Help with elliptical secondary source?

Hi,

I'm looking at sources to buy an elliptical secondary (around 2.14")
for my first Dob project. I see a lot of variation in prices ...
reading some Google archives gives me the impression that the
secondary quality doesn't matter overmuch. Is this the right
impression? I'm interested in thoughts and recommendations.

Options so far:
Orion 2.05"/2.28" - around $23-$25, ? refl, ? wave
University Optics 2.14" - $59, 94% refl, 1/10 wave
Astrosystems 2.14" - $80, 94% refl, 1/10 wave
Protostar 2.14" - $99, 94% refl, 1/10 wave
Discovery 2.14" - $119, ? refl, ? wave

I'm leaning toward either UO (because I had good experiences buying EP
from them), or toward Astrosystems (they are a local company to me).

Thanks in advance,
Nate Perkins
Fort Collins, CO

(Nate Perkins) wrote:

I'm looking at sources to buy an elliptical secondary (around 2.14")
for my first Dob project. I see a lot of variation in prices ...
reading some Google archives gives me the impression that the
secondary quality doesn't matter overmuch. Is this the right
impression? I'm interested in thoughts and recommendations.

What's your primary? If it's a 1/30th wave Zambuto then find something
as awesome as possible and pay the premium, if it's a home-made 1/4
wave then a top-grade secondary might not be the best use of your
cash.


Tim
--
Foo.

Tim Auton wrote in message . ..
(Nate Perkins) wrote:

I'm looking at sources to buy an elliptical secondary (around 2.14")
for my first Dob project. I see a lot of variation in prices ...
reading some Google archives gives me the impression that the
secondary quality doesn't matter overmuch. Is this the right
impression? I'm interested in thoughts and recommendations.

What's your primary? If it's a 1/30th wave Zambuto then find something
as awesome as possible and pay the premium, if it's a home-made 1/4
wave then a top-grade secondary might not be the best use of your
cash.

It's certainly not a Zambuto! It's Chinese (blush).

I'll take your advice and go with a lower cost secondary.

Thanks, Nate

Nate Perkins wrote in message
. ..
Tim Auton wrote in message
. ..
(Nate Perkins) wrote:

I'm looking at sources to buy an elliptical secondary (around 2.14")
for my first Dob project. I see a lot of variation in prices ...
reading some Google archives gives me the impression that the
secondary quality doesn't matter overmuch. Is this the right
impression? I'm interested in thoughts and recommendations.

What's your primary? If it's a 1/30th wave Zambuto then find something
as awesome as possible and pay the premium, if it's a home-made 1/4
wave then a top-grade secondary might not be the best use of your
cash.

It's certainly not a Zambuto! It's Chinese (blush).

I'll take your advice and go with a lower cost secondary.

Thanks, Nate

Do not take that advice please. If you can , meaning if you have the time
and will , please learn a little about testing telescopes and mirrors.
There's plenty of free info on the internet , and testing devices can be had
or built easily and for very little money and not much effort . Then measure
your primary and decide if it's crap , great or in between . There are lots
of people out there who are holding some piece of paper from an optician
that states their mirrors have a Strehl of 1.001 or whatever and have a very
elitist attitude . These people are deluding themselves that they have a
lambda/1million telescope , when they really are at best having a 1/4 or 1/8
instrument . Do yourself a favor and learn to test yourself, since you want
to build a dob, and not have to rely on assumptions or other people's
representations , especially people who have a vested interest and
consequently a large bias . Also regarding big name secondaries do yourself
a favor and search the saa history, there have been numerous negative
experiences with various brands. Learn which were these and how to measure
those types of problems.
After that, shop around and buy , but be ready to measure and send back if
it doesn't comply with the official bragging (loosely called specs by
vendors) .

best regards,
matt tudor

"matt" wrote in message ...

....
If you can , meaning if you have the time
and will , please learn a little about testing telescopes and mirrors.
There's plenty of free info on the internet , and testing devices can be had
or built easily and for very little money and not much effort . Then measure
your primary and decide if it's crap , great or in between . There are lots
of people out there who are holding some piece of paper from an optician
that states their mirrors have a Strehl of 1.001 or whatever and have a very
elitist attitude . These people are deluding themselves that they have a
lambda/1million telescope , when they really are at best having a 1/4 or 1/8
instrument .
....

Hi Matt,

Good advice. I have a fair amount of practice with the star test
method as described by Suiter, and have used enough scopes to know an
average optic from a good one (but to be honest I can't practically
tell a good optic from a great one). I've been observing for a few
years now (first with a C8 and lately with an MN61). At my
intermediate skill level, I am still more limited by observing
technique, viewing conditions, and scope collimation/cooldown than I
am by the difference between good and great optics.

Mostly I'm interested in building a Dob for the challenge of it ... as
an opportunity to experiment with a different design and learn new
techniques. Learning to do benchtop evaluation of a mirror would sure
fall into that category.

Cheers,
Nate

Whatever you do dont buy Parks.



Nate Perkins wrote:

Hi,

I'm looking at sources to buy an elliptical secondary (around 2.14")
for my first Dob project. I see a lot of variation in prices ...
reading some Google archives gives me the impression that the
secondary quality doesn't matter overmuch. Is this the right
impression? I'm interested in thoughts and recommendations.

Options so far:
Orion 2.05"/2.28" - around $23-$25, ? refl, ? wave
University Optics 2.14" - $59, 94% refl, 1/10 wave
Astrosystems 2.14" - $80, 94% refl, 1/10 wave
Protostar 2.14" - $99, 94% refl, 1/10 wave
Discovery 2.14" - $119, ? refl, ? wave

I'm leaning toward either UO (because I had good experiences buying EP
from them), or toward Astrosystems (they are a local company to me).

Thanks in advance,
Nate Perkins
Fort Collins, CO

On 23 Nov 2004 11:48:10 -0800, (Nate Perkins)
wrote:

Tim Auton wrote in message . ..


What's your primary? If it's a 1/30th wave Zambuto then find something
as awesome as possible and pay the premium, if it's a home-made 1/4
wave then a top-grade secondary might not be the best use of your
cash.

I'll take your advice and go with a lower cost secondary.

I do not necessarily agree with this. You do NOT want the diagonal to
be the weakest link in the chain. If your primary is smooth with say
..25 wavefront undercorrection, it's still a fairly good mirror. If you
buy a flat with too much curve too it, you will immediately notice
astigmatism in high power star images, reducing your .8 strehl
optic to well below that number.

A high quality diagonal in that size is not *that* expensive, and I
for one have never regretted the extra effort in making, or in your
case, buying a high quality optical component. What if you upgrade to
a better primary some day? You'll likely lament the taking the cheap
route back in the "good old days":-). Try Protostar; consistantly
recommended by critical observers and ATMs.

D Chaffee

Do not take that advice please. If you can , meaning if you have the time
and will , please learn a little about testing telescopes and mirrors.

Great advice. You should check if your local club can assist you in
evaluating optics. Our club periodically runs a mirror making workshop
where there are a number of folks who have the skills and equipment to check
your mirror.

In any case, I would buy a good quality secondary. We have always had good
luck with Protostar, but I am sure that others are also good.

Dennis