undercorrected scope: anything I can do?

When the front retainer ring is too tight, it warps the front element -
its as simple as that. (Roland can elaborate if he wants on the aberrations
this would introduce) Ive seen numerous examples - it seems to be a
favorite trick of " senile old phart refractor experts" guaranteeing their
refractors are "top notch", because they are experts and worked on the
scope, being advertising FOR SALE. Why this syndrom ever got started, only God
knows! You should also look for the faint blue traces of
LOCTITE, the thread locking glue many of these experts also use when
torquing down front retainer rings ... to protect the Alamo!
Jerry



"Clayton E. Cramer" wrote:

"Chris1011" wrote in message
...
Thanks for all the suggestions. The spacer that is currently in between
the
two lens is 2mm height, 129mm inside diameter, and 1mm thick. Where could
I
find some equivalent spacers that were 1mm and 1.5mm high?

If this is a round aluminum ring, it would be extremely difficult to get
one
made like that in a thinner dimension. You can buy shim stock at McMaster
Carr
in just about any size. Ask for plastic shims. Don't use metal shim stock
since
it will scratch. 3 shims spaced evenly around the lens will hold the
elements
in alignment. Once you determine the right thickness spacer, you should
glue
them in place with a very thin layer of rubber cement.

Roland Christen

By adjusting the tightness of the front retaining ring, I was able to
improve the optics
from "really awful" to "disappointing." I can now get a reasonably decent
image at 190x
on Saturn (Cassini's Division all the way around, and at the ansae, actually
black, not
gray; more detail on the planet itself). Outside focus I still don't have
anything that
qualifies as proper diffraction rings, but they might be diffraction rings
when they grow up!

I will try to find some shim material tomorrow in 1mm, 1.5mm, and perhaps a
couple
of other thicknesses in the 1mm to 2mm range tomorrow, and see if I can
improve the
optics enough that I can either consider myself happy with it, or at least
not feel
guilty selling it to someone else.

Photon Instruments never responded to my email requesting either assistance,
or a price
for purchasing a replacement objective. I've gotten more help, and more
quickly from
Roland Christen--and I don't even own a Astro-Physics product!

When the front retainer ring is too tight, it warps the front element -
its as simple as that. (Roland can elaborate if he wants on the aberrations
this would introduce) Ive seen numerous examples - it seems to be a
favorite trick of " senile old phart refractor experts" guaranteeing their
refractors are "top notch", because they are experts and worked on the
scope, being advertising FOR SALE. Why this syndrom ever got started, only God
knows! You should also look for the faint blue traces of
LOCTITE, the thread locking glue many of these experts also use when
torquing down front retainer rings ... to protect the Alamo!
Jerry



"Clayton E. Cramer" wrote:

"Chris1011" wrote in message
...
Thanks for all the suggestions. The spacer that is currently in between
the
two lens is 2mm height, 129mm inside diameter, and 1mm thick. Where could
I
find some equivalent spacers that were 1mm and 1.5mm high?

If this is a round aluminum ring, it would be extremely difficult to get
one
made like that in a thinner dimension. You can buy shim stock at McMaster
Carr
in just about any size. Ask for plastic shims. Don't use metal shim stock
since
it will scratch. 3 shims spaced evenly around the lens will hold the
elements
in alignment. Once you determine the right thickness spacer, you should
glue
them in place with a very thin layer of rubber cement.

Roland Christen

By adjusting the tightness of the front retaining ring, I was able to
improve the optics
from "really awful" to "disappointing." I can now get a reasonably decent
image at 190x
on Saturn (Cassini's Division all the way around, and at the ansae, actually
black, not
gray; more detail on the planet itself). Outside focus I still don't have
anything that
qualifies as proper diffraction rings, but they might be diffraction rings
when they grow up!

I will try to find some shim material tomorrow in 1mm, 1.5mm, and perhaps a
couple
of other thicknesses in the 1mm to 2mm range tomorrow, and see if I can
improve the
optics enough that I can either consider myself happy with it, or at least
not feel
guilty selling it to someone else.

Photon Instruments never responded to my email requesting either assistance,
or a price
for purchasing a replacement objective. I've gotten more help, and more
quickly from
Roland Christen--and I don't even own a Astro-Physics product!

When the front retainer ring is too tight, it warps the front element -
its as simple as that.

Of course, all us old timers know that trick. If you have astigmatism in the
individual glass elements, either due to the way they were held during
polishing, or because the glass was made from moldings and have internal bad
homogeneity, you can sometimes warp the elements to show a reasonably round
Airy disc. If the oval is in one direction, just putting small spacers at 90
degrees to the oval under the retaining ring can sometimes bring the image into
roundness. I did this once with a corrector plate on a large SCT which resisted
all other efforts to find a null by rotation of the 3 elements. Be careful,
though, you can crack the glass if too much pressure is applied.

Roland Christen

When the front retainer ring is too tight, it warps the front element -
its as simple as that.

Of course, all us old timers know that trick. If you have astigmatism in the
individual glass elements, either due to the way they were held during
polishing, or because the glass was made from moldings and have internal bad
homogeneity, you can sometimes warp the elements to show a reasonably round
Airy disc. If the oval is in one direction, just putting small spacers at 90
degrees to the oval under the retaining ring can sometimes bring the image into
roundness. I did this once with a corrector plate on a large SCT which resisted
all other efforts to find a null by rotation of the 3 elements. Be careful,
though, you can crack the glass if too much pressure is applied.

Roland Christen

Of course, all us old timers know that trick. If you have astigmatism in the
individual glass elements, either due to the way they were held during
polishing, or because the glass was made from moldings and have internal bad
homogeneity, you can sometimes warp the elements to show a reasonably round
Airy disc.

Are there other possible causes of astigmatism in a refractor, misalignment of
some sort, or is it pretty much necessarily due to bad glass ??

jon

Of course, all us old timers know that trick. If you have astigmatism in the
individual glass elements, either due to the way they were held during
polishing, or because the glass was made from moldings and have internal bad
homogeneity, you can sometimes warp the elements to show a reasonably round
Airy disc.

Are there other possible causes of astigmatism in a refractor, misalignment of
some sort, or is it pretty much necessarily due to bad glass ??

jon

Are there other possible causes of astigmatism in a refractor, misalignment
of
some sort, or is it pretty much necessarily due to bad glass ??


For a small lens, astigmatism is caused primarily by either inhomogeneity
inside the glass or a surface polish that has a saddle shape caused by uneven
support of the glass during polishing. Excessive pressure of the retaining ring
can also cause astigmatism, but this can be removed by backing off the ring to
loosen the pressure on the glass. Misalignment of the two (or 3) elements will
result in coma and lateral color.

For a large and heavy lens, astigmatism can be the result of uneven support of
the rear surface in the lens cell. This is generally not a problem in a small
lens.

Roland Christen

Are there other possible causes of astigmatism in a refractor, misalignment
of
some sort, or is it pretty much necessarily due to bad glass ??


For a small lens, astigmatism is caused primarily by either inhomogeneity
inside the glass or a surface polish that has a saddle shape caused by uneven
support of the glass during polishing. Excessive pressure of the retaining ring
can also cause astigmatism, but this can be removed by backing off the ring to
loosen the pressure on the glass. Misalignment of the two (or 3) elements will
result in coma and lateral color.

For a large and heavy lens, astigmatism can be the result of uneven support of
the rear surface in the lens cell. This is generally not a problem in a small
lens.

Roland Christen

For a small lens, astigmatism is caused primarily by either inhomogeneity
inside the glass or a surface polish that has a saddle shape caused by uneven
support of the glass during polishing. Excessive pressure of the retaining
ring
can also cause astigmatism, but this can be removed by backing off the ring
to
loosen the pressure on the glass. Misalignment of the two (or 3) elements
will
result in coma and lateral color.

For a large and heavy lens, astigmatism can be the result of uneven support
of
the rear surface in the lens cell. This is generally not a problem in a small
lens.

Roland Christen

Thanks for taking the time to clarify the causes of astigmatism for me.

Best wishes

jon

For a small lens, astigmatism is caused primarily by either inhomogeneity
inside the glass or a surface polish that has a saddle shape caused by uneven
support of the glass during polishing. Excessive pressure of the retaining
ring
can also cause astigmatism, but this can be removed by backing off the ring
to
loosen the pressure on the glass. Misalignment of the two (or 3) elements
will
result in coma and lateral color.

For a large and heavy lens, astigmatism can be the result of uneven support
of
the rear surface in the lens cell. This is generally not a problem in a small
lens.

Roland Christen

Thanks for taking the time to clarify the causes of astigmatism for me.

Best wishes

jon