Saturday, January 26, 2019

Meade ETX125 easy collimation

I have noticed that my ETX125 airy disk developed two hot spots which mean it got out of collimation a bit.



My favorite for eons Weasner website has a lot of information on that but it's mostly "don't do that yourself", "be super careful", and very little on the actual subject with quite a conflicting tips. So here is how I've done it:

  1. Instead of removing 4 side screws first, just unscrew three on the back of the plastic EPs housing (remove the focusing knob first). 
  2. Point OTA to Zenith and pull it out and up shaking and twisting a bit. 
  3. Make a photo of the set screws on the back of the OTA (you may also number screws with a marker for the convenience of remembering manipulations). 
  4. Get two Allen wrenches fitting two different sizes of set screws (screwdriver hex bits have no handle to clearly indicate the rotation angle so keys are preferable). 
  5. Sit with the OTA in front of bright light (e.g. a window) such as you can easily look down to your feet between knees (you will have to rig something to hold the OTA with its front down if you don't trust your knees). 
  6. Put a piece of white paper on the floor between feet (Letter/A4 size is sufficient). 
  7. Hold the OTA low and tight between your knees so you can see the paper and other reflections in the mirrors inside from the EP side (if the OTA is too close use reading glasses, the farther the OTA the easier to see imperfections in the concentricity). 
  8. Undo all three no-head (smaller) screws for one full turn (they are pushing down screws). 
  9. Using wide-flat-head screws collimate by pulling the mirror cage up or down (driven down by gravity as there are no springs there). 
  10. The idea is to achieve a perfect concentric view in the EP hole, including your eye pupil, so keep the head slightly up to have the eye in the light too. 
  11. You shouldn't need to rotate wrenches for more than 1/2 of the turn maximum to fix a slight misalignment from the factory position. That will minimize the chance you ruin the on-axis alignment of the MAC scheme. But if it seems that you have to, try to return that screw back to the original position and try to play with another one. 
  12. When done, screw down smaller push screws back, watching for any shift in the concentric picture. Try to achieve an equal finger tight tension on them little by little going around. 
  13. Star test with Polaris (or with electric pole isolator sun's reflection) by fitting the OTA back into the EPs housing. Remember to use at least 8mm EP to watch the airy disk (better 6 or even 4mm). As the out of focus Newtonian collimation method doesn't work well with MACs. Better to use the EP above the right angle mirror than the back inverting prism as the latter breaks the disk a bit. Bad seeing is not an excuse, just sit and watch for at least 5 minutes, there will be good enough moments to judge the result. 
  14. If problems found in the airy disk (I had none), point to the disk's issue in the FOV with your finger's shadow in front of the OTA after defocusing and locate the screw closest to that feature. 
  15. Repeat from #5 above but try to detect the possible error in the concentricity on the suspected side (or opposite to it) first. Move your head to try revealing that (as the only error source is the wrong position of the head). 
  16. Re-test (go back to #13).
A dedicated Cheshire could be of help here as well, however, I have found the direct viewing method sensitive enough from the first attempt.

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