Full-thickness mirrors, still best?

Remember the 6:1 thickness ratio good mirrors had
in the 1970s? According to articles I've seen,
they still hold their shapes better than thin mirrors
with complex supports. Yet I doubt anyone uses them
for telescopes anymore. Maybe because of cooldown?
Does anyone have information about support systems
that can actually allow a thin mirror to keep from
flexing out of spec?
-Rich

Are you wanting a thin mirror because it is lighter or are you looking for a
set-up to support an existing mirror? There has been a lot of progress made
in the "waffle" (as I like to call them) mirrors. They are rigid with half
the weight. (I am approximating but we have a 28" waffle mirror that I can
actually pick up myself and I am no hulk.)

--
Dawn Baird-Chleborad
www.wodenoptics.com
www.astronerds.com

"RichA" wrote in message
...
Remember the 6:1 thickness ratio good mirrors had
in the 1970s? According to articles I've seen,
they still hold their shapes better than thin mirrors
with complex supports. Yet I doubt anyone uses them
for telescopes anymore. Maybe because of cooldown?
Does anyone have information about support systems
that can actually allow a thin mirror to keep from
flexing out of spec?
-Rich

While there may be other designs that work equally well, the ones I'm
most familiar with are the mirror cells used by Starmaster and
StarStructure. They provide plenty of support for 'thin',
large-aperture mirrors. Take a look at their designs and you'll note
the attention to supporting the mirror's EDGE.

I have a particulary fine 24" Zambuto mirror (1.6" thickness) in my
Starmaster that is used primarily for planetary observations and, as
such, regularly operates at extremely high magnifications.
The images are simply remarkable and exhibit no aberrations that would
result from flex or stress.

Mike Harvey

There are three issues going on here. One relates to the bending
moments of a mirror, the other relates to how quickly the mirror
becomes useable in a telescope. The third issue is that it is basically
impossible to get inexpensive pyrex in thicknesses greater than 2"

Sheet pyrex comes out of the oven in 41" sheets just over 2" thick
(2.125). Obtaining a blank thicker requires custom casting $$$ of the
glass blank. The original pyrex is ground down to 2" to make the back
flat enough for the support structure and to prepare the front for
spherical grinding.

Thick glass takes a lot longer to cool than thin glass. A 2" thick
mirror can take several hours to reach thermal equilibrium. A similar
1" thick blank will cool in about 1/3rd of this amount of time.

Finally, programs like PLOP have shown that amateurs can build very
thin mirrors quite successfully and support them with similar wavefront
errors as thicker glas somewhat less well supported. I have heard of
20" mirrors of 7/8" thickness being ground, polished and mounted in
scopes. This is thinner than 1:20. Prior to PLOP mirrors over 20" were
invariably supported with 27-point cells. PLOP indicates that 2" thick
25" mirror can be successfully supported with a 6-point cell.

So, not only can the thickness of the blank be reduced (savign money,
cooling faster, less strain on the human) these mirrors can be just as
successful at delivering excellent images as thicker blanks, often
better due to the faster cooling rates of the thinner glass.

Google PLOP Lewis
Read "The Dobsonian Telescope" Berry and Kriege

Mitch

Mike wrote:

While there may be other designs that work equally well, the ones I'm
most familiar with are the mirror cells used by Starmaster and
StarStructure. They provide plenty of support for 'thin',
large-aperture mirrors. Take a look at their designs and you'll note
the attention to supporting the mirror's EDGE.

I have a particulary fine 24" Zambuto mirror (1.6" thickness) in my
Starmaster that is used primarily for planetary observations and, as
such, regularly operates at extremely high magnifications.
The images are simply remarkable and exhibit no aberrations that would
result from flex or stress.

Mike Harvey

I'd like to hear more about your Starmaster, its high on my list of choices.
In a year or two i plan to purchase something in the 18" range, and I'm
still asking around about what various people who have decent scopes think
especially if they've had them awhile and gotten to really know their pro's
and cons.
Thanks
Eric

Hi Eric...

This is my THIRD Starmaster!
I bought my first one as a replacement for my 18" Obsession and
'graduated' from it to a 20" and then the 24".
Over the past few years I've seen more examples of this brand than any
other at the star parties I attend.

Starmaster came along at the right time with innovations that no other
Dobsonian could match...real, removeable, mirror cells in place of the
(IMO) simply awful "slings" that Obsession used...
simpler and quicker setup/takedown and (most importantly) GOTO and
tracking.The superb Zambuto mirrors were a bonus!

I would not hesitate to recommend a Starmaster. But DO get the
GOTO/tracking! The ability to observe an object for long periods of
time without having to manually move the scope is like having 4 more
inches of aperture...you just see more!
If there are budgetary considerations, my opinion is that you'd be
better off with a 14.5" or 16" WITH the Skytracker than with an 18"
withOUT!

You should also take a look at the StarStructure scopes. They are new
(and less expensive) but I've used several and found each to be
excellent.These scopes (like the Starmasters of 5 years ago) have
innovations that no one else is offering.

My Discovery 12.5" has a mirror thickness of 2.5", a 1:5 ratio.
So some companies still use thick mirrors.

See http://www.foothill.net/~sayre/22-in...cular.htm#Cell
for a detailed description of a 27-point mount for a 22" mirror that's
1.6" thick.

Clear skies,
Shneor Sherman

On 17 Feb 2005 08:02:20 -0800, wrote:

My Discovery 12.5" has a mirror thickness of 2.5", a 1:5 ratio.
So some companies still use thick mirrors.

Apart from the support structure needed to keep a thin mirror flat,
I wondered how such a mirror could be supported during final figuring
and polishing. The mirror making machines must be more elaborate than
when they were used to make thick mirrors that were self-supporting.
-Rich

Mike wrote:

Hi Eric...

This is my THIRD Starmaster!
I bought my first one as a replacement for my 18" Obsession and
'graduated' from it to a 20" and then the 24".
Over the past few years I've seen more examples of this brand than any
other at the star parties I attend.

Starmaster came along at the right time with innovations that no other
Dobsonian could match...real, removeable, mirror cells in place of the
(IMO) simply awful "slings" that Obsession used...
simpler and quicker setup/takedown and (most importantly) GOTO and
tracking.The superb Zambuto mirrors were a bonus!

I would not hesitate to recommend a Starmaster. But DO get the
GOTO/tracking! The ability to observe an object for long periods of
time without having to manually move the scope is like having 4 more
inches of aperture...you just see more!
If there are budgetary considerations, my opinion is that you'd be
better off with a 14.5" or 16" WITH the Skytracker than with an 18"
withOUT!

You should also take a look at the StarStructure scopes. They are new
(and less expensive) but I've used several and found each to be
excellent.These scopes (like the Starmasters of 5 years ago) have
innovations that no one else is offering.

Thanks for the info, i really love hearing about these scopes, if only
i could find a star party nearby where someone had one set up....

Thanks
Eric