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5" minor axis Secondary Mirror Testing

Started by hbastro, 05/11/2014 01:58PM
Posted 05/11/2014 01:58PM | Edited 05/11/2014 02:00PM Opening Post
I have a small optical shop in my garage that includes couple of very old plano interferometers, a 6" Davidson Fizeau, and a larger 10" fused silica test plate setup, both use the green mercury line at 546nm.

I recently purchased a 5" minor axis secondary mirror for an astrograph I am making. I took a look with the 10" testplate and it looks reasonably close to the guarnteed 1/10 wave by eye.

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hbastro's attachment for post 58374
Posted 05/11/2014 02:14PM | Edited 05/11/2014 03:15PM #1
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

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hbastro's attachment for post 148398
Posted 05/11/2014 02:27PM | Edited 05/11/2014 03:17PM #2
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...
Posted 05/11/2014 02:37PM | Edited 05/11/2014 03:18PM #3
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.

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hbastro's attachment for post 148400
Posted 05/12/2014 10:35AM | Edited 05/12/2014 10:36AM #4
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...

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hbastro's attachment for post 148402
Posted 05/12/2014 08:52PM #5
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

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hbastro's attachment for post 148404