Posts Made By: Dave Erickson

Next the fringe centers were estimated for the dark fringes, and a line through each was drawn using a straight edge.

The maximum deviation from the straight line was then noted with a dot. If it deviates on both sides of the fringe centerline two dots are used. The fringe spacing was then measured using calipers and the deviation dots are measured as well. The PV Surface wavefront is a simple ratio of these measurements scaled by wavelength and a factor of 0.5 for surface and 1.0 for wavefront

I noticed that the fringe spacing was different along the major axis being progressively wider from one edge to the other. This is an indication of different power in the fringe orthogonal direction thus astigmatism.

Based on these measurements this mirror is about 1/7 wave at 546nm in the minor axis and nearly 1/3 wave along the major axis.

It is being reworked...

Lastly I took it to a clients shop and used a 6" zygo measuring its performance at 45 degrees, because this is how it will be used in the astrograph. The wavefront error was input to the Zemax model for the astrograph and confirmed that the mirror was not of sufficient quality for the project.

Hi Rod,
Thanks for posting the link...

Dave

Attached are a couple of annotated Zemax FFT Diffraction Ensquared Energy plots that show clearly the impact of a bad secondary mirror for this astrograph.

Plots are for the full spectrum of an Astrodon Luminance filter and the full Field of a an SBIG STIL11000M camera

While the degraded performance is still respectable it is significant, and for such a simple optic not worth the hit...

Well...this is the slower system, but for reasons other than Sky Quality concerns expressed. With more than 2 years of data taken every 10 minutes on clear nights there is little doubt as to the quality of the site. And yes I have more than 35,000 allsky images. Average seeing is <1.5 arc seconds. Average Sky Quality on clear moonless nights is better than 21.9 mag/arc seconds toward Polaris. The prior 6 years I was imaging in the So Cal area and most dark sky sites in the south west, including New Mexico and Arizona.

The reason it is only F/3.2 instead of F/2.6, like the 8" it is replacing, is the physics of narrow band filters. 5nm BPF's don't work so well at cone angles faster than F/3, and F/3 is a real stretch for them. The only filters I've measured that perform at those angles are from Astrodon, because of the way they are centered "slightly long" at near 0°angle. I've only measured the 3 sets I own.

In the 8 years the prior 8" F.2.6, image attached, has been operational it’s only been collimated once after I dropped it on the 10" secondary mirror mount...

So with the slower system I am not concerned with collimation, and with another passive a thermal design, focus with temperature is not a concern either...

Why a fast system...As much aperture as possible at a given image scale... The 45" focal length image scale almost takes advantage of the 1.5 arc second seeing, when the 5.4 micron pixel camera is used. I'd planned a larger 16" - 18" system but neither will fit in the 6' dome....Dave

Hi Keith,
Not yet Hope to image it soon...
Dave

Hi Keith,
Well its been a bit frustrating. First I made equipment and configured the 2 domes for comet ISON, the comet of the century. Next I configured things for the spectacular meteor shower. I am a little frustrated by our collective lack of ability to predict things. So its back to a more flexible setup for the summer constellations... The 8" F/2.6 and a 8300C one shot color camera for comets and a dual astrograph, in design, for wide field imaging 9X12 degrees at a time, attached is a sketch of the astrograph...

I hope to post some images soon...

Regards,
Dave

I finally had some good weather while on site and was able to reconfigure things and get a wide angle camera operational. Here is a single 10min exposure of the Pipe Nebula taken Saturday night while at the observatory. Its 14X19 degrees.
Regards,
Dave

Agreed resolved to the core and lots of crisp stars, very nicely done Floud!