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#1 biggles

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Posted 26 October 2013 - 04:44 AM

Hi fellow astronomers, I need to remove the back off my de-forked Meade LX90 10" for inspection. This is something that I would not attempt without REALLY good reason!...here's the problem.....

Some months ago it went in to have the inside of the corrector plate cleaned to remove a build up fungus (unfortunate emergency storage in foam lined wooden box in leaky shed! :p )
This was done very well and the scope was put back together by the Meade qualified engineer here in the U.K and re-colemated.
I have EVERY REASON to belive that it left their shop into my hands in tip top condition, so I now think something has happened to it over a four day period when it was placed corrector side down (mirror at the top) on a friends floor waiting to be carefully transported by me to my home.

The symptoms:

1. I set up useing the Meade 2" diagonal ( been used for years with no problem) and can focus on the trees at the bottom of the garden.

2. It is impossible in it's present state to focus on the Moon or stars.
As I wind the focus anti-clock wise I can seen the badly de-focused image of the star or Moon start to come to focus (but it's WAY out of focus). It is worth mentioning that at this stage of the problem I,ve started with the focus knob fully clock wise, and as I have moved the focus knob anti clock wise. While doing this the defocused image moves a lot around the eye piece field of view, which I don't recall happening before.
Just before I reach full anti clockwise travel with the focus knob the de-focused image splits into two (looking like a defocused image of a close binary!)

3. By removing the diagonal and very carefully inserting and holding an eyepice 91.25" 30mm...same as used in the diagonal)into the open back focus can be reached.

I can not beleve that this is a mistake when the scope was re-asembled and left the engineer, and I am wondering if somebody tried to move the scope and dropped it, or maybe hit it with a vacum cleaner or something.

So what I need is advice/info on how to remove the back so I can see if something is wrong internally.

All advice will be greatly appreciated!

p.s returning it to the engineer is not posible due to short term un-employment, a trip to take scope to them and another to get it back is 400 miles!

#2 jrcrilly

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Posted 26 October 2013 - 10:31 AM

Sounds as though something is limiting the primary mirror travel. How many turns of the focus knob are there between the extremes? Sometimes people remove the finderscope bracket and put the mounting screws back in to store them. Without the bracket, the screws are too long and extend into to path of the mirror; that's one possibility.

#3 Geo.

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Posted 26 October 2013 - 11:12 AM

The only way to the rear cell is through the front. Meade bonds the cells to the aluminum tube. Removing the corrector for cleaning will have no effect on the focusing mechanism.

Turning the focus knob counter clockwise (CCW) pushes the primary away from the rear of the scope. Infinity focus is achieved at some point before the knob is turned fully CCW. Near focus is fully CW with the primary pulled back. The 2120 has a near focus of somewhere about 100'.

Soooo, I would first look at the focuser. This is basically a stationary nut that rotates about a threaded shaft that is in turn connected to the primary carrier. Meade uses a simple housed stirrup that slips over a pin on the carrier's arm. It can become dislodged, particularly in the 10" OTA when transported without the primary retension bolt (2"-1/4"-20).

Use these intructions to remove the focuser for inspection.

http://karmalimbo.co...nstructions.pdf

#4 Tophat3

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Posted 26 October 2013 - 11:13 AM

I agree, do you have any hang ups when you turn the focuser?

If not I would like to add a suggestion that you may have a collimation issue.

Before tearing into it, I would suggest looking up " Gross miscollimation under adverse circumstances" just google it and it will bring it.

This will give you step by step guidance through the process. You may have to read it a time or two to feel comfortable with it, but it works like a charm if you are uncomfortable with the collimation process.

I suggest waiting for a clear night and taking it out and focus on a bright star a following the instructions and if its a collimation issue, which I suspect it is, this will work.

Your scope could have indeed been knocked out of collimation even though it was collimated before it left the shop.

Please let us know how it turns out.

Thx

Mark

#5 biggles

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Posted 26 October 2013 - 03:06 PM

Thanks for the replies guys,

John C, that is a possibility, as the tube is bonded to the cell I can remove all bolts from the rear and just for test purposes use smaller bolts for rear section of dove tail.

George, I will look at the focus in the morning.

Tophat3, the focus knob moves reasonably freely, I will count the turns in the morning and report back.
As you say, it could be a collimation issue. Again I will check tomorrow first with an artificial star, then as soon as possible a real star.
Thanks for the idea of googeling for instructions, I shall do this directly after posting this reply.

Thanks again guys I will report back results.

#6 biggles

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Posted 28 October 2013 - 03:43 PM

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!
Thanks John C for pointing me in the right direction.
:grin:

#7 Tophat3

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Posted 28 October 2013 - 04:52 PM

Biggles

Glad to hear your up and running again, and thanks for getting back to us.

Good call John, glad your around to help.






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