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Posted by Gavin Blakeman on March 30, 2002 18:09:50 UTC

I have started star testing a 200mm (8") F8.05 dob that I have just completed construction of.
I ground the mirror and am testing with an unsilvered mirror. (Pyrex)

I only have 2 ep at this time
40mm orion plossl (40)
17mm orion plossl (94)

When I look at Jupiter with the 40mm I can clearly see the main red bands and the 4 moons. The image is also sharp.

When I use the 17mm on Jupiter, the image is less sharp.

Looking at stars, the image is pinpoint with both the 17mm and the 40mm when I defocus i see that there may be some astigmatism. (does not initially defocus as a round image)

Q1: Is the less sharp image with jupiter at higher magnification because of:
a) the uncoated mirror
b) collumination not correct
c) a fault with the mirror
d) a fault with the ep
e) Improper parabolosiation of the mirror?

Q2: Is the astigmatism due to
a) The mirror
b) colluminaiton

I live in a big city, so there is lots of light pollution. I am waiting to get a brigher star into view to look at the astigmatism again.

Thanks for any input

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