Reflector optical quality vs focal ratio?

In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability. I know faster reflectors require better eyepieces and
precise collimation for maximum performance but I'm not sure this is
the optical compromise Suiter mentioned. Regarding optical quality, a
primary mirror is usually specified as having a level of surface
accuracy or may be specified as diffraction limited. My particular 8"
F6 primary mirror is specified as diffraction limited. Yet when I
observe through any plossl eyepiece, the view will be sharp in a
region around the center of the FOV and will deteriorate towards the
edges. Consequently, I say my telescope optical system, when using any
plossl eyepiece, is not diffraction limited. Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

-Bob

In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability. I know faster reflectors require better eyepieces and
precise collimation for maximum performance but I'm not sure this is
the optical compromise Suiter mentioned. Regarding optical quality, a
primary mirror is usually specified as having a level of surface
accuracy or may be specified as diffraction limited. My particular 8"
F6 primary mirror is specified as diffraction limited. Yet when I
observe through any plossl eyepiece, the view will be sharp in a
region around the center of the FOV and will deteriorate towards the
edges. Consequently, I say my telescope optical system, when using any
plossl eyepiece, is not diffraction limited. Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

Hi Bob,

I believe it was Suiter who defined "diffraction limited" as a social
contract g

I'm not sure everyone is using the same words in the same way, and that is
probably part of the problem. By "optical quality" do you mean how well made
the scope is made, ie how close it is to the intended design --- while the
design may be lousy, it is made to that design?

A f/5 mirror is harder to make than an f/8. The departure from a sphere is
more pronounced and harder to figure. It is harder to produce without
introducing a lot of surface roughness. It is harder to test/measure, which
is necessary for producing a good figure. So if someone offers both an f/5
and an f/8 at the same price, the f/8 is probably better quality.

But even if both mirrors are perfect, the f/8 will give a better image
across the FOV. The f/5 will have a good deal more coma. Additionally, the
steeper light cone is harder on eyepieces. And, assuming equal illumination
of the field, the f/5 will have a larger CO, robbing some of the contrast.

Is that what you were asking?

Clear Skies

Chuck Taylor
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Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/
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Bob,
What you have stated is the classic optical opinion, which may be
correct for all I know. But my own experience is that if a Visual
Paracorr is used, a Newtonian parabolic mirror can give fine images down
to f/4.5 or a bit below. You might want to visit the Tele Vue web site and
look at the data they have graphed for the Paracorr correction, and make
up your own mind, and also check out the Starmaster web site; their scopes
are optimized for the Paracorr.
Ciao,
Bill Meyers

Bob Barnard wrote:

In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability. I know faster reflectors require better eyepieces and
precise collimation for maximum performance but I'm not sure this is
the optical compromise Suiter mentioned. Regarding optical quality, a
primary mirror is usually specified as having a level of surface
accuracy or may be specified as diffraction limited. My particular 8"
F6 primary mirror is specified as diffraction limited. Yet when I
observe through any plossl eyepiece, the view will be sharp in a
region around the center of the FOV and will deteriorate towards the
edges. Consequently, I say my telescope optical system, when using any
plossl eyepiece, is not diffraction limited. Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

-Bob

Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

I think your definition of diffraction limited is not commonly accepted.

I believe normally diffraction limited only means that given the design of the
optics, the optics are sufficiently well made so that the image quality is
limited by diffraction.

A parabolic Newtonian such as your 8 inch F6 scope has coma and there is a
radius inside which the image will be diffraction limited.

If one uses an eyepiece with sufficient magnification and an appropriate field
of view, then the entire image can indeed be "diffraction" limited.

Consider this...

Your Newtonian is an 8 inch F6 scope, 48 inch Focal length. I have an 8 inch
F6 scope as well as a 12.5 inch Scope that is F4.06, which is nearly the same
focal length, 50.7 inches. I normally use it with a Paracorr.

When comparing the performance of this scope,

Do I compare it to an 8 inch F6 scope, which means it has 2.5 times the mirror
area and something like 1.56 times better resolution???

Compared to the 8 inch F6 scope, it provides much better planetary images,
splits tighter doubles, galaxies and all DSOs are far more spectacular and it
still fits in a compact car.....


Or do I compare it to a 12.5 inch F6 scope?

In that case the 12.5 inch F6 scope will not that will not have the same
problem with coma but at the same time, it will have a 75 inch OTA which will
cause a number of other problems, ie it won't fit in any car I own, the OTA
will be much heavier, more awkward.

Bottomline:

Scopes are compromises, trade offs. Protability, cost, optical quality,
mechanical quality. That fast focal ratio buys a lot of portability and
useablity and when compared to a smaller scope it is definitely lights up the
sky in the way that a smaller scope cannot.

jon

"Bob Barnard" wrote in message
m...
In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability.

What chapter, and section?

In context of coma, this will certainly be true. I don't think there is any
other aberation that is associated with a paraboloidal mirror surface that
will be specifically increased with a deeper curve. Although the negative
effects on eyepieces will increase.

Hi, Jon,
I agree with your post entirely. And you speak from experience.
Bill Meyers

Jon Isaacs wrote:

Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

I think your definition of diffraction limited is not commonly accepted.

I believe normally diffraction limited only means that given the design of the
optics, the optics are sufficiently well made so that the image quality is
limited by diffraction.

A parabolic Newtonian such as your 8 inch F6 scope has coma and there is a
radius inside which the image will be diffraction limited.

If one uses an eyepiece with sufficient magnification and an appropriate field
of view, then the entire image can indeed be "diffraction" limited.

Consider this...

Your Newtonian is an 8 inch F6 scope, 48 inch Focal length. I have an 8 inch
F6 scope as well as a 12.5 inch Scope that is F4.06, which is nearly the same
focal length, 50.7 inches. I normally use it with a Paracorr.

When comparing the performance of this scope,

Do I compare it to an 8 inch F6 scope, which means it has 2.5 times the mirror
area and something like 1.56 times better resolution???

Compared to the 8 inch F6 scope, it provides much better planetary images,
splits tighter doubles, galaxies and all DSOs are far more spectacular and it
still fits in a compact car.....

Or do I compare it to a 12.5 inch F6 scope?

In that case the 12.5 inch F6 scope will not that will not have the same
problem with coma but at the same time, it will have a 75 inch OTA which will
cause a number of other problems, ie it won't fit in any car I own, the OTA
will be much heavier, more awkward.

Bottomline:

Scopes are compromises, trade offs. Protability, cost, optical quality,
mechanical quality. That fast focal ratio buys a lot of portability and
useablity and when compared to a smaller scope it is definitely lights up the
sky in the way that a smaller scope cannot.

jon

Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

Most fast reflectors, F/4.5 to F/4 have a diffraction limited field of
less than 2mm in diameter, beyond that coma takes over. This in fact
is the main tradeoff when it comes to those huge light buckets, with a
tiny diffraction limited field these reflectors are in fact optically
better suited for planetary work rather than the widefield low power
DSO's.

Thanks,
Vahe

Diffraction Limited is not a supported standard, even though some will pay

lipservice to it being 1/4 wave standard while running away at the same
time.

Our locoal Suiter expert is David Knisely. You need to be asking him.

Jerry




Bob Barnard wrote:

In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability. I know faster reflectors require better eyepieces and
precise collimation for maximum performance but I'm not sure this is
the optical compromise Suiter mentioned. Regarding optical quality, a
primary mirror is usually specified as having a level of surface
accuracy or may be specified as diffraction limited. My particular 8"
F6 primary mirror is specified as diffraction limited. Yet when I
observe through any plossl eyepiece, the view will be sharp in a
region around the center of the FOV and will deteriorate towards the
edges. Consequently, I say my telescope optical system, when using any
plossl eyepiece, is not diffraction limited. Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

-Bob

Bob Barnard wrote:
In "Star Testing Astronomical Telescopes", Suiter wrote any reflector
with a focal ratio less than F5 is compromising optical quality for
portability. I know faster reflectors require better eyepieces and
precise collimation for maximum performance but I'm not sure this is
the optical compromise Suiter mentioned. Regarding optical quality, a
primary mirror is usually specified as having a level of surface
accuracy or may be specified as diffraction limited. My particular 8"
F6 primary mirror is specified as diffraction limited. Yet when I
observe through any plossl eyepiece, the view will be sharp in a
region around the center of the FOV and will deteriorate towards the
edges. Consequently, I say my telescope optical system, when using any
plossl eyepiece, is not diffraction limited. Considering this, I
suspect maybe a reflector with a focal ratio lower than F5 may never
be diffraction limited (have a sharp edge to edge FOV) at the eyepiece
regardless of the eyepiece/barlow/paracorr used. Does this make sense
and is this the optical compromise tradeoff for portability?

Well, the optical "compromise" has several aspects. The first is the ability
to make a high-quality paraboloidal surface at shorter f/ratios. It is more
difficult to make an accurate surface for focal ratios under f/5 than it is
for something longer like f/8. If cost is an issue, for the shorter f/ratios,
optical quality may suffer a bit when less expensive short f/ratio mirrors are
used. Shorter f/ratio mirrors of good to excellent quality are available, but
you will pay more of a premium price for them. The second aspect is the more
stringent collimation requirements for the shorter f/ratio system, and the
third it the increasing effects of the aberration known as "coma" on the
quality of off-axis images. The collimation can to some extent be dealt with
by using more expensive mirror cells and better support, while the coma can at
least be reduced to some extent by the use of a coma corrector such as the
Paracorr. Short f/ratio telescopes can be diffraction limited at the center
of their fields of view, but usually (unless corrective steps are taken), the
quality of star images will tend to decline in the outer portions of the
field. Some of this can also be due to the use of less-complex eyepiece
designs such as Plossls, which can have a hard time dealing with a short light
cone. Clear skies to you.

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

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* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
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"CLT" not@thisaddress wrote in message
...

I believe it was Suiter who defined "diffraction limited" as a social
contract g

Pretty funny but I was surprised to read Suiter actually likes the term
"diffraction limited" because it means the 1/14 wave RMS Marechal limit has
been met, which he states is superior to the Rayleigh limit.

I'm not sure everyone is using the same words in the same way, and that is
probably part of the problem. By "optical quality" do you mean how well
made
the scope is made, ie how close it is to the intended design --- while the
design may be lousy, it is made to that design?

I thought about that too and in this case optical quality refers to how well
the design (focal ratio) performs at the eyepiece. Suiter made the comment
about optical quality in the context of setting your expectations for star
test performance based on the type of telescope you have.

A f/5 mirror is harder to make than an f/8. The departure from a sphere is
more pronounced and harder to figure. It is harder to produce without
introducing a lot of surface roughness. It is harder to test/measure,
which
is necessary for producing a good figure. So if someone offers both an f/5
and an f/8 at the same price, the f/8 is probably better quality.

But even if both mirrors are perfect, the f/8 will give a better image
across the FOV. The f/5 will have a good deal more coma. Additionally, the
steeper light cone is harder on eyepieces. And, assuming equal
illumination
of the field, the f/5 will have a larger CO, robbing some of the contrast.

Is that what you were asking?


Yep, thanks for encumbering me with more knowledge!

-Bob