I suspect I have a telescope disaster!

[Gina]

No, I haven't take the end off - only the focuser.  And that was many years ago.

[JamesF]

Sorry, I meant Freddie.  He has an MN190 in his signature, so it's entirely possible he's had it apart and knows what's actually inside.

James

[Gina]

Ah, I see.

[Peter Drew]

Chin up Gina, if all else fails I would be surprised if the coating problem has much impact on imaging results. I too. would prefer to remove the cell complete without disturbing the corrector provided that, as already said, there are potentially loose nuts. I have one of these telescopes, I'll have a close look tomorrow morning to try and see what method of attachment is used.   😀

[Gina]

Thank you Peter 😀

I've been out to the observatory and managed to remove one of the dome head screws.  They go into a tapped hole in the lens cell.  The holes in the scope tube are much larger than the screws so I'm wondering if position is critical.  I also removed the front six screws which evidently hold the lens in place.  I was able to remove the ring but not the lens - I didn't try any force.  The other five dome head screws won't budge so I've put a drop of penetrating oil on each one.  These are steel screws and I suspect the lens cell is aluminium so they could be corroded in.

[Freddie]

The cell slides into the tube a few cm. The screws go through holes in the tube and screw into the cell so there are no nuts or whatever to worry about.

[Gina]

Is there any problem with the position of the cell in the tube or does the cell just rest up against the end of the tube?

[JamesF]

6 minutes ago, Freddie said:

The cell slides into the tube a few cm. The screws go through holes in the tube and screw into the cell so there are no nuts or whatever to worry about.

That's helpful to know.  Could well make things much easier.

James

[Gina]

I'll see if the remaining screws will come out tomorrow when the oil has had a chance to penetrate.  I might have to drill the screws and use an "easy-out" (I have a set but not sure if the smallest is small enough).  On a normal engineering job I'd apply heat but not near glass.

Edited July 1, 2019 by Gina

[Ships and Stars]

Looks nasty, but I bet it cleans up fairly well. No idea on dismantling ones of those, but as a former motor mechanic, my advice is mark and index everything, and if you have something with adjustment screws or threaded retaining rings etc, mark the alignment before dismantling and then count the turns as you remove it so it goes back close to where it was, if not bang on. I'm sure it can be salvaged! Awaiting a triumphant post!

[kippford]

Can you get a replacement for that glass?

[Gina]

No idea but it sure wouldn't be cheap!

[kippford]

Hopefully cheaper than a new scope though.

[Gina]

Maybe but not necessarily.  Scopes are almost mass produced.

[Gina]

Well, I've just used the scope to view Jupiter (if it was Jupiter and not a very bright star!).  8mm eyepiece and 3x Barlow but all I could see was a point of light.  Guess I need much more power!

[Freddie]

There are no adjustment screws or retaining rings etc just screws, holes in the tube for the screws to go through and threaded holes in the cell for the screws to screw into. Just mark the cell and tube so that the rotational position is set and see if you have a gap between the tip of the tube and the cell. I didn’t, the cell just pushes in until it sits on the upper rim of the tube.

[Gina]

I checked for a gap between tube and cell and there wasn't one.

Do you have any suggestions for budging the screws?

Edited July 1, 2019 by Gina

[Freddie]

9 minutes ago, Gina said:

I checked for a gap between tube and cell and there wasn't one.

Do you have any suggestions for budging the screws?

Yep, go to bed and wait for the penetration fluid to do it’s job!!

[wimvb]

Dielectric coatings on glass are very sturdy nowadays. With modern ion assisted coating technology, they have the same density as bulk material. Unlike in the past, when coatings were grained and would absorb moisture, and adhesion to the underlying substrate could be poor. So it's unlikely that the coating came off. Cleaning the glass as you would clean a mirror should work ok.

Some years ago, sgl member @Gunnar had a thread on replacing the focuser of his MN190, and I believe that at one point he removed the corrector plate. His posts are still available here.

Regarding rusting EQ8 bolts, that's not a surprise. Whenever two different metals meet (including "stainless" steel), electrochemistry kicks in, causing corrosion.

[Nigella Bryant]

Sorry to read about your scope, hope it's easily remedied by a good clean.

[wimvb]

Here's the old thread.

 

[wimvb]

If I'm not misstaken the corrector cell is put together as follows.

6 rounded screws hold the cell into the metal tube, just as the primary mirror cell.

Recessed screws in the side of the aluminium cell keep the corrector glass centered.

Screws on the front hold a ring that keeps the corrector plate in place in its cell.

To remove the corrector plate, you'd remove the front ring, and slightly loosen (one of) the recessed screws on the side. The corrector should then be able to just fall out of its cell. Mark the plate and cell as was advised previously, before you remove it. You should probably also remove the secondary mirror holder if you want to clean the glass properly. Otherwise water could get trapped between the (coated) glass and metal parts or screws. The position of the secondary mirror is critical with this scope. AfaIk, it should not be moved axially, only rotated and/or tilted for collimation.

I haven't tried this myself. It's just my conclusion after examining my own scope and reading online, so I may be wrong.

Btw, just as a precaution, I have now taped two small silica gel bags on the inside of my MN190 dust cover. 

Edited July 1, 2019 by wimvb

[Geoff Barnes]

10 hours ago, Gina said:

I'll see if the remaining screws will come out tomorrow when the oil has had a chance to penetrate.  I might have to drill the screws and use an "easy-out" (I have a set but not sure if the smallest is small enough).  On a normal engineering job I'd apply heat but not near glass.

No disrespect to you Gina, but you don't look like the most powerful person in the world. Do you know a big strong man who could loosen those screws for you?  

[Gina]

Yes, I do, but I suspect he would use too much force and break something.

[Tomatobro]

Try taking advantage of the different expansion rates of the steel screws and the aluminium holder. Hold a soldering iron on the head of the screw to warm it up then try to loosen the screw.