Thanks.Joseph.
-------------- Original message from John Mahony <jmmahony@...>:
--------------





----- Original Message ----

> From: Charlie Miller <cmiller@...>
>
> You are correct. At higher power you are seeing the center of the FOV
> where coma is smaller but since you are using higher power, the coma
> that is there is magnified more so it looks like just as much.
>
> Since coma is a by-product of a parabolic surface, coma increases at a
> non linear rate as you move away from the center of the field,
> increasing quickly at first and then increasing at a declining rate as
> you get further away.

If it increases at a declining rate as you move further from the center, that
implies that it would be less noticeable at low power.

-John


> At low power the coma near the center is not visible but the increasing
> amount of coma at the edge is visible.
>
> John Mahony wrote:
> >
> >
> >
> > ----- Original Message ----
> >
> > > From: Charlie Miller
> > >
> >
> > >
> > > All parabolic mirrors have coma as one of the optical abberations of
> > the
> > > design...
> > >
> > > 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.
> >
> > But at higher power you magnify the coma. I used to think that coma
> > increased "faster than linear" as you move away from the center of the
> > FOV, so indeed you'd see more coma at lower power, but it turns out
> > the effect is roughly linear, so you see just as much coma at high power.
> >
> > -John
> >
> > > 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]
> > > >
> > > >
> > >
> > >
> > > ------------------------------------
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> >
> >
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>




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