Problem with my Meade LX Quartz Drive optical tube.

[Quirq]

Hey everyone,

I have a telescope, I believe it was made a VERY long time ago, maybe in the 80's.

It has worked fine before, but it doesn't work anymore.

It is a Schmidt-Cassegrain OTA. I use a super plossl 20mm. The telescope is an 8" with a focal length of 2000mm I believe. F/10?

I have some photos through the finder, and then through the telescope.

(Note: If I take out the eyepiece I can still see the image formed clearly inside as it's reflected into my eyes. Just when the eyepiece is added and looked through, it goes... Funny).

Here are the images:

Through finder;

Through telescope eyepiece;

Any help would be much appreciated.

Kind Regards,

Zade.

[damnut]

Hi Zade,

The 20mm ep will give a 100X. Try a longer focal length ep which will yield a lower magnification and see if you can achieve focus.

Can't really tell from the image whats wrong.

[Peter Drew]

I would think that the "problem" with the main telescope is that it is not correctly focused.

[ronin]

The trees are too close.

The image formed will be too far back and there is not enough travel on the focuser to accomodate this.

Scope wasn't designed for trees at 800yds, was designed for M31 at 2.5 million light years

[Quirq]

I've tried it pointing at the Moon, and it was perfectly aligned to it. I had turned the focus knob all the way left til it didn't go anymore and all the way to the right til it didn't go anymore as well. I tried also with stars, doing the same thing to no avail.

I'll see if I can get another ep to use, and hopefully it will work...

[John]

As it's an old scope I wonder if the primary mirror has become detached from the focus knob mechanism somehow ?. When you turn the knob the main mirror moves slowly back or forward to bring the scope to focus. If the mirror is not attached to the knob then you won't be able to bring the scope to focus.

[Quirq]

I can clearly see the primary mirror moving forward and back when I turn the focus knob. So that works...

Kind Regards,

Zade.

[Oddsocks]

Hi Zade,

I remember my old 10" LX200 would not reach focus if the eyepiece was placed directly in the rear cell of the OTA, it would only focus if the diagonal was fitted as well to bring the eyepiece further out from the OTA.

William.

[Quirq]

It's very weird though, because it used to work as is. It didn't need a diagonal at all. Would there be anything wrong with the primary mirror? Does it need collimating?

Kind Regards,

Zade

[Oddsocks]

If it was badly out of collimation then you would only see a dark shadow or a crescent through the eyepiece but you said when you look through the aperture without the eyepiece then you can see a normal image.

The picture you took through the eyepiece is so strongly blurred and that centre spot looks for all the world like the ventilation hole in the eyepiece transport protection end-cap.

Can you post a picture without eyepiece fitted looking through the telescope aperture and another two looking at both ends of the eyepiece only, maybe part of the eyepiece assembly is missing?

William.

[Quirq]

If you go to my Imgur here, you should see all the photos I took.

http://imgur.com/a/uAydi

Kind Regards,

Zade

[Oddsocks]

Hi Zade,

Your latest pictures show that the view through the back of the telescope is normal and the collimation is good enough to bring an image to focus, since you checked the mirror is moving over the full range, lock-to-lock then I think you can assume the telescope is not faulty.

This leaves the eyepiece and from your pictures it looks like a Meade Plossyl, the barrel is very short so this makes me think it is a high power eyepiece and I know from experience that these won't reach focus directly at the visual back of the telescope, at least they would not with my old LX200 which is basically the same OTA that you have.

Since it was working in the past then it is always possible that it may have been knocked and one of the eyepiece elements has been dislodged making it impossible to reach focus.

The higher power eyepieces are more easily put out of focus by a knock than the low power eyepieces as the intra-lens spacings become more critical as the focal length of the eyepiece is reduced

There are a couple of simple tests to make to check the focus distance and the optical tube, aim the telescope at the horizon in the daytime, adjust the telescope mirror to somewhere in the middle of its adjustment range, don't fit the eyepiece in the telescope, now hold a piece of white card over the rear telescope aperture and gradually move the card away from the rear cell, at some point an image of the horizon will appear sharply focussed on the card, if you turn the focuser knob one way the focal point will move toward the telescope, turn the knob the other way and it will move away from the telescope. This will show for certain that the telescope is optically ok.

Now turn the focus knob to bring the image to a sharp focus on the card when it is held around four to six inches from the rear of the telescope, swap the card for the eyepiece, but held in your hand, don't fit it to the telescope, hold the eyepiece in front of your eye and line it up with the telescope cell aperture and move your head toward or away from the aperture. If the eyepiece is ok then I would expect the image in the eyepiece to come to focus around 4", 100mm, from the back of the telescope, if it does then the eyepiece is ok and it needs the diagonal fitted to the telescope to get the correct distance from the back of the telescope, if it does not come to focus at all then the eyepiece must be faulty and I would suggest trying a longer focal length eyepiece of around 15mm to 26mm.

From all that you have said so far I think the eyepiece must be suspect but the tests above will prove it for certain.

Hope this helps.

William.

[Quirq]

I shall definitely try that out tomorrow afternoon. Thanks very much for the response, much appreciated

Kind Regards,

Zade.

[Quirq]

I tried just then, but I didn't see any image formed on the card.

Oddly, If I do insert the ep in the end of the OTA, and keep my head a foot away from it, I can see the image formed IN the ep...

Kind Regards,

Zade

[Oddsocks]

Hello Zade,

Well, from your results it would suggest that the mirror is not moving far enough to bring the prime focus outside the body of the OTA.

The fact that with the eyepiece fitted and your eye about a foot from the telescope and you can see an image that means (i think) the prime focus is well inside the body of the telescope.

With my old LX200 I could image with a web cam at prime focus at a distance of around two feet from the back of the OTA when the mirror was at it's maximum range of movement.

Now I sold my LX200 years ago and my memory is not great these days so when you reply to this post I will rely on hopefully another SGL member picking up the thread to answer if necessary.

With the mirror fully towards the front of the telescope you should be able to focus an image on the skyline of those mountains that I guess are around five miles away? on to the white card held at a distance of around two feet from the back of the OTA.

But...I, can't for the life of me remember if the mirror is fully forward to move the prime focus further out the back of the OTA or if the mirror is fully back so try it with the mirror at both limits.

Now I suspect that something is jamming the full movement of the mirror, again, from memory I think you should be able to turn the focus knob at least fifteen turns from end to end, so turn the knob fully anti-clockwise, mark the knob with a marker pen or piece of sticky tape and then count the number of turns in the opposite direction until the limit is reached, then post your findings here, or, e-mail Meade support and ask them what is the number of turns for the spindle of the focuser for your OTA supposed to be.

It may be just something as simple as the grease used to lubricate the focuser spindle having gone hard over time and this is stopping the mirror from moving over the full range, or, if fitted, the rubber "O" ring buffers used as end stops on the focuser spindle have perished and are jamming the spindle thread at one end.

Unfortunately the only way to check this out is a strip down of the OTA and for the old Meade telescopes with steel screws that rust this is not an easy task.

If you search on-line you will find loads of articles on how to disassemble and carry out maintenance on the mirror support and focuser mechanism.

Let us know how you get on.

William.

[Oddsocks]

Here is a link to an X-ray CT scan, PDF document on Cloudy Nights, of a Schmidt Cassegrain OTA similar to yours, you can see how the mirror focuser mechanism works to move the mirror forwards and backwards.

Helps to give you an idea how the telescope is constructed internally and what may be stopping your mirror from moving over the full range (if that is the case).

http://www.cloudynig...ents/ctscan.pdf

William.

[Peter Drew]

Just to confirm, moving the mirror backwards focuses the telescope closer and vice versa.

[Quirq]

I can turn the focus knob, exactly 43 times. Lock to lock.

Kind Regards,

Zade.

[Oddsocks]

Hi Zade,

The only thing I can think of that is left, since you can not bring an image to prime focus outside the OTA with the focus knob at either limit, is that the mirror support arm connecting the focuser spindle to the mirror support bush is either bent or loose so that the mirror can not move physically far enough.

I read a few posts on Cloudy Nights forum about five or so years ago where the grease had gone hard on the mirror support bush, the mirror had become stiff to move and the focuser arm had bent backwards towards the rear of the telescope OTA, this, over time, gradually moved the focus point further forward into the OTA so that eventually it would not focus on a camera anymore.

There were also a few posts where the mirror support arm had become loose and the user had reported a "clunk" when the OTA was tipped down towards the ground and the mirror flopped backwards and forwards, this could also be a reason for the failure to reach focus.

If you are absolutely sure that you can not project a sharp image of the distant horizon on to the white card held behind the OTA with the focuser knob turned to either limit then the only explanation I can think of is either of the above and you will have to remove the rear cell of the OTA and check what is happening inside, either the mirror support arm has become loose or it is bent.

If you do remove the rear cell then you need to make a mark on the rear cell flange and a corresponding mark on the wall of the OTA so that you get the correct alignment again when you reassemble.

The rear cell and mirror is the heaviest part of the OTA so be aware it needs careful handling as you pull the rear cell away from the rest of the OTA.

If any of the steel screws that hold the rear cell to the telescope tube are rusted then you need to squirt a bit of penetrating solution such as "Plus Gas" on to the screw heads and leave for twenty four hours to soak in before trying to remove the screws, any that do shear off can be drilled out and re-tapped, many Meade stockists keep replacement screw sets that are made of Stainless Steel but you can buy them cheaper from most good machine shops or motor bike spares dealers etc.

If the mirror support bush grease has gone hard and bent the focuser arm then you need to straighten the arm, clean of all the old grease and re-grease with a so called "High Vacuum Bearing Grease" which you can get from most vacuum pump/bearing suppliers, normal grease is not recommended because it will gradually evaporate within the OTA and leave a coating on all the optical surfaces.

That is the limit of my knowledge Zade, I will leave the post to other SGL members who may have a different idea, post back what you find and any other questions you might have, also there are many more Meade owners and support groups in the USA so it might be worth posting your questions on a US based forum like Cloudy Nights.

Hope this is of help.

William.

[Quirq]

Hey William,

Thanks very much for your help, I sure hope I can get this working.

If not I was planning on upgrading (Considering how old it is).

No matter what the turn out, thank you a whole bunch for the very detailed, and helpful answers. Very much appreciated.

Kind Regards,

Zade.

[Quirq]

I did however try the focus test again.

This time in a dark room with a flashlight instead.

I can get the light to the smallest point (the prime focus I'd assume) which is outside of the eyepiece, no more than an inch or an inch and a half away from the eyepiece.

So I'd say it's definitely focused, just outside of the eyepiece...

Kind Regards,

Zade.

[Oddsocks]

This might help you Jade,

Here are a few images I took this afternoon to help you visualise what an image at prime focus looks like.

The telescope I used is an 80mm refractor with NO eyepiece fitted.

The image (inverted) is of a building around a hundred metres away and the trees are about half a kilometre away, the weather has been cloudy/overcast all day so the image is not very bright but I think you can see what to expect with your Meade SCT under similar conditions, your image though should be much brighter as you have greater aperture.

 

Here is a picture of the set up used to make the prime focus image:

 

And finally here is an image of the view through the focuser barrel looking at the same scene:

 

So, if your telescope focuser is moving the main mirror over the full adjustment range then you should be able to project a similar image, without the eyepiece fitted, at a similar distance.

If you can not project an image like this without the eyepiece fitted then it must indicate that the prime focus is well inside the body of the OTA for some reason and this is why you can not achieve focus with your eyepiece.

William.

[Oddsocks]

Zade,

Apologies for the mis-spelling of your name in the last post, "less haste, more speed" I think the old saying went...

Best of luck sorting the problem.

William.

[Quirq]

I just realised, that I already have a diagonal fitted. I'm somewhat new to the word, so forgive me about that.

I haven't tried it without the diagonal, so I probably should.

Sorry about that

Kind Regards,

Zade.

[Quirq]

So again I tried that test, and It didn't show anything on the white card. My setup looked the same as yours but alas.

Very unsure of the problem now, but thanks anyway.

Kind Regards,

Zade.