Telescope Reviews: I Broke My LX90 Corrector Plate

I have deforked my 10" LX200GPS a couple of times. Somewhere I have a write up that explains how I make sure it aligned to the base but I’ll try to rewrite it quickly here. At some point I’ll put the full version on my website. But in my opinion the fork mis-alignment is not as big a deal as people make it out to be unless it is really large. The mirror shift on a SCT is usually greater than the fork mis-alignment, but in any case here is how I realign the OTA to the base.

I use a Howie Glatter 2" green laser collimator inserted in the visual back. The laser is configured for single dot mode.

The scope is then set on the tripod. Be sure the scope’s base is as level as possible with the corrector pointing up as close to vertical as possible and lock the ALT axis clutch. The AZ clutch needs to be unlocked so it can spin freely. Remove the covers at the base of the scope so you can access the fork arm adjustment screws.

The green dot is now projected on the ceiling. Adjust the focus so you get the smallest laser doughnut projection you can on the ceiling. Don’t lock the mirror. Tape a piece of paper on the ceiling so the laser projection falls on the center of it.

Spin the scope about the AZ axis by hand, the laser will trace a circle on the ceiling as the scope rotates. The key is to reduce the diameter of the circle, but to do this you first have to be sure the ALT axis is truly vertical.

Using the knob on the fork arm adjust the position of the ALT axis and spin the scope by hand about the AZ axis to reduce the diameter of the laser projection on the ceiling. Repeat the process until the circle is as small as you can make it. Use the paper to mark the projection position to gage your progress if needed.

Now the fun part. Once the ALT Axis has been adjusted so the OTA is truely vectical the projection should only move a little, and that motion should be tied to the fork mis-alignment. So with the forks sitting squarely in front of you (one on the left, and one on the right) slowly spin the scope 180 degrees and carefully watch which way the circle moves. Mark the position of the outer most part of the projection on the edge of the circle. Now rotate the scope 180 degrees and mark the other side of the circle. Now find the center between those 2 marks and mark that center location on the paper on the ceiling.

Very carefully loosen the screws on the base of the forks just enough so you can adjust the height of the forks. Do not loosen them so much as to allow the forks to freely move. You may have to gently tap the forks in order to move the center of the laser projected doughnut so it is centered over the center mark on the ceiling. If the laser in to the left of center you will have to lower the right fork arm or raise the left. Depending on how far off of center the OTA is you may have to do both. But be careful not bump the position of the tripod or you will have to start all over.

Once the center of the doughnut is centered over the center mark on the ceiling, spin the scope 180 degrees about the AZ axis to verify the diameter of the circle stay as stationary. Repeat the circle marking and fork adjustment until the circle’s movement cannot be reduced any further, but you should only need one or 2 iterations.
Tighten down all the screws. Check that the projection hasn’t moved and you’re done.

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-Marcus

Earth is the insane asylum for the universe!
http://www.jatobservatory.org

Domed, 12" LX200R on a Paramount ME - currently mothballed.

Edited by JAT Observatory (04/15/09 04:10 PM)