Meade ETX90 falling apart, HELP!!

[bendiddley]

Just received a second hand Meade ETX 90 and what I think is possibly the secondary mirror?? has just dropped off when I turned it upside down and is now resting on top of the primary mirror. See photos. Can anyone please tell me what part has fallen off and how I can fix this? Cheers.

[Peter Drew]

This isn't the secondary mirror, it's the secondary baffle which is normally glued to the inner face of the corrector lens. The secondary is an aluminised spot on the inner surface of the corrector. The whole front cell should unscrew and then you can retrieve the baffle and carefully glue it back on. Don't let the detatched baffle rattle around on the primary mirror as it will scratch the coatings. 

Edited January 7, 2018 by Peter Drew
Typo

[Ruud]

Yes, the baffle of the secondary. The secondary itself is the silvered spot. There might be a base for the baffle still glued to the corrector plate, and the baffle might have unscrewed itself from that, or the baffle was glued directly to the corrector plate. 

This is a tricky problem! If you open the telescope you could in principle glue the baffle back in place, or screw it back on, but then you'd have to reassemble the telescope well enough so that it can still be collimated. That's the tricky bit. 

You need to find a dedicated Meade ETX website, maybe some youtube videos of people servicing their ETX.

[symmetal]

It's just the plastic light baffle which was stuck to the secondary mirror which has fallen off. The secondary mirror is the reverse side of the silver spot in the centre of the corrector plate and can't come off or move. You'll need to glue it back on. The front corrector plate can be removed by inserting a tool in the two holes on opposite sides of the corrector plate surround and unscrewing counterclockwise. Make a note of the corrector plate orientation with respect to the tube by marking the edge with a permanent marker or similar.

Edit: Oooh! beaten to it. 

If it's like the ETX-125 the baffle was held in place by a ring of double sided foam tape which has now failed. You'll probably have to remove all the sticky stuff left on the baffle and secondary mirror. For the secondary mirror I used copious amounts of isopropanol.

Edited January 7, 2018 by symmetal

[rl]

What has dropped off is the light baffle around the secondary..the important part (the secondary mirror) is the silvered spot on the back face of the meniscus. It was presumably glued on. 

The baffle has fallen on to the primary...hopefully it's too light and too soft to scratch the surface. 

The retaining ring for the meniscus should simply unscrew if you very carefully twist it using some pins/ nails in the 2 holes. You may have to make up a tool specially to move the ring; they can get tight with age and dust etc. 2 nails in a piece of wood with the right spacing will reduce the risk a lot. Spend some time preparing for this...it's worth an hour making up the tool and ensuring the OTA is firmly held without being bent. 

You should be able to simply remove the baffle and the scope will still work, but it will be prone to off-axis light getting in and spoiling contrast. If you're feeling brave then glue it back in place. Hopefully those on this site with Meade Mak experience. There's bound to be a special type of glue...there are those on this site much better qualifies on the exact details.

When the job is done the retaining ring only needs to be nipped up not much more than finger tight. 

Good luck, RL

Sorry, looks like everyone else types faster than me...a lot of responses for a clear night!

RL

Edited January 7, 2018 by rl
too late!

[bendiddley]

3 minutes ago, Ruud said:

Yes, the baffle of the secondary. The secondary itself is the silvered spot. There might be a base for the baffle still glued to the corrector plate, and the baffle might have unscrewed itself from that, or the baffle was glued directly to the corrector plate. 

This is a tricky problem! If you open the telescope you could in principle glue the baffle back in place, or screw it back on, but then you'd have to reassemble the telescope well enough so that it can still be collimated. That's the tricky bit. 

You need to find a dedicated Meade ETX website, maybe some youtube videos of people servicing their ETX.

Thanks. When you say "you'd have to reassemble the telescope well enough so that it can still be collimated" do you mean the process which symmetal is saying in his comment... "The front corrector plate can be removed by inserting a tool in the two holes on opposite sides of the corrector plate surround and unscrewing counterclockwise. Make a note of the corrector plate orientation with respect to the tube by marking the edge with a permanent marker or similar."?

[bendiddley]

1 minute ago, rl said:

What has dropped off is the light baffle around the secondary..the important part (the secondary mirror) is the silvered spot on the back face of the meniscus. It was presumably glued on. 

The baffle has fallen on to the primary...hopefully it's too light and too soft to scratch the surface. 

The retaining ring for the meniscus should simply unscrew if you very carefully twist it using some pins/ nails in the 2 holes. You may have to make up a tool specially to move the ring; they can get tight with age and dust etc. 2 nails in a piece of wood with the right spacing will reduce the risk a lot. Spend some time preparing for this...it's worth an hour making up the tool and ensuring the OTA is firmly held without being bent. 

You should be able to simply remove the baffle and the scope will still work, but it will be prone to off-axis light getting in and spoiling contrast. If you're feeling brave then glue it back in place. Hopefully those on this site with Meade Mak experience. There's bound to be a special type of glue...there are those on this site much better qualifies on the exact details.

When the job is done the retaining ring only needs to be nipped up not much more than finger tight. 

Good luck, RL

 

RL

Thanks. How do I work out where the exact spot is to glue it back on?

[rl]

The narrow end of the baffle will just cover the secondary mirror. I expect you will see the remains of the glue as a guide. It should be concentric.

[Peter Drew]

You don't need to remove the corrector from its cell, the whole cell unscrews and will screw back on retaing the orientation of the optics. As mentioned, the baffle is glued on concentric to the aluminised spot.  

[symmetal]

2 minutes ago, bendiddley said:

Thanks. How do I work out where the exact spot is to glue it back on?

Just concentric with the secondary mirror circle as best you can. It's not critical as only the centre portion of the secondary is actually used. I used a couple of tiny dabs of araldite to hold mine back on. Probably not the best solution but it's not coming off again. 

Alan

[Ruud]

Yes, everything has to go back exactly the way it was. I saw a youtube video on servicing an SCT and there were thin cork shims, for instance, that had to go back in exactly the same spots.

I looked again at the pictures. I think that on the inside of the secondary there are still traces of the glue that came loose.

You'll need to find the right glue too. Look for an expert website. There's one for Celestron SE telescopes so I imagine there will be one or two for the ETX. Also check on Cloudy Nights.

Edited January 7, 2018 by Ruud

[symmetal]

12 minutes ago, Peter Drew said:

You don't need to remove the corrector from its cell, the whole cell unscrews and will screw back on retaing the orientation of the optics. As mentioned, the baffle is glued on concentric to the aluminised spot.  

That's how I did my 125, but all the isopranol containing the dissolved seconary glue was collecting around the edge and seeping around the 'rubber' ring holding the corrector plate. In the end I decided to remove the plate from the cell as it was much easier to clean.

Alan

Edited January 7, 2018 by symmetal

[MarsG76]

I see that the question and solution have been delivered, but how is the primary mirror after the ring hit it?

 

 

 

[bendiddley]

4 minutes ago, MarsG76 said:

I see that the question and solution have been delivered, but how is the primary mirror after the ring hit it?

 

 

 

I can see some marks, could be scratches or could be glue residue. Any ideas how I can remove the primary to inspect it and clean?

[MarsG76]

1 minute ago, bendiddley said:

I can see some marks, could be scratches or could be glue residue. Any ideas how I can remove the primary to inspect it and clean?

On regular SCTs the back end is held in place with screws, you'd simply remove those screws and remove the whole back end allowing access to the mirror... mind you tho it might be different with muskatov designs. 

[Ruud]

This might give you some clues, but there will be differences between this telescope and yours.

 

[bendiddley]

17 minutes ago, Ruud said:

This might give you some clues, but there will be differences between this telescope and yours.

 

Cheers, thanks. I'll have a look through this.

[bendiddley]

Anyone got any ideas how I can remove the corrector plate without turning the scope upside down after I have removed the locking ring? As the baffle is currently resting on top of the mirror surface, if I turn the scope upside down the baffle is going to roll across the surface of the mirror before it drops on to the inside of the corrector glass.

[bingevader]

Is the baffle still in the same position as the original photo?

Can you hook it out with a long hooky thing?!

[bendiddley]

Yes but to get the corrector plate off I need to turn the scope upside down which will make the baffle roll across the mirror  or is there another way of getting the corrector plate off without turning the scope upside down?

[michaelmorris]

Another important thing to make note of the exact position of the front cell (or corrector plate on its own - which ever you decide to remove) relative tube at the point it comes off. This will help you make sure that you start screwing it back together from the right starting position.

[Peter Drew]

I would lay the scope down on its side so that the detatched baffle is halfway down the tube. If you can arrange it such that the front end is overhanging a table you should be able to get a good grip on the front cell to unscrew it, mine comes off fairly easily. 

[bendiddley]

On 07/01/2018 at 19:48, Ruud said:

Yes, everything has to go back exactly the way it was. I saw a youtube video on servicing an SCT and there were thin cork shims, for instance, that had to go back in exactly the same spots.

I looked again at the pictures. I think that on the inside of the secondary there are still traces of the glue that came loose.

You'll need to find the right glue too. Look for an expert website. There's one for Celestron SE telescopes so I imagine there will be one or two for the ETX. Also check on Cloudy Nights.

Just seen this, thanks for pointing out the glue ring! It might be a case of just pushing the baffle back on and it stays but if it's fallen off once it probably will again. Can anyone recommend me some glue please?

[RT65CB-SWL]

Oh dear Ben! that's a bit of a  after all the effort, work & time you have spent on it last month.

First thing I would do is give Steve at SCTelescopes a phone call  (+44 7934 063893) or send him an email before attempting a DIY repair.

If you can manage to push baffle back on, then you maybe able to let a thin cyanacrolite adhesive do what is called 'capillary action', (i.e. a drip of the adhesive follows the join). I do this to join to halves of a model aircraft wing together if they become damaged. Loctite used to do an adhesive specifically for repairing glass, though I don't know whether SuperGlue will do.

Edited January 9, 2018 by Philip R

[bendiddley]

17 hours ago, Philip R said:

Oh dear Ben! that's a bit of a  after all the effort, work & time you have spent on it last month.

First thing I would do is give Steve at SCTelescopes a phone call  (+44 7934 063893) or send him an email before attempting a DIY repair.

If you can manage to push baffle back on, then you maybe able to let a thin cyanacrolite adhesive do what is called 'capillary action', (i.e. a drip of the adhesive follows the join). I do this to join to halves of a model aircraft wing together if they become damaged. Loctite used to do an adhesive specifically for repairing glass, though I don't know whether SuperGlue will do.

Thanks Phillip. Actually this is a different scope to the one I had the problems with last month, luckily I sorted that one, now I have a new challenge to get my teeth into  Cheers for the advice. I'll look into that. Just thinking though, won't the glue spread onto the secondary mirror with the way you are suggesting??