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.