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Mewlon star test discrepancy...???

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#1 Scott

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Posted 05 May 2004 - 08:39 PM

My Mewlon 180 seems well collimated, however, the under focus and the over focus give me different results during a star test. One side of the out of focus is centered concentric rings...the other side is a few concentrics around a rather large dark, target like center. I am still getting decent detail on Jupitor on 6/10 skys. Any thoughts folks??

#2 Suk Lee

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Posted 05 May 2004 - 10:38 PM

If they're concentric on both sides, what you're seeing is artifacts of the optical design e.g. (I'm not saying yours does!) spherical aberration causes the rings to be asymmetrical around the focus point.

If your rings are concentric, don't worry about it.

Suk

#3 Scott

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Posted 06 May 2004 - 05:58 AM

They are both round and centered...but not the same. This scope has a small bit of mirror shift...could that have anything to do with it? Thanks Suk (is that a Mewlon 300 in your side photo?)

#4 Suk Lee

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Posted 06 May 2004 - 10:03 AM

Scott:

I've got a Mewlon 250 (couldn't spring for the 300 -- would have had to upgrade my mount as well...)

Mirror shift will cause collimation to change slightly as the optical axis tilts. At very high magnification (like, 400x) you'll probably see the (slight) effect when you re-center the star after shifting focus but again it wouldn't cause the dissimilar images on either side of focus.

How do you find the 180 compared to your C9.25?

Suk

#5 Scott

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Posted 09 May 2004 - 12:11 PM

Suk, I'm ready to list the 9.25 on Astromart...I'm simply not using it at all! I kept thinking that my interest in the Tak would wear off...and now I'm looking for a 210 or 250. I've been trying to use the system of collimation that they are using in Texas ...that is the the lining up of the reflection of the secondary and the secondary from 9' in front of the scope (I don't know if you do it this way?) but its not perfect ... I'm starting to think that the mirror shift is the Devil in the soup...I just purchased a better quality artificial star ...I'll let you know how it goes.

#6 Scott

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Posted 10 May 2004 - 09:04 PM

Suk...I think my collimation anxieties are gone for good. It's fairly easy to get a reasonable alignment on the Mewlon...but with this Digitec Dlx art. star, 400X plus was not hard. No tracking, clear sky, or heat problems...and I can take my time. I have to be honest and say that I choked over the price...but dissatisfaction and hand-ringing are costly as well. (I'm starting to sound like a salesman) In the end...It was worth every penny!

#7 Suk Lee

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Posted 18 May 2004 - 08:21 PM

Cool, I really like my PicoStar as well.

Suk


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