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SW MN190 Owners - Secondary rotation adjustment?


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Hi. Does any owner of a Skywatcher MN190 know the correct way of adjusting (rotating) the secondary mirror?

I have replaced the focuser with a Moonlite and when re-collimating noticed that the secondary was not centred (too high), and not quite circular. I've managed to lower the position of the secondary by carefully loosening the central attachment screw and taking up the slack by tightening the collimation screws:eek:.

As a test, I found if I rotate the corrector-secondary assembly a few degrees, I get a more circular profile of the secondary, so I'm pretty certain I do need to make a rotation correction. Just not quite sure how this shold be done.

Anyone done it?

Thanks

Adrian

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I've just done the exact same moonlite upgrade on my scope but I'm not certain that I've collimated properly and got stuck at the same place as you with the secondary rotation. I've not had access to my scope for a while since I've not been to my imaging site for a while, but I think I that there is a locking ring surrounding the secondary assembly in front of the corrector plate. It is not immediately obvious but once you know to look, it is apparent. It has a gnurled grip, but is not easy to turn with just fingers. You need to loosen that locking ring and then you can rotate the entire secondary assembly. You then tighten the locking ring and you should be set.

On my scope I haven't been able to loosen that ring yet because I didn't have the right tools when I tried and didn't want to try forcing it with the wrong tool and end up scratching the corrector plate or worse.

I'm interested to hear how you get on with this and I hope this helps.

Regards

David

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That secondary is a bit of a pain.

The way to align it correctly is to use a Cheshire. You will notice if you look carefully that the secondary has a centre ring on it (well mine has). This is where the cross hairs of your Cheshire should be looking at.

Whether the secondary is in the correct position is also done with the aid of the Cheshire. You are looking for the three mirror clips on the primary to be visible through the Cheshire and the reflection to be central in the sight tube. It is easy to get carried away with reflections - take your time.

As for rotating the secondary, the end cap of the secondary needs removing - your fingers will do this. The knurled ring under that needs to be loosened to rotate the mirror but be aware that once undone it can be a real pain to hold the secondary in the correct position whilst tightening it again.

If you cannot undo the ring then get a pipe wrench and carefully grip the locking ring and put pressure on it - that will probably be enough to rotate the secondary. I had to do that before the locking ring came loose and to be honest it is a better way of doing things.

I have found that collimating the 190MN to be a real pain and even when it appears to be visually perfect the stars can still have a slight hint of oval about them. Its the one thing that detracts from an otherwise brilliant instrument. Worth getting right though.

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Mick:

Thanks for the advice. A pipe wrench just millimetres from the corrector :). But you're right: I just tried it with my fingers and it's tight!

I had exactly the experience you described. Got it optically well collimated, with some experienced help, and got star images that were good, but not great, with some elongation at one corner and lower edge of field. Also the hot-spot and vignetting pattern was quite off centre.

With the corrector removed, I noted that the secondary was tilted way off what you might call a neutral tilt. I wonder if that was the result of the secondary being not centred in the focuser and slightly rotated, so collimating it like that had resulted in some extreme tilt adjustments to compensate.

David:

I'll let you know how it goes.

Adrian

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David, after installing the Moonlite, I found the outline of the secondary in the drawtube to be off-centre ... like the focuser needed to be higher up the OTA, or the secondary moved further from the corrector. If you haven't done so, I would suggest checking that.

One thing that helps to judge centring: cut a strip of white paper the right size to line the inner surface of the end of the drawtube. When you look through a collimation cap or peephole, the dark outline of the secondary stands out more clearly against the brighter circle of the drawtube.

Adrian

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One thing that helps to judge centring: cut a strip of white paper the right size to line the inner surface of the end of the drawtube. When you look through a collimation cap or peephole, the dark outline of the secondary stands out more clearly against the brighter circle of the drawtube.

Adrian

As I said, the reflections are very confusing and the reflection of the primary only just sits on the secondary. It would be such an easy mod for manufacturers to put a slit each side of the OTA so a strip of thin white plastic could be put across the tube behind the secondary. You are so right about it being easier to understand what is going on when you can see the outline of the secondary - its a shame you have to dismantle the thing to do it.

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Mick, two slits would be a neat way to do it from the outside - what an excellent idea! I suppose the thinking stems from standard Newtonian design where you can just reach into the tube to do these things. Manufacturers probably don't stop to think of the additional problems that arise when you can't do that.

The rolled up tube of paper inside the drawtube does help a bit though and is easy to do from the outside.

Adrian

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When i next get time to play with the collimation of my MN190, I am thinking I will remove the corrector plate (marking the alignment before I do so), and then inserting various bits of coloured paper into the tube like people recommend doing for normal Newt collimation. I will then refit the corrector plate and try to properly align the secondary mirror vertically in the tube and rotationally. Once I'm happy with the results, I'll take off the corrector plate and remove the inserts before closing it up.

Hopefully the secondary alignment is a job that will not need to happen very often once done correctly, and future collimations will simply need to tweak the secondary tilt screws and the primary alignment as normal.

Does anyone see a problem with this approach?

David

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David,

The whole corrector, its mounting and the secondary assembly comes off as a unit. Unscrew the six black round head screws that go through the OTA radially and into the side of the corrector mount. Then grasp the white end ring around the top of the OTA and ease it off the tube. It 'plugs in' to the top of the OTA can be quite tight to remove. The whole assembly lifts out complete with the secondary and its mounting attached. It's heavy!

If you need to move the secondary closer to the primary, as I did, it can be done, within limits, by carefully loosening the central countersunk screw at the centre of the 3 collimating screws. Just do it a very small amount and immediately take up the slack by tightening up the collimation screws. Do it in small stages and you can lower the secondary a few mm (or go the other way to raise it).

BEWARE though! If you loosen the central screw too far, the secondary and it's mount will pop off completely when you run out of threads - the centre screw has a compression spring on it pushing down on the secondary mount. So do this with the corrector assembly removed and the secondary supported, and don't go too far. I ran the screw out all the way to see how far I could safely loosen it and found that I ran out of threads while the collimation screws were still not screwed in as far as they would go. I intend to fit a longer central screw (it's M5 thread) so I'm not depending on just the last few mm of threads with it in its new lowered position.

As well as the central holding screw and the 3 collimation screws that push against it, you'll see a fourth round-headed screw, off centre and inside the circle of the 3 collimation screws. I've seen it described as a locking screw, but in fact it seems to act just as a locating peg. It goes into a plain unthreaded hole in the secondary mount - not screwed into or holding anything. I think the idea is that it just stops the secondary from rotating out of position in the event that you loosened off the collimation screws too much.

I'm learning as I go but hope this helps.

Adrian

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  • 1 month later...

Hi,

Just seen this. The off center screw does act as the location lug for the secondary to stop it rotating. This secondary collimation has had my attention for a while now but I think I finally cracked it.

The rotation is very critical and I found the only way of accurately setting it was using a camera and checking for even illumination and equal vignetting in the corners. For fine adjustment I rotated the whole corrector assembly within the confines of the OTA screw holes. Even a millimeter can make a difference ! Start with the threaded secondary holes central in the ota holes (they are a few mm larger in diameter.

This is a quote from my OAS thread on the subject.

Have taken a different approach. Using some of the collimating instructions in 'How to build a Dobsonian' I realigned the secondary laterally along the tube length. I then used a camera to accurately set secondary rotation so the centre of the flat was even radially. The secondary was then accurately aligned using a laser and the whole process repeated till perfect both visually and using camera/collimator. Rounded off with a Cheshire as a double check.

It looks good but it's looked good before. Will see what the results look like tonight on Jupiter and with an image if the cloud gives me a chance.

PS. The laser is a Hotech and looks to be perfectly adjusted.

The results did look good but bad weather and guiding has thwarted further checks so far.

Link to the saga here: Help - How to correctly collimate a Newtonian... with a twist

Hope that helps

Robert

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