Big Bertha Is So Ugly She Needs a Mask

Or at least the mirror does. I put on a mask that covers the outer 1" of the mirror--reducing it to a 15.5" reflector--and it made a noticeable improvement. At 222x, Saturn was now sharp--although Cassini's Division was still not the inky blank it should be. I can't claim that I could see any more detail on Saturn and in the rings than I can with my 8" f/7 reflector or my 5" f/9 refractor--and even the image scale isn't dramatically larger.

Adding the aperture mask to the front of the scope gave me a bit more detail at higher power--but 285x was about as high as it would go, before fuzzing away. The problem is no longer a classic turned edge, unless you use "edge" in a rather warped sense of the word.

Using Procyon for the star test now reveals what would appear to be an undercorrected mirror. The combination a turned edge (essentially, a form of extreme localized overcorrection) and an undercorrected rest of the mirror, unsurprisingly, produces a less than impressive image. I suppose that I can make a mirror mask that takes it down to 12", and see if it makes much of a difference.

It might make sense to have the mirror refigured, but I must confess that I am losing my enthusiasm for big Dobsonians for planetary work. Even with a really excellent mirror (and there are some big Dobsonians that truly have excellent mirrors), the lack of tracking capability (without adding one of the platform trackers) means that at high power, you are constantly repositioning the scope to track an object. A big Dobsonian is well suited to deep sky objects, where high magnification is neither necessary nor desirable, and where a less than perfect mirror is usually not a problem. I suspect that I will use it for that purpose primarily--and I will say, M42 at 160x through Big Bertha is quite breathtaking. I will be curious to see what M51 (the Whirlpool Galaxy) looks like.