Building a Refractor?

Hello All:

I've built a few Dobs scopes over the past several years. Now
William-Bell has a new book coming out titled, "Building a Refractor
Telescope". I'd love to try this! Does anyone have any experience
with a project of this type? If so please post. What does one do
about coating the lens?

Thank you,

Kurtis

"Space Traveler" wrote in message
ups.com...
Hello All:

I've built a few Dobs scopes over the past several years. Now
William-Bell has a new book coming out titled, "Building a Refractor
Telescope". I'd love to try this! Does anyone have any experience
with a project of this type? If so please post. What does one do
about coating the lens?

Thank you,

Kurtis,

I am looking forward to the book too, and hope it addresses getting
coatings. In reality, coatings are not going to make a huge difference for
most designs, especially if you are doing lunar and planetary work, but it
would be nice to have the option.

I would think standard MgF coatings would be available and not be too
expensive. The multicoatings are, I understand, considerably more
expensive.

Sending a lens out for coatings is always a bit risky. Most coating
application involves heating the lens, and I would think you'd want to take
care to carefully finish all edges and bevels to be free of chips to reduce
the chance of breakage.

I think the Schupmann book will be a bit more interesting. Now there is a
real "ATM" telescope.

Clear skies, Alan

On Tue, 26 Dec 2006 10:17:13 -0500, "Alan French"
wrote:

I am looking forward to the book too, and hope it addresses getting
coatings. In reality, coatings are not going to make a huge difference for
most designs, especially if you are doing lunar and planetary work, but it
would be nice to have the option.

I don't follow. This is precisely when you _will_ need very good
coatings. It doesn't matter much for DSOs, but when you look at bright
objects with uncoated optics you see myriad ghost reflections. Uncoated
achromats are largely useless for lunar and planetary viewing. They will
work okay with dimmer objects, although ghost images from stars may
prove irritating.


I would think standard MgF coatings would be available and not be too
expensive. The multicoatings are, I understand, considerably more
expensive.

Sending a lens out for coatings is always a bit risky. Most coating
application involves heating the lens, and I would think you'd want to take
care to carefully finish all edges and bevels to be free of chips to reduce
the chance of breakage.

These days, coating processes don't necessarily involve significant
heating of the optic. Sputtering techniques are cold- I've had plastic
components multicoated this way. The last time was nearly 10 years ago,
and the cost was about $500 for a run, which worked out to about $5 per
element. The problem with sending a single item (or just a few) is that
unless there are other components in the run, you pay a lot by absorbing
all the setup costs.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

"Chris L Peterson" wrote in message
...
On Tue, 26 Dec 2006 10:17:13 -0500, "Alan French"
wrote:

I am looking forward to the book too, and hope it addresses getting
coatings. In reality, coatings are not going to make a huge difference
for
most designs, especially if you are doing lunar and planetary work, but
it
would be nice to have the option.

I don't follow. This is precisely when you _will_ need very good
coatings. It doesn't matter much for DSOs, but when you look at bright
objects with uncoated optics you see myriad ghost reflections. Uncoated
achromats are largely useless for lunar and planetary viewing. They will
work okay with dimmer objects, although ghost images from stars may
prove irritating.

It depends on the design. If ghost reflections are a problem, then it
should be coated. I had an old Lohmann Brothers 5" f/15 achromat from
around 1910 that was uncoated, and there was no problem with ghost
reflections.

I would think standard MgF coatings would be available and not be too
expensive. The multicoatings are, I understand, considerably more
expensive.

Sending a lens out for coatings is always a bit risky. Most coating
application involves heating the lens, and I would think you'd want to
take
care to carefully finish all edges and bevels to be free of chips to
reduce
the chance of breakage.

These days, coating processes don't necessarily involve significant
heating of the optic. Sputtering techniques are cold- I've had plastic
components multicoated this way. The last time was nearly 10 years ago,
and the cost was about $500 for a run, which worked out to about $5 per
element. The problem with sending a single item (or just a few) is that
unless there are other components in the run, you pay a lot by absorbing
all the setup costs.

It's good to know there are alternatives.

Clear skies, Alan

In article , Chris L
Peterson wrote:

Uncoated
achromats are largely useless for lunar and planetary viewing.

So would you say all those old Clark refractors are largely useless on
the moon and planets?

--
Joe Bergeron

http://www.joebergeron.com

Alan French wrote:
"Chris L Peterson" wrote in message
...
On Tue, 26 Dec 2006 10:17:13 -0500, "Alan French"
wrote:

I am looking forward to the book too, and hope it addresses getting
coatings. In reality, coatings are not going to make a huge difference
for
most designs, especially if you are doing lunar and planetary work, but
it
would be nice to have the option.

I don't follow. This is precisely when you _will_ need very good
coatings. It doesn't matter much for DSOs, but when you look at bright
objects with uncoated optics you see myriad ghost reflections. Uncoated
achromats are largely useless for lunar and planetary viewing. They will
work okay with dimmer objects, although ghost images from stars may
prove irritating.

It depends on the design. If ghost reflections are a problem, then it
should be coated. I had an old Lohmann Brothers 5" f/15 achromat from
around 1910 that was uncoated, and there was no problem with ghost
reflections.

I would think standard MgF coatings would be available and not be too
expensive. The multicoatings are, I understand, considerably more
expensive.

Sending a lens out for coatings is always a bit risky. Most coating
application involves heating the lens, and I would think you'd want to
take
care to carefully finish all edges and bevels to be free of chips to
reduce
the chance of breakage.

These days, coating processes don't necessarily involve significant
heating of the optic. Sputtering techniques are cold- I've had plastic
components multicoated this way. The last time was nearly 10 years ago,
and the cost was about $500 for a run, which worked out to about $5 per
element. The problem with sending a single item (or just a few) is that
unless there are other components in the run, you pay a lot by absorbing
all the setup costs.

It's good to know there are alternatives.

Clear skies, Alan

Do ghosts show up with elements that are not equi-convex? Wasn't there
a way to apply a rudamentary single-layer coating on a lens using
chemicals at home? Didn't "straw coating" originate this way?

On Dec 26, 7:53 am, Chris L Peterson wrote:
On Tue, 26 Dec 2006 10:17:13 -0500, "Alan French"
This is precisely when you _will_ need very good
coatings. It doesn't matter much for DSOs, but when you look at bright
objects with uncoated optics you see myriad ghost reflections. Uncoated
achromats are largely useless for lunar and planetary viewing. They will
work okay with dimmer objects, although ghost images from stars may
prove irritating.

Chris,
How many types of uncoated achromats have you looked through?
My homemade uncoated 6.6" f/19 air spaces fraunhofer exhibits NONE of
the
problems you cite and is as far from useless as you can imagine for L/P
work-
with stark blackish backgrounds to boot. The Baker achromatic design is
also
free from detectable ghost reflections in an uncoated state. Even the
unoiled Littrow
and equal R1, R2-R3 radii, which does indeed show a ghost image, is
hardly useless for L/P work.


Dan Chaffee

On Wed, 27 Dec 2006 03:08:00 -0500, Joe Bergeron
wrote:

So would you say all those old Clark refractors are largely useless on
the moon and planets?

Ok, not useless. But they aren't very good by modern standards (at
least, not the one I've seen).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 27 Dec 2006 02:13:51 -0800, "decaf" wrote:

Chris,
How many types of uncoated achromats have you looked through?
My homemade uncoated 6.6" f/19 air spaces fraunhofer exhibits NONE of
the
problems you cite and is as far from useless as you can imagine for L/P
work-
with stark blackish backgrounds to boot. The Baker achromatic design is
also
free from detectable ghost reflections in an uncoated state. Even the
unoiled Littrow
and equal R1, R2-R3 radii, which does indeed show a ghost image, is
hardly useless for L/P work.

Hi Dan-

I regret using the term "useless"- that was far too strong. But in
general, most achromat designs will show ghosting effects. I guess many
of these older designs were specifically optimized to minimize ghosting
simply because AR coatings hadn't been developed. However, given the
wide availability of inexpensive and effective coatings today, the
optical designer has much more flexibility in his design. Are you aware
of any fast achromat designs that would be suitable for uncoated use?

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On Wed, 27 Dec 2006 08:36:32 -0600, Chris L Peterson
wrote:

On 27 Dec 2006 02:13:51 -0800, "decaf" wrote:

Chris,
How many types of uncoated achromats have you looked through?
My homemade uncoated 6.6" f/19 air spaces fraunhofer exhibits NONE of
the
problems you cite and is as far from useless as you can imagine for L/P
work-
with stark blackish backgrounds to boot. The Baker achromatic design is
also
free from detectable ghost reflections in an uncoated state. Even the
unoiled Littrow
and equal R1, R2-R3 radii, which does indeed show a ghost image, is
hardly useless for L/P work.

Hi Dan-

I regret using the term "useless"- that was far too strong. But in
general, most achromat designs will show ghosting effects. I guess many
of these older designs were specifically optimized to minimize ghosting
simply because AR coatings hadn't been developed. However, given the
wide availability of inexpensive and effective coatings today, the
optical designer has much more flexibility in his design. Are you aware
of any fast achromat designs that would be suitable for uncoated use?

Chris,

I was under the impression that an uncoated objective was too
far removed from the image plane to seriously contribute to
ghosting. If anything, only uncoated eyepieces, and those
of specific designs (even when coated) were prone to this
problem.

It seems that the curves of the objective would have to be just
right to form a reflected image anywhere near the direct image.
Is this not the case?

Thanks,
Larry G.


--
Your mind is a terrible thing to waste - TURN OFF YOUR TV!

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