Colour Delivery in SCT's vs Newts etc.

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.

Have to correct this: actual wavefront errors are about
twice larger, 1/2.7 wave in the blue, and 1/1.6 wave in red. On
average, F/C blur size is 1.6 times the Airy disc
diameter (0.013mm), which compares to ~3 times for
a 4" f/12 achromat. This means that a 16" SCT has
nearly twice lower color error, comparable to that of
a 4" f/22 achromat. I don't think anyone would notice
effect of this little chromatism on color saturation.

Vlad

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.

It is not a simple scenario. Presumably, SCTs are assembled so that the
min correction criterion (1/4 wave) is satisfied. So it can be, and
most likely will be, either overcorrected or undercorrected at its best
location to begin with. Now, every change in separation between the
primary and secondary will result in change of both, back focus
distance and correction level. Back focus distance changes approx. as
dm^2,
with "d" being the change in mirror separation, and "m" the secondary
magnification, while correction error induced in an f/2/10 system with
spherical secondary boils down to less than 1/20 wave error for 1mm of
change in mirror separation (d=1), which according to dm^2 results in
~25mm axial focus shift. A 2" focus extention would result in nearly
1/10 wave overcorrection induced. Depending on what is the amount and
sign of correction error at the "design focus", this would either
worsen or improve on scope's
correction. If it is, for instance, 1/4 wave overcorrected
at the "design focus", it would worsen to ~1/2.9 wave.
If it was 1/4 wave undercorrected, it would improve to
nearly 1/7 wave.

Vlad

Roger Hamlett wrote:

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.

I once asked Dick Buchroeder designer of the meade mak about this
'mythical' design position and he basically said there was so much variability

in the scopes (off the assembly floor) you could basically forget about any
socalled 'design position'.



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.

That's about what a Meade tech told me once. 4 inches behind the rear port....

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...

and yet focal reducers are standard equipment.


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

I will try a mask...
jerry

Roger Hamlett wrote:

"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

"jerry warner" wrote in message
...


Roger Hamlett wrote:

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.

I once asked Dick Buchroeder designer of the meade mak about this
'mythical' design position and he basically said there was so much
variability

in the scopes (off the assembly floor) you could basically forget about
any
socalled 'design position'.


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.

That's about what a Meade tech told me once. 4 inches behind the rear
port....

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...

and yet focal reducers are standard equipment.
Yes. Though remember that if fitted as 'assumed', directly to the rear
port, the back-focus requirements at the F*.63 (for example), are only
5.6", which is not far displaced from the 'design' spacing. Where I see
problems, is with people having a combination like a Crayford focusser,
flip mirror & reducer, giving optical lengths out beyond perhaps a foot.
Normally the image improvements from the field flattening effect of the
reducer/corrector, 'outweigh' the very slight degradation to the scopes
performance, but with these very long optical trains, the effects can
become more significant... :-)


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