Frequently
Asked Questions
Do OA Newts have
the same coma as their “Parent Mirror”?
Though
that notion has often been suggested, look at it this way; if you took a 10”
f/4 mirror and cut out a 4” f/10 ON AXIS mirror from the center
would that mirror have the same coma? The answer is of course no. Although,
this is not to say a 4” f/10 OA paraboloid has the same coma as our theoretical
4” f/10 on axis. Decentration does effect coma, just not as much as has been
suggested. From calculations I’ve made it seems an OA f/10 has about the same coma
as a 4” f/6.5 on axis parabola.
Is a laser
collimator useful for OA Newts?
Yes,
however it is imperative that the scope be in as good alignment as possible using
the technique(s) described in Use and
Care. Once you are satisfied a good alignment has been achieved you’ll need
to fashion an “over the aperture” mask, like you would for a solar filter. But
instead of having solar blocking material, use a translucent film, which your
laser can be projected onto from inside the tube. Place the mask over the tube
end, being sure to rotational index the mask with a mark that lines up with the
focuser. Next turn the focuser barrel in as far as possible. Place laser
pointer into focuser, tightens the setscrew, and turn on the laser. There
should be a dot projected onto the film. Mark that point with indelible ink,
and ta-da! In theory you should be able to bring the scope back to near perfect
alignment by merely tweaking the (3) primary mirror cell adjusters and moving
the laser dot to the original mark on the film. Now the caveat here is that I
have never tried this method, and have only ever heard of it being successfully
employed by a handful of OA owners. However there is no reason to think it
would not work. Because I’ve aligned so many OA newts it’s second nature and
I’ve never felt the need to try that above described technique.
My OA scope is in excellent alignment,
but the secondary tip does not lineup at 12:00 o’clock per Use and
Care?
Over
the years building and testing OA`s after first being very hung up on that
issue, I’ve found it doesn’t seem to matter. It represents small errors in the
primary’s rotational position, and perhaps small errors with the overall
secondary alignment. But the bottom line is that the OA newt’s converging light
bundle is not especially steep, making it relatively insensitive to focal plane
alignment errors. And ultimately you are just “steering” the optical sweet spot
to the center of the field of view by adjusting the primary. So relax and enjoy
the view!
Are field
flattening lens such as Paracorr useful
with OA Newts?
I would reply with caveat in place on this one too in that I
have no personal experience with coma correcting optics. However I do have user
reports supporting the notion that they do indeed offer some field
flattening/coma correcting qualities.
Can the OA-4 really
be used at 255X without the image breaking down?
For lunar observing
no doubt, seeing contingent. I routinely use my OA-4 at 204x (5mm Vixen
Lanthanum) and once used someone’s 4mm (255x) on Saturn. The image was starting
to get dimmer, but no image breakdown.
Optical Consultant, Mike Palermiti, took his OA-4 to the absurd extreme
for his June 2000 review
of the OA-4. While no one would find those magnifications useful for normal
observing, it does stand as testimony to the optical quality of OA newts.
Can an OA newt
be retrofitted with a Crayford Type focuser?
It can be done,
but as always the rule is don’t hit the secondary with the focuser barrel. The
ATS (Across Tube
Secondary) was designed with that in mind, however you must trade-off a bit
of aperture to accomplish it. In a nutshell; the ATS version is simply a way to
employ 2" Crayford type focusers to the OA design using the same size
"parent" mirror. The compromise being that in order to make the
design work without obstructing on the tip of the secondary mirror you must
being willing to give of a bit of aperture with ATS, but in return you get a
bit shorter tube and the benefits of a 2" focuser.
Is DGM Optics
going to offer the Across
Tube Secondary configuration again?
Possibly,
however it’s tough rationalize giving up the aperture on one hand, but on the
upside is the Crayford and the somewhat shorter tube the ATS offers.
