Re: [ATM] Conical mirror question

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Mike & Thomas, etc.,

I have found the center post conical (at least the 10" that I have in 
Dream's shop) by Royce easy to support.  They can be simpler than a 
floating cell but that highly depends on the thickness of a plano blank in 
comparison and therefore the number of floating points one must use.  If an 
8" or 10" plano blank are full thickness, ie: 1.3" and 1.7" thick 
respectively, then a three point cell is all that is needed for these two 
apertures when full thickness.  However, one is left with more mass in the 
glass if you go with full thickness in a plano-backed blank.  Higher mass 
means more time wasted in trying to get that glass to ambient 
temperature.  The 10" x 1.7" example mentioned produces a blank that is 
roughly 9.75-10.25 lbs (depending on the focal ratio).  The Royce 10" 
center post conical that Dream is using for a customer's telescope:
http://www.dreamscopes.com/pages/cells/y-cell-010-RFR.htm

weighs 7.0 lbs with the large threaded rod, washers, spring and nut (f5 
focal ratio).

A better example is to compare a 10" plano-backed mirror that has the same 
mass as Bob's conical.  Then it is likely that more than three points are 
necessary to support the thinner plano-backed mirror.

The surface area to volume distribution isn't even for plano-backed mirrors 
either.  The edge is thicker than the center because of the sagitta/focal 
ratio.  Only a meniscus has the same thickness across the entire area of 
the glass.  Conicals are used routinely in all sorts of applications, from 
cellular conicals to conical flats to conical primary mirrors.  A much 
bigger issue is the total mass of the glass.  Thermal equilibrium usually 
far exceeds any other errors in a properly built system.  Until thermal 
equilibrium is either modified or truly conquered, all other errors in a 
system are hidden behind this gross error.

Modifying the equilibrium can be accomplished with air flow across the 
surface of the mirror.  This has been mentioned routinely on the ATM 
group.  This greatly decreases the height of the boundary layer.  This is a 
good solution while the mirror is trying to reach equilibrium.  But the 
ultimate goal is to get the optic to true equilibrium because once that is 
reached, air flow across the primary is a moot point (unless structural 
elements are contributing thermals).  Air flow across the primary can help 
the optic reach equilibrium but the entire back half of the glass/mirror is 
lagging far behind, if no additional air flow is provided there.

In terms of holding the conical and collimation, take a look at the 
following photos:
http://www.dreamscopes.com/pages/cells/y-BP%26cell-010-01.htm

Supporting a mirror at the CG is one of the best ways to hold an optic like 
this, if possible.  You can see from the following Ronchi photo, at the top 
left of the following page, that there does not appear to be any strain 
caused by Bob's center post on this 10" f5 example of his work:
http://www.dreamscopes.com/pages/cells/y-BP%26cell-010-03.htm

Supporting an optic at the center, at least partially, has some 
advantages.  It eliminates the need for edge supports.  These can be 
extremely difficult to produce accurately, in a true "floating" 
manner.  They also need to stay away from the front/optical surface of the 
mirror, otherwise they will cause deformation.  Partial support at the 
center can also be used to prevent the optic from moving forward 
axially.  Another benefit of the central hub is that it allows the contact 
points on a floating cell to truly float.  They do not need to be hard 
attached to the back of the optic.  You can see an example of a perforated 
16" that uses a plano-backed substrate supported by an 18-point floating 
cell and a center hub at the bottom of this page:
http://www.dreamscopes.com/pages/06/AstrographN-18-point.htm

Royce's 8" and 10" are not perforated all the way through, as you can see 
on the 10" example Dream has.  So one can use products like Jim Fly's 
CatsEye collimation tools or single dot lasers.  His 12.5" and 16" are 
perforated all the way through, according to Bob.

Bob shipped very explicit directions on how to tighten the bolt on his 
center post conicals.  It is NOT done with a wrench....  That amount of 
force would cause all kinds of issues.  The spring on his design is in 
there for a reason.  It is very simple to see when the bolt is not tight 
enough.  If any cushions were added, either between the back washer and the 
mirror or in place of the steel washers, they will only allow the optic to 
deflect.  The mirror is thickest at the center and any slight variations in 
the mating surfaces of the washer versus the back of the mirror are 
irrelevant.  It will not pass all the way through the thickest portion of 
the glass.  Which is in the secondary shadow anyway.

History has shown that almost every design profile of a mirror has been 
used across different optical designs successfully.  From plano, to 
conical, to cellular.  From Newtonians to Cassegrains.  Within each of 
these main categories are subdivisions where everything under the Sun has 
been tried.  History also shows that one telescope succeeds, yet another 
fails, even though they are "identical."  Telescopes are complex systems 
due to the great number of variables.  Execution of a design or concept is 
extremely important but is rarely given the scrutiny and discussion it 
warrants.  This is why one person can make a telescope system work and 
another will have mountains of frustration.  Blanket statements about 
telescopes can help guide some initially but they can also cause 
misconceptions to grow.

My experience with this 10" f5 center post conical from Royce is that it 
does not appear to have any problems.  I also find it magnitudes easier to 
support than say a 9-18 point floating cell that not only requires much 
more time to design but also much more time and therefore expense to 
fabricate.  Is it the end all be all of a design?  Not necessarily.  But I 
do think it accomplishes what Bob was trying to do: offer amateurs an 
easier way to support their mirrors and at the same time yield a sturdier 
primary (less prone to shifting laterally, axially and tip/tilt)?  Yes.

Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833



At 03:48 AM 4/22/2006, you wrote:
>Hello,
>
>Has anyone purchased a conical mirror?
>
>I was looking at this website - seems to be some very well made
>mirrors for Dob building, they charge about $50 more for a conical mirror.
>
>http://www.rfroyce.com/conical/
>
>Any reason why it would not be better to go with a conical?  The
>*single bolt* mount to the cell seems pretty nice for amateur
>builders, makes the cell construction easier - and the "info" page on
>the website says that it is much sturdier and needs less collimating
>than standard thickness mirrors.
>
>Less weight also, that's kind of nice.
>
>What downsides am I missing here?
>
>Thanks,
>Mike
>California


For one I would question the thermal properties, even with the conical
cross section, the surface area to volume distribution isn't even, the
centre section will take longer than the outer zones to reach
equilibrium.

The conical mirror blank is best suited for Cass type systems (in my
opinion anyway) where the central baffle tube is also the mirror
retainer, you still need a cell to allow you to collimate the system, so
your not gaining all that much by going this route. The only real gain
is no mirror clips to warp the mirror, but that bolt he show on his site
will do that too if your not careful, or worse break the whole thing, I
didn't see any cushioning between it and the mirror, just a couple of
steel washers.

Any way that my $0.02 worth, they are a good idea in the right
application...

Cheers, Thomas.


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