Refractor collimation - a revelation?

[parallaxerr]

I took about a bit of astro fettling last weekend, my primary goals were to flock the dew shield of my 120mm f8 achro and to do something about the red diagonal I have which really doesn't match the scope.

The diagonal turned out well, a bit of adhesive vinyl and a black paint pen have transformed the exterior whilst the inside got the gray foam side pads replaced with flock. It appears much darker to look through now, hopefully a bit of extra contrast?

Whilst removing the dew shield to flock though, I soon became aware of a rattly objective. I know this is no major cause for concern normally, however it sounded too loose for my liking. I fashioned up a tool to loosen the objective lens retaining ring, which oddly was very tight. After standing the tube on the focuser end and doing the tube tap dance, I gently nipped the ring back down to snug up the lenses.

All week though I've been concerned that the scope may be out of collimation so did a bit of reading up. It turns out the scope was supplied with a collimation eyepiece which I had kind of disregarded thinking it was a standard accessories pack and the collimation ep was likely meant for a newt. Up the attic I went to recover said ep.

Having performed a preliminary inspection with the collimation ep I discovered quite a bit of mis-collimation. Two donuts were visible as expected (reflections of the lens surfaces) however they were only about 50% overlapped, i.e. far from concentric. Fortunately the OMNI XLT 120 had a collimatable lens cell so I took the plunge and got the tools out, only to discover more "looseness" in all but one of the collimation screws. 

Interesting that previous start testing returned reasonable results, though very soft rings I thought. This meant I had to test very close to focus, perhaps this hid the mis-collimation as racking further in or out just produced colourful blobs.

An hour or so of fettling, including focuser alignment and I seem to have a nicely collimated scope now! Initial daytime observations seem to indicate a fair reduction in CA which I always though was a bit high in this scope. It may also go some way to explaining the poor performance on Mars recently.

Of course a repeat star test is now required, but the forecast suggests I won't be doing that any time soon! Really looking forward to and hoping for some improvements during the next session!

I've added a photo of the view through the collimation ep after adjusting but it's hard to make out the pattern. Suffice to say it was nowhere near round before!

Edited October 24, 2020 by parallaxerr

[John]

Good stuff Jon !

Many of the lower cost refractors that I've owned have required colimation to some degree. I use a 2 stage process: focuser optical axis first (laser colimator for this) and once I'm happy with that, the tilt of the objective with the cheshire as you have done.

Hopefully you will see some benefits in the views in due course

 

 

[parallaxerr]

4 minutes ago, John said:

Good stuff Jon !

Many of the lower cost refractors that I've owned have required colimation to some degree. I use a 2 stage process: focuser optical axis first (laser colimator for this) and once I'm happy with that, the tilt of the objective with the cheshire as you have done.

Hopefully you will see some benefits in the views in due course

 

 

I'm just glad I got the Celestron with the collimatable cell! Had it been a SW I guess I'd have been stuck.

Will invest in a laser too I think. I did my best to align the focuser with the collimation ep, but there wasn't quite enough adjustment in the 3 screws to get it bang through a full rotation, but it's only a smidge out with the focuser at 180°. Having said that I rarely rotate the focuser any more as I mount the scope lower and the collimation is spot on with the focuser upright. 

I also checked with the diagonal in and there was no change, so hopefully I've achieved alignment throughout the full optical train.

[John]

You can tilt the focuser and objective cell by shimming with tape but having proper collimation adjustment makes life easier.

 

[parallaxerr]

Happy to report that tonight's first light with the Omni 120 (since collimating) was a great success!

Much better snap to focus, scope took higher mag, sharper image.....allround better EVERYWHERE! Star test showed better results outside of focus where it was mushy before and X400 on a bright star showed a very nice airy disc.

I tested out my new 27mm eyepiece on Andromeda and despite the bright sky due to the Moon rising, I could definitely make out more than previous attempts.

Star of the show was Mars. The scope held X200 rock solid for over an hour before dew started to hamper proceedings but in that time I observed lots of surface albedo, the southern polar cap, despite being small and all with good colour.

Super happy with the result of collimating the frac, it's really shown what it's capable of now.