removing the primary mirror from an 8" LB

Brian W (Brian) · #1 29-07-2009, 04:01 PM

Hi all, we have owned our 8" LB for about a year and it is beginning to need cleaning. Our major problem is that both my wife and I are seriously mechanically challenged. Our minor problem is that where we live in the Philippines we -are- the local astronomy club.

Any and all advice will be greatly appreciated.
Brian and Myra

White Rabbit · #2 29-07-2009, 05:07 PM

Hi.

Your in luck because it's easy to do. If its like the 12", then it's a piece of cake.

All you have to do is turn the part with the mirror upside down and take the six or so screws on the side of the scope then slide the whole thing out. Dont bother taking the mirror off of the cell as that will be a pain to put back together, just wash the mirror, fan and all. Make sure you dont put the fan on though until its dry. Remember to rinse the mirror with demineralized water when your done.

PCH (Paul) · #3 29-07-2009, 07:24 PM

I agree, it is a piece of cake.

Although I did take the mirror cell off the rear fan assembly by means of the six (black) screws reachable from inside the lower half of the dob.

Putting it back was fairly easy too. Be aware you will have to roughly align the primary, using the adjustment screws on the rear fan face, to get the secondary approx central when you look through the focuser. Mine was way off at first, and as it was my first time (cleaning the mirror that is) I had to think twice before realising why the secondary appeared so off-centre.

But that done, it collimated nice and easily.

Cheers,

Brian W (Brian) · #4 29-07-2009, 09:22 PM

Thanks for the replies. The collimation will be doable without to many blasphemes... I hope. But remember I did say -severely mechanically challenged-. When I have the mirror turned upside down and take the screws out what stops the mirror from accepting the call of gravity and falling to the floor? And how do I put it back in without touching the mirror surface? Or am I misunderstanding the upside down part?
Brian

Robh (Rob) · #5 29-07-2009, 09:45 PM

Slightly different approach. Put the whole bottom OTA the right way up on the floor (mirror at bottom). See the line (join) running down the back of the OTA. Under it, put a small texta mark on the bottom plate so you can assemble it exactly the same way. Take out the six screws then lift the OTA case off the mirror cell. As said below, you can wash the mirror without removing it from the cell.

Regards, Rob.

Brian W (Brian) · #6 29-07-2009, 10:30 PM

Hi Rob, that seems a lot safer to this poor old duffers way of thinking... no sliding, no gravity and much less chance of dropping anything. Thanks

Robh (Rob) · #7 29-07-2009, 11:00 PM

Brian,
I'd leave the plastic cover on the mirror till you've got it all apart. Just to play it safe.

Regards, Rob

Brian W (Brian) · #8 30-07-2009, 03:28 PM

Wise advice Rob and it is exactly where it is. Raining too much to get off the mountain today so no chance to get down to pick up the cotton and distilled water. But we have taken the chance to wrap the aluminum struts in flat black cloth. Hopefully in the next week or so the monsoons will cooperate and we will be able to get the supplies in for the cleaning.
Brian

erick (Eric) · #9 30-07-2009, 03:54 PM

If you cannot get distilled, de-mineralised should be OK.

But, why don't you collect a nice plastic bucket full of that lovely monsoon rainwater! Even better?

But not during a lightning storm - then you get dilute (very) nitric acid.

Brian W (Brian) · #10 30-07-2009, 05:28 PM

Hi Erik, as the old engineer put it... the theory is good the practical is not so good.... Regrettably all of the rain that falls here is severely acidic. @ 100 million people in the Philippines and at least 80 million cook with charcoal.
Brian

Brian W (Brian) · #11 31-07-2009, 02:00 PM

Hi all, thank you all for your suggestions. here's whats happening... (1) marked everything for easy reassembly. (2) stripped the scope with the scope right side up except for the cooling fan which should be ok to soak and if it isn't I have never actually used it anyway. (3) currently soaking primary in filtered water and one small drop of dish-washing liquid. (4) when Myra gets home from town with the sterilized cotton batten we will either swirl or dry depending upon how the mold has come off in the soaking. (5) swirl and dry the secondary and then put it all back together.

Let you all know how it works after a clear night.
Brian

erick (Eric) · #12 31-07-2009, 02:04 PM

Quote:

Originally Posted by Brian W

... Regrettably all of the rain that falls here is severely acidic......

Ooops!

jjjnettie (Jeanette) · #13 31-07-2009, 02:52 PM

I can understand your anxieties, we all went through it the first time we cleaned our mirrors.
I'm sure you'll have no problems at all.

Brian W (Brian) · #14 01-08-2009, 10:37 PM

Hi all thanks for the advice and encouragement. Myra and I have got everything back together and we have also taken the opportunity to flock the wee beast. All that is left is to collimate. Though it was not as badly off as I had feared it would be.

Not to make toooo big a deal our of it but this has a definite 'rite of passage' feel to it.
Brian

Brian W (Brian) · #15 01-08-2009, 10:39 PM

One more quick question... all of you Australian types are some kilometers distance from where?

JethroB76 (Jeff) · #16 02-08-2009, 11:01 PM

Quote:

Originally Posted by Brian W

One more quick question... all of you Australian types are some kilometers distance from where?

from you