and buying the perforated flat would take a custom order and I couldn't afford that.

 

 

Clear skies!
Thomas, Denmark



#22 Diego

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Posted 07 January 2015 - 05:18 PM

Making a Pfund telescope track objects requires a simple 3D vector-sum algorithm in the control computer software. You can't drive the Pfund flat with
conventional Dobson alt-az software. Oh you can try, but it won't track anything. With this algorithm, and accounting for atmospheric refraction, the Pfund will track an object all night. The image does rotate over time, however, and this field rotation would have to be added to the algorithm, along with a precision computer-controlled rotation stage to keep from getting arcs around the edge of the FOV rather than points.
Mike


Regarding the tracking of the scope, if the flat was mounted on an equatorial fork aligned with the polar axis, would it then track only requiring movment in RA?

#23 Mike I. Jones

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Posted 07 January 2015 - 06:55 PM

Right, Diego. What you describe is a heliostat, which is an equatorially mounted Pfund. The outer gimbal axis is aligned to the pole and driven at the same rate and direction. The flat declination is angled to keep the reflected beam aimed right up the polar axis, giving a rotating but non-translating image.

A good thread on this was started by "Optinuke" here on CN on Sept 2 2014 over in the Solar Observing forum.
Mike

#24 Jeff Morgan

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Posted 08 January 2015 - 01:01 AM

This is a nice overview chart of the 18" Wren-Marcario Accessible Telescope (WMAT) at the McDonald Observatory Visitor Center.  I designed all the optical systems for it.  By using two matching primary mirrors, we were able to cover the entire sky except for small keepout zones around each primary.  Carl Zambuto made both 18" f/8 fused silica primaries (incredibly accurately, BTW), and the 24" perforated flat received final figuring at the Zygo Corporation to a flatness well below 1/10 wave P-V.  Las Cumbres Observatory made the tracking mount, and it points and tracks like a dream.  The staff at McDonald say this telescope consistently gives the sharpest images on the mountain.  We dedicated it in July of 2010, and even had several very excited wheelchair-bound visitors look through it on the inaugural night.  It was a wonderful project and team to work with.
Mike

Amazing set-up!

 

I'm curious if one did not want to install two identical primaries and could accept 1/2 sky coverage if the steering and primary mirrors could not be oriented east-west instead of north-south?


Edited by Jeff Morgan, 08 January 2015 - 01:02 AM.


#25 Chriske

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Posted 08 January 2015 - 03:06 AM

I wanted to build one as well, for a 6" f/10 mirror I still have lying about somewhere, but I am no mirror maker and buying the perforated flat would take a custom order and I couldn't afford that.

 

 

Clear skies!
Thomas, Denmark

Thomas,

 

You could make that flat yourself. The only extra's you need to make a flat is a few extra blanks.

And there's no need to drill a hole in that flat. We've put a secondary mirror in that scope, the same way as in a newt. The only thing: it's not a real Pfund this way.




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