I have a couple more questions about this, Ed. Are you going to post
a prescription of the doublet specs?(after checking patents)
     Would this corrector work with -ANY- F/3 mirror?
Just think- a squatty-looking 30" F/3 .....!!!!! Or a 10" mounted on an
SVP mount!
Mark
***************
--- In lockwood_optics@yahoogroups.com, "Ed Jones" <ejones@...> wrote:
>
> I uploaded a file on a Paracorr "like" coma corrector that I designed
> in about 5 minutes for a 20 inch F/3 primary. The doublets are
SF2/BK7
> pairs about 1.4 inches in diameter and 2 inches from focus. I have no
> idea what the real Paracorr is but one could be made to work. Final
F#
> is 3.8.
>
> Ed Jones
>

Thanks for creating this place, Mike. We are all indepted to you. Looks
like there's some interesting stuff going on already, with F/3s and
correctors!
Mark

Mark,

You're very welcome.  No debt is owed.

I'm just trying to put out a good product, expand the range of
acceptable focal ratios, make mirrors that cool in a reasonable time,
and dispel some myths that others have benefited from.  I'm pleased
that such experienced people have chosen to join and contribute to
this group.

I'd like to thank those who have posted comments about observing
experiences they have had, especially Gary Beyerstein for his kind
words regarding Rick and I and for taking the time to type them.  I
hope we can meet at Chiefland some time in the future.

I enjoy feedback about how my optics are performing.  It's important
to my work, but my ego does not rely upon glowing reviews.

What I enjoy more is hearing about the observing experiences people
have had while viewing the universe with a quality instrument, and how
those experiences helped them make new friends under the stars.
People read those stories or share those great views and that
motivates them to go outside and observe.  That help the hobby grow,
and we need that.

I talked to Al Nager earlier today.  I let him know that the 20" F/3
scope actually exists now and works fairly well (last time we talked
it was not assembled yet), I sent him a link to the article I wrote,
and I encouraged him to design a Paracorr for F/3 and up.  He was
interested, receptive, and generous with his time.  If you're reading
this, Al, thanks for taking the time to talk to me, and for having an
open mind regarding fast optics.

I don't believe that cameras will every capture quite the detail the
human eye can when it comes to planetary observing, so a quality
visual instrument will never go out of style.  The unknown right now
is how to strike a balance between visual performance and the ability
to image faint objects quickly with a high-sensitivity camera (which I
think may play a large and valuable part in the future of public
astronomy).  I know the 20" F/3 will teach me more on this subject,
with the aid of a good steady Florida atmosphere.

I'll be at Chiefland Feb 17-19 (or 20th), and at WSP for most of the
week following.  I look forward to meeting a lot of people, and
sharing some great views through the eyepiece and from the cameras
that many have volunteered to bring by.

   Mike Lockwood
   Lockwood Custom Optics

--- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@...> wrote:
> Thanks for creating this place, Mike. We are all indepted to you.
> Looks like there's some interesting stuff going on already, with
> F/3s and correctors!
> Mark

Mike,

Yeah we kinda do owe you for this, at least I do.

You've helped me some on CN (BTW I'm Biff). I've only been into
telescopes a for about 2 1/2 years and am quickly finding out that I'm
more interested in the optics end of it then actually using the
instrument. I've found myself paying close attention to a small
handful of people I consider the some of the best of the best... and
you are in that group. So thanks for all you've taught me so far, even
if you don't realize it.

So far I've only completed a 10" f/7.25 pyrex that I still have to
determine if it's a winner before I send it off to the coaters.
FigureXP says it's a ringer (1/40 wave RMS) but I'll believe that when
I see it in a star test and I've read that FigureXP is very optimistic
which makes me NOT very optimistic. Right now I've started a
14.7"x0.8" F/5 quartz from recolabs and if that works out I think I
might try a 20" f/3 like what you're working with. Yes I know it's a
crazy proposition for a third mirror but I think like the challenge
more than expecting a finished product.

Glad to hear that Al is considering making a paracorr for f/3 and that
yours is doing well with present solutions. I am surprised to hear you
talking about using it for planetary work. Not due to mirror quality
but more so the size of secondary that a f/3 would demand. I'm really
happy to hear that it's working for that too.

Unfortunately I won't have much to offer to this group for some time
until I get some more experience so I thought I'd take this
opportunity to say 'thanks'.

Take care,
Ryan



--- In lockwood_optics@yahoogroups.com, "Mike Lockwood"
<parabola30@...> wrote:
>
> Mark,
>
> You're very welcome.  No debt is owed.
>
> I'm just trying to put out a good product, expand the range of
> acceptable focal ratios, make mirrors that cool in a reasonable time,
> and dispel some myths that others have benefited from.  I'm pleased
> that such experienced people have chosen to join and contribute to
> this group.
>
> I'd like to thank those who have posted comments about observing
> experiences they have had, especially Gary Beyerstein for his kind
> words regarding Rick and I and for taking the time to type them.  I
> hope we can meet at Chiefland some time in the future.
>
> I enjoy feedback about how my optics are performing.  It's important
> to my work, but my ego does not rely upon glowing reviews.
>
> What I enjoy more is hearing about the observing experiences people
> have had while viewing the universe with a quality instrument, and how
> those experiences helped them make new friends under the stars.
> People read those stories or share those great views and that
> motivates them to go outside and observe.  That help the hobby grow,
> and we need that.
>
> I talked to Al Nager earlier today.  I let him know that the 20" F/3
> scope actually exists now and works fairly well (last time we talked
> it was not assembled yet), I sent him a link to the article I wrote,
> and I encouraged him to design a Paracorr for F/3 and up.  He was
> interested, receptive, and generous with his time.  If you're reading
> this, Al, thanks for taking the time to talk to me, and for having an
> open mind regarding fast optics.
>
> I don't believe that cameras will every capture quite the detail the
> human eye can when it comes to planetary observing, so a quality
> visual instrument will never go out of style.  The unknown right now
> is how to strike a balance between visual performance and the ability
> to image faint objects quickly with a high-sensitivity camera (which I
> think may play a large and valuable part in the future of public
> astronomy).  I know the 20" F/3 will teach me more on this subject,
> with the aid of a good steady Florida atmosphere.
>
> I'll be at Chiefland Feb 17-19 (or 20th), and at WSP for most of the
> week following.  I look forward to meeting a lot of people, and
> sharing some great views through the eyepiece and from the cameras
> that many have volunteered to bring by.
>
>   Mike Lockwood
>   Lockwood Custom Optics

Ok, this is my follow-up on a discussion with Al Nagler at Tele Vue
last fall about revising the ParaCorr for faster applications.

I've looked for my notes but I've misplaced them somehow. :(  The
reason I wanted to use those was due to the nonlinear nature of the
conversation I wanted to follow the track of it in explaining what the
results were, but I'll just do the best I can.  There may be some
stuff of interest about the alternative approaches and the nature of
the retail market that any such product has to address, but it will
ramble.

I simply called Tele Vue to ask about whether they were planning to do
anything for coma correctors for faster mirrors, as I wasn't coming up
with anything too useful and the commercial application of f/3.3 and
below of large/thin mirrors in novel ultralight structures was
otherwise a natural application.  After explaining this to about one
person, Al came on the phone.  This wasn't too surprising, but then we
chatted for about an hour and a half. :)

Digression 1) For those who haven't made the connections yet - in an
ultralight design made to take full advantage of thin & light mirrors
(20x1.1" f/3.3 on quartz for example) you can have balance issues
related to shifting upper end loads.  Large alt bearings help there.
A low COG helps a lot.  A very fast mirror gives you a low COG, and
the math works out for the structure, without the need to incur added
weight on the bottom end.  On a traditional Dob you can use virtual
counterweights (AKA springs) but an ultralight made with flex rocker
and ring base won't go there SFAIK.  Tracking can help in stability,
with active control of destabilizing forces, but the best way to gain
stability is by proper design - wide stance with minimal moment arms
on the load transmission structure.  For the later Mel Bartel's
trilateral (AKA TriDob) provides the best possible solution, as the
altitude and azimuth bearings can be constructed from single
components, if desired.

One of the first things I mentioned to Al was that the ParaCorr
prescription wasn't available.  He allowed as how that was true.  And
I pointed out that it could be reverse engineered.  He said nobody
has, and it's been on the market for a long time.  :)  I explained a
bit about what I was looking for, and some of the discussions I've had
with Steve Kennedy about performance with fast mirrors, and star
testing, for which he leaves the ParaCorr out altogether due to the SA
it introduces. And a few other things.  And he said they'd look into
updating the design a while back and that his son had done some work
on that but they'd just kind of left it there as the market was still
well served by the existing product.

However we were talking about a new market.

We then proceeded to toss back and forth different ways of getting to
what was needed for much faster mirrors. His first train of thought
went something like defining a focuser aperture size and specific
working distance in front of the FP at which to build in a dedicated
coma corrector.  Since with sub f/3.5 or so mirrors the performance
without coma correction was going to be severely compromised anyway it
might as well be a permanent part of the OTA.  Done this way it can
have no effect on the FL, though, unlike the ParaCorr, which buys some
working space by lengthening the FL by (IIRC) about 15%.

Quickly it becomes obvious that there are four problems with this: 1)
Cost. 2) Volume run. 3) Standardization.

Volume is low at least in the beginning, and possibly in the long run,
which drives up cost for tooling and short production runs.  Plus
these are large elements, which drives up the cost even more.

Standardization is even more troublesome - multiple commercial
mirror/telescope makers would have to agree (!) on a specification to
incorporate into the design of the OTA - although the device doesn't
itself alter the FL it shifts the FP physically.  And focusers to
accommodate it, and then of course you probably want to have the
ability to do imaging with the setup (hey, fast, flat field, well
corrected!).  It remains an option, but a very expensive one.

Digression 2) An offshoot of the dedicated coma corrector is perhaps a
bit subtle but Al picked it up right away - the primary can be be
corrected to a conic that isn't -1 (paraboloid) in order to facilitate
the overall performance.  This can also be done with the existing
ParaCorr to correct for the residual SA.  Although that's somewhat
appealing, it has the disadvantage that the primary then is locked
into the current coma corrector product forever.  Not a step I'd want
to take.

The fourth problem (ah here it is!) is catch-22.  The MINIMUM
production run on something like that just described (a housing with
elements to a new prescription) is about a hundred units.  And unless
you have, oh, about a hundred telescopes already built to take
advantage of the corrector, there's no place for them to go.  But to
sell those 'scopes you need the new corrector.  Well, enough of that.

So there we hemmed and hawed for a while and talked about a bunch of
other stuff, as I recall, which was quite entertaining and obviously
lateral, because then Al says, HOWEVER, one could just add an element
or two to the existing production ParaCorr at minimal expense.  And
although it would still add the 15% to the effective FL would that
really be a problem?

Well no, when you think about it. After all, one is spec'ing the
mirror to the structure in these cases, to make step-less viewing
available at zenith, and that FL can be anything.

The advantage of an enhanced design, besides being tailored for the
coma of an f/3 or so, would be to reduce the SA significantly.  This
is what you'll notice with the existing corrector at faster f/ratios,
for which it wasn't really designed.

Digression 3) If you look at the hand-drawn chart dated 7/89 at
http://www.televue.com/images/accessories/Paracorr%20Graphs.jpg on the
Tele Vue website, indeed it shows curves for f/3.5, and indeed it
improves the field, but not much of that corrected curve ever gets
below diffraction limited.  Hence room for improvement.  Mike Lockwood
can probably address this directly. ;)

The cost would be comparable to the existing ParaCorr in production
quantity, I'm at least somewhat sure. :)  And the availability of
experimental versions of this yet-to-be-designed item will allow those
who are working with what I'll call next-generation mirrors to field
test and refine that design is pretty cool.  This is the time to talk
to TeleVue about what f/ratios might realistically be produced in such
a generation - I see from 20" f/3.3 to 28" f/2.5 as yielding the same
EP height when deployed.  But that's where I left it.  After all, one
still has to produce such mirrors reliably and with the quality of
more tractable f/ratios. ;)

Now, Al did mention something else - the 2nd Ethos released (there
were two out at the time) has less than 1/100th wave SA at f/5. (!)
He suspects (and so it's proving, I believe) that this will work quite
well at f/3-f/4 ratios.

And yet another thing - Al said (though I can't find it) that the Tele
Vue web site had on it for 15 years or so that "The ideal telescope is
a 16" f/4.5 - with a ParaCorr".  He allowed as to how it's time for
that to change - "The ideal telescope is a 20" f/3.3 - with a
ParaCorr2" - or whatever we crazy folks come up with.  :)

It took longer to write this then to talk about it, and that's about
all I recall that seems germane, so have fun with it.  Also be be
aware that Al was quite emphatic about this: he really wants to hear
from people who are going this way!

Best,
Mark Cowan
Salem, OR

I know you have pushed the envelope with this F/3 venture. Can I ask
if the intended goal is as a general multi-purpose visual/imaging
that's super compact? If visual is to play a significant part, do
envision usage in the area of 10-15x/inch as the norm?
Mark

I'm wondering about a few details in your conversation.
************
     We are thinking about mirrors on the short side of F/3-4. It
looks like we need a coma corrector that eliminates coma, and
maintains reasonable quality taking SA into account.  In view of
that, to maintain image plane integrity, we are talking about using
a Paracorr or similar -most-, if not, -ALL- of the time -REGARDLESS-
.  It would be interesting to hear Uncle Al's take on the complexity
of the project if we in fact, -DID- use mirrors with corrections of
smaller than -1.0 (Hyperboloids). It might be possible just to
change an element or two to the existing Paracorr. I made 2-  8"
hyperboloids for the Rosin astrographs,(courtesy of Mike Jones) and
they were around F/4, and @ -1.72~. They were straightforward, and
came out suprisingly well, with no real issues. Performance specs
for the field it was capable of were extremely respectable as
compared to a typical F/4, or F/5-6 for that matter! Scaling up to
20" or so could throw in a concern or two with this idea, but I
think it would be worth exploring.

        I knew I saw that drawing before! Optimum is 16" F/4.5....
hope a 20" ap isn't too far from that ideal.  :)

Next, I wonder if the short focus corrector designed for F/3.~ would
in fact supercede the Paracorr and encompass all that it's presently
capable of as well??? (barring issues of the strength of the
correction being used on the primary.)

    Finally, would the std Paracorr work -BETTER- for a sub- F/4
mirror if we just increased the correction a bit; say 20-25%? I'd be
really interested in that particular idea. The Paracorr exists
already, and an additional bit of aspherization isn't difficult.

    Just food for thought,
Mark

Mark,

--- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@...> wrote:
> I know you have pushed the envelope with this F/3 venture. Can I ask
> if the intended goal is as a general multi-purpose visual/imaging
> that's super compact?

The goal is to give the same quality of visual image as slower
telescopes over most of the field, while being more compact and
providing shorter exposure times for imaging.

> If visual is to play a significant part, do
> envision usage in the area of 10-15x/inch as the norm?

I envision usage in the 3-50x/inch range, depending on what object is
being viewed and what the atmosphere will allow.  The 20" F/3 mirror
is quite good and should handle some serious power.

   Mike Lockwood
   Lockwood Custom Optics

Mark,
   The "Paracorr like" is 2 inches from focus similar to Televiews
paracorr which is about 45 mm from focus I've heard.

Ed

--- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@...>
wrote:
>
> Ed, is the Paracorr "like" corrector in front of the secondary
flat,
> or in the focuser???
> Mark
> ********
>
> --- In lockwood_optics@yahoogroups.com, "Ed Jones" <ejones@>
> wrote:
> >
> > Mike,
> >   <<Why not just develop an optimized coma corrector to go in a
2"
> > or 3"  focuser for use at or near F/3?>>
> > Actually exploring that option I found a Paracorr "like"
solution
> > that uses common glass.  It would interesting to compare it a
> > Paracorr but I don't have the prescription. I guess the only
> > criticism is that it would need a bigger flat, over 5 inches.
> >
> > Ed
> >
> > --- In lockwood_optics@yahoogroups.com, "Mike Lockwood"
> > <parabola30@> wrote:
> > >
> > > Ed,
> > >
> > > Why not just develop an optimized coma corrector to go in a 2"
> or
> > 3"
> > > focuser for use at or near F/3?
> > >
> > > That solution would be less expensive, it is not an integral
> part
> > of
> > > the instrument and thus could be moved from telescope to
> telescope.
> > >
> > > If anyone on this group can suggest sources for coma
correctors
> > that
> > > perform better than the Paracorr at F/3 and are practical to
use
> > with
> > > eyepieces like Naglers and Ethos, I would like to hear about
> it.
> > It
> > > will be difficult to beat the Paracorr for the price.
> > >
> > > The 3" Keller correctors are fairly difficult to get and are
> > > expensive.  What else is there?
> > >
> > >   Mike Lockwood
> > >
> >
>

Andrew,

--- In lockwood_optics@yahoogroups.com, "Aurigema, Andrew N.
(KSC)[ASRC AEROSPACE]" <andrew.n.aurigema@...> wrote:
> I have to go with recomending dedicated scopes for each task.  What
> makes a scope good for viewing ( keeping the system light and
> portable, putting the eyepiece at a comfortable place and making it
> easy to change out eyepieces )  is not condusive to the requiremts
> of imaging (massive regidity to hold up all the mass of the cameras
> and guiders, great tracking capability and remote control for long
> duration imaging to capture the dim photons ).

Good points - let me clarify - when I say imaging I'm talking about
high sensitivity cameras like the Stellacam or Mallincam which are
taking under one minute exposures, usually more like 10 seconds.

I'm not talking about serious astroimaging, because this scope is
alt-az and has no field de-rotator (but it is possible to do that with
software), and the focuser won't handle a 10-lb camera.  It will,
however, handle a DSLR.  See John VeDepo's article here:
   http://www.starmastertelescopes.com/imaging.htm

> ps... i would love to view thru one of them f/3 scopes if
> you are thinking of bringing one down.

Checking out the 20" F/3 is one of the main goals of this trip, as is
relaxing.  It will be set up in your backyard in about a week, and you
will be enjoying the colors in M42.

   Mike Lockwood
   Lockwood Custom Optics

Something that I think will be important with the F3 and 3.3 scopes is
having plenty of in-focus for focusing the Mallincam or Stellacam.
With the F 3.3 mirror and using the focal reducer and a couple of
spacers, one would be able to achieve about F 2.1, as long as there
was plenty of in-focus to reach focus. That is incredibly fast and
short integration times of 7 or 14 seconds would be plenty on an alt
az scope. I need to do some research and find out what software would
account for field rotation and correct it. With the F3 and F3.3 we are
certainly entering a new era of observing - visually and video assisted!

Duane

--- In lockwood_optics@yahoogroups.com, "Mike Lockwood"
<parabola30@...> wrote:
>
> Andrew,
>
> --- In lockwood_optics@yahoogroups.com, "Aurigema, Andrew N.
> (KSC)[ASRC AEROSPACE]" <andrew.n.aurigema@> wrote:
> > I have to go with recomending dedicated scopes for each task.  What
> > makes a scope good for viewing ( keeping the system light and
> > portable, putting the eyepiece at a comfortable place and making it
> > easy to change out eyepieces )  is not condusive to the requiremts
> > of imaging (massive regidity to hold up all the mass of the cameras
> > and guiders, great tracking capability and remote control for long
> > duration imaging to capture the dim photons ).
>
> Good points - let me clarify - when I say imaging I'm talking about
> high sensitivity cameras like the Stellacam or Mallincam which are
> taking under one minute exposures, usually more like 10 seconds.
>
> I'm not talking about serious astroimaging, because this scope is
> alt-az and has no field de-rotator (but it is possible to do that with
> software), and the focuser won't handle a 10-lb camera.  It will,
> however, handle a DSLR.  See John VeDepo's article here:
>   http://www.starmastertelescopes.com/imaging.htm
>
> > ps... i would love to view thru one of them f/3 scopes if
> > you are thinking of bringing one down.
>
> Checking out the 20" F/3 is one of the main goals of this trip, as is
> relaxing.  It will be set up in your backyard in about a week, and you
> will be enjoying the colors in M42.
>
>   Mike Lockwood
>   Lockwood Custom Optics
>

My name is Bob Kirschenmann and I've finally gotten around to joining
this group.  I have a bit of background in fabricating optics,
telescope components, and improbable stories.  I'll be lurking here and
watching.  If you post something and feel you are being watched...it's
me so keep it clean.

Hi Mike,  have fun in Florida.

See Ya,
Bob

Mark,

I tried to forward some stuff to your "mdhopt..." Yahoo email but it
wouldn't take it, so I hope the questions got answered.

--- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@...> wrote:
>
>    I'm wondering about a few details in your conversation.
> ************
>     We are thinking about mirrors on the short side of F/3-4. It
> looks like we need a coma corrector that eliminates coma, and
> maintains reasonable quality taking SA into account.  In view of
> that, to maintain image plane integrity, we are talking about using
> a Paracorr or similar -most-, if not, -ALL- of the time -REGARDLESS-

This is true except for on-axis solely.  Even the disc of Jupiter is
large enough to start to show degradation at the edge with a fast
enough mirror.

> .  It would be interesting to hear Uncle Al's take on the complexity
> of the project if we in fact, -DID- use mirrors with corrections of
> smaller than -1.0 (Hyperboloids).

You can ask directly, certainly.  There didn't seem to be anything to
it _other_ than making the mirror to a different conic.  But I
wouldn't want to lock a premium mirror into use with a corrector
system that quite likely changes down the road.


>It might be possible just to
> change an element or two to the existing Paracorr. I made 2-  8"
> hyperboloids for the Rosin astrographs,(courtesy of Mike Jones) and
> they were around F/4, and @ -1.72~. They were straightforward, and
> came out suprisingly well, with no real issues. Performance specs
> for the field it was capable of were extremely respectable as
> compared to a typical F/4, or F/5-6 for that matter! Scaling up to
> 20" or so could throw in a concern or two with this idea, but I
> think it would be worth exploring.

I'm interested in all workable methods.  What Al was talking about was
changing the prescription a bit (with the existing housing) and adding
an extra element or doublet to the design.  He said they'd already
done some design work with that but without looking it up he couldn't
say what the results were like - still it was the direction they were
heading once there was a market to sell into to.

>
>        I knew I saw that drawing before! Optimum is 16" F/4.5....
> hope a 20" ap isn't too far from that ideal.  :)
>
> Next, I wonder if the short focus corrector designed for F/3.~ would
> in fact supercede the Paracorr and encompass all that it's presently
> capable of as well??? (barring issues of the strength of the
> correction being used on the primary.)

Somehow I doubt that - it was definitely something that would be an
addition to the product line and special purpose for that (it would
cost more obviously).  I suspect that designing it for sub f/4 with
some optimal range will mean it works less well at longer f/ratios,
but that's really only a guess.


>
>    Finally, would the std Paracorr work -BETTER- for a sub- F/4
> mirror if we just increased the correction a bit; say 20-25%? I'd be
> really interested in that particular idea. The Paracorr exists
> already, and an additional bit of aspherization isn't difficult.

Mel Bartels just recently discussed his experience with a 13" f/3 and
ParraCorr testing he did on the ATM list.  He says, counterintuitive
as it seems to me, that it didn't _add_ SA but it reacted to SA that
the primary had.  He refigured it multiple times while exploring
figuring techniques at that speed and I believe had a good handle on
what the actual correction was on the glass each time, for comparison.
  I'm still musing about this one. ;)

>
>    Just food for thought,
> Mark
>

Appreciate your comments, sorry to take so long to get back to the posts.

Best,
Mark

> Mel Bartels just recently discussed his experience with a 13" f/3 and
> ParraCorr testing he did on the ATM list.  He says, counterintuitive
> as it seems to me, that it didn't _add_ SA but it reacted to SA that
> the primary had.  He refigured it multiple times while exploring
> figuring techniques at that speed and I believe had a good handle on
> what the actual correction was on the glass each time, for comparison.
>  I'm still musing about this one. ;)

*****************************************
     This might have been an experimental failure, simply because the
degree of correction, like the astrograph mirrors, have a very narrow
range where the particular correction at a predetermined focal length
will actually work. A percent or two of over/under-correction will
baloon the plots.
     This can be realised when you think about doing just a typical
paraboloid, where the focal length can vary an inch or two, and
correction likewise, a few percentage points. Doesn't make a difference
in the end when used as a Newt. Add a corrector, whether a Mak type, or
a multi-element similar to a coma corrector or field flattener, and
that goes right out the window. Collimation is more critical, and the
corrector will only work well with a specified focal length range and
correction that's far narrower. You'd have to be one of these design
gurus with Zemax or similar -sophisticated- program to obtain that
narrow range/correction parameter limits. Somehow, I don't think you
can just try hit-or-miss corrections.
    I'm with you; in that I'd still give more thought about this
particular Paracorr adaption to a hyperboloid idea as workable. But it
cannot be approached with a casual unsophisticated MO! I think it would
work. I can't quite understand his comment about SA...?
     If you make a paraboloid; sure, you can use it as a Newt. But in
accepting an F/3~ focal ratio, you have to use -SOMETHING- with that
particular mirror to correct abberations. You have "locked in" the
mirror to the corrector by choosing the fast mirror to begin with.  :)
Mark

Thinking about it some more, the SA Mel spoke of could be addressed
by "bending" the corrector achromats. One possibility.
    The astrograph design works with 2 separated elements with that
hyperboloid. The hype could simplify the design job. -OR- you could
investigate what a spherical mirror would require; or other ellipses(?)
Spherical mirrors this fast would be relatively easy to make.
M.

Mark,

I tried to forward some stuff to your "mdhopt..." Yahoo email but it
wouldn't take it, so I hope the questions got answered.

--- In lockwood_optics@yahoogroups.com, "mdhopt53" wrote:
>
> I'm wondering about a few details in your conversation.
> ************
> We are thinking about mirrors on the short side of F/3-4. It
> looks like we need a coma corrector that eliminates coma, and
> maintains reasonable quality taking SA into account. In view of
> that, to maintain image plane integrity, we are talking about using
> a Paracorr or similar -most-, if not, -ALL- of the time -REGARDLESS-

This is true except for on-axis solely. Even the disc of Jupiter is
large enough to start to show degradation at the edge with a fast
enough mirror.

> . It would be interesting to hear Uncle Al's take on the complexity
> of the project if we in fact, -DID- use mirrors with corrections of
> smaller than -1.0 (Hyperboloids).

You can ask directly, certainly. There didn't seem to be anything to
it _other_ than making the mirror to a different conic. But I
wouldn't want to lock a premium mirror into use with a corrector
system that quite likely changes down the road.


>It might be possible just to
> change an element or two to the existing Paracorr. I made 2- 8"
> hyperboloids for the Rosin astrographs,(courtesy of Mike Jones) and
> they were around F/4, and @ -1.72~. They were straightforward, and
> came out suprisingly well, with no real issues. Performance specs
> for the field it was capable of were extremely respectable as
> compared to a typical F/4, or F/5-6 for that matter! Scaling up to
> 20" or so could throw in a concern or two with this idea, but I
> think it would be worth exploring.

I'm interested in all workable methods. What Al was talking about was
changing the prescription a bit (with the existing housing) and adding
an extra element or doublet to the design. He said they'd already
done some design work with that but without looking it up he couldn't
say what the results were like - still it was the direction they were
heading once there was a market to sell into to.

>
> I knew I saw that drawing before! Optimum is 16" F/4.5....
> hope a 20" ap isn't too far from that ideal. :)
>
> Next, I wonder if the short focus corrector designed for F/3.~ would
> in fact supercede the Paracorr and encompass all that it's presently
> capable of as well??? (barring issues of the strength of the
> correction being used on the primary.)

Somehow I doubt that - it was definitely something that would be an
addition to the product line and special purpose for that (it would
cost more obviously). I suspect that designing it for sub f/4 with
some optimal range will mean it works less well at longer f/ratios,
but that's really only a guess.


>
> Finally, would the std Paracorr work -BETTER- for a sub- F/4
> mirror if we just increased the correction a bit; say 20-25%? I'd be
> really interested in that particular idea. The Paracorr exists
> already, and an additional bit of aspherization isn't difficult.

Mel Bartels just recently discussed his experience with a 13" f/3 and
ParraCorr testing he did on the ATM list. He says, counterintuitive
as it seems to me, that it didn't _add_ SA but it reacted to SA that
the primary had. He refigured it multiple times while exploring
figuring techniques at that speed and I believe had a good handle on
what the actual correction was on the glass each time, for comparison.
I'm still musing about this one. ;)

>
> Just food for thought,
> Mark
>

Appreciate your comments, sorry to take so long to get back to the posts.

Best,
Mark




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but if the paracorr is expecting an SA3 corrected
> input, it would not surprise me if the star test
gave "interesting" results
> when used before the primary had been fully corrected.
>
>
>
> Just musing, as you said.
>
>
>
>
>
> Scott Milligan
>*********************************

    So do you think it plausible that an area of compromise could be
found with a prolate? You have zemax, last I knew........!
M.

--- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@...>
wrote:
>
> Thinking about it some more, the SA Mel spoke of could be addressed
> by "bending" the corrector achromats. One possibility.
>    The astrograph design works with 2 separated elements with that
> hyperboloid. The hype could simplify the design job. -OR- you could
> investigate what a spherical mirror would require; or other ellipses
(?)
> Spherical mirrors this fast would be relatively easy to make.
> M.
>

What happens with a corrector is that when you try and correct for
coma, without adding so much astigmatism that you destroy the image
that way, is that you add Spherical Aberration to the system.

This is a direct consequence of the equations. Wynne proved it
mathematically.  (He is the guy that developed the corrector design
used on the big professional observatories.)

The more elements you add, the less additional SA you add. The closer
to the primary (further from the eyepiece) the less SA you add.
You can balance the SA by overcorrecting the primary (makeing it
hyperbolic), like in a hyperbolic astrograph.

Coma correcting an "overcorrected" (hyperbolic) primary, is very easy.
Coma correcting a parabolic is quite a bit more difficult. You have
to balance the effect of additional SA with how much coma you are
correcting, to get minimum spot sizes.

Correcting a fast spherical mirror is REALLY REALLY HARD. You are
pushing the equations the wrong way. (Not impossible, but really
difficult to get it within tolerance.)

Several people have tried. For slower mirrors, this can reasonably be
done.  There was an older Sky & Tel article by someone who did it (I
forget if it were Jones, or Johnson - sorry).  Tasco had a telescope
that did this too, with only moderately good reviews on how well it
actually worked. (There is an article on web, where someone took one,
took out the corrector, and flexed the mirror to make it parabolic,
in order to get a better image.)  There was briefly another
commercial scope, called something like an "argonaut", that did this
too, but obviously was not a lasting commercial success.

All of the new "advanced" coma corrected 2-mirror telescopes, (there
are several different brands at this time, using slightly different
methods) are all based in overcorrecting SA before it hits the
cluster of corrector lenses. Under current technology, this seems to
be the aproach that is creating commercial products that are lasting
in the market.

Scott Beard

Yes, if the corrector could achieve higher performance by using a
conic other than -1 and it was a permanent part of the OTA.

No, if a new corrector design comes along that is optimized for a
straight paraboloid, and exceeds the performance of the first combined
system (standard ParaCorr + adjusted primary).  The straight
paraboloid is to my mind a much easier target for commercial corrector
design at sub f/4, which appears to the era coming up. <g>

Best,
Mark


>"You can ask directly, certainly. There didn't seem to be anything to
>it _other_ than making the mirror to a different conic. But I
>wouldn't want to lock a premium mirror into use with a corrector
>system that quite likely changes down the road."
>************
>    My personal take, it'd be more "premium" with the correction and
>have a better image plane with the correction/corrector designed in
>at the beginning. (more money?!)
>
>M.

It also obviously matters what the spacing is - Mel said he was using
it at the maximum out, and Al mentioned several things about adjusting
the spacing, which I wrote down at the time but have misplaced. :(
IIRC though he seemed to feel there was an intermediate position that
was actual optimal, and it was near the maximum, and of course depends
on the particular EP.  He used that to explain the curved focal plane
effect I passed on from other reports.

I see what you mean about the corrector interacting with a mirror that
wasn't quite at the target conic of -1, at least in broad strokes.
That makes some sense of the otherwise inexplicable result.

To recast it a different way - the high angles and lateral spread of
the focusing cone could be interacting across the mirror, so that when
the primary isn't quite there it reads as having more aberration than
would be expected?  I've got the headache from hell today, but that
statement at least capsulizes for me what might happen, but I can't
think of an easily testable analogy.  And perhaps it's exactly what
you just said?

Best,
Mark

--- In lockwood_optics@yahoogroups.com, "Scott Milligan"
<starzkey@...> wrote:
>
> Mark wrote:
>
>
>
> "Mel Bartels just recently discussed his experience with a 13" f/3 and
> ParraCorr testing he did on the ATM list. He says, counterintuitive
> as it seems to me, that it didn't _add_ SA but it reacted to SA that
> the primary had. He refigured it multiple times while exploring
> figuring techniques at that speed and I believe had a good handle on
> what the actual correction was on the glass each time, for comparison.
> I'm still musing about this one. ;)"
>
>
>
> I'm speculating that what Mel was seeing was a consequence of what
the lens
> design literature refers to as "induced aberration".  Basic idea is that
> whenever a system has large spherical aberration in an airspace, ray
mapping
> errors occur at the downstream surface which cause it and subsequent
surface
> refractions not to "behave" as designed.  The two element Offner
corrector
> for conicoids can be viewed as one example where this effect is actually
> used to improve a design, but if the paracorr is expecting an SA3
corrected
> input, it would not surprise me if the star test gave "interesting"
results
> when used before the primary had been fully corrected.
>
>
>
> Just musing, as you said.
>
>
>
>
>
> Scott Milligan
>

Interesting - what kind of hyperbolic coefficients would make this
work for the test examples?  It's not _that_ hard to build it in, up
to a point obviously, and if the system that results is close to
flawless, as Mark H. says, it has commercial appeal in spades,
possibly enough to offset the expense.

Best,
Mark


--- In lockwood_optics@yahoogroups.com, "scott_beard2000"
<indianola@...> wrote:
>
> --- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@>
> wrote:
> >
> > Thinking about it some more, the SA Mel spoke of could be addressed
> > by "bending" the corrector achromats. One possibility.
> >    The astrograph design works with 2 separated elements with that
> > hyperboloid. The hype could simplify the design job. -OR- you could
> > investigate what a spherical mirror would require; or other ellipses
> (?)
> > Spherical mirrors this fast would be relatively easy to make.
> > M.
> >
>
> What happens with a corrector is that when you try and correct for
> coma, without adding so much astigmatism that you destroy the image
> that way, is that you add Spherical Aberration to the system.
>
> This is a direct consequence of the equations. Wynne proved it
> mathematically.  (He is the guy that developed the corrector design
> used on the big professional observatories.)
>
> The more elements you add, the less additional SA you add. The closer
> to the primary (further from the eyepiece) the less SA you add.
> You can balance the SA by overcorrecting the primary (makeing it
> hyperbolic), like in a hyperbolic astrograph.
>
> Coma correcting an "overcorrected" (hyperbolic) primary, is very easy.
> Coma correcting a parabolic is quite a bit more difficult. You have
> to balance the effect of additional SA with how much coma you are
> correcting, to get minimum spot sizes.
>
...

The reason I mentioned it, was the forementioned Rosin astrograph.
The hype enabled Mike to design the corrector to lie about 2-2.5"
inside focus, and only used 2 elements total. The corrector was fairly
simple to make.(Ed Jones made 6 sets, afaik) Considering the
performance potential and the simplicity, it seemed to be an excellent
solution with imaging and visual useages. Never seen a Paracorr up
close, but I don't imagine it was as simple as this astrograph
corrector.
Mark
***************
--- In lockwood_optics@yahoogroups.com, "Mark Cowan" <toolontop@...>
wrote:
>
> Interesting - what kind of hyperbolic coefficients would make this
> work for the test examples?  It's not _that_ hard to build it in, up
> to a point obviously, and if the system that results is close to
> flawless, as Mark H. says, it has commercial appeal in spades,
> possibly enough to offset the expense.
>
> Best,
> Mark
>
>
>

The optimum hyperbolic coefficient depends entirely on how far up-
stream (away from the eyepiece) you will be placing the corrector.
I understand that 1.3 is a common coefficient used, for designers
wanting the corrector to be near the secondary, so they use the same
spider to mount it.

The further up-stream the corrector is, the more hyperbolic the
primary needs to be, but the easier it is to get good correction, and
the lenses have less curvature. Further upstream you also create less
astigmatism, which for correctors can be a deal-killer.

If you balance the corrector-created SA with overcorrection in the
primary, suddenly the corrector becomes very simple: It becomes a
simple 2-lens, zero total power. Without power, there is no Chromatic
Aberration, and both lenses can be the same type of glass [usually
BK7 or UBK7 'cuz its so easy to get and work with]. (Added bonus-
since both are the same glass, there is no secondary color - which is
usually what drives one have to use exotic glasses.]

In this configuration, there is a simple formula for determining the
lens prescriptions.  The journal articles by the guys that make this
for big observatories typically report in their articles that they
preliminarly sized it using the simple equations, and then ray traced
to refine the design, and discover that they didn't need to make any
adjustements.

There are three other offshoots of this, that pertain to this
conversation:

1) many of the big observatory scopes are now being made with
hyperbolic primaries. They will set them up as a modified R-C,
tweaking the hyperbolic coeffient of the primary, so that it will
easily accept one of these correctors for prime-focus work.  That way
they get a two-fer, with very easy to make correctors.

2) any configuration before the corrector that over-corrects SA, will
allow this approach to be taken.  There are a number commercial
scopes now starting to be marketed that are "advanced comma corrected
Dall-Kirkams".  All they have done is increase the correction on the
primary mirror, to over-correct for SA. (They make it less of an
oblate spheroid, and bring it closer to a paraboloid). A simple 2-
lens, one glass, corrector then takes care of all the coma.  A DK,
with all the simplicity of an easy to make secondary, and easy to
collimate (don't discount how very much easier a DK is to collimate
than any other Cass scope), yet gives you comma free images like a RC.

If you give yourself flexibility in where the corrector is placed, a
modified comma-free DK with parabolic primary and spherical secondary
is possible using this method. This approach could also give you a
two-fer.  [A project for that extra old longer-focus parabolic mirror
you have lying around in the workshop, gathering dust?]

3)When this approach is applied to spherical primaries, the reverse
happens. Rather than the primary being overcorrected, it is
undercorrected.  You can correct SA with a two-lens corrector, but it
is at the expense of ADDING comma to the system

In general, if you can balance the SA between the scope and
corrector, the corrector becomes very easy.  If you can't (such as a
parabolic primary), then you have to add extra lens elements to make
up for the imbalance.  Such as the parracor, with four elements, and
still some SA, but certainly very tolerably small amount. (I love my
parracor - so don't get the wrong idea.)

As mentioned in other posts, it will probably take a couple of more
elements to get the parracor optomized for F3.  -this is a great, and
versitile thing to do. I hope they do so.

Once you understand the interplay between SA in the main scope, and
SA in the corrector, all kinds of very reasonable to build, wide
field scopes become possible.

Scott Beard



--- In lockwood_optics@yahoogroups.com, "Mark Cowan" <toolontop@...>
wrote:
>
> Interesting - what kind of hyperbolic coefficients would make this
> work for the test examples?  It's not _that_ hard to build it in, up
> to a point obviously, and if the system that results is close to
> flawless, as Mark H. says, it has commercial appeal in spades,
> possibly enough to offset the expense.
>
> Best,
> Mark
>
>
> --- In lockwood_optics@yahoogroups.com, "scott_beard2000"
> <indianola@> wrote:
> >
> > --- In lockwood_optics@yahoogroups.com, "mdhopt53" <harrymark7@>
> > wrote:
> > >
> > > Thinking about it some more, the SA Mel spoke of could be
addressed
> > > by "bending" the corrector achromats. One possibility.
> > >    The astrograph design works with 2 separated elements with
that
> > > hyperboloid. The hype could simplify the design job. -OR- you
could
> > > investigate what a spherical mirror would require; or other
ellipses
> > (?)
> > > Spherical mirrors this fast would be relatively easy to make.
> > > M.
> > >
> >
> > What happens with a corrector is that when you try and correct
for
> > coma, without adding so much astigmatism that you destroy the
image
> > that way, is that you add Spherical Aberration to the system.
> >
> > This is a direct consequence of the equations. Wynne proved it
> > mathematically.  (He is the guy that developed the corrector
design
> > used on the big professional observatories.)
> >
> > The more elements you add, the less additional SA you add. The
closer
> > to the primary (further from the eyepiece) the less SA you add.
> > You can balance the SA by overcorrecting the primary (makeing it
> > hyperbolic), like in a hyperbolic astrograph.
> >
> > Coma correcting an "overcorrected" (hyperbolic) primary, is very
easy.
> > Coma correcting a parabolic is quite a bit more difficult. You
have
> > to balance the effect of additional SA with how much coma you are
> > correcting, to get minimum spot sizes.
> >
> ...
>