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Is 1/4 wave good enough for a rich field?



 
 
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  #1  
Old November 21st 03, 06:11 AM
donutbandit
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Default Is 1/4 wave good enough for a rich field?

I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.
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  #2  
Old November 21st 03, 08:23 AM
Bill Becker
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Default Is 1/4 wave good enough for a rich field?


"donutbandit" wrote in message
...
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


I don't know but....if the total correction for the "system" was 1/4 wave,
that wouldn't be too bad at all but...
for the primary itself to be 1/4 wave, it doesn't sound very promising.

Best regards,
Bill


  #3  
Old November 21st 03, 02:08 PM
BllFs6
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Default Is 1/4 wave good enough for a rich field?


IFFFF your only gonna use the mirror as a rich field scope....1/4 wave is
fine......


particularly if your not using extremely well corrected eyepieces like naglers
and coma correctors like a paracoor....

I could go into all kinds stuff regarding the eyes resolving power, airy
(hairy) disks, MTFs etc etc....

but the bottom line is....for rich field (ie lower power/large exit pupil)
viewing, unless you got eyes like superman...you AINT GONNA see any difference
between a 1/16 wave mirror and one as worse as possibly a FEW waves of
error....

Ive actually toyed with the idea of making a poor mans high performance rich
field scope that would give very NICE views on a pretty good budget....

make the mirror F8 (sorta) or so.....use a 50mm plossl or perhaps one of the
higher performance eyepieces at a slighter shorter focal length (and drop the F
ratio accordingly)...

make the mirror spherical....AND let the secondary diagonal mirror be
undersized so that IT defines the aperture ....

So, for example....you would use an 8 F6 spherical mirror with an UNDERSIZED
secondary so that anywhere across the eyepieces field of view it acts like a 6
inch F8.....so there would also be a fully illuminated field for a 50mm
plossl....

Actually, for nice rich field viewing....proper /well done baffling of the
telescope tube, a scope that is easy and comfortable to use, and low scatter
eyepieces are probably WAY more important than mirror accuracy....

take care

Blll
  #4  
Old November 21st 03, 02:14 PM
Ed Majden
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Default Is 1/4 wave good enough for a rich field?


"Bill Becker" wrote in message
...

"donutbandit" wrote in message
...
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on

it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


I don't know but....if the total correction for the "system" was 1/4 wave,
that wouldn't be too bad at all but...
for the primary itself to be 1/4 wave, it doesn't sound very promising.

Best regards,
Bill

Take these surface accuracy claims with a grain of salt! Test the
mirror yourself. Unfortunately in most cases, it is up to you to prove the
claim incorrect. Even a good 1/8 wave mirror will act poorly if the surface
is not properly polished. Also what does the claim mean. Is it
peak-to-peak or wave front????
Ed
Ed


  #5  
Old November 21st 03, 02:20 PM
Martin Brown
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Default Is 1/4 wave good enough for a rich field?

In message , donutbandit
writes
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


The fast focal ratio and geometric aberrations are more likely to hurt
image quality than whether or not the image is perfectly diffraction
limited. Few eyepieces will tolerate fast f4 light cones and your
secondary will necessarily be rather big. The latter factor cuts both
ways.

It is probably worth a try if the mirror is reasonably cheap. Don't
expect great planetary details though!

Regards,
--
Martin Brown
  #6  
Old November 21st 03, 02:34 PM
Roger Hamlett
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Default Is 1/4 wave good enough for a rich field?


"donutbandit" wrote in message
...
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.

Unfortunately, you can't tell from this figure.
'1/4 wave', is one of these phrases that is often used, and doesn't really
mean very much. Firstly, you need to know if this is RMS, or peak. But, even
with this extra data, there is a problem. It is possible (for instance), to
have a mirror, which has a small 'bump' in the middle, making it '1/4 wave',
yet the majority of the surface, is fantastic. Conversely, another mirror
could meet the same criterion, and be covered in imperfections. If you
actually have numeric values for the RMS, and peak errors, you can get a
very good idea. The other number that can help to 'quantify' the overall
quality, is the Strehl ratio.
It is possible for some 1/4 wave mirrors to be the basis of a fine
instrument. I have actually seen one scope, that on test, would have
probably only 'scraped home' as 1/2 wave, yet is superb (it had the central
hump described above).

Best Wishes


  #7  
Old November 21st 03, 02:45 PM
Tony Flanders
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Default Is 1/4 wave good enough for a rich field?

donutbandit wrote in message ...

I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


I'm sure that you would never know the difference between 1/4 wave and
1/8 wave at 40X. But this suggests that the mirror wasn't fabricated
very carefully, and there are other defects besides wavefront error
that *would* be noticeable at 40X, particularly turned edge and a
rough surface. Both of those would show up as loss in contrast.

Besides, why restrict yourself to 40X, when a decent mirror could
easily get to 200X? Even if you only used it 5% of the time, it
would be an awfully nice ability to have.

- Tony Flanders
  #8  
Old November 21st 03, 03:20 PM
Jon Isaacs
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Default Is 1/4 wave good enough for a rich field?


I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


I wouldn't worry to much at low powers because Coma and eye aberations from the
large exit pupil will probably be the big factors in determining the quality of
the views.

The problem with F4 scopes is that you run out of exit pupil quite quickly.
Given an maximum exit pupil of 7 mm, this means that an eyepiece over 28 mm
will be wasting aperture.

Now there maybe nothing wrong with wasting aperture but at F4 your are dealing
with heavy duty coma and require some pretty good eyepieces. A coma corrector
will help but now you have are operating at F4.65 and it is costing a few
hundred bucks if you don't aleady have one.

Those big views are not so impressive if the FOV is filled with Coma both from
the mirror and from the eyepieces.

From my point of view, a 6 inch F5 mirror or a 5 inch F5 scope is a better
choice.

At low powers the 5 inch scope will provide equivelent brightness for a given
eyepiece, and a 6 inch scope can be used with longer focal length eyepieces to
achieve the same field of view without exceeding the exit pupil considerations.

For visual use, i think the only justification for a F4 scope is to make it
more compact, and in this case, the trade off is just not worth it.

jon who owns a 5 inch F5 scope and thinks it makes a nice widefield scope
with a 32 mm WF eyepiece.



  #9  
Old November 21st 03, 04:04 PM
Michael A. Covington
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Default Is 1/4 wave good enough for a rich field?


"donutbandit" wrote in message
...
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.


Yes. Build the telescope and enjoy it.


--
Clear skies,

Michael Covington -- www.covingtoninnovations.com
Author, Astrophotography for the Amateur
and (new) How to Use a Computerized Telescope



  #10  
Old November 21st 03, 10:44 PM
Bettrel
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Default Is 1/4 wave good enough for a rich field?

I agree with those that say go ahead and give it a try. It doesn't sound as if
much money will be involved, so why not try it? Since you're interested in
rich field use, I doubt the optics, in whatever way they are "1/4 wave", will
be much of a problem. Yes... coma will be there, but it seems to bother some
people more than others and what's the harm in building the telescope to find
out whether or not the coma is objectionable. If we all spent our time trying
to figure out on paper or computer what the "perfect telescope" would be and
didn't try any design that didn't fit perfection, how much actual observing
would get done?
 




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