PST ponderings

[JamesF]

Some will know I have a fairly heavily hacked about PST.  One of the things I'm not really happy about from an imaging point of view is the vignetting.  Mostly I think that's down to the size of the blocking filter and the distance from the camera with my current setup.  I believe the light cone is just too wide at the point where it passes through that filter.

I have been wondering whether it might be possible to split the two halves of the eyepiece holder and somehow remount the blocking filter in an adapter with an M42 thread on the outside, so the filter could be placed only a few millimetres in front of the camera sensor.  The remainder of the eyepiece holder (with the mini-ERF or whatever you wish to call it) could then screw into the front of that and I'd make an extension to go in front of that to make up the "lost" spacing in the optical train.

I'm not sure if the eyepiece holder will come apart into the right sorts of pieces for that to work and mine is going to need rather more than a couple of baby boas to split it by the feel of things, so before I make the effort does it even sound like a reasonable thing to attempt?

James

[Merlin66]

Hmmm

A possible "mod", but you may have to separate the two sections of the eyepiece holder, then remove the blocking filter from the top section and find ways and means of re-mounting it into a suitable holder VERY close to the chip.

Which camera are we considering here?

[JamesF]

Probably my ASI120MM, but I'd not discount attempting to use a 1/2" sensor at some point.

A rough measurement with my vernier calipers suggests that I could probably get the filter to within 5mm of the sensor using this method.  At the moment the calipers suggest that the sensor is about 40mm behind the filter.

I suspect that at 40mm from the sensor the blocking filter is not large enough to accommodate the entire light cone.  At 5mm from the ASI120MM sensor, I think the sensor size will be the problem not the size of the filter.

James

[JamesF]

Actually, having looked at it again I'd say the sensor is currently at least 45mm behind the filter, perhaps closer to 50mm.  I could reduce that by using a QHY5L-II, but even then the spacing would still be between 15mm and 20mm.

James

[JamesF]

Whilst I think about it, does the blocking filter holder screw into the top section of the eyepiece holder?  If so it may not be a problem to make an adapter with an M42 thread for the camera on the outside, and a thread to match the filter holder on the inside.  If it's just held in with loctite or something else horrible like that it may well be more tricky.

James

[Merlin66]

Yeah, there's usually the overload of "Mexican" Loctite on the joint......

The blocking filter is held in place with a locking ring - you'll see that more clearly when the parts are separated.....

[JamesF]

Time to get "inventive" then.  I threw away a rubber tree tie at the weekend.  That would probably do a good job of gripping the holder.  I'll have to see if it's found it's way to the bin-men yet.  I could probably drill another hole in the top of the holder and put a bar through that and the existing eyepiece retaining screwhole, too.  That might give enough purchase to loosen it.

James

[Peter Drew]

On blocking filters that I have dismantled the retaining ring was secured by a black silicone type material. The 5mm  blocking filter in the PST is actually square and does not have its own cell, it is just trapped between the retaining ring and the main body. 

[keithatrochdale]

And it is possible to open the 5mm hole to 6mm if you are careful.

[moriniboy]

BF5 photo if this helps

[JamesF]

Thank you.  It's certainly food for thought.

I was doing some calculations earlier.  The largest angular diameter of the Sun appears to be 32.7 degrees, which gives an image diameter of about 3.8mm on the camera sensor.  Call it 4mm to fit in proms and so on.  If the blocking filter is 45mm in front of the camera sensor I estimate it needs to be at least 8.05mm diameter for a view that is not vignetted.  At 50mm I reckon it needs to be 8.5mm.

Assuming those figures are close to correct, I'd say the blocking filter needs to be within 11mm of the sensor for an view that is not vignetted, or within 22mm if the filter could be opened out to 6mm.  Without modification of some sort I reckon even a QHY5L-II that can slide down inside the eyepiece holder is not going to work out perfectly because the sensor is quite some distance back from the front edge of the housing.  Assuming those figures are correct is perhaps not that safe a step to take though

I reckon 10mm would easily be achievable with a custom filter fitting for the ASI120 though.  If the threads are the right size it might even fit in the existing hole for the optical window.  I really am going to have to take the eyepiece holder apart

James

[Zakalwe]

Actually, having looked at it again I'd say the sensor is currently at least 45mm behind the filter, perhaps closer to 50mm.  I could reduce that by using a QHY5L-II, but even then the spacing would still be between 15mm and 20mm.

James

The front (the ring holding the IR block) of the QHY5L-II screws off. I junked mine as I needed to get the camera in closer to the body of my OAG. That could save a good few mm.

[JamesF]

The front (the ring holding the IR block) of the QHY5L-II screws off. I junked mine as I needed to get the camera in closer to the body of my OAG. That could save a good few mm.

I'll look into that.  I had a feeling it might, but I didn't want to attempt it without behind sure bits wouldn't fall out

James

[JamesF]

And I've just managed to unscrew the two halves of my PST eyepiece holder

The baby boa wasn't going to do it, but as luck would have it an order of 45cm soft rubber tree ties arrived today.  Wrapping one of those around each half of the holder gave me enough purchase to break the stupendous amount of threadlock.

I'm slightly annoyed to discover once opened that the inner faces of both the ITF and BF5 filter are far from clean.  There's a lot of fine dust on the inside face of the ITF, and some large particles on the inside face of the BF5, including one large one bang in the middle

James

[Zakalwe]

I'll look into that.  I had a feeling it might, but I didn't want to attempt it without behind sure bits wouldn't fall out

James

The sensor is mounted on a small daughterboard, which is screwed to the ally body of the camera. The body of the camera touches the sensor at the back and acts like a heatsink to draw heat out of the sensor. It's quite a clever design and there's no chance of anything falling out or coming loose.

[JamesF]

The sensor is mounted on a small daughterboard, which is screwed to the ally body of the camera. The body of the camera touches the sensor at the back and acts like a heatsink to draw heat out of the sensor. It's quite a clever design and there's no chance of anything falling out or coming loose.

That's rather neat.  It was always a concern for me that the camera didn't really seem to have a way to shed heat from the sensor, but in some respects that's better than the ASI120.

James

[Zakalwe]

That's rather neat.  It was always a concern for me that the camera didn't really seem to have a way to shed heat from the sensor, but in some respects that's better than the ASI120.

James

Yeah, it's a clever design. The camera has an internal "bulkhead" with a raised dimple. The daughterboard that the sensor is mounted on has a hold in it to allow the dimple to touch the back of the sensor. So the ally body of the camera acts as a heatsink.

[JamesF]

Well, that was unexpected!

I didn't really internalise the fact from moriniboy's photo though in retrospect it's obvious.  I've just abused a pair of dividers by using them as a pin spanner (I will make a proper adjustable pin spanner one day, honest) to remove the filter retaining ring.  I was then surprised to find that the BF5 filter itself is a block made of what appears to be two completely different materials cemented together.

I've measured it with the vernier calipers and it looks to be exactly 6mm across.  I don't think I'd dare open the holes out to 6mm.  Even 5.5mm might be sailing a little close to the wind unless I could do it in the lathe.

I assume the housing for the lens is superglued into the upper half of the eyepiece holder.  Certainly looks like it, though there is also a lip inside the lower half that I assume is also to hold it into the upper half.  Could just be that the threadlock has got into places it shouldn't have.  I think the best bet for moving the filter closer to the camera sensor would be to remove that housing so it can all be used as a unit.  It does mean though that it's going to have to be exceptionally close to the sensor.  I'd estimate that the filter is about 4mm thick.  With about 2mm either side for the housing and retaining ring, that doesn't leave much of my 11.1mm  On the other hand, it will be considerably better than it was.

James

[moriniboy]

I just drilled it out, first to 5.5mm then 6.0mm.

Side by side comparison with std version

[JamesF]

I'm amazed you don't get any "leakage" or diffraction effects around the edge of the filter.  Perhaps they're not all quite the same size.  Mine is definitely spot on 6mm across though, and in fact on the back edge there's a small amount of damage to the filter.  I'll see if I can get a picture.

James

[moriniboy]

There was a bit of leakage so I just built up the edges of the BF with some Araldite and painted it black.

I would stick to 5.5mm just to be sure if you don't fancy that.

By the way I now have a BF10 diagonal on my PST mod scope as I found even the BF6 did not provide enough aperture.

[JamesF]

Somehow, fixing it with Araldite really doesn't appeal

I don't currently have a diagonal at all on my PST imaging setup, but one day I think that will have to change.  Anyhow, here are the photos of my BF5.  Apologies for the quality.  I forgot how utterly pants the digital macro is on this camera

Despite the quality of the photos I think it's quite clear how raw the back edges of the filter are.  Looks like someone cut it out with a chisel

James

[moriniboy]

I did not mind altering that BF as it was part of a PST (minus etalon) I got for £20 of eBay so if it messed up I would still have the original BF.

Stick to 5.5mm and you will have no issues I would say.

If you want to try the BF6 out I will post it to you, just pm me.

[sungazer]

Hi All. I drilled mine out to 5.5mm so as to leave a safe margin, this helps , I find with a slightly larger field of view, plus I have fitted a new ITF filter which brings the PST to as good, or even better, than when new. John.

[JamesF]

If you want to try the BF6 out I will post it to you, just pm me.

That's very kind of you, thank you.  I'll see how I get on with things and get back to you if that looks like it might be helpful.

James