Coronado PST “Stage 0.5” mod

A short while ago I purchased a slightly damaged PST for solar Ha viewing. 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:


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:



The M50 female thread was cut by turning the lathe by hand using the sweat of the fear of messing up a lot of turning work as lubricant. Once done I fitted it to the focuser:


And now for the moment of truth… fitting the “gold tube”. Perfect:


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.


Turns out it uses the same thread.


And now here’s the entire unit assembled:


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.

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16 Responses to Coronado PST “Stage 0.5” mod

  1. Alan Foley says:

    Hey I just seen your mod, not bad, did it turn out ok.

    • james says:

      Works very nicely. It’s a huge improvement on the focusing mechanism that is fitted to the PST as standard.

  2. Alan Foley says:

    Thanks James for your quick reply, I think I might look into this.


  3. Alan Foley says:

    James what thickness is your 2″24 tpi thread to M50x1 thread adapter.

    And can a Coronado BF5 or BF10 be used instead of using the PST eyepiece holder in a Skywatcher diagonal.

    • james says:

      There’s no thread adapter specifically. It’s a completely new mounting plate with an M50x1 thread to fit the gold tube instead of the original 2″ SCT fitting. Making up a collar with one thread on the inside and the other on the outside would have made life very easy, but they’re too similar to make it possible, at least with my turning skills 🙂

      The plate I made is about 15mm excluding the flange that sits inside the focuser body. I worked it backwards in fact. Using PTFE tape the thread on the gold tube can be packed out so it will just about hold in the original focuser mounting plate. I then put in the diagonal and found focus with a few different eyepieces. By looking at how close the focuser came to the limit of travel I decided my plate needed to be a few millimetres thicker than the original because for some eyepieces I’d want to use I was almost against the “outwards” focuser stop.

      I see no reason that it shouldn’t be possible to use a BF5 or BF10. That might mean the design needs changing a little to move the focuser and gold tube further apart as I assume they would have a shorter optical path than my diagonal. In turn that might make the project somewhat simpler as given enough room it might be possible to make an extension tube of the correct length with an M50x1 female thread on one end to fit to the gold tube and a male SCT thread on the other to fit in the original mounting plate.

      • Alan Foley says:

        Thanks James, I’m already working on it.
        I have the PST and just purchased a s/h Moonlite SCT focuser.
        I’ll think about whetrer I use a BF or the original eyepiece holder in a cheap diagonal.
        I have a BF15 that I can test it with but it belongs to another solar scope, but it will give me an idea of which way I will go with the diagonal.

        Thanks, I’ll report back.

  4. Alan Foley says:

    James I just used Priceparts adapter builder to order a adapter for the PST to SCT focuser, does this look right.

    Custom adapter, Coronado PST Etalon Filter to GSO 2 inch Crayford Focuser for SCTs, L=25.0 mm


    • james says:

      I can’t be certain, but if 25mm is the additional length you think you need then it sounds like it ought to be the right sort of thing.

  5. Alan Foley says:

    Changed my mind, they want $185 Aussie dollars.
    I contacted a local machinist and he can do one for $50

  6. Alan Foley says:

    James I have a PST and

    S/H Moollite focuser coming from the U.S.
    BF10 coming from the U.S.
    SME40 double stack coming Sydney Australia

    Just having a problem with the threaded adapter from etalon to SCT thread, I won’t pay PreciseParts prices, I have a local machinist but he needs the Etalon and Focuser to be able to check the threads first.
    Just wish I could order one off the shelf.


    • james says:

      Probably sensible to wait until you have all the parts anyhow. Then you can be sure of the thickness you actually want the adapter.

      If you haven’t already you also need to consider how you’ll mount the finished OTA. There’s another post somewhere here with a photo of mine, using a pair of Munsen rings lined with felt to mount the OTA to a dovetail.

  7. Alan Foley says:

    You’ll have to tell me how do I determine the length of the adapter.

    I’ve got 2 of each of these clamps that I will decide which to use, I can mount them to a Vixen dovetail bar.

    • james says:

      I’m sure they should do the job. The steering damper mount looks very smart 🙂

      I estimated the thickness of my adapter by fitting the “gold tube” to the focuser having wrapped the thread in a load of PTFE tape. It holds, but only just. Fortunately an M50x1 thread is just fractionally smaller than the standard 2″ SCT thread. Then I found focus with the diagonal and eyepieces I felt I was likely to use most and decided that to have them focus around the mid-range of the focuser travel I’d need my plate to be a few millimetres thicker than the standard one.

      As you’re using a BF10 I imagine the optical path will be shorter than mine which could make the process a little more tricky. If you can’t get focus at all I’d probably add an extension in the drawtube in front of the BF10 and repeat the process. Then the length of your adapter should be the length of the extension plus whatever offset you need to be able to get your eyepieces to focus somewhere near the middle of the focuser travel. I think 🙂

      This second method is probably more awkward, but the positive side is that you almost certainly only need a tube machining with the M50 female thread on one end and male SCT thread on the other and can use the existing SCT mounting plate. I had to go with a complete replacement mounting plate because the required length was so short. The internal and external threads would have overlapped and there’s not sufficient difference between the two for that to work. Not with my limited turning skills, anyhow 🙂

  8. Alan Foley says:

    Good news bad news, Astroparts in England are making me an adapter but the BF10 won’t be available for a month.

    I can still use the BF15 to set it all up till the BF10 arrives.

    Now I just gotta wait till everything turns up.

  9. Alan Foley says:

    Still impatently waiting for the adapters to arrive.
    Can I post a photo of my setup here when I complete it.

  10. Alan Foley says:

    Finally got it all together, I used a non rotating 2” helical focuser and had a 55mm extension tube made up to go between the PST etalon and the helical focuser and a BF10.
    Got 2 x 2” clamps off eBay to hold the gold tube bolted them to a dovetail and a Televue Sol finder on the side.
    Viewing and focusing is so much better with this setup.



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