Shoddy PST build quality :(

[JamesF]

At least, that's how it looks to me at the moment...

I've been working on a PST mod today (more of that in a separate post) that involves completely dismantling the eyepiece and then remantling it slightly differently.  After machining up the part I wanted I was doing a final test for fit when I noticed something odd about the body of the BF5 filter unit.  Here it is (apologies for the rubbish picture.  I wanted to get it close up and the macro mode is pants on my compact camera)

The filter itself sits in that recess in the middle and is held in place by a retaining ring with a hole in the centre the same size (5mm diameter) as the one in the part pictured.

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.

I shall have to take my other eyepiece holder apart and see how that looks.  Either way I think I'm going to have to invest in some milling tools and a suitable slide for the lathe, or a BF10, or another used PST (or at least the eyepiece holder thereof).  I shan't be keen on the latter unless it's very cheap as there's no way to check that it isn't the same as mine without taking apart bits that are threadlocked together in the factory

James

[moriniboy]

Ouch not good news.

Have you checked the pentaprism yet, my original (Mexican) one had a fracture in the corner?

[JamesF]

Fortunately I have done away with the pentaprism (and in fact the entire "black box" bit) by fitting the tube to a proper focuser and screwing the eyepiece holder into the top of a standard diagonal.

James

[JamesF]

I've just finished taking apart my second eyepiece holder.  I'm not sure it's perfect, but it's considerably better than the first.  I also discovered that the BF5 itself has all its edges cut beautifully square, unlike the first where the back edges of the BF5 show a lot of chipping:

No wonder PSTs have a reputation for being highly variable in quality.

James