The K-20 Project

The K-20 Project

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2/20/2007 - The Secondary Mirror arrives!!

My 3.1 inch Secondary Mirror arrived from Galaxy Optics today complete with interferogram and overall Peak-to-Valley (P-V) and RMS values. The P-V wavefront error is the distance from the highest poiny to the lowest point on the test wavefront relative to the reference wavefront.  Due to the fact that this value is obtained from the two worse interferometric data points on the mirror, it must be taken with some question as there are literally thousands of data points across the entire surface of the mirror.  Two mirrors with the same P-V value may in fact have very different optical quality.  If the data is obtained from both mirrors at a high zone BUT the first has the high zone in the center and second at the edge, the second mirror will actually have lesser optical quality because the overall surface area of a high zone at the edge is larger then at the center.  Lost yet?  OK, most optic shops now report the RMS wavefront error.  This method uses data from thousands of interferometric points across the ENTIRE surface of the mirror and thus produces a much more accurate representation of the mirror's optical quality. By common convention an optic with RMS wavefront error of 0.0712 wave (~1/14 wave) or less is considered diffraction limited.  The RMS of this secondary is 0.009 and therefore it is diffraction limited.

ENTRY PICTURES


RMS value = 0.009 Anything less then 0.0712 is considered diffraction limited. The coating is 98% Reflectivity Enhanced Aluminum, 5 Dielectric Layers over Al with a top sealing layer of pure quartz.	Interferogram of the secondary mirror from Galaxy Optics. Only basic optical quality data can be deduced from this image without being a trained optician. If you'd like further detail of what is here, let me know.

This is a picture of the Maple Veneer I will be laminating in a jig to form a curved focuser board. The curve creates a very rigid yet LIGHTWEIGHT board that fits to the inside radius of the secondary cage and focuser base.	Multi-ply Hardwood Core, Hardwood Veneer Baltic Birch plywood. This will be used on the secondary cage rings, mirror and rocker boxes and ground board. It is very rigid and finishes beautifully with just polyurethane.