For those of you that have an older ETX, and have done a collimation check that failed, please consider the following BEFORE you collimate.

by Ken Toliver (ktoliver@cox.net)

 

Check your primary mirror mounting FIRST.

 

I will not repeat the process for collimation; it is covered very well here on this site. All the same steps are followed; however, once you have removed your OTA from the diagonal mirror housing (3 screws), BEFORE you start turning those Allen head screws that adjust your primary mirror, do yourself a favor and inspect your primary mirror mounting.

 

I had already separated the OTA tube from the diagonal mirror housing. I was ready to boldly start turning those Allen head screws; then I thought…why not just give that primary mirror a quick dusting with compressed air before I start? It will take less than a minute and as long as I’m careful NOT to touch the mirror, I’m good. All you have to do is unscrew the primary mirror base from the big blue OTA tube. It is threaded in the same way that the lens cover is.

 

Here is what I found:

 

1)     The primary mirror base was not even tight in the OTA tube! It was at least 1/8 of a turn loose – that would have the same effect as the primary lens being rotated out of alignment!

2)     Once the mirror base was removed, I found that the mirror itself was not tightly held in place by its retaining ring. There are 2 small washers (fiber or cork) below the retaining ring that had desiccated and shrunk a bit over the years.

 

To tighten the retaining ring (plastic, of course) requires a spanner tool that 99% of us will not have. I had to man up and use a small punch and an even smaller hammer to carefully tap it around until tight.

 

Afterwards my collimation test passed with flying colors and I never had to touch those Allen screws. My stars are pinpoints of light now.

 

REMEMBER:

1) Be very careful and unscrew the primary mirror base slowly until it is free

2) Slowly pull the mirror base straight backward out of the tube until the baffle tube clears the OTA completely

3) DO NOT TOUCH THE PRIMARY MIRROR

 

Hopefully this can help somebody avoid going through a long collimation session.