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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?

I got brave this evening and disassembled the eyepiece holder to the point I’d reached as described a few days ago. I then examined the BF5 housing and decided that it appeared to fit into the eyepiece holder from the top. This seemed counter-intuitive given the design of the lower half of the holder which appears to be intended to trap the housing against the upper half. Anyhow, as there was no obvious means to gain purchase on the housing to turn it, I thought I’d give it a bit of a push up through the holder to see if anything happened.

Somewhat unbelievably given that just about everything else that comes apart on the PST is smothered in threadlock, it gave quite easily and I was able to push the housing out of the top of the eyepiece holder, leaving me with this:

There’s a flange on the housing that rests on top of a shoulder inside the eyepiece holder. Interestingly, the flange is quite a snug fit in the bottom of the holder whereas it’s very loose at the top, so the inside of the holder must be tapered.

So, my plan is now to turn a retaining ring with a T2 thread on the outside and either a 1,.25″ nosepiece or a thread to fit the lower half of the eyepiece holder on the inside, with a shoulder to hold the entire BF5 unit central in the ASI120 thus:

The measurements I’ve taken from the camera and BF5 unit are:

ASI120 sensor to flange: 12.5mm
ASI120 flange to case: 7.5mm
ASI120 sensor to case: 5mm
BF5 housing depth: 8.5mm
BF5 retaining ring thickness: 2mm

This would give a “sensor to front of BF5” distance of 11.5mm. That’s 0.4mm more than I think I need to get an image with no vignetting, but perhaps worth a go nonetheless. If I drill the retaining ring out to 5.5mm then it will hopefully mean that any vignetting occurs at the edges of the sensor and doesn’t affect the actual image of the Sun.

Following on from my thoughts that I might have to move the blocking filter on my modded PST closer to the camera sensor, I decided to take it apart to see what I might be dealing with.  Though I’ve got rid of the “black box” on mine, the holder is still exactly as it came off:

First step was to split the barrel into its two component parts.  Easier said than done.  Everything is welded together with thread-lock and it just didn’t want to budge.  By coincidence I had some large soft rubber tree ties arrive this afternoon and by wrapping one around each half of the barrel I was eventually able to get them to turn without having to resort to tools.  This is the inside of the bottom half.  Despite the fact that it cannot have been apart since it was assembled at Meade’s factory, the inner face of the ITF was disgustingly dusty.  So bad, in fact, that I think it’s quite clear even in this photo.

And the top half, with the retaining ring for the BF5 blocking filter visible:

I unscrewed the retaining ring to find that the filter is actually a block of material rather than a sheet as I was expecting:

It’s loose, just sitting in a slight circular depression inside the body:

Apologies for the quality of these next few pictures.  I forgot how poor the digital macro is on the camera I was using.  These show three views of the blocking filter.  You can see how raw the back (camera side, rather than objective side) edges of the filter are 🙁

There were some large dust particles on the gold face of the filter, too.  I took the opportunity to clean the faces, but I think perhaps it’s partially scratched (again, how?  It’s never been opened since it left the factory).

It seemed to make sense to have a go at cleaning the ITF too, so I unscrewed the ITF body from the bottom section of the eyepiece holder:

It’s threaded, but also held in with some nasty rubbery adhesive as well.  The “inside” of the ITF unit has a retaining ring in, so I also removed that and dropped out the ITF itself:

Another poor photo, but there’s some writing on the side:

The ITF is “outside up” in this photo.  The upper face goes towards the objective.  I gave the ITF a light clean with a microfibre cloth and left things there.

I estimate that the blocking filter is about 4mm thick.  I measured across the sides with vernier calipers and they were 6mm, but I wasn’t going to touch either optical face.  Allowing 2mm for the retaining ring and another 2mm for the housing, that doesn’t leave me with much to play with if I want to have the filter no more than 11.1mm from the sensor to avoid vignetting.  I’m going to have to give this a fair bit of thought, I think.  Worth it, given that the alternative is probably $500-worth of BF-10 diagonal 🙁