Explorer 130m Mirror Replacement

[allotmenteer]

I have recently purchased a second hand 130m and have now found the mirror has a small crack in it (pitfalls of not knowing what you are looking at when starting out!). A replacement is a pretty good price but not sure what is involved in swapping it our.

The image doesn't seem to be affected as far as I can see so thinking if it isn't totally broken......

Does anyone have any experience of replacing this type of mirror and advice?

Thanks in advance

Mark

[rikroe666]

Hi Xylene2011Lola

do you have any pictures you can post? Its quite easy to take the mirrors out, if its the primary mirror you are talking about? just take out all the small screws at the mirror end of the tube, i think there's about 5, then carefully pull the whole mirror cell out. once its out you will see 3 clips with 2 screws in each, unscrew all of these and the mirror will just pull out. just be very careful not to touch the mirror and have the telescope horizontal when working on it so the mirror doesn't fall on the floor or your toes.

Rick

[earth titan]

Are you sure its cracked? These are thick old bits of glass and cracking them is tough. Chipping edges off is easy though. I thought I had a cracked mirror - turned out to be a hair.

[rikroe666]

Are you sure its cracked? These are thick old bits of glass and cracking them is tough. Chipping edges off is easy though. I thought I had a cracked mirror - turned out to be a hair.

True, this is most likely.

I once knocked my old helios 130 over on its mount in my kitchen and it landed mirror side first, it put a dent in my wood floor but the scope was fine. It is as earth titan said  probably is just a hair or cobweb.

[allotmenteer]

Thanks - looking down the tube I can see a 'starburst' crack about 10 mm across (thought is was some form of contamination initially) looks like a screwdriver or the like has been dropped on to it. (Primary mirror).

Thanks Rick - this is encouraging, could imagine dis-assembling it and finding it's glued together and might never get it back together again (good description of about 50% of my DIY fixes ;-) ).

I imagined the mirror to be a flimsy plastic thing but if it is substantial I guess the geometry would not be affected and so maybe it's OK after all ?

Thanks

Mark

[rikroe666]

Thanks - looking down the tube I can see a 'starburst' crack about 10 mm across (thought is was some form of contamination initially) looks like a screwdriver or the like has been dropped on to it. (Primary mirror).

Thanks Rick - this is encouraging, could imagine dis-assembling it and finding it's glued together and might never get it back together again (good description of about 50% of my DIY fixes ;-) ).

I imagined the mirror to be a flimsy plastic thing but if it is substantial I guess the geometry would not be affected and so maybe it's OK after all ?

Thanks

Mark

yeah they are pretty thick, not sure on that mirror but i guess it would be near an inch thick.

its pretty daunting the first time you take one apart but its the best way to learn.

if i was you if the views through it are ok i wouldn't bother changing the mirror.

Rick

[allotmenteer]

I decided to go ahead with disassembling this as when I looked closely the mirror was very dirty and was keen to see how the cell was put together. Hope to get some help with collimating it too so will be able to get it re-set after cleaning. Good plan so far.

Have now removed the first of the 4 screws around the outside of the tube and found that the nuts (which I assumed were captive) are not so they drop off into the tube. unfortunately my arm is about 2" short to hold the nut on the inside and wouldn't be looking forward to me holding the nut and my wife trying to put the screw back in.

Any suggestions very gratefully received - if I remove the 3 non-adjustment screws on the base does this release the mirror (doesn't seem likely). Don't want to remove any more bits and find I can't get them back - I am only one screw down so far.

Maybe should have left well enough alone!

Cheers

mark

[rikroe666]

Hi mark

The 3 screws on the base are just to lock the mirror in place when collimated so i wouldn't take them out. 

I have never taken one apart that had nuts,on the few i have done the screws normally screw in  to the mirror cell.

if you cant reach the nuts it will be quite hard to put it back together.

You could take the secondary mirror Assembly off(same screws around the tube, but the other end) so its easier  to get your arm in if you really need to clean the primary mirror? 

Rick

[allotmenteer]

Thanks again - have got the secondary off to try to reach in (without success) guess they must have some people with long arms in the factory to build them.

Very frustrating though!

Do you just use a clean dry cloth to clean the mirror?

Mark

[Peter Drew]

No Mark, never clean a telescope mirror with a dry cloth. You need to use a swab of good quality cotton wool soaked in a very weak solution of washing up liquid and water and very gently pull the swab over the surface using just its weight. Change swabs regularly until the mirror is clean and then rise off with distilled water. Leave the mirror on edge to dry. 

[John]

Do you just use a clean dry cloth to clean the mirror?

Mark

You need to take some care not to damage / micro scratch the coatings of the mirror.

The approach is to blow any surface debris off the mirror with a puffer before you touch it with anything else. You then need to rinse the mirror in a sink with distilled water and 100% lint free cotton wool. Just use the weight of the soaked cotton wool in outward strokes discarding each piece of cotton wool after a couple of strokes. Then rinse the mirror off again with it on it's side in the sink and its edge on a towel. The corner of a kitchen towel can be used to absorb any drips of distilled water that don't just run off.

It's a delicate business and the mirror does need to be out of the tube for this.

Edit: Peter got there first !

[bogdog]

you have 6 screws on the end plate of your scope, 3 are locking screws and 3 are adjusting screws to adjust mirror

there should also be 4 screws on the side rim of mirror cell holding it onto tube

i think you have to remove the 4 screws holding the bottom cell to the tube out and wiggle the whole assembly off

once you have it off you can put the bolts back on where they fell off from

[allotmenteer]

unfortunately the nuts for the 4 bolts are not captive so cannot see how to replace them if removed as need to reach down inside the tube to hold them and then replce the screws from the outside.

Thanks for the cleaning advice but probably not an option if I can't get the mirror out.

Don't think I am missing anything obvious?

Mark

[Peter Drew]

You can always bond the nuts to the inside of the tube before replacing the mirror cell, use the screws to locate them in the correct position. 

[allotmenteer]

Thought about something like that but think theory and practice might be a bit different (for me anyway) - can imaging bonding the screws in too and having even more of a mess - might look for a second hand tube and will email SW again next week to see what they say.

Cheers

Mark

[John]

There was a thread on this a couple of years back but I'm not sure what the outcome was:

http://stargazerslounge.com/topic/162294-sw-explorer-130-primary-mirror-removal/

[allotmenteer]

Thanks, seems to reflect my conclusions. Maybe can replace the bolts with slightly larger ones and tap them in.

Anyone got a spare 130 OTA in their shed!

Mark