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Equipment Discussions >> Cats & Casses

Pages: 1
Mark9473
Postmaster
*****

Reged: 07/21/05

Loc: 51N 4E
HELP - my focusser is stuck! new
      #5677946 - 02/13/13 04:18 PM Attachment (20 downloads)

The mirror focussing unit on my Mewlon has become stuck and I'm not sure what to do to get it running again.

Last time I used it, couple of days ago, I ran it to its end to see how much focus room I had left with various binoviewer configurations, and then left it there. Today I found the focusser to be *very* stiff and the image wasn't moving. Only gave it probably half a turn or so. Brought in the scope to warm up before opening it, and now it's completely stuck. Only tried hand force.

Any help from the experts here would be really appreciated!


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Larloch
member


Reged: 06/29/08

Loc: Hungary
Re: HELP - my focusser is stuck! new [Re: Mark9473]
      #5678787 - 02/14/13 05:29 AM

Since I had some experience in disassembling a Mewlon (mine), I can give you some hints.

I suspect the problem is one or both of the following:
-- the primary cell has been stuck on the baffle tube.
-- the focuser knob's moving mechanism stuck for whatever reason.

As you can see, all of this is can be resolved to perhaps one thing: the grease used within them.

The "only" problem is to determine what went wrong you have to remove the whole primary cell. This can be tricky, but it is pretty straightforward. Here are some key points:
-- always mark the original positions.
-- remove the primary mirror.
-- there are screws under the screws in the cell. This also means...
-- ...don't force anything. If something doesn't move, look for an another screw. Takahashi was nice enough to hide some in the cell.
-- if you have removed the primary mirror, removed all screws and everything, and the cell still doesn't come out (or at least, goes out hardly), then we found problem #1.
-- if the cell goes out easily, then the problem is with the focuser.

When you put back everything an extremely great care must be taken, otherwise you get a mirror shift. I know since I have about 60" from the none. Count the rotations while screwing back. I also advise to do this with an ATM friend. He might have more tools and ideas in solving this problem.

I hope this helps somewhat.


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Mark9473
Postmaster
*****

Reged: 07/21/05

Loc: 51N 4E
Re: HELP - my focusser is stuck! new [Re: Larloch]
      #5678792 - 02/14/13 05:37 AM

Thanks for your tips Gabor. In the meantime I have done the following:
when I removed the hex screw at the top of the focusser mechanism, the focus knob turns freely. I was expecting that this would allow me to screw the treaded rod downward, but this is not the case, it stays in place (though freely rotating).

Therefore I am now thinking it has to be the focusser knob mechanism. Any tips for that? I noticed that the smaller of the two brass rings at the bottom of the mechanism (see photo above) turns with the threaded rod. Is it supposed to do that?

Would appreciate your further comments.


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Larloch
member


Reged: 06/29/08

Loc: Hungary
Re: HELP - my focusser is stuck! new [Re: Mark9473]
      #5679307 - 02/14/13 12:03 PM

Hmm... While I never disassembled the focuser -- or at least, I don't remember --, I say it definitely worth a try.

You can even remove the whole focusing assembly, if you feel something is still not right. With great care, you can't really mess up the scope. My only advice here is make sure you support the primary cell from below with something. Only the focuser keeps it at that position on the baffle tube.

Either way, the danger of introducing a mirror shift is still lurks in shadows


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Mark9473
Postmaster
*****

Reged: 07/21/05

Loc: 51N 4E
Re: HELP - my focusser is stuck! [Re: Larloch]
      #5679347 - 02/14/13 12:21 PM

I was thinking of just lifting the primary over the baffle tube and out of harms way, but I have no idea how to start at getting the focusser assembly out of the back plate.

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Mark9473
Postmaster
*****

Reged: 07/21/05

Loc: 51N 4E
Re: HELP - my focusser is stuck! new [Re: Mark9473]
      #5679880 - 02/14/13 04:51 PM

Just a quick note that a firm clockwise twist with a gripping tool has released the focusser knob - all is back to normal now.

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Larloch
member


Reged: 06/29/08

Loc: Hungary
Re: HELP - my focusser is stuck! new [Re: Mark9473]
      #5679914 - 02/14/13 05:20 PM

Look for small (1,5 mm) screws on the primary mirror holder. These are keeping the primary on the baffle. The bottom ones are hiding another screws.

Be very careful, though. Always check the mirror, wheter it is loosened or not. The last thing you want is to make it slip off the baffle, or touching it. Also, I advise to arrange a meeting with an ATM-er acquaintance, for additional help.

Good luck!


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Larloch
member


Reged: 06/29/08

Loc: Hungary
Re: HELP - my focusser is stuck! new [Re: Larloch]
      #5679918 - 02/14/13 05:22 PM

Wow, I'm really glad to hear that everything is fine now!

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