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Focuser issue with Skywatcher Skymax 127 mak.


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Since I've had this scope which I bought s/h I've felt that the focus adjustment wasn't quite right, but it seems to be getting worse, and it's proving more difficult to nail focus than I think it should be.

If I go past focus then wind it back, I seem to have to turn the knob back much further than where focus actually was to get past focus and bring it back again, so I get the impression that the position of the knob in relation to it being in focus seems to change. Also sometimes the change from being out of focus to being in focus is more of a step than a smooth transition.

I have checked that the knob is not moving on the shaft and the grub screws are tight.

Have to say it's quite annoying and getting critical focus isn't the easiest task at the best of times anyway.

Any ideas or remedies greatly appreciated.

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Sounds like backlash somewhere in the system.

First thing i would try is loosen the grub screw & see if the knob will push a little further onto the shaft?

It's a long time since i took mine apart,so i forget the exact workings, but somewhere on this site there was an excellent stripdown guide.(hopefully that won't be required)

I find my focuser works very well, though i supplement it with a helical focusser for fine tuning @ high power.

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Hi

Thanks, I had already done that, I removed the knob, shaft had small indents where the screws had been tightened onto, so rotated and tightened in to fresh section of the shaft. But that hasn't made any difference.

Thanks for the link, it seems a task not to be undertaken without good reason, especially without the accompanying photographs!  However I did try to unscrew the back plate just to see if it would move, but failed miserably.

Also I did find this article :  http://scopeviews.co.uk/SW127Mak.htm

Where it does touch on the same issue I'm experiencing, unfortunately he doesn't explain or delve into this problem any further than the following:

The focuser is typical of a Maksutov – a knob protruding from the back-plate moves the primary mirror. This arrangement gets a lot of bad press because in some cases you get a lot of image-shift when changing focus direction

That being the case I'm surprised there is not more to be found documenting the problem or reports of others with the same issue.

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The focus mechanism consists of a threaded Rod rigidly connected to a bulkhead which is bonded to the the back of the primary mirror, both of which are rigidly connected to the outer of two baffle-tubes. The knob turns a threaded nut also on that shaft but fixed to the rear of the ota. As you rotate the knob, the threaded Rod travels in or out through the nut at the knob, dragging the primary mirror arrangement up and down with it. It’s crude but it should work well.

it may be that the attachment of the threaded rod at the bulkhead inside has worked loose, which would explain the behaviour. Unfortunately to fix or check that you will have to do some dismantling.

if you do decide to dismantle, DO NOT DO SO by first undoing the 3 sets of collimating screws at the back. Those screws actually hold the mirror/baffle in place inside the tube, as well as collimate, and detaching them will set the mirror free to drop on to the corrector plate and/or dangle off the threaded Rod.

seerch for “reverse engineering the skymax 180” and you’ll find my thread about dismantling the 180. The way your tube attached to the ota will be different from mine, but the basics of how the focuser works and how the mirror assembly is attached will be similar.

I believe on a 127 the main tube actually screws directly on to the rear lump via a thread.

cheers Magnus

 

Edited by Captain Magenta
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Thanks so much for that information Magnus, your post/thread on the 180 is superb, such a lot of time and effort into producing it, I'm sure it's been  great help to many with questions or issues with these scopes.

If I do eventually decide to venture inside the tube, these instructions will be invaluable.

One question at this point, not related to the focuser but the secondary mirror, I've been led to understand that it's best not to try and clean these, so can I ask what your method, cloth and cleaning fluids were to complete that task?

Many thanks for your help.

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The secondary mirror and corrector plate are coated in SiO2, i.e. quartz. It’s pretty hard stuff! After removing the secondary baffle I used cotton swabs paintbrush and acetone to remove glue residues, even (especially) superglue disappears with that. Once the glue has finally dissolved, I treated it as if it was any other lens surface and cleaned again with acetone-cotton swabs and pure water.

however if you’re not going to remove the secondary baffle, then I shouldn’t worry about cleaning the secondary. I only did it because my baffle was seriously off centre so I pulled it off and had no choice but to clean up.

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Hi Magnus

No I don't intend to do it now, it was just for future reference, but very interesting to know that it's okay to use acetone on the mirrors.

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