Telescope Reviews: Figuring out which area of damaged mirror to mask

Thanks for the analyses!

Correction: The secondary is 1.75" m.a.

The chip itself is takes up less than half of the area of the masked portion. However, my experiments with a point source of light and viewing the magnified reflection of my eye at 2*FL showed that the mirror was distorted up to around twice the width of the chip (the chip is about 3" long and 1.5" wide--I had to mask out a slightly gibbous area of diameter 3.5"). I didn't build a proper Foucault tester because the distortion was showing up so nicely as it was--the blob would stretch into a teardrop when it got nearer to the chip, but not nearer all the other edges.

It's usable at 200X on the moon, and up to about 133X on everything else, and best below 100X. Last night, Jupiter on this scope was much less sharp than on my Coulter 8" F/4.5 (though Jupiter was a bit lower down--not a controlled experiment), but I did clearly see two cloud belts at all magnifications I tried up to 133X, but they were hard to see at 200X. (The Coulter was showing sharp images at 150X, and good images at 225X.) Still, like I said, it's a travel scope, and I think it'll pull in faint fuzzies sufficiently well for my purposes. (The only DSOs I've tried it out on so far--and that was with the old, larger mask--were M31/M32/M110 and the double cluster. The Double Cluster looked really nice--the second nicest view from my red skies backyard ever. M31 was core-only, as is normal for an 8" in my backyard. M32 was obvious and definitely non-stellar. I may have caught a glimpse of M110, which is better than I've done with my Coulter 8"--but I am more experienced than when I last turned my Coulter on M31/32/110 in red skies. In any case, the views of M31/M32/M110 were no worse than in my Coulter, while the Double Cluster was decent, though the latter comparison is skewed in that the only memory I have of the Double Cluster from that site with the Coulter was of an exceptionally lovely view of it.)

To add to the mess, now a semicircular area of 5mm x 10mm along one edge of the secondary, on the side opposite to the chip on the primary, is blackeded out. As Mark said, it sounds like a matched set. :-) (When I was buying, I knew about the problem with the primary but not about the problem with the secondary.)

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Coulter Odyssey 13.1" split-tube
Coulter Odyssey 8"
Home-made 7.8" F/4 dobsonian travel scope
Home-made 68mm F/5.3 achro (typically used as finder on 13.1")
Skymaster 15x70
BPTs4 8x30
32mm Plossl, 30mm Rini, 27mm Kellner, 13mm Hyperion, 6mm TMB/BO Planetary, Owl 2X Barlow
Palm TX with AstroInfo and RescoViewer