And while I love my Discovery telescope, especially after some modifications I made to it, I none-the-less had to admit that the 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.
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 was my eyes -- it wasn't. This left me with the conclusion that my Newtonian's primary mirror may have astigmatism.
Best thing 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 astigmatism was gone, and views were crisp. I checked the system critically on some close doubles, and things looked nearly textbook.
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. 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.