The constant rain this weekend meant indoor activity was called for, so I started work on my new PST mod to move the BF5 filter closer to the camera in an attempt to reduce or perhaps even completely remove the vignetting that’s often a problem when imaging with this scope. More detail on that in another post, but in simple terms my plan was to remove the BF5 unit entire and make up a spacer to trap it against the casing of my ASI120MM when the 1.25″ nosepiece is screwed in.
So I turned up the spacer from a piece of aluminium and when testing it for final fit, noticed something rather odd about the BF5 housing. Here’s a picture of it. Apologies for the poor quality. My compact camera isn’t great at these sorts of photos.
The design of the PST is such that the filter is barely big enough for the part of the light cone corresponding to the image of the Sun to fit through that 5mm hole and comments about vignetting when attempting to image with the PST are common. It’s really very tight, which means the construction needs to be quite accurate for it to work. But look again at the picture. Notice that the inner section is not actually concentric with the outside? The hole is central and the filter will sit central, but the retaining ring won’t be. Which means the two holes won’t line up. Measuring the difference I reckon that the hole in the retaining ring ends up about 1/6th of its width off the optical axis. And indeed when I reassemble the two parts without the BF5 in place it’s quite obvious that they don’t line up. I can’t see any way that isn’t going to have a noticeable effect on the image.
Fortunately I have a second unit (I use one with a diagonal for visual, and one “straight through” for imaging). When I took that apart it was nowhere near as bad, though hardly perfect by the standards of industrial production. I also noticed quite a significant difference with the BF5 itself. The second one was very cleanly cut with nice square edges all around. The first one looks like this:
All the “back” edges of the filter are damaged.
The worst thing about this is that unless you take apart all sorts of bits that Meade/Coronado never meant you to (because it’s all glued together with threadlock and takes some work to get apart), you can never find out these problems exist. How many people are not getting the performance they should from their PST because of problems they’re totally unable to diagnose, I wonder?