Colour Delivery in SCT's vs Newts etc.

Frankly, this is what Ive heard also. Its not the optics per se in fact
our lx200 has a nice primary and when you lock down the priamry
carefully so as to keep things oin axis, then collimate, the scope is
quite crisp.... but with poor contrast. Mechanically the scope is a dog.
jerry
======================================
Hi Jerry, Well, I'll buy that. I would assume that there shouldn't be
differences between the optical quality, or the time spent to match
optics between an 8" LX-200-16"-LX-200, and one would also think that
closer attention to detail and quality would be spent on thier flagship
SCT scope.

No doubt these same maladies also affect the Celestron C-14 also, in
that countless owners try to devise ways of getting away from mirror
flop, as the mirror flop definitely has an effect on GOTO Pointing, CCD
Imaging, and maintaining a high degree of collimation which as we all
know is paramount to achieving top performance from any SCT.

Many C-14 owners also have resorted to OTA internal flocking, and I've
even heard mentioned the flocking of the Baffle Tubes as well on SCTs.
Pehaps this is mod also needs to be done on the Meades to enhance
performance?

Though, for the price of what these top of the heap SCT's cost, maybe
areas like this should have a bit more sophistication implemented into
them like a better Primary Baffle-Mirror Support.

I imagine on an 8" SCT, the primary mirror is not so heavy-bulky to be
that great of an issue, but on 14"-16" SCTs, even though these mirrors
are not full thickness, and are cast in a roughly concave state first
before grinding-figuring-polishing, they still must weigh a substantial
amount. Mark

Also, I recall a dealer many years ago once telling me about Meade's
claim of using an over size primary mirror versus Celestron, and I think
Meade's claim was more light gathering power, (and they in other words
professed that they were giving you more than celestron was) but this
dealer stated that all the oversize mirror succeeded in doing, was
scattering light, and reducing contrast.

Was said the light that reflected from the outermost perimeter of the
oversize primary really never made it to the eyepiece to be of any
benefit, and that any reduction in baffle length to perhaps accept said
light would certainly not be a benefit to contrast.

I'm not sure if this is an issue anymore with any of the current Meade's
SCT's, but perhaps it is?
Mark

VcDy wrote:
Why, would a 16" lx200 be stingy in its color delivery vs. a 17" f/4.5
Coulter (which is great) ? Is there something about the sct design
which
inherently retracts colour delivery vs say larger refractors, newts, and

maksutovs.

Several have suggested its the size of the sct at question...

SCT color error does increase in proportion to its aperture.
For a common commercial configuration (f/2/10, neutral zone
at 0.866 radius) chromatic blur diameter is given approximately by
dD/73 in mm,
with "d" being the difference in refractive index vs. that
for which the corrector is optimized, and D the aperture diameter.
So a 16" SCT has chromatic blurs larger by a factor of 2 vs. an 8".

Also, SCT corrector with the 0.866 radius neutral zone is optimized
for photography, not visual. It brings together best focus for the
optimized wavelength and circles of least confusion of other colors.
While the circle of least confusion is smallest of all blurs between
paraxial and marginal foci, it has twice the wavefront error of the
so called "best" or "diffraction" focus, which is a common
focus for other (than the optimized) colors when the neutral zone is
at 0.707 radius. For instance, a 16" f/2/10 SCT with 0.866 radius
neutral zone, corrector refractive index ~1.5 at the optimized
~550nm, would have ~0.016mm blur diameter in the blue F-line (d~0.003)
and ~0.027mm blur diameter in the red C-line (d~0.005). It translates
into 1/5 wave wavefront error in the blue, and 1/3.1 wavefront error
in the red. Should the neutral zone be placed at 0.707 radius, the
color wavefront errors would be 1/10 wave and 1/6.2 wave, respectively.

Obviously, even at 16" aperture, the color error is still relatively
small, even more so considering lowered eye sensitivity. It may affect
color saturation of smallest resolvable low-contrast details, but not
likely very significantly. More likely culprit for lower color
saturation
over an entire object is a higher amount of scattered light (from rough
surfaces and/or and compromised glass homogeneity of the corrector),
and less efficient baffling.

Vlad

Thanks Roger...
jerry

Roger Hamlett wrote:

"jerry warner" wrote in message
...
Frankly, this is what Ive heard also. Its not the optics per se in fact
our
lx200 has a nice primary and when you lock down the priamry carefully
so as to keep things oin axis, then collimate, the scope is quite
crisp....
but with poor contrast. Mechanically the scope is a dog.
jerry
The comment about chromatic aberration being 'all withing the Airy disk',
is however not actually true.
If you ray trace a typical SCT, using a BK7 corrector, with the optics
laid out in the current 'mass production' form, chromatic aberration, does
spread beyond the size of the Airy disk, but mainly at the extreme blue
end of the spectrum. If you try the experiment of ray tracing and setting
the focus on 'green' light, then add back other colours, leaving out just
red, and violet, the plotted spread remains tiny. Adding red, spreads it a
tiny amount (from 10.4um spot size to just over 15um). However add back
the violet, and the spot size jumps up to 34.8um. The corrector, only
introduces a tiny level of chromatic aberration, but because of where it
is, the error is amplified by the secondary, and even if focussed
perfectly for each colour, there is a slight spherochomaticism shown,
which comprises the largest part of the total aberration. The focus 'shift
as a result of chromatic aberration is almost nil, but the increase in
spot size is significant.
On most SCT's, this is one of the 'lesser' faults, but it is still there,
and detectable.
The commonest thing degrading images, is collimation (90% of SCT's, only
have 'adequate' collimation, rather than really good levels), followed by
degradation caused by focussing significantlty away from the scope's
'design' position (this makes a big difference, with it often being
suprising how good images become when an attempt is made to get these two
factors close to 'right').

Best Wishes

Mark D wrote:

THe corrector plate (the only refractive part of the scope) will do
some
color aberration but that amount won't be visible in the image as it is
all within the Airy disc for the error.
More important is the quality of the reflective surfaces as well as the
corrector putting the image to a poorer quality than that of a
newtonian
reflector. While the Coulter scopes were often of poor quality, you can
get a really good one on occasion. Same thing with the SCT design
although Meade did tend to do the 16" a lot better than their smaller
scopes. You'll probably get a 1/8 wave accurate scope in the 16" size
which is good enough that you'll never see any real problems.
======================================

Bob, I've heard the exact opposite about Meade's 16" SCT's, in that
they are all mostly real dogs, particularly in comparison to thier
smaller SCT's. Mark D.

Mark I appreciate your post. Looking over optical designs in general
and realisations of those designs, color and contrast always go together
and where crisp color rendition is missing one usually (always?) finds
low contrast coupled with light scatter (the light scattered from any number

of sources including poor polish on optical surfaces).

Having never had our lx200 apart I cannot confirm an oversized mirror, but
the
seller told us it was (as a selling point!). I had nothing to do with buying
this
scope in fact it probably would have been the last scope on Earth I would
ever have spent (the bucks$$$) the club spent on this 'project'. The scope
has been "controversial" ( to say the least) since its acquisition.

Mirror flop in this beast is unbelievable. The scope is basically useless
without
locking the mirror down and even then you never are as sure as you would
like to be about where (relative to axis) the mirror is being locked in at.
Ive
tried every normal remedy to no avail - others likewise. Nevertheless I
continue to work this scope if only to get to know it - like a client! It's
pathologies are ubiquitous and a case study, I say with humor!

Finally while roaming carbon stars several wekends ago the issue of color
came up. Our prominent club engineer (Rockwell etc) suggested size alone
accounted for this and I knew he was wrong. I am sharing everything here at
saa with him and being the great fellow he is (he) is very interested in all
of the
comments posted here. (John is a great person and a good friend).

So, is it worth tearing what John has dubbed (the *******!) apart to flock
and this and that ... when there are a million other things we both would
rather
being doing. I think I will defer this to next year unless I get the urge to
pull
the corrector and "flock away".... some rainy evening.

Thanks Mark, and all.
Jerry Warner









Mark D wrote:

Also, I recall a dealer many years ago once telling me about Meade's
claim of using an over size primary mirror versus Celestron, and I think
Meade's claim was more light gathering power, (and they in other words
professed that they were giving you more than celestron was) but this
dealer stated that all the oversize mirror succeeded in doing, was
scattering light, and reducing contrast.

Was said the light that reflected from the outermost perimeter of the
oversize primary really never made it to the eyepiece to be of any
benefit, and that any reduction in baffle length to perhaps accept said
light would certainly not be a benefit to contrast.

I'm not sure if this is an issue anymore with any of the current Meade's
SCT's, but perhaps it is?
Mark

nick wrote:

VcDy wrote:
Why, would a 16" lx200 be stingy in its color delivery vs. a 17" f/4.5
Coulter (which is great) ? Is there something about the sct design
which
inherently retracts colour delivery vs say larger refractors, newts, and

maksutovs.

Several have suggested its the size of the sct at question...

SCT color error does increase in proportion to its aperture.
For a common commercial configuration (f/2/10, neutral zone
at 0.866 radius) chromatic blur diameter is given approximately by
dD/73 in mm,
with "d" being the difference in refractive index vs. that
for which the corrector is optimized, and D the aperture diameter.
So a 16" SCT has chromatic blurs larger by a factor of 2 vs. an 8".

Also, SCT corrector with the 0.866 radius neutral zone is optimized
for photography, not visual. It brings together best focus for the
optimized wavelength and circles of least confusion of other colors.
While the circle of least confusion is smallest of all blurs between
paraxial and marginal foci, it has twice the wavefront error of the
so called "best" or "diffraction" focus, which is a common
focus for other (than the optimized) colors when the neutral zone is
at 0.707 radius. For instance, a 16" f/2/10 SCT with 0.866 radius
neutral zone, corrector refractive index ~1.5 at the optimized
~550nm, would have ~0.016mm blur diameter in the blue F-line (d~0.003)
and ~0.027mm blur diameter in the red C-line (d~0.005). It translates
into 1/5 wave wavefront error in the blue, and 1/3.1 wavefront error
in the red. Should the neutral zone be placed at 0.707 radius, the
color wavefront errors would be 1/10 wave and 1/6.2 wave, respectively.

Obviously, even at 16" aperture, the color error is still relatively
small, even more so considering lowered eye sensitivity. It may affect
color saturation of smallest resolvable low-contrast details, but not
likely very significantly. More likely culprit for lower color
saturation over an entire object is a higher amount of scattered light

There we go. This is a beautiful analysis Vlad - Thanks. So the obvious
place one can go is to baffling and any ordianry thing one can do to
improve contrast.

Thanks - Jerry


(from rough
surfaces and/or and compromised glass homogeneity of the corrector),
and less efficient baffling.

Vlad

"jerry warner" wrote in message
...
Mark I appreciate your post. Looking over optical designs in general
and realisations of those designs, color and contrast always go together
and where crisp color rendition is missing one usually (always?) finds
low contrast coupled with light scatter (the light scattered from any
number

of sources including poor polish on optical surfaces).

Having never had our lx200 apart I cannot confirm an oversized mirror,
but
the
seller told us it was (as a selling point!). I had nothing to do with
buying
this
scope in fact it probably would have been the last scope on Earth I
would
ever have spent (the bucks$$$) the club spent on this 'project'. The
scope
has been "controversial" ( to say the least) since its acquisition.
One thing worth saying, is why not try masking the mirror?. If you cut a
cardboard ring, whose outside diameter matches that of the mirror, make a
hole in this about 1/2" smaller, cover the front face with black felt, and
tape it onto front of th mirror, with the tape only going to the edges of
the primary, it is always removable without any harm, and prevents
reflection from this area.
I'd partially suspect that the 'reason' for this 'feature', was that Meade
had problems producing the primaries without a significant turned down
edge, especially on the larger scopes, and making the primary oversized,
took most of this outside the normally used area. I had the excuse a while
ago, to do a Foucault test on the primary from such a scope, and the outer
1/2", was significantly out of shape.

Mirror flop in this beast is unbelievable. The scope is basically
useless
without
locking the mirror down and even then you never are as sure as you would
like to be about where (relative to axis) the mirror is being locked in
at.
Ive
tried every normal remedy to no avail - others likewise. Nevertheless I
continue to work this scope if only to get to know it - like a client!
It's
pathologies are ubiquitous and a case study, I say with humor!
When you think of the weight of the bit of glass involved, it is not
suprising that there is a significant 'slop', but unfortunately the levels
on some scopes are so bad, that they should never have passed 'QA'. Sad.
:-(

Finally while roaming carbon stars several wekends ago the issue of
color
came up. Our prominent club engineer (Rockwell etc) suggested size alone
accounted for this and I knew he was wrong. I am sharing everything here
at
saa with him and being the great fellow he is (he) is very interested in
all
of the
comments posted here. (John is a great person and a good friend).

So, is it worth tearing what John has dubbed (the *******!) apart to
flock
and this and that ... when there are a million other things we both
would
rather
being doing. I think I will defer this to next year unless I get the
urge to
pull
the corrector and "flock away".... some rainy evening.

Thanks Mark, and all.
Jerry Warner
In the past, on the LX200-12, which I then had, I had cause to take the
scope apart for other reasons, and did 'flock' it at the same time (as
well as adding a mirror lock to the shipping bolt hole). With this done,
and using a very short 'micro' focusser outside the scope to avoid adding
too much length to the optical path, I collimated with the primary
'locked', and imaged/viewed without ever releasing it. The views through
an eyepiece, were noticeably 'better', I think partially because the
locked primary allowed really critical collimation to be achieved, which
had never been possible, given how much the mirror moved before, and the
areas round bright objects, did seem to have slightly less scattered
light, which I felt was possibly from the flocking. However this was of
course 'subjective', rather than an objective test.

Best Wishes

Back to something that Roger wrote [and I missed the first time around]:

The commonest thing degrading images, is collimation (90% of SCT's, only
have 'adequate' collimation, rather than really good levels), followed
by
degradation caused by focussing significantlty away from the scope's
'design' position (this makes a big difference, with it often being
suprising how good images become when an attempt is made to get these
two
factors close to 'right').

I have a 5" CAT. I thougth I had good images with good collimation until I
compared my views with another 5" CAT. same OTA, just different mount and
vintage. The other definitely has superior images.

What is this about focussing near the scope's 'design' position? How does
one determine this? In my case, since they are the same OTA, likely this
focus issue is not the reason for the image quality difference.

Comments?

Thanks.

Jim


Best Wishes

Mark D wrote:

THe corrector plate (the only refractive part of the scope) will do
some
color aberration but that amount won't be visible in the image as it
is
all within the Airy disc for the error.
More important is the quality of the reflective surfaces as well as
the
corrector putting the image to a poorer quality than that of a
newtonian
reflector. While the Coulter scopes were often of poor quality, you
can
get a really good one on occasion. Same thing with the SCT design
although Meade did tend to do the 16" a lot better than their smaller
scopes. You'll probably get a 1/8 wave accurate scope in the 16" size
which is good enough that you'll never see any real problems.
======================================

Bob, I've heard the exact opposite about Meade's 16" SCT's, in that
they are all mostly real dogs, particularly in comparison to thier
smaller SCT's. Mark D.

I have a 5" CAT. I thougth I had good images with good collimation until
I compared my views with another 5" CAT. same OTA, just different mount
and vintage. The other definitely has superior images.
What is this about focussing near the scope's 'design' position? How
does one determine this? In my case, since they are the same OTA, likely
this focus issue is not the reason for the image quality difference.
Comments?
Thanks.
Jim
======================================
Hi Jim, this is a very good point to bring up, and I have for years
also wondered about this?

This seems to be a bit of a "grey'area" in giving a clear cut
explanation, or what acessories are a "go", and what is a "no go".

I would assume the designers of said scopes would've taken into account
what "standard" acessories such as standard Visual Backs, the various
Camera Adapters, and the normal range of eyepieces to be used with such
an instrument.

I've always wondered about this with the C-14" OTA for instance, and
what detrimental effect, if any, would be caused by the implementation
of Secondary Crayford Focusers attached to the Rear Cell, and then the
addition of 2" Diagonals, etc.

I'm thinking with the additional length of these acessories, that now
the optimal FL of the scope has been compromised.

Perhaps others with a bit more knowledge of the subject than I have can
comment, and offer some guidlines, and what the detriment will be from
deviating from the designer's specs? Mark

"Jim Hewitt" wrote in message
...
Back to something that Roger wrote [and I missed the first time around]:

The commonest thing degrading images, is collimation (90% of SCT's,
only
have 'adequate' collimation, rather than really good levels),
followed
by
degradation caused by focussing significantlty away from the scope's
'design' position (this makes a big difference, with it often being
suprising how good images become when an attempt is made to get these
two
factors close to 'right').

I have a 5" CAT. I thougth I had good images with good collimation
until I
compared my views with another 5" CAT. same OTA, just different mount
and
vintage. The other definitely has superior images.

What is this about focussing near the scope's 'design' position? How
does
one determine this? In my case, since they are the same OTA, likely
this
focus issue is not the reason for the image quality difference.

Comments?

Thanks.

Jim
The design of an SCT, has the second mirror correcting for the spherical
aberration caused by the first. It only does this properly at one mirror
spacing. Now Meade have never quoted a distance for this, and in fact one
example a friend tested, seemed to have the best corrected point at the
rear port of the scope (silly!). However I'd hope this is the exception.
Celestron have at times quoted that they attempt to correct the scope for
the setup with their 'T' adapter, and a 35mm camera, which puts the best
corrected point about 4" behind the rear port, and I'd expect Meade to be
similar. You can test for this, using a Ronchi tester, and a set of
extension tubes. If you try the tester at different distances behind the
scope, focus, then defocus by the same amount in each position, the spot
where the lines are straightest is the best corrected spot. Small
differences have little effect, but when you see people with a foot of
accessories behind the scope, and especially using focal compressors as
well, the optical lengths involved can be huge, and take the performance
below that of a much 'worse' scope...
My own C11, had the best corrected point just under 5" behind the rear
port, which is a very good compromise position, since this is just
slightly less than the backfocus needed for the *.63 focal compressor if
mounted tight to the scope, and just slightly more than is needed for a
1.25" diagonal.
How are you doing the final collimation test on your scope?. The last
'tweak', really is tiny, and does make a suprising difference. Thierry
Legault's page about this, is the 'bible' abou this aspect of the SCT. :-)

Best Wishes