Parabolid 4" Mirror?

Apogee has two 4" f10 mirrors available. One is listed as spherical, the
other parabolic. It is my understanding that smaller mirrors don't need to
be parabolic; that spherical is just fine and dandy. It's only a few
dollars difference in price, but I'm wondering if there is any practical
difference when it comes to the image formed by the mirror.

Is it worth an extra five bucks for a parabolid 4" mirror? Will there be
*any* noticable difference in the image?

Thanks!

-J

--
Submergo ergo sum

-J posted:

Apogee has two 4" f10 mirrors available. One is listed as spherical, the
other parabolic. It is my understanding that smaller mirrors don't need to
be parabolic; that spherical is just fine and dandy. It's only a few
dollars difference in price, but I'm wondering if there is any practical
difference when it comes to the image formed by the mirror.

Is it worth an extra five bucks for a parabolid 4" mirror? Will there be
*any* noticable difference in the image?

A 4 inch f/10 spherical mirror should work fairly well in an astronomical
telescope. You might notice a very slight difference in its star test, but in
practice, it will be more difficult to notice the difference in overall
performance between a spherical and a paraboloidal 4 inch f/10 mirror. Its at
significantly larger apertures or shorter focal lengths that you have to go to
parabolizing. Here is some common approximate data for a truly "diffraction
limited" spherical mirror system:

The minimum f/ratio goes as the cube root of the mirror diameter, or the
DIFFRACTION-LIMITED F/RATIO: F = 6.675(D)**(1/3). For example, the typical
"department store" 3 inch Newtonian frequently uses a spherical f/10 mirror,
and should give reasonably good images as long as the figure is smooth and the
secondary mirror isn't terribly big. For common apertures, the following
approximate minimum f/ratios for Diffraction-Limited Newtonians using
spherical primary mirrors can be found below:

APERTURE F/RATIO FOR DIFFRACTION-LIMITED SPHERICAL MIRRORS
-----------------------------------------------------------------------------
3 inches f/9.6 (28.8 inch focal length)
4 inches f/10.6 (42.4 inch focal length)
6 inches f/12.1 (72.6 inch focal length)
8 inches f/13.4 (107.2 inch focal length)
10 inches f/14.4 (144 inch focal length)
12 inches f/15.3 (183.6 inch focal length)

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

-J posted:

Apogee has two 4" f10 mirrors available. One is listed as spherical, the
other parabolic. It is my understanding that smaller mirrors don't need to
be parabolic; that spherical is just fine and dandy. It's only a few
dollars difference in price, but I'm wondering if there is any practical
difference when it comes to the image formed by the mirror.

Is it worth an extra five bucks for a parabolid 4" mirror? Will there be
*any* noticable difference in the image?

A 4 inch f/10 spherical mirror should work fairly well in an astronomical
telescope. You might notice a very slight difference in its star test, but in
practice, it will be more difficult to notice the difference in overall
performance between a spherical and a paraboloidal 4 inch f/10 mirror. Its at
significantly larger apertures or shorter focal lengths that you have to go to
parabolizing. Here is some common approximate data for a truly "diffraction
limited" spherical mirror system:

The minimum f/ratio goes as the cube root of the mirror diameter, or the
DIFFRACTION-LIMITED F/RATIO: F = 6.675(D)**(1/3). For example, the typical
"department store" 3 inch Newtonian frequently uses a spherical f/10 mirror,
and should give reasonably good images as long as the figure is smooth and the
secondary mirror isn't terribly big. For common apertures, the following
approximate minimum f/ratios for Diffraction-Limited Newtonians using
spherical primary mirrors can be found below:

APERTURE F/RATIO FOR DIFFRACTION-LIMITED SPHERICAL MIRRORS
-----------------------------------------------------------------------------
3 inches f/9.6 (28.8 inch focal length)
4 inches f/10.6 (42.4 inch focal length)
6 inches f/12.1 (72.6 inch focal length)
8 inches f/13.4 (107.2 inch focal length)
10 inches f/14.4 (144 inch focal length)
12 inches f/15.3 (183.6 inch focal length)

--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 10th Annual NEBRASKA STAR PARTY *
* July 27-Aug. 1st, 2003, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"David Knisely" wrote in message news:dVf3b.2218

A 4 inch f/10 spherical mirror should work fairly well in an astronomical
telescope. You might notice a very slight difference in its star test,
but in
practice, it will be more difficult to notice the difference in overall
performance between a spherical and a paraboloidal 4 inch f/10 mirror.

Excellent. Thanks for the response. I think with the price difference
being about equal to the optical performance, I'll get the parabolic. If
there is the potential for better viewing, and it costs me five bones, I'll
consider that a worthwhile risk.

Thanks again.

-J


--
Submergo ergo sum

"David Knisely" wrote in message news:dVf3b.2218

A 4 inch f/10 spherical mirror should work fairly well in an astronomical
telescope. You might notice a very slight difference in its star test,
but in
practice, it will be more difficult to notice the difference in overall
performance between a spherical and a paraboloidal 4 inch f/10 mirror.

Excellent. Thanks for the response. I think with the price difference
being about equal to the optical performance, I'll get the parabolic. If
there is the potential for better viewing, and it costs me five bones, I'll
consider that a worthwhile risk.

Thanks again.

-J


--
Submergo ergo sum

"On Thu, 28 Aug 2003 17:13:25 GMT, in sci.astro.amateur you wrote:




Excellent. Thanks for the response. I think with the price difference
being about equal to the optical performance, I'll get the parabolic. If
there is the potential for better viewing, and it costs me five bones, I'll
consider that a worthwhile risk.

In focus images will show no improvement with a parabolized
4" f/10 over a spherical one of that size and aperture. Long focus
spheres can actually be smoother than paraboloids, which is paramount
for planetary work.

Dan Chaffee

"On Thu, 28 Aug 2003 17:13:25 GMT, in sci.astro.amateur you wrote:




Excellent. Thanks for the response. I think with the price difference
being about equal to the optical performance, I'll get the parabolic. If
there is the potential for better viewing, and it costs me five bones, I'll
consider that a worthwhile risk.

In focus images will show no improvement with a parabolized
4" f/10 over a spherical one of that size and aperture. Long focus
spheres can actually be smoother than paraboloids, which is paramount
for planetary work.

Dan Chaffee

"Dan Chaffee" wrote in message

In focus images will show no improvement with a parabolized
4" f/10 over a spherical one of that size and aperture. Long focus
spheres can actually be smoother than paraboloids, which is paramount
for planetary work.

Smoothness is a function of fabrication, right, so that's kind of a crap
shoot, anyway.

All other things being equal (quality, etc), which would you prefer?

Thanks!

-J


--
Submergo ergo sum

"Dan Chaffee" wrote in message

In focus images will show no improvement with a parabolized
4" f/10 over a spherical one of that size and aperture. Long focus
spheres can actually be smoother than paraboloids, which is paramount
for planetary work.

Smoothness is a function of fabrication, right, so that's kind of a crap
shoot, anyway.

All other things being equal (quality, etc), which would you prefer?

Thanks!

-J


--
Submergo ergo sum

"Jerome's Sock Puppet" wrote in message news:IX24b.232227$Oz4.62941@rwcrnsc54...


Smoothness is a function of fabrication, right, so that's kind of a crap
shoot, anyway.

Parabolizing a surface runs the risk of introducing roughness. Some
long focus paraboliods are indeed smooth, but long focus spheres
tend to be more consistantly so. Parabolizing is an attempt to
bring third order spherical aberration to zero, but the difference
between a sphere and a paraboliod in this case is so small that
the gain spherical correction is often offset by an increase in
higher order aberrations, both symmetrical and asymmetrical, and slight
roughness.

All other things being equal (quality, etc), which would you prefer?

4" f/10 sphere. Most attempts at parabolizing such a mirror not only
raise the above possibilities, but slight overcorrection as well.

DC