Posted 08 June 2015 - 05:22 PM

The Celestron Powerseeker 127EQ is a telescope that the internet loves to hate. It is an example of the controversial Jones-Bird (or Bird-Jones) design, and is 5" of aperture for less than $175. Both of these facts have associated pros and cons. I received this scope as a gift this past Christmas. I would like to document the modifications that I have made to the scope to improve its performance and usability, as examples to others. This thread is not intended as a stage for yet another debate of this scope's merits.



Faster/Easier Collimation



As shipped from the factory, the primary mirror lacks a center spot, and collimation is adjusted Phillips-head adjustment and lock screws on the back of the mirror cell. Neither of these features are conducive to collimation in the field. Additionally, the mirror cell's angle is stabilized during collimation by sets of compressed rubber o-rings instead of metal springs, resulting in a limited range of primary mirror angle and unsteady adjustment behavior.



First step: add a black center spot and ring reinforcer to the primary mirror. There are numerous tutorials elsewhere for how to do this, so I won't re-write one here.







Note that the rubber o-rings have already been removed from the mirror cell in this photo. They were carefully peeled/cut off with a pen knife.



Second step: replace collimation adjustment and lock screws, and replace rubber o-rings with springs. The inspiration for this step was niuno15's how-to on Astronomy Forum. The springs were sourced from a Home Depot spring assortment.







The replacement adjustment screws are 1" long 10-32 wing screws, and the replacement lock screws are 1/2" long 10-32 thumb screws. The difference in head design lets me easily feel which screws are which without taking my eye away from the eyepiece while collimating on a star.







Improve Focuser Performance



Even with the improvements above, the scope was having trouble maintaining collimation. Collimation would qualitatively change with different eyepieces, and different collimation methods (laser vs. reference star) wouldn't agree in their results. The culprit was angular slop in the focuser tube. Heavier eyepieces would pull it to the side and skew collimation, and the heavy laser collimator would skew the tube while inserted. Inspiration and guidance for this fix came from here.



First, we remove the focuser assembly from the OTA, and disassemble it fully. Notable pieces include: the shiny silvered plastic draw tube, the stipple-painted plastic focuser base, the metal focuser pinion and adjustment knobs, and the Bird-Jones corrector doublet. (One note: when re-assembling the focuser, I advise others to only screw in the corrector lens finger tight. This way, one can (very) carefully reach into the tube and unscrew the corrector by hand for collimation purposes)

Materials and tools:

Goo Gone, to remove sticky grease from focuser tube, focuser base, and pinion.

White Lithium Grease to re-grease focuser tube rack and pinion

Plastic collar stays from men's dress shirts, to be used as shims

Krazy glue, or other cyanocrylate-based glue

200-grit sandpaper (not shown)

Teflon Tape (which would prove unnecessary)





The focuser base contained a single ribbed plastic shim that did little to hold the focuser draw tube steady. It was removed, and both the focuser rube and base were de-greased with the Goo Gone. Below you can see exactly how much of a gap there is between the outer diameter of the draw tube and the inner diameter of the focuser base.





To fix this, we add the plastic collar stays as shims around the inside of the focuser base. Here they are being test-fit:







After being trimmed to length and glued into place:







The fit with the shims as-is was extremely snug, to the point of nearly getting stuck. Since the focuser draw tube's gear rack is plastic, and the pinion is metal, a fit that is too tight will lead to broken rack teeth. Thus, it was necessary to sand down the shims with the sandpaper until the draw tube moved more easily. Once a snug-but-movable fit was achieved, white lithium grease was applied to the focuser rack teeth, and the focuser reassembled. Here is the draw tube in place, with the focuser reassembled, after sanding:







While some similar rebuilds use Teflon tape for additional shimming and friction reduction inside the focuser base, I did not notice a significant improvement in smoothness.



The focuser was re-attached to the OTA, and the scope collimated. Now the collimation is much more reliable, star and laser methods agree, and the focuser more reliably holds a constant focus with the increased resistance from the shims. There is no longer any noticeable slop of the focuser tube inside the base.





