Coronado PST BF5 positioning mod

A few days ago I posted about the problems with vignetting when imaging with a PST and decided that for me a solution might be to move the BF5 unit as close to the camera sensor as possible so that the light code is as small as possible when it reaches the filter. To that ends I completely stripped apart the eyepiece holder and pushed the BF5 unit out of the eyepiece:


(In fact, this is one unit assembled from the best parts of two as some of the parts were very poorly-made as I’ve also already posted about.)

I stripped everything off the front of my ASI120MM so that the filter would be able to fit right in front of the sensor:


The filter unit rests on the front of the camera body nicely, but needs to be kept central, so I turned an aluminium spacer to fit over the top of the filter body, leaving the flange on the back resting against the spacer:



The outer diameter of the spacer was made to fit snugly inside the back of the T2 to 1.25″ nosepiece that goes on the front of the camera:


So now, when the nosepiece is screwed on the filter is trapped centrally in front of the sensor between the nosepiece and the camera body:


The camera now goes into the top of the reassembled eyepiece holder just as it used to, but only the ITF is present now.

I hope this should massively reduce the vignetting effects for me. I haven’t yet had the courage to drill the retaining ring out to 5.5 or even 6mm, but that may come if I’m not satisfied with the result. Unfortunately it looks like there’s not going to be much chance to test it in the near future if the weather forecast is to be believed 🙁

I did consider making a more complex spacer that would incorporate a threaded section allowing the camera to be fitted directly to the bottom half of the eyepiece holder (that has the ITF), but that would reduce the length of the optical path by up to 30mm and I don’t have that much to play with at the focuser. It would also have the effect of moving the ITF further back in the optical train when the camera was in focus. I’m not sure it’s a good idea to do that as it would concentrate more energy on a smaller area of the ITF.

What has since occurred to me is that I could probably do the same mod for my Atik 314L+. I know of at least one person who uses this camera for solar imaging with a PST and it does have the advantage of fitting the entire disc in a single frame, so I think I shall give that a try too.

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