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JamesF

PST stage 0.5 mod

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Not wishing to stump up the price of a Tal 100RS just yet, but wanting to make life somewhat easier for imaging and not being entirely impressed with the prism focuser I cast about for other ideas on how the PST could be made easier to use. One that seemed to come up a few times was to replace the "black box" with an SCT focuser and that seemed like it would be worth a try. The obvious problem is that the SCT focusers use a 2" 24 tpi thread and the etalon of the PST has an M50x1 thread. I decided I'd sort that out once I'd got all the bits together.

Finding a reasonably-priced SCT focuser proved tricky as all the Skywatcher and Revelation ones appear to be sold out, but I was offered a Revelation one second hand and was pleased to give it a new purpose:

revsct.jpg

When it arrived I found that the etalon couldn't really be made to fit the SCT thread and by bodging the two together with an unpleasantly large amount of PTFE tape that the focus point was still sufficiently close to the focuser to mean that making up an additional adapter would have moved the focus point too far inside the focuser. So there was only one thing for it: push my newly-learnt machining skills to the limit and make a replacement fitting for the focuser. Fortunately the Revelation SCT focuser has an SCT fitting that is intended to allow the focuser to be rotated. Undoing the screws meant that would come out. I took some measurements and ordered a nice big chunk of 3.5" diameter aluminium bar from that ebay.

At this point I should perhaps make a few admissions. I've never turned anything this diameter before (I only put hands on a lathe for the first time at the start of this year). I've never turned a thread on anything before. And I'm entirely self-taught. If anyone is thinking "recipe for disaster" at this point, I'd not blame them :)

After a bit of faffing about trying to turn something that was big enough for the chuck to almost foul the topslide, a very fast education on the use of changewheels and thread turning and a good deal of "turning by the seat of one's pants" I have this evening produced this:

part1.jpg

part2.jpg

The M50 female thread was turned by hand using the sweat of fear of messing up a lot of turning work as lubricant :)

Once done I fitted it to the focuser:

revpst.jpg

And now for the moment of truth... fitting the "gold tube". Perfect:

joined.jpg

By now more than one person is probably thinking "Yeah, but what about the eyepiece holder?" Well, I'd read that it could be removed from the "black box" and had the same thread as the holder on a Celestron 1.25" diagonal. I happened to have one and it works. Unfortunately the Celestron diagonal is a prism and the thread fouls the prism slightly, so I took apart a Skywatcher diagonal.

skywatcher.jpg

Turns out it uses the same thread.

pstdiagonal.jpg

And now here's the entire unit assembled:

complete.jpg

Obviously there's still more to do, like testing it, for a start :)

I also need to make up some mounting rings for it. I have some more aluminium on order for that. Then it will need a finder which I'll probably fit to the top of one of the mounting rings. And now I'm not so scared of thread-cutting I'm going to make up a 2" fitting for the focuser that takes the eyepiece directly without needing a diagonal and I'll use that for imaging.

Thus far though I'm feeling very pleased with myself :D

James

Edited by JamesF
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Oh, I need to find some matt black barbeque paint to spray on the inside of the new fitting too. I must remember to do that.

James

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Very neat bit of work! Have you attempted to separate the eyepiece holder from its extension tube yet? On mine it made it possible to focus a camera without needing a Barlow.

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Very neat bit of work! Have you attempted to separate the eyepiece holder from its extension tube yet? On mine it made it possible to focus a camera without needing a Barlow.

I haven't, no. Ideally I'd like to get hold of a second eyepiece-holder to use for imaging so I have a diagonal (plus eyepiece-holder) for visual and an extension with eyepiece holder for imaging. Even with just the one in the diagonal I don't think there'll be any need to split the holder because there's enough inward focuser travel to use a camera anyhow.

That's the idea, at least :)

James

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Ingenious! :-)

Look forward to hearing the results, looks great!

Stu

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It looks very tidy James. Have you measured the optical train to see if you can reach the 200mm mark for visual use, or have you looked through it to see if you can reach focus visually.?

If it works, it would be a much cheaper alternative to Borg helicals.

Edited by allcart

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Well done!

This type of mod has been done successfully before, but not yet "popular" enough to be listed in the PST mod "Options". I will however mention it in the next revison.

Look forward to your first light!

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I checked it when I had the tube fitted to the focuser with lots of PTFE tape around the thread. Focus point with an eyepiece was near the mid-point of the focuser travel. My part is about 2mm thicker than the standard one so I had room to make a deep enough thread, but that still leaves plenty of room for focus.

Whilst I wait for the aluminium to arrive for the rings I'm tempted to bodge up something in wood so I can play with it, err, do some testing :)

James

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Well done!

This type of mod has been done successfully before, but not yet "popular" enough to be listed in the PST mod "Options". I will however mention it in the next revison.

Look forward to your first light!

It's unfortunate that it's not possible just to make up a 2" to M50x1 converter to go in the front of the focuser, but once you've got something long enough to do the job I think you'd be in danger of running out of focuser travel again. Unfortunately the threads are sufficiently similar in size that the male and female sections can't overlap and the etalon has got to come at least 10mm further forward, probably nearer 15mm. A low-profile 2" to 1.25" adaptor might claw some of that back, but I'm not sure it would be enough.

If you have the facilities to make up your own adaptor or know someone who can though, I think it's quite a neat non-destructive solution and an awful lot cheaper than the stage 1 mod (though obviously that has other advantages).

I'm quite excited about testing it out. I really want to take it on holiday next month so as long as I can get some sort of tube rings sorted before then I might just go with a simple gnomon type solar finder for the time being and make something neater up later.

James

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]Good to go then James. Check out your local plumbing wholesalers They might have some pipe clamps that will do the job. Something like this.

post-25425-0-25790300-1370297157.jpg

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Oooh, Munsen rings. Yes, I'd not thought of those. They'd make an ideal short-term solution. There are some 54mm brass ones on ebay that would do the job nicely when lined with felt or cork. They have M10 fittings, but I could turn up a "top hat" sleeve to fit inside that would take a bolt small enough to pass through existing holes in a dovetail.

James

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Right, sorted. I can just use a long thin tube as a finder for the time being I think.

James

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I made a finder out of 3 lengths of overflow plumbing pipe about 4 inches long. Build it pyramid fashion with a space between the bottom 2 and glue the 3rd one on top. I used a wooden spoon handle for a spacer until the glue set. Fasten onto tube with a rubber band and it always sits straight along the tube. All you need then is a stopper with a small hole at the front and a bit of tape across the back end. Mark the sun spot on the tape when you have the scope aligned with the sun the first time, and it will work everytime.

You could make the top tube with larger plastic pipe if you have some. I always seem to have odd bits of plumbing knocking around.

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I like that idea. Very simple, very neat. I shall hunt down some pipe tomorrow.

James

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My goodness!!! I'm expecting some mighty fine images to come out of this baby after all that sweat and toil :)

Well done!

Alexandra

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When it gets that far I shall without doubt be begging the benefit of your experience with the processing, Alexandra :)

James

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The aluminium for making the mounting rings has arrived, but thanks to Allan's suggestion of using Munsen rings I now have it on a dovetail already:

dovetailed.jpg

So tomorrow we are good to go. Apologies for the unexpected monsoon weather arriving overnight, people.

James

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I should add some details, I guess...

The rings are 54mm internal diameter Munsen rings in brass. That's too big for the "gold tube", but I needed to line them with something anyhow and happened to have some offcuts of some self-adhesive stuff that people put on the bottoms of table feet and so on so they don't scratch wooden floors. It's like self-adhesive felt, but much thicker and more robust. I've used that to line the rings and they fit very nicely.

The mounting for the Munsen ring is M10, and in the end I just drilled out two M11 holes in a dovetail at a suitable distance apart. The dovetail is currently held on by a couple of M10 bolts I happened to have in the workshop, but I've ordered some M10 domed cap allen bolts to do the job properly.

I'm quite excited about using it for the first time.

James

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I'm very impressed with this James. The focuser looks the biz and even the munsen rings don't look out of place on the gold tube, Overall, I think you did an excellent conversion. :cool:

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Looking forward to hearing how you've got on with this mod. I have a spare Dual-speed Crayford Focuser which I may well use if you find the results are significantly better than the basic PST diagonal.

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I finished making up the imaging adapter this evening. Here it is:

adaptor.jpg

I turned it from a 70mm length of 42mm diameter aluminium bar. One end is turned down to 31.75mm to fit the focuser (and has a tapered undercut). The other end has an M33x0.75 thread down the inside to accept the eyepiece holder. In fact, I threaded the entire thing all the way through (two different diameters, obviously) to help combat reflections. I'm also going to spray it with matt black barbeque paint.

James

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And here it is fitted to the focuser. Now I just need some decent sunshine to test it to check I can get focus. I think I should be able to as the distance from the end of the etalon to the back of the adapter is 125mm and the eyepiece holder is a further 57mm giving me 182mm. The camera needs 13mm, so there should be a about 5mm spare. I did choose to err on the side of making it too long on the grounds that it's easy enough to turn it down shorter, but somewhat more tricky to make it longer :)

adaptor-fitted.jpg

James

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