Thanks so much.Joseph
-------------- Original message from Charlie Miller <cmiller@...>:
--------------




All parabolic mirrors have coma as one of the optical abberations of the
design. It is not a side effect of a poorly manufactured mirror. A
perfectly made parabolic mirror will still have coma. What is coma?
Coma causes star images to become small, comet shaped images as the star
image moves away from the center of the field of view. The only star
image that is perfect is the one that is exactly at the center of the
field of view.

The faster the f-ratio, the worse the coma. At higher power, you are
only looking at the very center of the field of view and coma is
minimal. The lower the power, the wider the field of view and the more
coma you see since the amount of coma increases with off axis distance.
So with a fast f-ratio mirror and low power you get a double whammy from
coma.

The Televue Paracor is designed to introduce "negative coma" so that it
cancels the normal coma of the parabolic mirror. As such, it can only
perfectly correct coma for one specific f-ratio but in reality it works
quite well over a range of f-ratios. So DOBs from f/4.0 and above do
quite well with a Paracor. An f/4.0 or faster DOB is often considered
unusable without a Paracor and many people would go as far as saying
anything below f/5 requires a Paracor to have acceptable image quality
but that is a very subject argument.

A new problem, of sorts, pops up with Starmaster's f/3.3 systems. Those
very fast systems may require Televue to build a "Paracor II" to be
optimized for that much faster f-ratio. That is not to say a standard
Paracor won't work at f/3.3, it will. But it won't provide as fully a
corrected field at f/3.3 as it does at f/4.5. At TSP 2009, I looked
through a 22" Starmaster f/3.3 with a 17mm Ethos eyepiece + Paracor. I
was seriously impressed. So the idea of even better image correction
with a new Paracor design was tantalizing.

Also, this is not to say that collimation is not an issue. The faster
the f-ratio, the more sensitive the image quality becomes to
collimation. Perfect collimation is always the first step toward
getting the best images from your scope. The best Paracor and best
eyepiece in the world won't provide high quality images if your scope is
not collimated accurately.

Charlie Miller
CTO, PBXcentral, Inc.
512-744-1510

Michael Hrivnak wrote:
>
>
> I've owned an 18" f/4.5 Starmaster for a little over a year, and it's
> my first
> big dob. Should I be using a paracorr?
>
> I have noticed that especially at higher magnifications, I can never
> get stars
> to be quite pinpoints. I've generally blamed it on the humidity in NC or
> imperfections in collimation. What visual symptoms should I see that
> could be
> corrected with a paracorr?
>
> Thanks,
> Michael
>
> On Monday 06 July 2009 22:10:44 Joe Castor wrote:
> > A paracorr is a "required" option. At 4.3f, coma is a given effect,
> but is
> > easily corrected with a TeleVue paracorr.
>
> [Non-text portions of this message have been removed]
>
>




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