Near the close of the comfortable observing season a few years ago, just before it got too cold for me, I had to admit something dreadful about my 6 inch f/5 Newtonian. I’ve always considered it, and the similar Celestron Omni XLT 150mm, to be nearly the perfect telescope design. Compact, simple, relatively inexpensive, and good for a wide range of observing.
And while I love my rich field telescope, especially after some modifications I made to it, I none-the-less had to admit that the high magnification images it presented were often sub par.
I particularly noticed this when I did side by side comparisons between it and my ETX 90 Maksutov. I used high resolution targets like Jupiter and Saturn as comparison objects. I was disappointed that the ETX, with only 3.5 inch aperture, would give as good or better images as the larger 6 inch reflector. That wasn’t what I was expecting.
Did I need different eyepieces, more expensive ones that would work better with the short focal ratio of the telescope? Did I need a new objective mirror? Did I need a new and better Barlow lens? Did I simply need to accept poor images at high power because the telescope was a rich field, and resort to only using the instrument for general star gazing? I decided to get to the bottom of the problem, hoping that some kind of affordable accessory purchase would help me improve the performance.
With some critical observing on some close double stars, I observed what appeared to be astigmatism. I did the usual tests, rotating the eyepiece to see if the astigmatic image rotated with the eyepiece — it didn’t. I used my ETX on the same objects to see if I saw the same problem, suggesting it was my eyes — it wasn’t. This left me with the conclusion that my Newtonian’s primary mirror may have astigmatism.
A bit of reading told me that it was possible that I still had an alignment problem, though I’d tried very hard to get the collimation right.
I’d used a homemade Cheshire eyepiece to help me get it right, following the procedure on the Reflector Telescope Collimation webpage. As the animation above shows, I tried to adjust the secondary to put the dot I’d painted on the center of my primary, right on the cross hairs. Then I aligned the primary to get the reflected cross hair coincident with the centered primary dot.
Even so, I wanted to be certain about the alignment, because it had been a difficult procedure due to the short focus of the telescope. Buying a new primary mirror would have been expensive, and I didn’t want to do that on incomplete evidence. So for much less than the cost of a new primary, I bought a Celestron Collimation Eyepiece to more precisely adjust my telescope’s collimation.
For alignment, it’s the best thing that I’ve ever done. I found, using the Celestron device, that I’d been more often than not getting the secondary alignment wrong. My homemade Cheshire device was too short to reveal the error. The poor secondary alignment was causing the apparent astigmatism. Now, with the better alignment, the apparent astigmatism was gone, and views were crisp. I checked the system critically on some close doubles, and things looked nearly textbook.
Above is an image that provides proof of properly operating optics. It is an image of the lunar crater Tycho taken through the same f/5 Newtonian that had been giving so much collimation aggravation. It was taken with a Homemade Web Cam Astrocamera looking through the aforementioned 6 inch f/5 rich field telescope. The image is a stack of around 20 frames, and shows clearly that the images produced by the 6 inch, when properly collimated, are tack sharp.
The bottom line is, if you have a Newtonian reflector — especially a short focus one — don’t do as I did and just assume that you’ve got the knack for optical alignment. Collimation of short focus Newtonians isn’t necessarily all that straight forward. It’s not that expensive to get a Collimation Eyepiece, or for a bit more money a laser collimator, like the Orion LaserMate Deluxe II Telescope Laser Collimator. You, like me, may be surprised that your skills aren’t as good as you thought. I figure that over the years, probably 80% of the time my reflectors have been misaligned enough to cause some degradation of performance.
So don’t agonize over or tolerate poor telescope performance. Get your collimation eyepiece in hand, go to the Newtonian Collimation Tutorial, review the material, and get your Newtonian performing like it should.