LX200 focusing issue fixed .

[Rusty2]

I'm writing this in the hope that it may help someone else with the same frustrating issue .

To cut a long story short , I was having ongoing focusing issues with my 12 inch LX200 . After years of trying to figure it out I used a Farpoint laser collimator to find the true centre of the primary optical axis . Upon doing this I found that my SCT was unable to be properly collimated due to the limited movement of the corrector plate and poor manufacturing .

About 4 or 5 years ago I bought a Meade 12 inch LX200 ACF .
The moment I looked through the scope for the first time I was disappointed . I couldn't seem to get it in proper focus .
It was suggested that I probably needed a good focuser . After installing the moonlite focuser the same thing happened . Couldn't get it into focus .

After trawling through the astronomy forums I found the possible problem . Collimation .
I bought a Hotech laser collimator .

During the collimation process I reached a point where I had to centre the secondary mirror to the primary optical axis . I noticed an immediate problem . I could get it close but not perfect . I couldn't push the corrector plate and secondary mirror far enough in one direction to properly centre the mirror . I assumed this was inexperience and didn't think much of it .

After years of collimating and recollimating with no success , I looked around for advice and found that a field flattener/ reducer might help and bought a Celestron F.63 x flattener/corrector . It didn't work and apparently was never going to work . I gave up on the LX200 and bought a proper astrophotography scope .

Faced with the prospect of having to sell the scope I tried one last time to see what was wrong .

This time I used a Farpoint laser collimator . A single centered laser with one straight beam of light . I unscrewed the adjustment screws on the focuser so the focuser was flush with the primary optical axis barrel . I then bought a piece of corflute and faced the telescope face down on the corflute and traced the telescope diameter and cut it out of the corflute . I then removed the corrector plate and secondary mirror and replaced it with the corflute and turned on the laser . The laser produced a red dot on the corflute which showed the true centre of the primary optical axis .
I burnt a quarter inch hole in the corflute with a soldering iron and covered it with a red circle sticker . I then burnt a 5 mm hole in the sticker .

The exterior plastic adjustment plate of the secondary mirror has been manufactured in a way that you can see the centre point so I poked a small hole in another red sticker and placed it over the centre point of the plate .

Removing the laser , I then sat the telescope on the floor so it was sitting on the focuser with the front of the telescope facing up . I inserted the corrector plate and looked through the corflute to find the centre of the corrector plate . It was around 6mm out and couldn't be adjusted any further . This was the problem I'd been looking for .

This would also mean that the scope was never properly collimated at the factory and never could be without machining of the exterior ring which was preventing the corrector plate from adjusting any further .

I had some special drill bits for grinding metal and ground down the exterior ring . After about 2 hours the secondary mirror was finally centered . I cleaned all the glass and tightened the corrector plate into position . I finished off the collimation procedure with the hotech collimator , found the secondary mirror was now centered and adjusted the secondary mirror to point to the centre of the primary optical axis .

I also bought a Lepus 0.62X Telecompressor to improve the clarity .
I have yet to use the scope due to the terrible weather but will post here when I do .

https://www.californiaskys.com/blog/...-ct-collimator
https://miltonhill.us/Tele/Meade%20L...ube/index.html
https://brazosvalleyastronomyclub.or...pe-repair.html

[The Mekon (John Briggs)]

Brilliant work Rusty. Way beyond "Beginner" level for this forum - I only caught the post as I replying to my thread.
Let's hope you get the Meade operating as it should,. It is a shame that so many of these scopes had problems from the factory.

[Nikolas (Nik)]

That's quite the task, have you tried looking at a distant object just to see if it works properly?

[Rusty2]

Hi John : )
Yeah it's a shame , more for the unsuspecting newbies . I wonder how many have been sold due to the too hard basket .

Hi Nick : )
I did shortly after and noticed around a 30% increase in clarity .
However the secondary mirror was so far off dead centre that I thought it might be clipped in the FOV from the internal centre tube . I started all over again .

I was thinking that in a perfect world , everything should be perfectly dead centre within the OTA .

I made some more corflute cutouts with centres marked and used the Hotech laser to see if I could align the eyepiece , the centre tube , the corrector plate position and the primary mirror all in one . I managed to align everything BUT the Primary mirror . That's the culprit , the primary mirror is not parallel with the OTA as you can see in the bottom right hand corner of the attached photo , but everything else is .

At about 2 this afternoon I also noticed that the wooden floor in my house is moving when I walk near the laser . I've had to take everything outside onto the concrete and start again . This will be the fourth time I've tried to collimate today .

[Rusty2]

Oops , forgot the photo .
The angles make it look weired but they're all centered except the primary mirror , bottom right .

[anthony2302749]

Hi Rusty2

I feel your pain, have you considered collimating your telescope on a star or an artificial star. This is how I collimate my all my SCT that I have owned (currently have a 12” LX200) and have sold when working for a lead telescope shop here in Melbourne. Using this method of collimation I have had excellent results, and yes the focus is sharp. Have a read of the link I have provided, it pretty good and goes into detail about what you should be looking for in regards to collimating a SCT.

http://www.astrophoto.fr/collim.html

Good luck.

P.S. did originally use a laser to try and collimate an SCT found that collimating on a star work way better once you understand what happening.

[Rusty2]

Hi Anthony : )
I did collimate on a star about 3 years ago but wasn't experienced enough to know exactly what to look for . I did centre the donut but the secondary may have been off . I appreciate the link and have taken notice , thank you ! That method will be the next experiment .

I assumed that the Primary mirror was tilting forward when in focusing position but I pulled the corrector plate off to measure the difference and there was no dramatic tilting (maybe half a millimetre) so I really don't know what's going on or which part of the scope to concentrate on .
I pulled off the Moonlight focuser and replaced it with the original Meade back and eyepiece holder in a process of elimination .

I couldn't wait till dark so I tried a light projected through the eyepiece and onto the wall . Not a precise way of doing things but you could see the shadow of the secondary get larger and smaller . I'll now wait till I have time to look at it again . The guiding on the other scope needs more refining .

[anthony2302749]

Ok, good start. So when you are collimating on a star or an artificial star (this can be made out of a simple light circuit and Al foil). Your main focus is the diffraction pattern around the secondary not the secondary itself. Use an eyepiece which give you a resonable high mag, start about 300x. The point of focus is just in or out of best focus (1/4 turn or less) and what you are looking for is how concentric the diffraction pattern is in regards to the secondary. Make sure the diffraction pattern is in the centre of the FOV. Now as the only collimation adjustment a SCT has is on the secondary, choose one give it a tweak CW/CCW, your choice. Re-centre the star look at the diffraction pattern again and if it looks more central your heading in the right direction, if it’s worst, tweak in the opposite direction, re-centre and repeat above process until you are satisfied that the diffraction pattern is concentric around the secondary.

Should note, a cheap way of creating an artificial star is to use a marble, the sun reflecting off it will give you a nice artificial star to work with, is mentioned in the LX200 manual, well the old ones I have seen.

Also if you can do the collimation during the day it will be easier no worries about tracking etc set up the telescope on you mount power up centre the artificial star and star the collimation process. For me usually in the garage looking across the road at my neighbours mail box. Doesn’t work with my LX200 12” due to the near focus being fur then that is I will be using a star.

The only big problem with the Meade 12” is it near focus, which is roughly 40, 50m. So if you can get a clear line of sight for at least 50m plus you should get focus an spend the day adjusting until you get a very nice tie concentric diffraction pattern to the point of a Google Airy Disk.

Other thing to be careful of is don’t loosen the collimation screws to much, I have heard painful stories of secondary falling on to the mirror. If you feel one screw getting to tight, back all of a full turn equally this will not effect the collimation to much. Link below for help on making an artificial star.

https://www.cloudynights.com/topic/4...tificial-star/

Also a good read if you want to dive into star testing.

https://shopatsky.com/products/star-...es-2nd-edition

Anyway keep working on it and again I feel you pain.

Good luck


P.S. kindly help from Zurich at the moment.

[anthony2302749]

Hi Rusty2

Thinking about your problems and understand your reasoning in regards to finding the mechanical axis of your SCT in hope of collimating the telescope, there are issues with regards to this. Laser out of alignment (can happen), visual back (focuser) not being square to the optical axis can lead to the belief that the mechanical axis is out of alignment and therefore collimation. What we should be looking at is the optical axis. It is true to say that the factory does align the mechanics along the mechanical axis and then collimate the telescope but collimation does not last due to rough shipping and other factors. Plus people who sell these telescopes must up their sales pitch with explanation about how to collimate and maintain these instruments.

So, anyway looking at the image of the correct plate in your first post, all the cork shims are in place and the telescope index is aligned correctly (white out paint on the corrector and tube). Important if you plan to remove the corrector ensure it is returned to the index mark, this was set in the factory.

Not knowing the history of the telescope e.g. new or second hand, this a good indication that the telescope is still in original factory condition other the then any tinkering with the collimation screws. I can assure you that the optic are correct so you are good there.

Been brushing up on my understanding of SCT optic but couldn’t find any info in layman terms to best describe them so I will let that hang for another time.
So collimation of an SCT is about aligning the optical axis with the centre of FOV. So for example if you are just going to do visual work then the collimation would include everything in the optical train e.g. focuser, star diagonal. Astrophotographer would be straight through.

As we want to align the optical axis we don’t need to worry about anything else, only need to ensure the star in the centre of FOV and the diffraction pattern is concentric. Keep this in mind collimation should be conducted on a star for best results. In my previous post I tried to explain this but I found that Celestron gives a good example on the support page including pictures, see link

https://www.celestron.com/blogs/know...limation-guide

If you can, perform this task during the day with and artificial star, it will give you time to examine your work and ensure you are on track and also zero out any frustration. When you are spot on and have a nice Airy Disk with the artificial star, then finalise your collimation on a real star, about 4th mag.

Once you have collimation and still want to use a laser to check. Place the laser in the back of the scope and mark the return beam with a dot. Only work if you have a 45degree opening on the laser.

Any real feel your pain and I hope the information I provided will help get you telescope into tip top shape.

Hang in there, Cheers

Anthony.

[Rusty2]

Hi Anthony !
I appreciate you taking the time to explain this for me .

I assumed that the laser would be the most precise way of checking alignment but seeing the immediate effects first hand using a star or artificial star removes the guesswork .

I understand what your saying and have read the links , thank you !
I'll see if I can make a fake star box : )

[Rusty2]

I made an artificial star out of a small cardboard box and a headlamp . I focused and defocused but couldn't see anything that remotely resembled any concentric rings , maybe I should have done it at night . That was yesterday .

My holidays are about to run out and I wanted this project finished before I went back to work . Resigning myself to night time collimation which may be weeks away I tried one last thing .

Using a Sony A7S markll , a HDMI cable and a Flysight monitor I was able to visibly see the effects of turning the collimation screws on my Meade 12 inch LX200 . There is a noticeable sweet spot that can be targeted for rough collimation .

This obviously isn't a precise method and will need to be properly collimated on a star but it has been reassuring for me to know that after 4 or 5 years of doubt , my telescope isn't a loss and is capable of producing a reasonable image .

For me , the best thing about it is that I could immmediately see what was going on without spending hours with a laser .

https://www.youtube.com/watch?v=tANL_igO2os

[anthony2302749]

Hi Rusty

Nothing wrong with your light circuit from what I can see other than the size of the hole where the light shines out.

The trick with creating an artificial star is getting the correct size hole in the aluminium foil. We are talking of a very small hole in the micron size.

The method of doing this is to cut out say 10 to 20 small sheets of aluminium foil (say 5cm x 5cm) and stack them together and place the stack on a hard surface like a table. Find the centre of the stack and use a very fine dressmaker needle and press it gently into the stack of aluminium foil (you don’t need to drill through all the way, just press it into the stack)

Once you have completed this task you will need to go through the stack and find a sheet with a small enough hole to do the job.

My recommendation is to start with the bottom most sheet and work towards the top. Hold each piece of aluminium foil one at a time up to a light and look for any light passing through the pinhole you created. General speaking you will go thorough some with no pinhole, once you detect light passing thorough a piece of aluminium foil, choose the second or third sheet after the initial one this should be the correct size for the artificial star.

Careful place the choosen piece of aluminium foil over the light circuit you create and centre and tape edges down. If all goes well you have an artificial star of the correct size. Remember you can repeat the above procedure as many times you like until you get something you’re happy with.

Not to sure how you went about looking at the artificial star on your first attempt. But you have adequate distance to bring the light into focus. Could you give me some details of what eyepiece/accessories you used on your initial attempt.

Make sure you follow the instructions on the link I provided when collimating

http://www.astrophoto.fr/collim.html

Hang in there.

Cheers Anthony

[Rusty2]

Hi Anthony !

I'm not too worried about it anymore mate .
Using the live view method was enough for me to know that there's nothing wrong with my telescope . Basically that's all I needed , just to know that my scope wasn't a piece of junk and was capable of a reasonable image .

I will be following the instructions from the astrophoto link you've shared with me and now that I know , I'll just wait till the stars come out again .

Again I appreciate you taking the time to guide me in the right direction . I'll post some pictures when the skies clear up , thank you and have a great weekend !

Rusty

[anthony2302749]

Hi Rusty

Did assume you were using a camera, so this piece of software should help a lot. I have used it myself and get excellent results and will be using it on my 12” once I get back home to Australia.

https://smallstarspot.com/metaguide/

[Rusty2]

Hi Anthony !

Thanks for the tip , looks like an interesting piece of software to use .

I now have the ASIair plus and can't go back to laptops and multiple programs .

I ordered 2 eyepieces yesterday to perform the proper collimation steps as set out in the link you shared . I'm sure it will work well and solve my little problem : )