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Konus-Synta achromat refiguring. Help!!



 
 
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  #1  
Old February 23rd 04, 05:47 AM
Miklos Szabo
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Default Konus-Synta achromat refiguring. Help!!

Hi everybody!

I have a Konus 5" f8.3 with not a sharp optics, in focus stars has
many bright rings...with green filter as well.
So I decided to experiment with the shims. The difficulty
is not only the under correction but with a "s" zone ~70%
Well the original shim is ~0.4mm. Changed it to 0.1mm I not
achieved anything....
Then I asked a friend to help measure the radii on the 4 surfaces.
Put the numbers in OSLO and I got overcorrection using BK7-F2 glasses.
Am I missing something or the glass has a irregularity that creating
the under correction?
If I can get the correct glass info I may decide to refigure one of
the surface... Pheraps R4 edge? Please dont tell me it is impossible,
I hapy
to lose the coating just to make the lens sharp at least in green!
R1=611.873mm
R2=353.286mm
R3=355.631mm
R4=1488.956mm
Any help, suggestion appreciated!
Thanks, Miki.
  #2  
Old February 23rd 04, 07:06 AM
Dan Chaffee
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Default Konus-Synta achromat refiguring. Help!!

On 22 Feb 2004 20:47:31 -0800, (Miklos Szabo) wrote:


I have a Konus 5" f8.3 with not a sharp optics, in focus stars has
many bright rings...with green filter as well.
So I decided to experiment with the shims. The difficulty
is not only the under correction but with a "s" zone ~70%
Well the original shim is ~0.4mm. Changed it to 0.1mm I not
achieved anything....
Then I asked a friend to help measure the radii on the 4 surfaces.
Put the numbers in OSLO and I got overcorrection using BK7-F2 glasses.
Am I missing something or the glass has a irregularity that creating
the under correction?
If I can get the correct glass info I may decide to refigure one of
the surface... Pheraps R4 edge? Please dont tell me it is impossible,
I hapy
to lose the coating just to make the lens sharp at least in green!
R1=611.873mm
R2=353.286mm
R3=355.631mm
R4=1488.956mm


Adding space between the elements will worsen the undercorrection.
I played with the radii you give and it seems the best design with
cheap crowns and flints for these radii would be BK 7-F3 with a
1.15mm space. The problem with unknown glasses is that you need the
melt data for the actual glass you have. At any rate, you could try
decreasing the spacing.
Since you know it is undercorrected, and since you are not dealing
with exotic glasses, it is not hard to refigure in terms of adding or
subtracting correction by working R1. At this point, it doesn't really
matter what the design is or glasses are, since you know the problem.
Trying to get rid of a zone is harder because you either have to know
which surface is producing the zone, or be very handy with a small
lap(not for an inexperienced tinkerer:-)

Dan Chaffee
  #5  
Old February 23rd 04, 09:50 PM
Bob May
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Default Konus-Synta achromat refiguring. Help!!

Getting rings around a bright star when at very high power is just looking
at the Airy Disc - go see what they look like before going any further.
If it isn't an Airy Disc that you are seeing, you have some very interesting
problems with the lens as optics generally don't work that way.
I suggest that you do a Ronchi or Knife Edge test to see what the optical
surfaces are like on a bright star. The Ronchi test should show straight
lines if the optical path is good and with the KE, the light from the star
should dim evenly across the aperture if the optics are good.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!


  #6  
Old February 24th 04, 05:42 AM
Miklos Szabo
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Default Konus-Synta achromat refiguring. Help!!

"Bob May" wrote in message ...
Getting rings around a bright star when at very high power is just looking
at the Airy Disc - go see what they look like before going any further.
If it isn't an Airy Disc that you are seeing, you have some very interesting
problems with the lens as optics generally don't work that way.
I suggest that you do a Ronchi or Knife Edge test to see what the optical
surfaces are like on a bright star. The Ronchi test should show straight
lines if the optical path is good and with the KE, the light from the star
should dim evenly across the aperture if the optics are good.


Thanks for the replay!

At high magnification IN FOCUS I can see the Airy disk +5 or 6
bright diffraction rings.
Of course the atmosphere make those rings dancing like ballerina on
ice....
I think this is a " extraneous junk (borrowing R.C.) " which is
destroying my contrast.
At medium and low mags. inside focus I can see the Outer 2 rings much
brighter than the outside of focus outer rings. Top of that there is
the 3rd. ring from
outside, doing the opposite what is the surrounding rings do, I guess
this is a zone error
@70% of aperture. I hope the description is not sound like garbage....
My hope is, if the under correction is coursing the major problem,
maybe a "slight"
polishing of R1 edge fixing the spherical error and live the zone
alone. Of course
the mgf2 coating is partially destroyed.

I appreciated this is a cheap lens, but cutting a long story short:
I started with baying the Vixen VC200L cassegrain. That was a big
mistake!
Open tube, 40% obstruction, thick vane, field corrector lens, made
sure that I never
see a star only a big snow bowl! Then I asked the dealer to change it
to a 5" refractor.
I can see the Airy disk now at least, but the resolution, despite the
colour aberration
should be better. Regard to " throwing good money after bad" : 2
teenager kids, mortgage
and a less than sympathetic wife, is not an option. ATM is. And if I
can fix just a bit
of the correction I can share the experiment with other less then
happy Synta achromat owners....
Thanks for reading, and please forgive me my less than perfect
English!
Miki.
  #7  
Old February 24th 04, 03:58 PM
Jon Isaacs
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Default Konus-Synta achromat refiguring. Help!!

Thanks for the replay!

At high magnification IN FOCUS I can see the Airy disk +5 or 6
bright diffraction rings.


What is high power?

Of course the atmosphere make those rings dancing like ballerina on
ice....
I think this is a " extraneous junk (borrowing R.C.) " which is
destroying my contrast.


At medium and low mags. inside focus I can see the Outer 2 rings much
brighter than the outside of focus outer rings.


The bottomline on a scope is not the star test but rather how it works when in
focus as an observational instrument..

It appears to me that this scope does a reasonable job, seeing 6 diffraction
rings at high power is a sign things are pretty good.

I am wondering if you just asking too much of this scope...

Jon "If it ain't broke, don't fix it" Isaacs
  #8  
Old February 24th 04, 09:59 PM
Dan Chaffee
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Default Konus-Synta achromat refiguring. Help!!

(Jon Isaacs) wrote in message ...

It appears to me that this scope does a reasonable job, seeing 6 diffraction
rings at high power is a sign things are pretty good.

I am wondering if you just asking too much of this scope...


I think not. Any scope of mine that showed 5 or 6 bright diffraction
rings
in focus would be headed back to the pitch lap. All my scopes have
strehls
at or above .8, and none shows that many bright rings, filtered or
not.

Refractors with the typical Faunhofer design, as this one is, are
aplanatic, meaning not only being comma free, but are nulled to 3rd
order spherical correction if all surfaces are spherical(easy for
machine polishers) AND if the glasses are close to the proper indices
and dispersion values. This is a problem in cheap, massed produced
refractors, and when these values are
off (especially for objectives this fast), errors in third order
correction and/or color correction are common. The former can be
overcome with figuring one of the surfaces--usually R1. A green light
strehl of at least .8 should not be too much to ask for considering
the ease and low cost of the fabrication of these massed produced
refractors. Resolving high contrast features near the
limits for the aperture should not be a problem for a short achromat,
but don't expect great contrast on planets--even with an error free
example of
one of these scopes.


Dan Chaffee
  #9  
Old February 25th 04, 07:38 AM
CLT
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Default Konus-Synta achromat refiguring. Help!!

It appears to me that this scope does a reasonable job, seeing 6
diffraction
rings at high power is a sign things are pretty good.


???????

I've never seen that in a scope I consider good and am not sure why I would
want to. To see 6 rings means a lot of light is being pushed out. IIRC, S&T
had a discussion on this years ago about some older (uncoated) scopes that
did this with (?)multiple reflections between r2 and r3(?)

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
Lunar Picture of the Day http://www.lpod.org/
************************************


 




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