Another possible PST mod

My PST has been repeatedly modded and now looks like this:


(though now with the addition of a motor to the focuser.)

I have been pondering of late however on the amount of vignetting that occurs when imaging and what might be done about it.

Looking up the maximum angular diameter of the Sun, I calculate that the maximum size of the solar disc on the camera sensor with this scope as about 3.8mm.  Say 4mm to allow for proms.  The BF5 blocking filter is about 45mm in front of the camera sensor in the optical train.  I calculate that in order to achieve an image with no vignetting at that point, the actual diameter of the filter would need to be about 8.05mm.

So, I’m thinking about possibly taking the eyepiece holder apart and making up an adapter with an M42 (T2) external thread to fit a camera that can hold the lens very close to the camera sensor.  The lower section of the eyepiece holder with the mini-ERF would then have to fit to the front of that.  I’d also need to extend the optical train in front of the ERF by the same amount as I’d removed by placing the filter and ERF so close to the eyepiece.

My calculations suggest that a 5mm filter needs to be at most 11mm from the sensor to give a view without vignetting.  I’m told that it may also be possible to open the filter out to 6mm, in which case I can probably go to 22mm from the sensor.

First however I need to defeat all the threadlock that’s liberally splashed over the inside of the eyepiece holder so I can take it apart and get all the filters out.

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