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Ratcatcher

Can anybody help?

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I recently (within the last twelve months) had the corrector plate (internal side) cleaned on my Meade 10" by a well known Meade dealer and they did a grand job.

The problem is that due to moving house and all that goes with that and a serious back condition, I was not able to test the scope for months, when I did I found it would focus on the trees at the bottom of the garden but not on a star. At all!

Phoned the dealer and it was 'we will have to have a look', but due to being un-employed a round trip of  192 miles, a possible repair bill (which I would have fought about) and another 192 mile trip to get the scope back,I don't at this stage whant to follow that route. (no pun intended!)

Do any of you know if there is a shop/repairer in this area?

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Not aware of a retailer around your area, and most retailers would send it off for repair anyway and likely to the same place.

Then being a "repair" there would be a charge and carriage+insurance both ways and the retailer would add in a cost.

If you took it back direct and they knew the circumstance there may not be a charge for rectifing.

Your fuel costs would be around £50-60 overall.

It half sounds as if the mirror is not in the right place, although the front corrector should have been removable without going near the mirror so no reason for it to have changed. Leaves the corrector being incorrectly positioned - too far in ? at a guess. That equally seems unlikely as having removed it I would have presumed they checked it for collimation. An artifical star would have shown that the focus was wrong.

May sound stupid but have you got an extra bit at the rear in place? In effect pushing the eyepiece back.

Anything preventing the diagonal seating correct.

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Hi ronin,  fuel costs are at £40, that includes both trips. I was afraid that that scopes would get sent to the same place!

I suppose what has irked me is the responce..'we'll have to have a look' ..and 'he would only have moved the corrector not touched the focus' , when in my opinion it should have been along the lines of 'sounds like something has gone wrong,..lets look see and sort it from there'

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Sorry for the double reply  ronin, will run outside and check the back of the scope, but I am running with the Meade 2" diaganal so should not be issues there as that is what was being used before the clean.... just checked and nothing to cause any problem.

I am wondering how they check the collimation...I would imagine that the scope is put in some kind of jig,..has that caused a problem of some sort...has the focus broken some wehre?

AS you say, biting the bullet and taking it back has to be the cheapest option.

 

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Could be a number of things:

- Collimation might well be out (perhaps by a long way).  That could have happened in transit or maybe it wasn't re-collimated after cleaning.  What sort of pattern do you get when you look at a de-focussed star through the eyepiece?  If the collimation is only slightly out you should get a series of bright concentric rings with a black 'hole' in the centre when you are either side of focus.  If only slightly mis-collimated then I don't think you'd be reporting focus problems.  If you get something looking like a bright crescent/moon shape when you go either side of focus that is a sign of severe mis-collimation and you might well have problems getting focus. Either way the solution is to re-collimate which is something you should be able to do without sending it back.

- If the corrector hasn't been re-aligned properly then you may get less than optimal views even when collimated, but I doubt you'd be reporting major focus issues in this case.  At the factory they rotate the corrector plate until they find the best alignment between the mirror figuring and the corrector.  Even if you rotate the corrector it should work reasonably well.  Normally Meade will put a small blob of white paint on the corrector and on the adjacent outer ring of the tube, with a black marker pen line drawn between the two to show the correct alignment from the factory tests.

- I don't know how the corrector was cleaned, but I suppose it is possible the secondary holder and secondary mirror were completely removed from the corrector plate.  If that was done then it is remotely possible that they were re-assembled the wrong way round and the corrector is now in back to front.  This would take a lot of carelessness but I suppose it might be possible.  You might be able to diagnose the problem by looking for the pair of alignment marks mentioned above.  If the one on the corrector is on the inside of the OTA then it has been put back in upside down.

- I suppose other problems are possible like damage to the secondary mirror (which could cause big issues), or misalignment of the primary, but why that would be touched when cleaning the corrector is beyond me.

If you do feel brave enough to check these things (and assuming the scope is out of warranty) removing the corrector is not that hard on most Meades.  It sound daunting but with a bit of care it is not a big deal:

- Lay the OTA flat on the floor (so nothing can fall too far), with something soft underneath and covering the floor in front of the corrector in case you drop it.  (I used a big fluffy towel, but obviously there is a risk of fibres getting on the corrector or in the OTA).

- Look at the front of the OTA and you will see a ring of allen head screws surrounding the corrector plate.  These hold down the black retaining ring, which will have the OTA name and scope specifications printed on it in white usually.

- Remove the allen head bolts in a crosswise manner (remove pairs opposite each other and then go round 90 degrees and remove the next pair).  Make sure you use a good quality allen key as the heads are prone to rounding out.  From the factory it usually requires a bit of torque to break the lock of the threads and then each screw should remove easily.

- Once you get to the last few bolts, make sure you grasp the secondary holder firmly to avoid the corrector falling out (an assistant would be helpful here).  The black retaining ring should lift out easily enough - make a note of its orientation before removing.  

- Now the corrector alignment marks should be visible as they will normally be hidden by the retaining ring.

- If you do not see any marks and decide to pull the corrector out, then you should make your own marks before doing so.  Use a fine permanent marker to make a small line on the corrector and on the outer ring to show how the corrector is aligned.  Provided you keep the line on the corrector small it will be under the retaining ring and won't have any effect on the view.

- Check the alignment of the two factory marks, and make sure they are both on the front of the corrector plate to be sure it was re-assembled properly.  The curve on the corrector plate is not at all obvious so it is really difficult to tell if it is the right way round by eye.  I wouldn't advise removing the secondary if you suspect it was damaged or re-assembled backwards.  Send the thing back to the shop that refurbed it and make them pay the courier and insurance costs.

- The other things to check are the secondary mirror - look for any damage or signs that it is not installed properly.  Avoid touching the secondary at all costs, as any marks or damage to the coating here can have a significant impact on the image (unlike marks or damage on the corrector or primary mirror, which would have to be huge to degrade the view appreciably).  Also take a look inside the tube to make sure the primary looks okay, that it is properly aligned and moving up and down the baffle tube when you turn the focus knob.

- Re-asseambly is just the reverse of the above.  Put the corrector back in by grasping the secondary holder, making sure the alignment marks line up.  Pop the retaining ring back in place, ensuring it is rotated to its original alignment (to avoid any problems with re-fitting).  Screw in the allen head bolts in pairs in the crosswise/90 degree manner (like doing up a car wheel).  Don't over-tighten the bolts - hand tight is sufficient.

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Just looked in the fridge (boredom) while thinking about what you described.

One quite horrible thought was:

They didn't collimate it after wards as they didn't remove the plate.

Could have (weird idea) removed the mirror assembly from the rear, cleaned the corrector (arm up tube) then replaced the mirror assembly. That way the corrector stays in place so no collimation check.

So now the mirror assembly can be in the wrong position and you may not get focus.

If you took it back could/would they do it while you wait in effect ?

Saves one round trip.

Checking collimation and focus would be at the same time, artifical star. You would have to defocus the artifical star which by implication means you would first have to focus it. So I would have thought that collimation and focus would come out of the same exercise. Unless as above the plate is not removed, so someone says no need to check collimation and therefore focus is not checked.

That would explain how the mirror may not be at the correct position now. Just seems an odd way of cleaning the inside of the corrector. However that may be the approved, recognised and easier method of doing it.

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Hi IanL, thanks for the detailed reply.

To give more information, due to my back issues I set the scope up (for only the second time since cleaning) this morning and tried to look at the moon.

Not something I would normally do during the day, but my back was up to the job and I have no distant land marks to focus on.

Focus not possible at any point ( focus knob wound fully in both directions ), so I removed the 1.25" ring on the diagonal ( using Meade 2" diagonal)

and very carefully lowered the 1.25" eyepiece into the diagonal and things started to come into focus .  

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That sounds rather odd.  SCTs have a pretty huge focus range so I can't really fathom why moving the eyepiece closer to the mirror diagonal would get you closer to focus.  Doesn't sound like a corrector plate or collimation issue to be honest.  Things to try:

- Check the primary mirror is actually moving when you turn the focus knob.  If you have someone who can help, just look down the tube and have them wind the focus all the way in one direction and then all the way in the other.  You should be able to see the mirror edges moving relative to the outer tube or the inner baffle tube.  It can look a bit distorted, but it is quite a large range of motion and should be obvious.

If the mirror isn't moving at all, there's your problem.  You don't say how old the OTA is; I don't know much about current models but the focus mechanism on older Meade SCTs is pretty basic.  There is a threaded rod which moves in and out of the OTA as you turn the focus knob.  The end of the rod has a hole which fits over a small pin on an arm sticking out from the mirror sled (which in turn fits around the central baffle tube).  It is possible for the rod to become detached from the arm in which case the mirror won't move (or may only move forward a bit if the rod itself pushes on the back of the mirror directly, but then won't come back again as the rod can't pull it).

On larger mirrors, there should be shipping bolt(s) which hold the mirror in place when it is being moved.  These prevent the mirror from moving in transit, avoiding damage to the mirror and the focus mechanism.  I don't know what size mirrors these are used on, certainly not on 8" mirrors, but maybe on larger ones.  You'd have had to remove the bolts when you first received the scope so you should know, and if you have them they should have been used to re-ship the scope.  If it is the focuser, the rod may simply have come off the pin - it is a bit fiddly to dis-assemble the focuser but it is a DIY job.  If the pin is broken or the rod is bend, you ought to get the dealer to repair it.

- Secondly try the scope without the diagonal if possible.  This may be hard if you have to get down low to look through the eyepiece; again if you can get any assistance it may help.  Assuming the mirror can move through the full range of focus then I'd suspect the diagonal as the problem.  As I have said, SCTs have a wide range of focus from somewhere inside the scope to a long way out of the back - adding a working diagonal should make no difference unless it is broken somehow.  Also try a different eyepiece if poss to eliminate that as the culprit.

@Ronin:  I think it would be pretty unlikely that they cleaned the corrector from inside by removing the rear cell assembly.  It would take about two minutes to remove and replace the corrector if you know what you are doing.  Removing the rear cell is not that hard either, but it is a lot heavier and I can't see any good reason why a supplier would do it that way, especially as it makes it a lot harder to access the corrector which it what they were cleaning.  I suppose anything is possible though.

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Have spoken to the dealer and they will give it the glance over while I wait...they are at a loss as to what has happened also.

The thing I am thinking about now is that the pin has become detatched on the focus rod,...this would account for the moon coming to near focus when I manually lowered the eyepiece into the diagonal.   

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Ian, I agree entirely. Removing the mirror assembly and cleaning from inside makes little sense, but would account for a change of mirror location and no check afterwards.

As said a somewhat weird thought. Worse is the idea that the option even occurred to me. :evil: :evil: :evil:

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 IanL, thanks for the input everybody, the more I think about this the more I think the focuser (pin) is the problem.

Well, the Mrs is back today from 'The Smoke'  so she can give a turn or two on the focus knob while I check that the mirror moves.

Will keep you all posted.

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Well,....problem solved...it turned out to be that one of the bolts I replaced after de-forking the scope was obstructing the primary mirror,..you live and learn!, been used for at least 2 years with no problems.
I did not notice at the start because I had removed all the bolts before inspection.

Been collimated now on artificial star and will be refined on a real star as soon as clear skies permit! :laugh: 

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