Tasco objective

So I've disassembled my Tasco's objective today. Turns out it doesn't
have a separator _between_ the two elements. It has two plastic
retainers above and below the two elements, so that the cell and screw
ring does not come in contact with the lens surfaces.

I can also see some Newton's rings between the two elements when in
contact, which change shape when one of the elements moves or slides
along the other.

It appears as through the elements are airspaced, but the contact is
pretty tight. The elements separate, but when placed together the
induced vacuum tends to keep them together.

I lifted the elements and placed some paper pieces at 120 degree angles
and did some viewing of some distant light sources. My impression was
that the paper spaced objective gave a slightly sharper definition
between closer points. That is, my impression was that the separation
limit increased somewhat.

Questions:
1) Is it right to lift and separate the two elements with paper, if the
objective is designed to be airspaced? Can performance improve that way?
Any test to objectively measure such a change?

2) Defocusing distant points' images, I get circles when the image is at
the center, but elipses when the image moves near the edge of the
Tasco's field, when I move the scope. Two months ago I sawed the tube in
order to shorten it and I might have placed its focuser slightly
non-perpendicular to its optical axis. Does this have to do anything
with the distortion?

Thanks in advance,
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

Ioannis wrote in message news:1091312829.703297@athnrd02...

I lifted the elements and placed some paper pieces at 120 degree angles
and did some viewing of some distant light sources. My impression was
that the paper spaced objective gave a slightly sharper definition
between closer points. That is, my impression was that the separation
limit increased somewhat.

Questions:
1) Is it right to lift and separate the two elements with paper, if the
objective is designed to be airspaced? Can performance improve that way?

It's quite possible that the manufacturer didn't get the spacing
exactly right when they installed the lens elements, so yes,
performance can be improved in some cases. Depending on the design,
changing the spacing can change the points at which certain
corrections are accomplished, such as the wavelength of optimal focus.

Beyond this, I'd better leave any explanations to the experts. :-)

Any test to objectively measure such a change?

Without special equipment, I don't know. Just experiment with
different amounts of spacing until you obtain what looks like the best
results, in terms of focus and color correction. With some designs,
you might not notice much of a difference, so don't worry about it too
much.

2) Defocusing distant points' images, I get circles when the image is at
the center, but elipses when the image moves near the edge of the
Tasco's field, when I move the scope. Two months ago I sawed the tube in
order to shorten it and I might have placed its focuser slightly
non-perpendicular to its optical axis. Does this have to do anything
with the distortion?

Hmmm...I'm not sure, but it sounds like your objective, eyepiece, or
diagonal might have some astigmatism and/or coma near the edges. If
it's only serious near the edges, then it's probably just a limitation
of your equipment. If your focuser were non-perpendicular, you would
see distorted images in the center of your field of view, so you're
probably alright there. To be able to tell with greater precision,
you'll need to perform this defocusing test with an actual star. Then
you'll be able to see several rings, one inside the other. If they're
all nicely centered, your collimation should be good enough.


- Robert Cook

Robert Cook wrote:
[snip]
Questions:
1) Is it right to lift and separate the two elements with paper, if the
objective is designed to be airspaced? Can performance improve that way?

It's quite possible that the manufacturer didn't get the spacing
exactly right when they installed the lens elements, so yes,
performance can be improved in some cases. Depending on the design,
changing the spacing can change the points at which certain
corrections are accomplished, such as the wavelength of optimal focus.

Beyond this, I'd better leave any explanations to the experts. :-)

Hold it, so I can understand what you are saying: I understand that *in
some* cases, the manufacturer of cheapo scopes may not get the spacing
exactly right, but in this case we are talking about a doublet which
seems to be *designed* to be "airspaced". For that matter, I am not sure
even if "airspaced" is the right term for this objective, since in my
mind the term seems to imply a *finite* air separation between the
elements. The elements I've got, come into almost "perfect" contact with
each other, as indicated by the Newton fringes I see when they are
placed together. Are such lenses really "airspaced" or does the term
indicate a different setup altogether?

Of course, "perfect contact" is a misnomer anyway, since there will
always be a very thin film of air between the elements, even when they
are designed to be in contact, unless some other substance (Canada
balsam or such) is used to hold them in place. This adds to my confusion :-(

To conclude, if I understand you right, you are saying that a
possibility exists that even such elements (designed to be in "perfect"
contact) can benefit from a (relatively larger) finite separation?

Thanks for your response,

- Robert Cook
--
I. N. Galidakis
http://users.forthnet.gr/ath/jgal/
------------------------------------------
Eventually, _everything_ is understandable

Ioannis wrote in message news:1091528403.77244@athnrd02...

Robert Cook wrote:
[snip]
It's quite possible that the manufacturer didn't get the spacing
exactly right when they installed the lens elements, so yes,
performance can be improved in some cases. Depending on the design,
changing the spacing can change the points at which certain
corrections are accomplished, such as the wavelength of optimal focus.

Beyond this, I'd better leave any explanations to the experts. :-)

Hold it, so I can understand what you are saying: I understand that *in
some* cases, the manufacturer of cheapo scopes may not get the spacing
exactly right,

All I can say is that they tend to get a lot of things wrong. :-)
Mass produced telescopes--even those costing many times more than
yours--typically require many adjustments to get into good working
order, if that is even possible. Sometimes the optics are fine out of
the box, sometimes they become acceptable after modification, and
sometimes they're just bad.

but in this case we are talking about a doublet which
seems to be *designed* to be "airspaced".

It *probably* is, otherwise the elements might have been cemented
together, but keep in mind that we're making an assumption here.

For that matter, I am not sure
even if "airspaced" is the right term for this objective, since in my
mind the term seems to imply a *finite* air separation between the
elements. The elements I've got, come into almost "perfect" contact with
each other, as indicated by the Newton fringes I see when they are
placed together. Are such lenses really "airspaced" or does the term
indicate a different setup altogether?

In most designs with which I'm familiar, the gap between the elements
is usually very small--on the order of 0.1 mm (approximately the
thickness of lightweight writing paper). However, if you can see
Newton's rings, then the size of the current gap must be fairly close
to the wavelengths of visible light, which is much tighter. It's
possible that the lens was meant to be assembled this way, but it's
also possible that it was designed to have a larger gap, and the
manufacturer didn't bother to account for this. Without hard data on
the design of the lens, you'll have to experiment with different gaps
to find out whether spacing is an issue.

Of course, "perfect contact" is a misnomer anyway, since there will
always be a very thin film of air between the elements, even when they
are designed to be in contact, unless some other substance (Canada
balsam or such) is used to hold them in place. This adds to my confusion :-(

As far as I know, such a lens would still be considered air-spaced,
since there is nothing but air, however little, between the elements.

To conclude, if I understand you right, you are saying that a
possibility exists that even such elements (designed to be in "perfect"
contact) can benefit from a (relatively larger) finite separation?

Yes, I believe that the *possibility* exists. Most of these lenses
are designed to have a small but visible separation, even though they
may fit together "perfectly." If you're willing to spend some time on
this (it was your idea! ;-) ), try a few different spacings, taking
notes on what effects they have on image sharpness, ease of focusing,
chromatic aberration, and the basic "star test" that I described
earlier (i.e. defocusing a star image). Pay attention to both the
center of the field of view and the edge, because changing the spacing
of the lens elements could cause one aspect of the field of view or
the other (or both) to improve or degrade. For example, the star test
you did earlier seemed to indicate noticeable astigmatism toward the
edge of the field of view, but not the center (did you use your new
eyepiece?). Determine whether increasing the spacing reduces or
worsens this aberration, and whether it has a positive or negative
effect on the center of the field of view.

From the data you will have gathered, decide which set of tradeoffs
you prefer, if any are noticeable. If you're lucky, you might be able
to improve the performance of your telescope to some degree, but don't
expect any miracles. :-)


- Robert Cook