testing a fine old coulter

I've been using an old (1980's) coulter 8 inch f-4.5 for some years now. It resides at the local observatory and is clearly better than a twin coulter purchased at the same time. I might test this one next. I have been cleaning the mirrors and thought what the hell I'll make a mask and foucault test her.

here's the result.

radius of curvature 1805mm
203mm

25mm plate mirror.

Looking at the mirror with a ronchi its a nice smooth looking parabola. It appears free of zones, edge isn't turned down and looks OK.

With the foucault tester it looks rather smooth in terms of figure again confirm no obvious high or low points.

Some minor roughness visible nothing to be too concerned about.

I made a mask to measure zones from .3,.6,.8 and.95 radius.

Prior to doing this I did'nt calculate the respective positions.

I took 3 sets of measurements and rotated the mirror for each set averaging out the measurements. I found little variation.

Using .3 as a starting point (therefore 0)I got 1.78mm at .6,3.81mm at .8 and 5.08mm at .95.

This is the average only the image was very stable and rather easy to read.

The mirror is therefore moderately over corrected as the correct measurements would be for a perfect parabola

0,1.56mm3.13mm and 4.66mm.

This could simply mean .3 is a low point but the scope does startest as slightly overcorrected.

Its a reasonable mirror for the price. coatings are still very good.

anyone got a program to calculate the wavefront? i suspect its under 1/4 wave but would be interested in knowing. I had a program on my last computer and forgot to transfer it.

Phil,

The program I use shows that your wavefront error is about 1/3.8 wave at a focus position with a maximum transverse aberration of 1.9 times the theoretical Airy disk radius, or about 1/2.8 wave at a focus position with a maximum transverse aberration of 1.4 times the Airy disk radius. The first pair of numbers represents a focus position for the best wavefront, the second for the smallest transverse aberration (i.e., smallest Airy disk).

I made the following assumptions:
Active diameter of 201mm (allowing for bevel)
Moving light source tester
Mask zone openings 12mm (~1/2") wide

I use a program named Genmir I found and adapted in the old Telescope Making magazine some years ago. It might have been done by Dick Suiter, but I'm not sure. It was published in Basic (!) and I have my own C version.

Hope this is not too discouraging but it is reasonable. At f/4.5, mirrors must be very precisely figured, and it doesn't have to be off by much to get it out of tolerance.

Best regards,
Doug Scobel

Phil,

After my previous post, I discovered that my Genmir program is making assumptions regarding the mask opening widths, and bases the wavefront and transverse aberration errors on those assumptions. The assumption is not that the mask openings are 12mm wide - rather, it is assuming that each opening picks up where the other left off. So the results I posted assume that the opening widths (going from the center out) are 12mm, 30.45mm, 20.3mm, and 13.3mm. Note that this affects the error calculations only - the mean radii of .3, .6, .8, and .95 were still used as the mean radius of each zone, for purposes of the slope calculation. The slope errors are extrapolated out to the edges of the mask openings, and so if the widths I listed above are larger than what they actually are on your mask, then they could be in error.

Please post the actual zone widths as they exist on your mask, and in the meantime I'll modify my program to let me input them directly. You should also carefully measure what the "active" diameter of the mirror is (i.e., omitting the bevel). I can then see if the numbers come out any different.

Regards,
Doug