My latest PST (BF5 positioning) mod

[JamesF]

I've been "thinking out loud" about the problems with vignetting when imaging with a PST and decided that for me the solution was 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.)

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.

James

[David Smith]

Nice work James. That lathe coming in handy, must be a real boon being able to knock-up a custom part for a project. Will be interesting to see the results when you next see the Sun.

[JamesF]

It's certainly handy, though I'm very much a beginner when it comes to using a lathe.  Working out the right order to perform all the different cutting operations to make anything of a complex design is not something that is yet coming naturally and I really need to get my head around making up my own cutting tools.

It occurred to me late last night that I could potentially make a full T2 to 1.25" adaptor to hold the BF5 and to fit the 314L+ and give that a whirl for solar imaging.  The advantage would be a sensor that should give me a full frame image.  The downside is the slower imaging speed.  But then capturing all the frames for a mosaic isn't that fast either, so maybe it wouldn't be an issue.

James

[lukebl]

Very nice work. If it's successful, then it's clearly hugely cheaper than the outlay on a BF10 or 15. Look forward to hearing about the results.

[JamesF]

Having a symmetric filter body that isn't chopping a chunk out of the light cone and a filter that isn't chipped will be a good start.  I'm quite miffed about that -- not with anyone who I've bought stuff off, but with the incredibly poor QA at Meade.  Makes me wonder how many other people are imaging using the PST and not really getting the results they're after perhaps because of a problem you can only find if you completely dismantle the telescope.

James

[Merlin66]

James,

An interesting mini-mod. Please remember that the original PST was not designed as an imaging system, it was visual only. Always tread cautiously when working with any solar filter system, the arrangement of the various elements - the etalon, the ITF (interference transmission filter -"mini erf") and the final blocking filter work as a package.

[JamesF]

I don't intend to use the scope without both the ITF and BF5 in place certainly, and I understand that the BF5 is there for a reason.  The only effective change should be that there's a greater separation between the two.  I don't believe the spacing is critical -- i've not been able to find any information suggesting that it is.

Spacing is in fact one of the reasons I haven't made a fitting to put the camera (plus BF5) straight onto the thread on the lower section of the eyepiece holder which I had considered doing at one point.  If I were to do that I think I'd end up having the ITF at a narrower part of the light cone when the camera was in focus, meaning more energy being concentrated on a smaller area of the ITF.  That doesn't strike me as a good idea at all.

James