undercorrected scope: anything I can do?

jerry warner wrote in message ...
you might open up the spacing a bit. It usually helps. But first be sure
the front retainer ring is not overly tight adding to the undercorrection.
I would check that first just to be sure. Sometimes these rings really
get snugged down - relieving the ring abit sometimes helps.
Jerry

I thought that increasing spacer distances produced more
undercorrection. Someone please let me know which is true because I
have an undercorrected refractor also and would like to find out which
is true. Thanks.





Hi Clayton,

There are three options I can think of. Two of them cost more than the scope
is worth.

(1) Buy a chromacor with correction.

(2) Buy a SAFIX

(3) Experiment with changing the spacing of the two elements of the
objective. You can check with some of the people on the Yahoo Chinese
Refractor list who have tried this.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

************************************
"Clayton E. Cramer" wrote in message
...
I bought a Photon Instruments 127mm achromat through Astromart in late
December. The rain, snow, and clouds were essentially continuous until
this
weekend. Now that I have had a couple nights of clear, relatively
unturbulent weather, I am disappointed.

I tried to star test--but the only diffraction rings were inside of focus
(where they were crisp and clear). There were NO diffraction rings the
same
distance outside focus. A helpful suggestion was to use a green filter to
suppress any purple. No good; still no diffraction rings outside
focus--just
a big blurry elliptical blob. This implies a severely undercorrected lens.

Performance on the planets is unimpressive. I set up my Televue Ranger
alongside; at 127x in the Photon, and 120x in the Ranger, they give
essentially the same image: you can just barely see one cloud band on
Saturn, and Cassini's Division is a gray stripe just barely visible around
the planet. At 190x, under skies with good transparency and little
turbulence, Saturn is just a big fuzzy blob. In short, a 127mm f/9
achromat
performs no better for resolution than a 70mm f/f6.8 semiapo. (Obviously,
the Photon does a better job of gathering light than the Ranger.)
Chromatic
aberration is no worse than I expected; actually, pretty good.

The person I bought it from had owned it less than a month; I notice that
the paint was chipped on one of the screws holding the tailpiece to the
tube. (He did warn me of this in advance.) I wonder if he tried to improve
the optics, discovered that he couldn't--and then couldn't return the
scope
to Photon because it was now damaged.

My suspicion is that short of replacing the objective, there is nothing
that
I can do but sell it, and buy something a bit better made. (Perhaps from
Tasco. :-)) I know that there are some tricks for a lot of optical
deficiencies; is there anything that can be done to correct this, or
should
I just take my lumps, and never buy something expensive on Astromart
again?

Clayton E. Cramer

jerry warner wrote in message ...
you might open up the spacing a bit. It usually helps. But first be sure
the front retainer ring is not overly tight adding to the undercorrection.
I would check that first just to be sure. Sometimes these rings really
get snugged down - relieving the ring abit sometimes helps.
Jerry

I thought that increasing spacer distances produced more
undercorrection. Someone please let me know which is true because I
have an undercorrected refractor also and would like to find out which
is true. Thanks.





Hi Clayton,

There are three options I can think of. Two of them cost more than the scope
is worth.

(1) Buy a chromacor with correction.

(2) Buy a SAFIX

(3) Experiment with changing the spacing of the two elements of the
objective. You can check with some of the people on the Yahoo Chinese
Refractor list who have tried this.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

************************************
"Clayton E. Cramer" wrote in message
...
I bought a Photon Instruments 127mm achromat through Astromart in late
December. The rain, snow, and clouds were essentially continuous until
this
weekend. Now that I have had a couple nights of clear, relatively
unturbulent weather, I am disappointed.

I tried to star test--but the only diffraction rings were inside of focus
(where they were crisp and clear). There were NO diffraction rings the
same
distance outside focus. A helpful suggestion was to use a green filter to
suppress any purple. No good; still no diffraction rings outside
focus--just
a big blurry elliptical blob. This implies a severely undercorrected lens.

Performance on the planets is unimpressive. I set up my Televue Ranger
alongside; at 127x in the Photon, and 120x in the Ranger, they give
essentially the same image: you can just barely see one cloud band on
Saturn, and Cassini's Division is a gray stripe just barely visible around
the planet. At 190x, under skies with good transparency and little
turbulence, Saturn is just a big fuzzy blob. In short, a 127mm f/9
achromat
performs no better for resolution than a 70mm f/f6.8 semiapo. (Obviously,
the Photon does a better job of gathering light than the Ranger.)
Chromatic
aberration is no worse than I expected; actually, pretty good.

The person I bought it from had owned it less than a month; I notice that
the paint was chipped on one of the screws holding the tailpiece to the
tube. (He did warn me of this in advance.) I wonder if he tried to improve
the optics, discovered that he couldn't--and then couldn't return the
scope
to Photon because it was now damaged.

My suspicion is that short of replacing the objective, there is nothing
that
I can do but sell it, and buy something a bit better made. (Perhaps from
Tasco. :-)) I know that there are some tricks for a lot of optical
deficiencies; is there anything that can be done to correct this, or
should
I just take my lumps, and never buy something expensive on Astromart
again?

Clayton E. Cramer

(3) Experiment with changing the spacing of the two elements of the
objective. You can check with some of the people on the Yahoo Chinese
Refractor list who have tried this.


An undercorrected refractor objective can be pulled toward overcorrection if
the airgap length between front and rear element is reduced. If the airgap is
already only .05mm, then there is nothing that can be done. You need to be able
to move the rear element at least .2mm to see any significant change in
correction.

If the lens spacing is very small, in the range of .05mm or less, then the only
way to affect the undercorrection would be to deepen R3 slightly. However, I
doubt that this is the problem. It may very well be that most of the lens is
well corrected, but that the outer 5 or 10mm is turned down (this is quite
common for high speed polished surfaces). This will result in a soft image and
the hard Fresnel rings inside focus and no Fresnel rings outside of focus that
you observed. In that case, you can experiment with masks around the periphery.
Start by masking off 5mm of the outer edge. If the pattern does not improve,
mask off a bit more. You may end up with a 120mm or even only 110mm, but it
will be sharper than what you have now.

Roland Christen

(3) Experiment with changing the spacing of the two elements of the
objective. You can check with some of the people on the Yahoo Chinese
Refractor list who have tried this.


An undercorrected refractor objective can be pulled toward overcorrection if
the airgap length between front and rear element is reduced. If the airgap is
already only .05mm, then there is nothing that can be done. You need to be able
to move the rear element at least .2mm to see any significant change in
correction.

If the lens spacing is very small, in the range of .05mm or less, then the only
way to affect the undercorrection would be to deepen R3 slightly. However, I
doubt that this is the problem. It may very well be that most of the lens is
well corrected, but that the outer 5 or 10mm is turned down (this is quite
common for high speed polished surfaces). This will result in a soft image and
the hard Fresnel rings inside focus and no Fresnel rings outside of focus that
you observed. In that case, you can experiment with masks around the periphery.
Start by masking off 5mm of the outer edge. If the pattern does not improve,
mask off a bit more. You may end up with a 120mm or even only 110mm, but it
will be sharper than what you have now.

Roland Christen

(3) Experiment with changing the spacing of the two elements of the
objective. You can check with some of the people on the Yahoo Chinese
Refractor list who have tried this.


An undercorrected refractor objective can be pulled toward overcorrection if
the airgap length between front and rear element is reduced. If the airgap is
already only .05mm, then there is nothing that can be done. You need to be able
to move the rear element at least .2mm to see any significant change in
correction.

If the lens spacing is very small, in the range of .05mm or less, then the only
way to affect the undercorrection would be to deepen R3 slightly. However, I
doubt that this is the problem. It may very well be that most of the lens is
well corrected, but that the outer 5 or 10mm is turned down (this is quite
common for high speed polished surfaces). This will result in a soft image and
the hard Fresnel rings inside focus and no Fresnel rings outside of focus that
you observed. In that case, you can experiment with masks around the periphery.
Start by masking off 5mm of the outer edge. If the pattern does not improve,
mask off a bit more. You may end up with a 120mm or even only 110mm, but it
will be sharper than what you have now.

Roland Christen

Roland Christen wrote:
An undercorrected refractor objective can be pulled toward
overcorrection if the airgap length between front and rear element
is reduced. If the airgap is already only .05mm, then there is
nothing that can be done. You need to be able to move the rear
element at least .2mm to see any significant change in correction.

If the lens spacing is very small, in the range of .05mm or less,
then the only way to affect the undercorrection would be to deepen
R3 slightly. However, I doubt that this is the problem. It may very
well be that most of the lens is well corrected, but that the outer
5 or 10mm is turned down (this is quite common for high speed polished
surfaces). This will result in a soft image and the hard Fresnel rings
inside focus and no Fresnel rings outside of focus that you observed.
In that case, you can experiment with masks around the periphery.

Start by masking off 5mm of the outer edge. If the pattern does not
improve, mask off a bit more. You may end up with a 120mm or even'
only 110mm, but it will be sharper than what you have now.

Roland, I wish that the people who come here, it seems, only to give
you a hard time would stop and read what you have to say on a fairly
uncontroversial topic like this. Maybe it would give them assurance
that you don't simply rest your case on your reputation.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Roland Christen wrote:
An undercorrected refractor objective can be pulled toward
overcorrection if the airgap length between front and rear element
is reduced. If the airgap is already only .05mm, then there is
nothing that can be done. You need to be able to move the rear
element at least .2mm to see any significant change in correction.

If the lens spacing is very small, in the range of .05mm or less,
then the only way to affect the undercorrection would be to deepen
R3 slightly. However, I doubt that this is the problem. It may very
well be that most of the lens is well corrected, but that the outer
5 or 10mm is turned down (this is quite common for high speed polished
surfaces). This will result in a soft image and the hard Fresnel rings
inside focus and no Fresnel rings outside of focus that you observed.
In that case, you can experiment with masks around the periphery.

Start by masking off 5mm of the outer edge. If the pattern does not
improve, mask off a bit more. You may end up with a 120mm or even'
only 110mm, but it will be sharper than what you have now.

Roland, I wish that the people who come here, it seems, only to give
you a hard time would stop and read what you have to say on a fairly
uncontroversial topic like this. Maybe it would give them assurance
that you don't simply rest your case on your reputation.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Roland Christen wrote:
An undercorrected refractor objective can be pulled toward
overcorrection if the airgap length between front and rear element
is reduced. If the airgap is already only .05mm, then there is
nothing that can be done. You need to be able to move the rear
element at least .2mm to see any significant change in correction.

If the lens spacing is very small, in the range of .05mm or less,
then the only way to affect the undercorrection would be to deepen
R3 slightly. However, I doubt that this is the problem. It may very
well be that most of the lens is well corrected, but that the outer
5 or 10mm is turned down (this is quite common for high speed polished
surfaces). This will result in a soft image and the hard Fresnel rings
inside focus and no Fresnel rings outside of focus that you observed.
In that case, you can experiment with masks around the periphery.

Start by masking off 5mm of the outer edge. If the pattern does not
improve, mask off a bit more. You may end up with a 120mm or even'
only 110mm, but it will be sharper than what you have now.

Roland, I wish that the people who come here, it seems, only to give
you a hard time would stop and read what you have to say on a fairly
uncontroversial topic like this. Maybe it would give them assurance
that you don't simply rest your case on your reputation.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Brian

This is an interesting little nugget. Can you
explain this further - I thought it was reasonably
reliable 8-(

Adam

--
Eschew obfuscation. Eliminate such idiom previous
to rejoining.
"Brian Tung" wrote in message
...


Just making sure you know that the relationship
between scintillation
and seeing is unreliable.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at
http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at
http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at
http://astro.isi.edu/reference/faq.txt

Brian

This is an interesting little nugget. Can you
explain this further - I thought it was reasonably
reliable 8-(

Adam

--
Eschew obfuscation. Eliminate such idiom previous
to rejoining.
"Brian Tung" wrote in message
...


Just making sure you know that the relationship
between scintillation
and seeing is unreliable.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at
http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at
http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at
http://astro.isi.edu/reference/faq.txt