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Can't focus after primary mirror clean


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Hi there

I recently removed the primary mirror from my Revelation 8" Dob as it needed cleaning.  There are simply 6 screws holding the mirror assembly in place, and after cleaning the mirror I just popped the assembly back in, making sure to put it back exactly where it was before (I marked one of the screw holes).

As you can imagine, after that I needed to collimate the scope.  I did this with a Cheshire, and had to make adjustments to the primary and secondary (the secondary has always been too 'high' ie too close to the top of the tube, so I took this opportunity to centre it under the focuser).

Tonight, I put the scope outside to cool down for about 30 mins, and then tested it by simply looking at a single star.

I've now got a problem where the focuser runs out of travel before the star is in focus.  Basically I can wind the focuser right in/down, but my view is still not in focus.  This didn't happen before I did the primary clean, and the collimation was close enough then for me to see things in focus ok.

Anyone had this issue and know what I should try to fix it?

Cheers

Baz

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After the six screws and cell removal, did you actually remove the mirror from the cell, or clean it  within the cell?  If not, and the cell went back in as previously marked, then the primary shouldn't be too far out!, needing only  careful tweaking to realign. However, you said that you have adjusted the secondary. I think there lies your error. 

 You haven't broke anything, but if your not focusing, then re-collimate again. Taking everything out (if your brave) and slowly re-fitting is a great way you get to know your scope better. Get that secondary mirror perfectly centred, non-elliptical under the focuser tube,  that's the first requirement. Then tilt / adjust to align with the primary mirror, then finally tweak the primary mirror. Good luck.

Edited by Charic
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You mention that you took the opportunity to centre the secondary to the focuser as it was too far up the tube before, I would think this was the correct thing to do as you do want the secondary central and circular in the focuser. This is why I'm thinking the primary height might need adjusting perhaps?

Edited by starfox
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After the six screws and cell removal, did you actually remove the mirror from the cell, or clean it  within the cell?  If not, and the cell went back in as previously marked, then the primary shouldn't be too far out!, needing only  careful tweaking to realign. However, you said that you have adjusted the secondary. I think there lies your error. 

 You haven't broke anything, but if your not focusing, then re-collimate again. Taking everything out (if your brave) and slowly re-fitting is a great way you get to know your scope better. Get that secondary mirror perfectly centred, non-elliptical under the focuser tube,  that's the first requirement. Then tilt / adjust to align with the primary mirror, then finally tweak the primary mirror. Good luck.

Hi Charic

I left the mirror in the cell when I cleaned it.  When I refitted the mirror cell and collimated, I backed out the 3 white knobs so that the adjustments via the 3 black knobs were unrestricted.  Once the primary was positioned I wound up the 3 white knobs so that they were finger-tight, and 'locked' the mirror in the cell.

I think you may be right, and I've munged things by tampering with the secondary.  Once I've got the secondary centred and non-elliptical under the focuser, won't tilting it then move it from being centered?

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You mention that you took the opportunity to centre the secondary to the focuser as it was too far up the tube before, I would think this was the correct thing to do as you do want the secondary central and circular in the focuser. This is why I'm thinking the primary height might need adjusting perhaps?

I can see where you're coming from with this, but I'm pretty sure that the primary hasn't moved up/down the tube from where it was originally seated.  I think I'm going to have to work backwards and try to undo the steps I made yesterday, which means starting with the secondary; I'll go right back to basics with that and get it properly centered under the focuser and if that doesn't help, I'll consider putting it back to its original 'high' position.  I'll tweak the primary again anyway, as I'm sure it'll need it...hopefully somewhere along the line I'll fix the issue!

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Baz. If you moved the secondary nearer to the primary it should have moved the focus further out from the tube which you would expect the focuser to travel outwards further?.

Hi Peter, yeah I can see what you're saying here, and it implies that I've moved the secondary the other way ie higher which has pulled the sweet spot 'into' the tube and out of reach of the focuser.  I'm definitely going to start by adjusting the secondary, as from everyone's responses I'm sure this is what's caused the issue.

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Does your scope need an extension in the focuser for visual use? Could the collimation be a red herring?

I don't think so as before I did the clean etc, the scope was focusing ok.

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(the secondary has always been too 'high' ie too close to the top of the tube, so I took this opportunity to centre it under the focuser).

If you have positioned the secondary further down then the focus will have moved outwards ( by the same amount) so you need more outward travel.

If the secondary is now central then either a small extension tube or the mirror needs to go downwards.

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Hi Charic

I left the mirror in the cell when I cleaned it.  When I refitted the mirror cell and collimated, I backed out the 3 white knobs so that the adjustments via the 3 black knobs were unrestricted.  Once the primary was positioned I wound up the 3 white knobs so that they were finger-tight, and 'locked' the mirror in the cell.

I think you may be right, and I've munged things by tampering with the secondary.  Once I've got the secondary centred and non-elliptical under the focuser, won't tilting it then move it from being centered?

......technically the secondary mirror  has to be central to the tube, that is  looking down the focuser tube. Once its centred you need to rotate the mirror in order to see the primary mirror,  this  task aligns the mirror so that it is seen as a pure  ( as pure as you can get )  circle, encompassing the visual appearance of the primary and the mirror clips? Once this is achieved, final alignment is achieved by  using the collimation adjusters  in the spider  assembly to accurately align the secondary to the primary.  At this stage 'tilting' as you mention,  is just a minuscule minor adjustment, it shouldn't move  your mirror from centred. It should still look centred. 

Our optics probably adjust in a similar manner. I start collimating by having the screws all tightened ( not overly - just enough to ensure that the fittings are central  ) and the lock-nuts, partially removed. Starting from a tightened position ensures that the secondary is central on its stalk, and on my primary, as  I only have 3 thin rubber washer grommets that hold the cell in adjustment, I find again that ensuring the adjusters are tightened, ensures that the primary is central to its mount. This also ensures that the screws are all fully home ( none will come loose - fall out ) and there is only one way to adjust the mirror, by gently unscrewing the adjusters. Works for me every-time.

Just take your time, re-read your methods of collimation, follow the right sequence,  and be patient. You will get it right eventually.

Edited by Charic
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I think you may be right, and I've munged things by tampering with the secondary.  Once I've got the secondary centred and non-elliptical under the focuser, won't tilting it then move it from being centered?

A good way to do this is centre the secondary under the focuser (using the spider vane screws and the for and aft screw) and then adjust the secondary tilt screws until its aligned. If the secondary now appears elliptical, loosen the tilt screws and slightly rotate the secondary on its holder and then re-tighten the tilt screws and adjust until its aligned again.

If the secondary now appears even more elliptical you've rotated the secondary the wrong way. So repeat the procedure but rotate the secondary in the opposite direction.

Repeat until the secondary when aligned, appears circular. You'll now have no rotational error and shouldn't need to adjust it ever again :-)

Good luck with sorting out the focusing issue. If the focus point is now further out from the focuser drawtube you can check by just holding the eyepiece further out. If that's the case then try an extension tube as others have mentioned.

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  • 7 months later...

A somewhat delayed reply from me, I admit, but I dragged the dob out of the shed earlier today, hell bent on fixing the focusing issue; I'd been ignoring it a bit following dismal results last time around.

Anyway, today I basically 'reset' the scope, in terms of moving the primary as far to the bottom of the tube as it'd go, and moving the secondary as close to the spider vanes as possible.  I then adjusted the primary to get the middle 'polo' aligned with the collimator eyehole, and then centered the secondary.  I then put the supplied extension tube (30mm I think) into the focuser, followed by the supplied 2" to 1.25" adaptor, and finally a 20mm EP.  I then lined up the scope with the furthest chimney pot away that I could find, and tried to focus.  What I discovered is that the focal point was outside the max focus limit, and slowly removing the EP from the focuser, pulling it away, then gave me focus, well outside the focuser (probably 2"+).  I then spent a good couple of hours experimenting with moving the primary and secondary (keeping alignment along the way) to see if I could draw the focal point into the focuser.  After much faffing about, I did manage to achieve focus, albeit with the extension tube not completely seated in the focuser.

A couple of hours on, a niggling question entered my mind - was the reason I was having so much trouble with focus due to the fact that the chimney pot I was using as a target, wasn't that far away?  For reference, it was the chimney of the house next door, so probably only about 60' away from where I was at the bottom of my back garden.

Anyone have any thoughts on this?  I really need some closure on this one!  I would do a star test this evening, alas the sky is full of clouds : /

Cheers

Baz

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Your proximity to the chimney could very well be your Waterloo. I try to use objects at least 1/2 mile away when I'm setting my finders. So I suggest that you find a more distant target. Or, best idea, wait until dark and use a star.

Please keep us posted.

Dave

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Strange!

If/when  the mirror is removed in one piece, with care and caution, its possible ( should be) to re-fit the mirror without any further collimation. You just have to be gentle!

First task, you must centre and make sure the secondary is concentric to the focuser, starting from the face of the spider is ok, you need to unscrew the bolt to allow the mirror to travel towards the primary, but stop when central to the focuser axis ( keep the scope level when carrying out this task, in-case the secondary falls and hits your primary.....Ouch!

Next, you need to  tilt  the secondary to  align with the primary, in order to see those mirror clips?

Lastly, you may need to adjust the tilt of the primary to align with the tool in the  focuser. Again, starting with the primary mirror tight against its housing is a good way to start. The adjustments are minimal.

Finally, you should only need one of the adaptors. You mention an extension tube?   My Skyliner has a 2" adaptor and a 2"-1.5"  I only use the 2" -1.5". You should do the same. One adaptor, one eyepiece. Don't install both adaptors with the eyepiece? I bet the extension is almost 2" in length? :embarassed:

Edited by Charic
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