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G 9.25 Collimation

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Posted by Kip Crawford on February 28, 2002 19:43:48 UTC

Collimation could be part of it. The unusual "inner" spike can be a few things. First of all it is very important to make sure the tube is properly cooled. You cannot do a proper determination of the problem unless the OTA is at ambient temps.

The "anomoly" you are experiencing sounds almost like a stress point on the primary mirror. SCT's on EQ mounts do a lot of turning sideways back and forth unlike a fork mount which is considered a more casual movement. Basically like a mirror "flop", the primary "floats". When moving the OTA to different positions, the mirror actually moves slightly. This, by the way is an astrophotographers "pain in the butt" type situation.

Collimating the secondary may be done successfully, however the worse case scenerio is to remove the primary from the OTA and inspect the focus mechanism. Sometimes during cold or warm outings the mirror clamps could be tighter than the others doing temperture contractions. This causes the mirror to be "on tilt" which gives half of the mirror surface the impression of being focused and the other not, hence the focusing problem at higher powers.

Just rememeber to cool the OTA thoroughly before doing any testing. Set the scope outside (covered) for 2 hours at least if possible.

Sometimes a really good "jostle" can muck things up. I myself handle a SCT OTA like a case of Nitro. One good bump can take things out of kilter. Good Luck!!

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