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Monday 7th Jan: My new T2 fitting has arrived from Telescope-Express. Though assembling it into something useful took some ingenuity. The first image shows the procession of parts in order of fitting. The two halves of the PST filter barrel had to be separated and screwed back together umpteen times! First I had to turn down the PST eyepiece socket to get it to fit inside the cheap, helical focuser. I sawed off 8mm and then reduced the diameter of what was left. Until the parts would just slide together with the helical focuser contracted to its minimum length. It was fine when expanded.
This machining was not remotely my intention when I ordered the new part. I wanted it to be easily put together by anyone using the PST filtration system on a donor telescope.Though anyone copying my set-up could simply saw off the eyepiece socket and file it smooth. Thereby saving themselves the lathe work. A ring of the original diameter should be left to center and locate the barrel in place. Great care should be exercised to avoid damaging the hideously expensive but tiny, PST filter optics!
The DSLR-T2 fitting TS-Optics, T2-D42 was meant to grip the PST's 36mm Ø filter barrel. Despite being in the center of the expected range of 30-42mm it was not nearly as positive as I had expected from studying the T-E website images. Being considerably oversized even for the 36mm barrel. It is sold as a digiscoping adapter.
I had to use the thickest rubber packing strip supplied with the fitting to get a firm enough grip on the 36mm diameter. The film of adhesive on the rubber strips was worthless and soon fell off. Not that it mattered much once the pressure was applied by the four screws on the compression band.The thickness of the rubber strip can be seen in the image alongside. The 36mm barrel diameter did not want to go in and when it was in and fully tightened it wanted to fall out! Each time I tried I had to split the two halves of the PST filter barrel [by unscrewing] to get it to go into the rubber ring.
Originally I had no focusing system on my H-alpha telescope. Which meant using the barrel and a bored out 2"-1.25" adapter as a crude push-pull, drawtube arrangement. Which made for a rather sloppy set-up when loaded with the binoviewer and two low power eyepieces on the end of further items. I could lift the binoviewer by at least half an inch [12mm] even though everything was strongly tightened up with maximum finger strength.
This image shows the modified, PST filter barrel now trapped between a 2":T2 adapter and the cheap, helical focuser. There being no real benefit from using the external, eyepiece clamping device as shown above. I just couldn't get it to hold the barrel firmly enough!
Anyone thinking of hanging a DSLR body from one of these clamping rings should take extreme care and use a very short safety lanyard to catch the camera body when it falls off the eyepiece! Which it probably will do IMHO! I certainly wouldn't trust it myself! Once the camera falls off, the telescope goes very light at one end. Then what?
I can well understand the need to protect the cosmetic finish of expensive eyepieces when a DSLR is used to take images through a complete telescope. The problem is that the variation in diameter for the two models of compression ring is simply far too large. After considerable thought I could rescue the situation with solid jaws instead of loose, rubber strips.
My PST filter barrel is certainly no longer in any need of cosmetic protection. So a more solid grip is indicated if I am still going to use it as intended. The TS fitting has a plastic coated, copper compression band. The problem is that the band is lifted right out of its groove as the four thumbscrews are tightened. Once that happens there is no longer any lateral location except by friction.
I am still trying to think of a suitable firmer material to form solid jaws inside the digiscoping adapter ring. Solid copper strip? Perhaps the answer is to increase the diameter of the barrel itself? Then the compression ring can do its job without leaving its groove. No compliant sandwich material required. I just need to turn a tube with a 36mm bore and 42mm exterior. The flanges on the end of the PST barrel will prevent longitudinal movement.
This adapter is probably just complete waste of time and money! Despite the expense and all the hours spent trying to make it work, I have decided not to use it. The modified PST filter barrel is now firmly secured without it. Though it took some slow and careful machining to make it fit inside the helical focuser. I was afraid of damaging the optical elements.
The PST barrel certainly won't fit the helical focuser, as standard, because of the PST's eyepiece socket's, length and large, external diameter. It is this socket which must be reduced in diameter to fit inside the threaded portion of the helical focuser. I could have left the socket longer than I did. Provided, I left a lip to grip with the components on either side. Greater length might have provided better alignment of the axis of the filter barrel.
I'll post some daylight images tomorrow showing my final[?] set-up. We shan't see any sun until at least Wednesday or even Thursday. With another windy couple of days forecast. So I'll have to wait to check the precise component spacing and solar, H-a, image quality.
The last image shows the inside of the CR150 tailpiece with new packing disk of 10mm thick aluminium. The cantilevered weight of the H-a components and binoviewer was dragging the whole lot out of the tailpiece despite its very tight, press fit.
Now the tailpiece is almost solid metal and the visible focuser disk can't be levered out any more. The three screws pass right through three thick rings of aluminium. I shall fit Nyloc nuts and paint the internal disk matt black when conditions allow.
In retrospect I could have machined a complete new tailpiece from solid metal. Though it would have taken many hours on my modest, vintage, S&B lathe.
This image shows the modified, PST filter barrel now trapped between a 2":T2 adapter and the cheap, helical focuser. There being no real benefit from using the external, eyepiece clamping device as shown above. I just couldn't get it to hold the barrel firmly enough! Anyone thinking of hanging a DSLR body from one of these clamping rings should take extreme care and use a very short safety lanyard to catch the camera body when it falls off the eyepiece! Which it probably will do IMHO! I certainly wouldn't trust it myself! Once the camera falls off, the telescope goes very light at one end. Then what?
I can well understand the need to protect the cosmetic finish of expensive eyepieces when a DSLR is used to take images through a complete telescope. The problem is that the variation in diameter for the two models of compression ring is simply far too large. After considerable thought I could rescue the situation with solid jaws instead of loose, rubber strips.
My PST filter barrel is certainly no longer in any need of cosmetic protection. So a more solid grip is indicated if I am still going to use it as intended. The TS fitting has a plastic coated, copper compression band. The problem is that the band is lifted right out of its groove as the four thumbscrews are tightened. Once that happens there is no longer any lateral location except by friction.
I am still trying to think of a suitable firmer material to form solid jaws inside the digiscoping adapter ring. Solid copper strip? Perhaps the answer is to increase the diameter of the barrel itself? Then the compression ring can do its job without leaving its groove. No compliant sandwich material required. I just need to turn a tube with a 36mm bore and 42mm exterior. The flanges on the end of the PST barrel will prevent longitudinal movement.
This adapter is probably just complete waste of time and money! Despite the expense and all the hours spent trying to make it work, I have decided not to use it. The modified PST filter barrel is now firmly secured without it. Though it took some slow and careful machining to make it fit inside the helical focuser. I was afraid of damaging the optical elements.
The PST barrel certainly won't fit the helical focuser, as standard, because of the PST's eyepiece socket's, length and large, external diameter. It is this socket which must be reduced in diameter to fit inside the threaded portion of the helical focuser. I could have left the socket longer than I did. Provided, I left a lip to grip with the components on either side. Greater length might have provided better alignment of the axis of the filter barrel.
I'll post some daylight images tomorrow showing my final[?] set-up. We shan't see any sun until at least Wednesday or even Thursday. With another windy couple of days forecast. So I'll have to wait to check the precise component spacing and solar, H-a, image quality.
The last image shows the inside of the CR150 tailpiece with new packing disk of 10mm thick aluminium. The cantilevered weight of the H-a components and binoviewer was dragging the whole lot out of the tailpiece despite its very tight, press fit.Now the tailpiece is almost solid metal and the visible focuser disk can't be levered out any more. The three screws pass right through three thick rings of aluminium. I shall fit Nyloc nuts and paint the internal disk matt black when conditions allow.
In retrospect I could have machined a complete new tailpiece from solid metal. Though it would have taken many hours on my modest, vintage, S&B lathe.
Click on any image for an enlargement.
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