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Main
Date: 16 Sep 2007 18:31:12
From: Quadibloc
Subject: The Aplanatic Gregorian: A Neglected Telescope Design?
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I've been doing some web searches about different types of telescopes.
As many of you know, the popular Schmidt-Cassegrain telescope design
was made by only three American companies - Celestron, Meade, and
Bausch and Lomb - and was beset by patent and trade secret litigation.
As a result, Bausch and Lomb is no longer in that field, and there are
no newcomers trying to enter it.
But inexpensive Chinese-made Maksutov telescopes are starting to
appear on the market, in sizes as large as 180mm (7 inches).
Things aren't perfect; nowadays, one's choice is between a
computerized *altazimuth* fork mount or a German Equatorial - the
equatorial fork mount, the ideal mount from an astrophotography
perspective, is nearly unavailable (presumably because it is a bit
more expensive, and price competition has become stronger).
In any event, one of the telescopes I came across was a modified
Maksutov-Cassegrain by Astro-Physics with an *ellipsoidal* primary
mirror, which was coma-corrected. It had a 10-inch aperture, so I
suspect it was quite expensive; it is no longer available, and the
company seems to make only small refractors now.
Another thing I came across was that the aplanatic (coma free) version
of the Gregorian telescope, unlike the Ritchey-Chretien, used two
*ellipsoidal* surfaces. So it would have been easier for an amateur to
grind and test such mirrors - the popularity of the Dall-Kirkham
design, for example, is due to the fact that traditional methods were
suitable for testing ellipsoidal figures, but not hyperboloidal ones.
Of course, not many people grind their own mirrors nowadays! And
grinding just one aspheric mirror is enough to give someone a working
Newtonian telescope, and so that would be enough for most amateurs in
the past. But given that there are now, and were then, many ATMs that
have experimented with advanced designs, I'm surprised I had never
heard of the aplanatic Gregorian until now.
The telescope design on this page
http://www.telescope-optics.net/Houghton4.htm
in figures 101 and 102 is also very impressive: although the use of
lenses with one flat surface means more lenses are required, it could
also mean that the telescope could be built entirely with "off-the-
shelf" parts!
John Savard
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Date: 19 Sep 2007 00:54:44
From: atasselli@hotmail.com
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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On 17 Sep, 03:25, Chris L Peterson <c...@alumni.caltech.edu > wrote:
> On Sun, 16 Sep 2007 18:53:51 -0700, RMOLLISE <rmoll...@hotmail.com>
> wrote:
>
> >> Things aren't perfect; nowadays, one's choice is between a
> >> computerized *altazimuth* fork mount or a German Equatorial - the
> >> equatorial fork mount, the ideal mount from an astrophotography
> >> perspective, is nearly unavailable (presumably because it is a bit
> >> more expensive, and price competition has become stronger).
> >I'd hardly call the fork ideal for astrophotography. It's great for
> >visual use in alt-az mode, but a pain to image with compared to a GEM.
>
> I agree with John that the equatorial fork is pretty much the ideal
> astroimaging mount- certainly far better than a GEM, which is usually a
> PITA when imaging. Of course, most of the fork mounts that people use
> aren't made as well as they could be, but that isn't an indictment of
> fork mounts, just the specific designs. You _can_ buy very good fork
> mounts, which will run circles around any GEM for imaging, but of course
> you'll spend twice as much (worth it, IMO). There's a reason that very
> few professional scopes use GEM mounts.
>
While agreeing on the fact that *some* equatorial fork mounts are
better than GEMs in principle (better mass distribution, less mass per
unit aperture and better dynamics) GEMs for the AA is more than
adequate given the size of the telescopes used and far more versatile,
given that you can use one for so many different telescopes both in
size and design (think refractors, cassegrains and newtons), something
that cannot be done with equatorial fork mounts. The reason why some
variations of the basic design for equatorial fork mount is used by
the pros (although only for smaller apertures nowadays) is because of
the sheer size of them and because there is no need for swapping OTAs.
BTW, for smaller scopes even pros are using GEMs (Bisque's comes to my
mind).
Andrea T.
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Date: 18 Sep 2007 20:27:06
From: Quadibloc
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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RMOLLISE wrote:
> On Sep 17, 10:24 pm, Quadibloc <jsav...@ecn.ab.ca> wrote:
> > As for the sizes that cost in five figures, I am not concerned with
> > them.
> I've yet to see a wedge that costs "five figures."
I meant the cost of the whole telescope. Obviously, if I can't afford
to buy the telescope, I won't need the wedge for it.
John Savard
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Date: 19 Sep 2007 03:57:42
From: Jim Jam Gee
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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WTF is a quadibloc?
"Quadibloc" <jsavard@ecn.ab.ca > wrote in message
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Date: 19 Sep 2007 12:28:57
From: John Savard
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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On Wed, 19 Sep 2007 03:57:42 GMT, "Jim Jam Gee" <JJG@cfc.org > wrote, in
part:
>WTF is a quadibloc?
I replied to that question not very long ago. QUADIBLOC stands for QUick
And Dirty Implementable BLOck Cipher, which is described at
http://www.quadibloc.com/crypto/co0407.htm
John Savard
http://www.quadibloc.com/index.html
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Date: 18 Sep 2007 12:27:29
From: RMOLLISE
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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On Sep 18, 6:18 am, Helpful person <rrl...@yahoo.com > wrote:
> On Sep 18, 12:10 am, "UI/FOX-Simpson Athletic Dept."<iowac...@ioway.org> wrote:
> > Quadibloc wrote:
> > > I've been doing some web searches about different types of telescopes.
>
> > > As many of you know, the popular Schmidt-Cassegrain telescope design
> > > was made by only three American companies
>
> > uhhh, you missed a few but whad da heck! Its American.
> > Averyone is entitled to bloviate.
>
> Actually acording to Wikipedia Schmidt was from Estonia and lived and
> Germany and Cassigrain was French.
>
> www.richardfisher.com
Yeah, but neither man ever made an SCT. The person to thank for that
design--at least for making the design feasible to mass produce--is a
Californian, Tom Johnson.
Unk.
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Date: 18 Sep 2007 08:18:16
From: AES
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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In article <1190118449.188873.297620@19g2000hsx.googlegroups.com >,
RMOLLISE <rmollise@hotmail.com > wrote:
>
> Yeah, but neither man ever made an SCT. The person to thank for that
> design--at least for making the design feasible to mass produce--is a
> Californian, Tom Johnson.
>
Johnson, or Johnston?
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Date: 18 Sep 2007 12:25:45
From: RMOLLISE
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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On Sep 17, 10:24 pm, Quadibloc <jsav...@ecn.ab.ca > wrote:
> RMOLLISE wrote:
> > On Sep 16, 9:06 pm, Quadibloc <jsav...@ecn.ab.ca> wrote:
> > > RMOLLISE wrote:
> > > > Are you aware that computerized "alt-az" forks can also be used in
> > > > equatorial mode with the addition of a wedge?
>
> > > Some of them, at least in the smaller sizes... no, that probably isn't
> > > accurate, and optional wedges are still available for the larger
> > > sizes, even if one doesn't see them advertised any more.
>
> > > I must resist the impulse to believe that if it isn't advertised in
> > > *this* months Sky and Telescope, it doesn't exist.
>
> > Right. You can get a wedge for a 16-inch fork mount LX200.
>
> > Not exactly... ;-)
>
> I understand there are limits. The *smallest* sizes actually include a
> built-in rod to adjust them to equatorial. Perhaps there are wedges
> for the 8 and 10 inch.
>
> As for the sizes that cost in five figures, I am not concerned with
> them.
>
> Of course, if we all *did* decide to assume that things not advertised
> in this month's Sky and Telescope don't exist - then they would have
> to put their ads back in again, and it would grow thicker! (The
> subject, of course, of another thread.)
>
> John Savard- Hide quoted text -
>
> - Show quoted text -
I've yet to see a wedge that costs "five figures."
This very fancy model for the 16 is 3500:
http://www.optecinc.com/optec_039.htm
Unk
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Date: 18 Sep 2007 04:18:25
From: Helpful person
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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On Sep 18, 12:10 am, "UI/FOX-Simpson Athletic Dept."
<iowac...@ioway.org > wrote:
> Quadibloc wrote:
> > I've been doing some web searches about different types of telescopes.
>
> > As many of you know, the popular Schmidt-Cassegrain telescope design
> > was made by only three American companies
>
> uhhh, you missed a few but whad da heck! Its American.
> Averyone is entitled to bloviate.
>
>
>
Actually acording to Wikipedia Schmidt was from Estonia and lived and
Germany and Cassigrain was French.
www.richardfisher.com
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Date: 18 Sep 2007 08:16:50
From: AES
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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In article <1190114305.894373.88080@g4g2000hsf.googlegroups.com >,
Helpful person <rrllff@yahoo.com > wrote:
> >
> Actually acording to Wikipedia Schmidt was from Estonia and lived and
> Germany and Cassigrain was French.
>
And what nationaity was Moire ???
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Date: 17 Sep 2007 23:10:35
From: UI/FOX-Simpson Athletic Dept.
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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Quadibloc wrote:
> I've been doing some web searches about different types of telescopes.
>
> As many of you know, the popular Schmidt-Cassegrain telescope design
> was made by only three American companies
uhhh, you missed a few but whad da heck! Its American.
Averyone is entitled to bloviate.
> - Celestron, Meade, and
> Bausch and Lomb - and was beset by patent and trade secret litigation.
> As a result, Bausch and Lomb is no longer in that field, and there are
> no newcomers trying to enter it.
>
> But inexpensive Chinese-made Maksutov telescopes are starting to
> appear on the market, in sizes as large as 180mm (7 inches).
>
> Things aren't perfect; nowadays, one's choice is between a
> computerized *altazimuth* fork mount or a German Equatorial - the
> equatorial fork mount, the ideal mount from an astrophotography
> perspective, is nearly unavailable (presumably because it is a bit
> more expensive, and price competition has become stronger).
>
> In any event, one of the telescopes I came across was a modified
> Maksutov-Cassegrain by Astro-Physics with an *ellipsoidal* primary
> mirror, which was coma-corrected. It had a 10-inch aperture, so I
> suspect it was quite expensive; it is no longer available, and the
> company seems to make only small refractors now.
>
> Another thing I came across was that the aplanatic (coma free) version
> of the Gregorian telescope, unlike the Ritchey-Chretien, used two
> *ellipsoidal* surfaces. So it would have been easier for an amateur to
> grind and test such mirrors - the popularity of the Dall-Kirkham
> design, for example, is due to the fact that traditional methods were
> suitable for testing ellipsoidal figures, but not hyperboloidal ones.
>
> Of course, not many people grind their own mirrors nowadays! And
> grinding just one aspheric mirror is enough to give someone a working
> Newtonian telescope, and so that would be enough for most amateurs in
> the past. But given that there are now, and were then, many ATMs that
> have experimented with advanced designs, I'm surprised I had never
> heard of the aplanatic Gregorian until now.
>
> The telescope design on this page
>
> http://www.telescope-optics.net/Houghton4.htm
>
> in figures 101 and 102 is also very impressive: although the use of
> lenses with one flat surface means more lenses are required, it could
> also mean that the telescope could be built entirely with "off-the-
> shelf" parts!
>
> John Savard
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Date: 17 Sep 2007 20:24:46
From: Quadibloc
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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RMOLLISE wrote:
> On Sep 16, 9:06 pm, Quadibloc <jsav...@ecn.ab.ca> wrote:
> > RMOLLISE wrote:
> > > Are you aware that computerized "alt-az" forks can also be used in
> > > equatorial mode with the addition of a wedge?
> >
> > Some of them, at least in the smaller sizes... no, that probably isn't
> > accurate, and optional wedges are still available for the larger
> > sizes, even if one doesn't see them advertised any more.
> >
> > I must resist the impulse to believe that if it isn't advertised in
> > *this* months Sky and Telescope, it doesn't exist.
>
> Right. You can get a wedge for a 16-inch fork mount LX200.
>
> Not exactly... ;-)
I understand there are limits. The *smallest* sizes actually include a
built-in rod to adjust them to equatorial. Perhaps there are wedges
for the 8 and 10 inch.
As for the sizes that cost in five figures, I am not concerned with
them.
Of course, if we all *did* decide to assume that things not advertised
in this month's Sky and Telescope don't exist - then they would have
to put their ads back in again, and it would grow thicker! (The
subject, of course, of another thread.)
John Savard
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Date: 17 Sep 2007 14:34:24
From: RMOLLISE
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
|
On Sep 16, 9:06 pm, Quadibloc <jsav...@ecn.ab.ca > wrote:
> RMOLLISE wrote:
> > Are you aware that computerized "alt-az" forks can also be used in
> > equatorial mode with the addition of a wedge?
>
> Some of them, at least in the smaller sizes... no, that probably isn't
> accurate, and optional wedges are still available for the larger
> sizes, even if one doesn't see them advertised any more.
>
> I must resist the impulse to believe that if it isn't advertised in
> *this* months Sky and Telescope, it doesn't exist.
>
> John Savard
Right. You can get a wedge for a 16-inch fork mount LX200.
Not exactly... ;-)
Unk Rod
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Date: 17 Sep 2007 09:06:58
From:
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
|
On Sep 16, 8:31 pm, Quadibloc <jsav...@ecn.ab.ca > wrote:
>
> Another thing I came across was that the aplanatic (coma free) version
> of the Gregorian telescope, unlike the Ritchey-Chretien, used two
> *ellipsoidal* surfaces. So it would have been easier for an amateur to
> grind and test such mirrors - the popularity of the Dall-Kirkham
> design, for example, is due to the fact that traditional methods were
> suitable for testing ellipsoidal figures, but not hyperboloidal ones.
>
> Of course, not many people grind their own mirrors nowadays! And
> grinding just one aspheric mirror is enough to give someone a working
> Newtonian telescope
Just as a matter of semantics, one never "GRINDS" any sort of aspheric
surface into a telescope lens or mirror. The act of grinding an
optical surface will almost always result in a very low quality sphere
(anywhere from 1 - 5 waves error). This surface, no matter how fine
ground it is, will never allow you to actually look at an image
because it is translucent, but not transparent. Upon many hours of
polishing, one ends up with a transparent surface (almost always a
sphere also because of the way the surfaces are lapped with the
polishing tool). Now, once the surface has been fully polished out can
one proceed to put a very accurate spherical or aspherical surface
onto the glass using various polishing methods (subdiameter polishing
tools or modified pitch surfaces on a large tool). This process is
called "figuring", and thus one would say more accurately:
"So it would have been easier for an amateur to grind, polish, figure
and test such mirrors"
Aspheric surfaces can, of course be generated by grinding, but the
result is totally unsuitable for the precision necessary for
astronomical surfaces. Furthermore, in order to actually polish this
type of aspheric ground surface, a very special form of polishing is
necessary - both very time consuming and quite difficult to do to any
sort of accuracy. It is infinitely easier to actually polish an
aspheric from a polished spherical surface (unless the required
aspheric is so huge that it cannot ever be polished in, but then it
would have zero application for an astronomical objective system -
maybe for exotic eyepieces or small camera lenses).
Rolando
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Date: 16 Sep 2007 19:06:50
From: Quadibloc
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
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I wrote:
> Another thing I came across was that the aplanatic (coma free) version
> of the Gregorian telescope, unlike the Ritchey-Chretien, used two
> *ellipsoidal* surfaces. So it would have been easier for an amateur to
> grind and test such mirrors - the popularity of the Dall-Kirkham
> design, for example, is due to the fact that traditional methods were
> suitable for testing ellipsoidal figures, but not hyperboloidal ones.
But like the Ritchey-Chretien and the Lurie-Houghton, this design
apparently works best for designs with a large secondary - and
therefore, a large central obstruction. So that is probably why it has
not attracted too much interest.
Of course, since it *is* a Gregorian, one could cheat and put a tiny
diagonal near the focus...
on this page,
http://www.telescope-optics.net/three-mirror.htm
they show a design with the diagonal pointing the other way, so that
there are three curved mirrors plus the diagonal!
John Savard
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Date: 16 Sep 2007 19:06:26
From: Quadibloc
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
|
RMOLLISE wrote:
> Are you aware that computerized "alt-az" forks can also be used in
> equatorial mode with the addition of a wedge?
Some of them, at least in the smaller sizes... no, that probably isn't
accurate, and optional wedges are still available for the larger
sizes, even if one doesn't see them advertised any more.
I must resist the impulse to believe that if it isn't advertised in
*this* months Sky and Telescope, it doesn't exist.
John Savard
|
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Date: 16 Sep 2007 18:53:51
From: RMOLLISE
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
|
On Sep 16, 8:31 pm, Quadibloc <jsav...@ecn.ab.ca > wrote:
> I've been doing some web searches about different types of telescopes.
>
> As many of you know, the popular Schmidt-Cassegrain telescope design
> was made by only three American companies - Celestron, Meade, and
> Bausch and Lomb - and was beset by patent and trade secret litigation.
> As a result, Bausch and Lomb is no longer in that field, and there are
> no newcomers trying to enter it.
>
> But inexpensive Chinese-made Maksutov telescopes are starting to
> appear on the market, in sizes as large as 180mm (7 inches).
>
> Things aren't perfect; nowadays, one's choice is between a
> computerized *altazimuth* fork mount or a German Equatorial - the
> equatorial fork mount, the ideal mount from an astrophotography
> perspective, is nearly unavailable (presumably because it is a bit
> more expensive, and price competition has become stronger).
>
> In any event, one of the telescopes I came across was a modified
> Maksutov-Cassegrain by Astro-Physics with an *ellipsoidal* primary
> mirror, which was coma-corrected. It had a 10-inch aperture, so I
> suspect it was quite expensive; it is no longer available, and the
> company seems to make only small refractors now.
>
> Another thing I came across was that the aplanatic (coma free) version
> of the Gregorian telescope, unlike the Ritchey-Chretien, used two
> *ellipsoidal* surfaces. So it would have been easier for an amateur to
> grind and test such mirrors - the popularity of the Dall-Kirkham
> design, for example, is due to the fact that traditional methods were
> suitable for testing ellipsoidal figures, but not hyperboloidal ones.
>
> Of course, not many people grind their own mirrors nowadays! And
> grinding just one aspheric mirror is enough to give someone a working
> Newtonian telescope, and so that would be enough for most amateurs in
> the past. But given that there are now, and were then, many ATMs that
> have experimented with advanced designs, I'm surprised I had never
> heard of the aplanatic Gregorian until now.
>
> The telescope design on this page
>
> http://www.telescope-optics.net/Houghton4.htm
>
> in figures 101 and 102 is also very impressive: although the use of
> lenses with one flat surface means more lenses are required, it could
> also mean that the telescope could be built entirely with "off-the-
> shelf" parts!
>
> John Savard
The reason Bausch in Lomb is not in the business is not because of the
patent infringement lawsuit brought by Celestron (which was quite
justified). That was against Criterion, whom B&L later bought out. The
major reason B&L and Criterion went out of business was that they
produced LOUSY SCTs.
I'd hardly call the fork ideal for astrophotography. It's great for
visual use in alt-az mode, but a pain to image with compared to a GEM.
Are you aware that computerized "alt-az" forks can also be used in
equatorial mode with the addition of a wedge?
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Date: 16 Sep 2007 20:25:58
From: Chris L Peterson
Subject: Re: The Aplanatic Gregorian: A Neglected Telescope Design?
|
On Sun, 16 Sep 2007 18:53:51 -0700, RMOLLISE <rmollise@hotmail.com >
wrote:
>> Things aren't perfect; nowadays, one's choice is between a
>> computerized *altazimuth* fork mount or a German Equatorial - the
>> equatorial fork mount, the ideal mount from an astrophotography
>> perspective, is nearly unavailable (presumably because it is a bit
>> more expensive, and price competition has become stronger).
>I'd hardly call the fork ideal for astrophotography. It's great for
>visual use in alt-az mode, but a pain to image with compared to a GEM.
I agree with John that the equatorial fork is pretty much the ideal
astroimaging mount- certainly far better than a GEM, which is usually a
PITA when imaging. Of course, most of the fork mounts that people use
aren't made as well as they could be, but that isn't an indictment of
fork mounts, just the specific designs. You _can_ buy very good fork
mounts, which will run circles around any GEM for imaging, but of course
you'll spend twice as much (worth it, IMO). There's a reason that very
few professional scopes use GEM mounts.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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