Good 80 or 90 mm Refractor for Imaging?

THe CCD Camera I will be using is black & white (no color). Will the
false color from the scope still be an issue?

Thanks.

It will probably be worse since CCDs have broader response than your eye.

jon

"justbeats" wrote in message
m...

31mm Nagler gives an amazing 4 degree FOV at ~20x (albeit with heavy
field curvature). Even a simple 26mm Super Plossl gives you a good 2
degrees FOV.

In summary - I think you'll find the Orion 80ED a good complement to

Just to clarify, what is your definition of field curvature?

My definition, and I believe correctly so, is the inability of the entire
field of view to be in focus at the same time. With a flat field (no
curvature), when the inner area of the eyepiece is sharply focused, so is
the outer area. Not saying it's the case here, but differentiating
aberrations that cause out of focus stars isn't trivial for the
inexperienced observer. It definitely takes time to learn what is what.

The problem I see with my F5 achromat, aside from chromatic aberration, is
generally astigmatism in the eyepiece. However, since Uncle Al (Nagler)
insists that his eyepiece designs behave well down to F4, the Naglers and
Panoptics do a good job at supressing astigmatism.

If you are seeing problems in the Nagler with an F7-ish refractor, you might
want to consider getting your eyes tested for astigmatism. I have a 35mm
Panoptic that I use in my 12.5" F4.8 Dob with Paracorr, and although I think
things look pretty good, there was always still a touch of astigmatism here
and there, which I would find myself chasing out with the focuser when I
would move my eye to look at a particular spot in the field. When I put on
my reading glasses, which also correct for my own eye astigmatism, it made a
_big_ difference. Eyepiece behavior that appeared similar to what one would
expect from field curvature, suddenly got a whole lot better. Now I can
bring stars to a much tighter focus across the entire field all at once. It
is simply marvelous, and the other night I spent just about the whole time
with the low power, wide field 35mm in the focuser. Very nice.

There are yet two other affects called pin cushion and distortion. On these
I am not exactly clear as to what are the accepted definitions, but I
understand distortion to be an edge aberation where a straight line (say a
utility pole) at the edge of the field bends inward at its center, and pin
cushion has the straight line bending outward at its center. These are not
necessarily obvious with star fields, although some people are
hypersensitive to it, as others are to coma. However once you move the scope
while looking in the eyepiece at the same time, it becomes quite obvious.

Just some thoughts on the subject.

Stephen Paul

"justbeats" wrote in message
m...

31mm Nagler gives an amazing 4 degree FOV at ~20x (albeit with heavy
field curvature). Even a simple 26mm Super Plossl gives you a good 2
degrees FOV.

In summary - I think you'll find the Orion 80ED a good complement to

Just to clarify, what is your definition of field curvature?

My definition, and I believe correctly so, is the inability of the entire
field of view to be in focus at the same time. With a flat field (no
curvature), when the inner area of the eyepiece is sharply focused, so is
the outer area. Not saying it's the case here, but differentiating
aberrations that cause out of focus stars isn't trivial for the
inexperienced observer. It definitely takes time to learn what is what.

The problem I see with my F5 achromat, aside from chromatic aberration, is
generally astigmatism in the eyepiece. However, since Uncle Al (Nagler)
insists that his eyepiece designs behave well down to F4, the Naglers and
Panoptics do a good job at supressing astigmatism.

If you are seeing problems in the Nagler with an F7-ish refractor, you might
want to consider getting your eyes tested for astigmatism. I have a 35mm
Panoptic that I use in my 12.5" F4.8 Dob with Paracorr, and although I think
things look pretty good, there was always still a touch of astigmatism here
and there, which I would find myself chasing out with the focuser when I
would move my eye to look at a particular spot in the field. When I put on
my reading glasses, which also correct for my own eye astigmatism, it made a
_big_ difference. Eyepiece behavior that appeared similar to what one would
expect from field curvature, suddenly got a whole lot better. Now I can
bring stars to a much tighter focus across the entire field all at once. It
is simply marvelous, and the other night I spent just about the whole time
with the low power, wide field 35mm in the focuser. Very nice.

There are yet two other affects called pin cushion and distortion. On these
I am not exactly clear as to what are the accepted definitions, but I
understand distortion to be an edge aberation where a straight line (say a
utility pole) at the edge of the field bends inward at its center, and pin
cushion has the straight line bending outward at its center. These are not
necessarily obvious with star fields, although some people are
hypersensitive to it, as others are to coma. However once you move the scope
while looking in the eyepiece at the same time, it becomes quite obvious.

Just some thoughts on the subject.

Stephen Paul

Stephen Paul wrote:
There are yet two other affects called pin cushion and distortion. On these
I am not exactly clear as to what are the accepted definitions, but I
understand distortion to be an edge aberation where a straight line (say a
utility pole) at the edge of the field bends inward at its center, and pin
cushion has the straight line bending outward at its center. These are not
necessarily obvious with star fields, although some people are
hypersensitive to it, as others are to coma. However once you move the scope
while looking in the eyepiece at the same time, it becomes quite obvious.

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

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

Stephen Paul wrote:
There are yet two other affects called pin cushion and distortion. On these
I am not exactly clear as to what are the accepted definitions, but I
understand distortion to be an edge aberation where a straight line (say a
utility pole) at the edge of the field bends inward at its center, and pin
cushion has the straight line bending outward at its center. These are not
necessarily obvious with star fields, although some people are
hypersensitive to it, as others are to coma. However once you move the scope
while looking in the eyepiece at the same time, it becomes quite obvious.

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

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 Tung) wrote in message ...
Stephen Paul wrote:
There are yet two other affects called pin cushion and distortion. On these

Distortion is variation in magnification with off-axis angle. If object

Sorry folks, my mistake. I'll check terminology in future! Where I
said "field curvature", I should have said "pincushion distortion"
(the first phrase just seems more appropriate in normal "english"
usage). With the 31mm Nagler, the 80ED is focussed across the entire
FOV, but lines get distorted towards the edges. Not a problem if
you're just enjoying a wide field view "eyes only".

Cheers
Beats

(Brian Tung) wrote in message ...
Stephen Paul wrote:
There are yet two other affects called pin cushion and distortion. On these

Distortion is variation in magnification with off-axis angle. If object

Sorry folks, my mistake. I'll check terminology in future! Where I
said "field curvature", I should have said "pincushion distortion"
(the first phrase just seems more appropriate in normal "english"
usage). With the 31mm Nagler, the 80ED is focussed across the entire
FOV, but lines get distorted towards the edges. Not a problem if
you're just enjoying a wide field view "eyes only".

Cheers
Beats

"Brian Tung" wrote in message
...

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

Thanks for the clarification... all though, I have to say, the pin cushion
in my wife's sewing kit is oblate. I also think it interesting how the
description of a curved line at the edge, gives the illusion that the bowing
out is a result of the center of the line getting thrust beyond where it
should be, rather than the corners of a square getting "sucked in" toward
the center.

You do realize of course that from your description, one might get the
impression that you have some experience putting square pegs into round
holes. g

-Stephen

"Brian Tung" wrote in message
...

Distortion is variation in magnification with off-axis angle. If object
A is twice as far from the axis as object B, then A should appear twice
as far from the center of the field of view as B. If it appears *less*
than twice as far, you have what is called barrel distortion, because if
you look at a square through such a system, the sides of the square will
be bulged outward, like a barrel.

If it appears *more* than twice as far, you have pincushion distortion.
If you look at a square through such a system, the sides of the square
will be bowed inward. I guess that reminds people of pincushions.

Thanks for the clarification... all though, I have to say, the pin cushion
in my wife's sewing kit is oblate. I also think it interesting how the
description of a curved line at the edge, gives the illusion that the bowing
out is a result of the center of the line getting thrust beyond where it
should be, rather than the corners of a square getting "sucked in" toward
the center.

You do realize of course that from your description, one might get the
impression that you have some experience putting square pegs into round
holes. g

-Stephen

Stephen Paul wrote:
Thanks for the clarification... all though, I have to say, the pin cushion
in my wife's sewing kit is oblate. I also think it interesting how the
description of a curved line at the edge, gives the illusion that the bowing
out is a result of the center of the line getting thrust beyond where it
should be, rather than the corners of a square getting "sucked in" toward
the center.

It amounts to the same thing. There is no "officially correct"
magnification, so the two interpretations are equivalent.

You do realize of course that from your description, one might get the
impression that you have some experience putting square pegs into round
holes. g

I might, rabbit, I might.

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