Quote:
Originally Posted by alistairsam
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Hi Alistair...
Gary's experience runs counter to my understanding of optics.
Anyway, before posting my thoughts on the subject, I felt I should cover my bases and contact Dave Rowe directly. This is his unedited response:
Quote:
Hello Clive:
Great to hear from you after all these years.
Thomas Baader sent me one of the original correctors and it performed well
and to specification in a small fast Newtonian. Since then I have not had
another opportunity to see or use another corrector. Thomas tells me that
there are many satisfied customers. He's an honest guy, so I believe
that's the truth.
The corrector is no more and no less sensitive to collimation that any
other coma corrector. Good collimation is necessary, of course, and
becomes very important for large fields and fast scopes.
The corrector in question may be incorrectly made, or it may be misused.
It's impossible to tell without studying the actual unit. Certainly it
should not move the focal surface 30 mm closer to the primary. If that's
true then I suspect that the corrector was improperly made. Additional
information could be useful, like an image from the optical system.
I designed the corrector for Thomas out of friendship. I did not and do
not receive remuneration of any form for the design. The long back focus
makes it very useful in situations where off-axis guiding is desired, it
has much better off-axis correction than the 2-element Ross corrector, and
its larger unvignetted field makes it useful for larger format cameras. In
my opinion it's an excellent design that has a solid place in
astrophotography.
My best,
Dave
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Perhaps more so on the 'cloudy nights' thread than here, I feel that a few
things were stated that intimate a questioning of Dave's design work, and
so I feel compelled to share a bit of background.
You might get a sense of Dave's credibility in the field of optics if you
explore his history. Dave's first attempt at telescope making was a 12" f4
concentric, flat field anastigmat. In the process Dave was so thorough with
his homework that he discovered an error in the formula accepted as
common knowledge in the public domain used to produce the aspheric on
the schmidt plate.
In my personal correspondence with Dave over the years where we
bounced a few corrector ideas back and forth (Dave being the brains side of
the conversation) I can state emphatically that for any given iteration that
was considered serious, the optical prescription included sensitivity and
tolerances for axial placement, tilt, decenter, ghosting etc) etc) and so I am
struggling to accept the assertion that the RCC1 is inherently flawed or
that MPCC design is functionally better. I would bet London to a brick
that this is not the case.
Suffice it to say, anyone who dismisses the quality of Dave's work (in a
public forum) should really have their ducks in a row first.
A few thoughts on the corrector:
The MPCC is a simple Ross type, implicit in that design of course is that you
need an over-corrected primary to compensate for the spherical aberration
introduced by the corrector. The RCC1 on the other hand has three lens
elements, which provide sufficient degrees of freedom to correct all the
field aberrations. It is functionally equivalent to the old Wynne style
corrector but without the slight magnification inherent in the original design.
The RCC1 is obviously a much better corrector than the MPCC (on paper at
least).
Now getting back to Gary's real-world experience with the RCC1...
My first clue is the 30mm error in the focal position. This tells me that
the corrector is operating way outside of its design parameters. If there
is a manufacturing fault, ie) something goofy going on with the lens
assembly, under those circumstances you might expect to see the
corrector changing the imaging scale and a dramatic loss of colour
correction. The other possible issue is the spacing between the lens
assembly and the image plane is incorrect.
In any event, the best (and fairest) way to resolve this would be for Gary
to contact Thomas Baader directly and give him the opportunity to address
each possibility in a process of elimination.
I have absolutely no doubt that he would do what it takes to get to the bottom of this.
regards,
~c
