Concerning Coma

I was searching for performance data on off axis newtonians, and this popped
up in Google.

http://www.opticalmechanics.com/about_coma.htm

Still not sure I agree that smaller apertures provide better resolution
under poor seeing, but at least I finally understand something about spot
diagrams for coma.

Enjoy, free associate, and comment at will,
-Stephen

Stephen Paul wrote:
I was searching for performance data on off axis newtonians, and this popped
up in Google.

http://www.opticalmechanics.com/about_coma.htm

Still not sure I agree that smaller apertures provide better resolution
under poor seeing, but at least I finally understand something about spot
diagrams for coma.

Thanks for the link. I didn't see where the page said it would provide
better resolution. It did say that it would defeat some of the effects
of poor seeing, which it will. However, it will also introduce some
other effects--namely, diffraction through a smaller aperture. This
diffraction may well make the seeing more difficult to detect, of course,
but at the cost of having a wider PSF.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

Good website. I modeled the 18" f/4.5 Newt James used with the 6"
aperture offset 6" in Y. I got the same spots he did in the Y
direction, but the spots in the X direction are totally different.
Vignetting of the parent is much more obvious in the X direction. The Y
spots are elongated, thus the tangential and sagittal MTF's are quite
different, with sagittal (in the narrow direction) being much higher.
The X-spots show better clipping of the flare and are more symmetrical,
and their MTF values match almost perfectly, only slightly lower than
the Y Sag MTF curve. The interesting thing I found was that at 71.5
cy/mm the average (Tan+Sag)/2 MTF over a 0.4º square format was pretty
much a figure of revolution, rather than showing higher performance in X
or Y. Also saw pretty much the same thing when I increased the primary
diameter to 30" (f/2.7) and offset the 6" pupil center 12". Not
something I realized about OAP's before. The blur circles more than
doubled in size for the 12" offset system for the same field angles, so
it pays to stay as close to the axis as possible.

Stephen Paul wrote:

I was searching for performance data on off axis newtonians, and this popped
up in Google.

http://www.opticalmechanics.com/about_coma.htm

Still not sure I agree that smaller apertures provide better resolution
under poor seeing, but at least I finally understand something about spot
diagrams for coma.

Enjoy, free associate, and comment at will,
-Stephen