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#1 MrJones

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Posted 22 August 2013 - 11:58 AM

How do I get the big black ring at the end of the OTA off of my Meade LT6 ACF? Reason: it prevents me from removing the mirror.

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#2 KerryR

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Posted 22 August 2013 - 12:06 PM

I believe Meade glues the front cell on. But, the mirror should be able to come out with it in place-- normally, there are 2 slots in the corrector cell that allows for this if the mirror/carrier is turned sideways. You won't be able to see the slots with the corrector retaining ring in place.

Meade may have changed this since the days of my 2005ish LX90 8"...

#3 Joe Aguiar

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Posted 22 August 2013 - 12:09 PM

can i ask why you want to remove the mirror? is something wrong?
joe

#4 rmollise

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Posted 22 August 2013 - 12:44 PM

One word: Don't. ;)

Unless disaster has happened, don't even think about it. "Disaster" is not "a little dust." Or even a lot.

#5 MrJones

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Posted 22 August 2013 - 12:58 PM

It has some astigmatism. I know people with larger ACFs have reseated their mirrors to fix this but it could just be a crooked baffle tube. I've exhausted all options to do with secondary and corrector alignment other than also reseating the secondary mirror so it looks like I need to take the mirror out. And it's only a LT6 and fun to play with. :) I might get a C8 to put on the mount anyway and I would not sell it with the astigmatism so not a huge loss if something goes wrong.

And @KerryR, I sure have tried to get it out with the big ring on it but it doesn't quite make it and the ring does seem glued.

#6 MrJones

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Posted 22 August 2013 - 01:19 PM

As per KerryR here's how they are removed on bigger ACFs, I can't seem to do this. link

#7 Joe Aguiar

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Posted 22 August 2013 - 02:16 PM

if its only near the egdes or just a tad i also say leave it as is
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#8 rmollise

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Posted 22 August 2013 - 03:24 PM

It has some astigmatism.


Be danged sure it is really astigmatism, or is not in your eyepiece or eyes instead of the scope.

#9 T1R2

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Posted 22 August 2013 - 03:31 PM

I 2nd that, be sure to wear corrective lenses it you got'em, specially at lower powers, you might not need them at planetary powers. just in case.

#10 MrJones

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Posted 22 August 2013 - 03:36 PM

It's unfortunately bad enough to be obvious. I'll get star test photos tonight if it's clear. Thanks everyone.

#11 jgraham

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Posted 22 August 2013 - 05:20 PM

I swore that I was having a string of bad luck with telescopes that suffered from astigmatism. It turned out that it was me, not the scopes. Ugh!

#12 MrJones

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Posted 22 August 2013 - 08:41 PM

I did an artificial star and lots of photos with different amounts of intra and extra focus. Collimation might be a little off but it's pretty difficult to get it any better. This first pair is just slightly on both sides of focus and close to what I see even in focus on a real star.

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#13 MrJones

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Posted 22 August 2013 - 08:42 PM

2

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#14 MrJones

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Posted 22 August 2013 - 08:42 PM

3

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#15 MrJones

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Posted 22 August 2013 - 08:43 PM

4

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#16 MrJones

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Posted 22 August 2013 - 08:43 PM

5

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#17 MrJones

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Posted 22 August 2013 - 08:55 PM

Didn't get the scale exactly the same on this pair but the set feels incomplete without it.

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#18 Ed Holland

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Posted 22 August 2013 - 09:50 PM

Can you do the "view from the front" test to check concentricity of the internal reflections and shadows? This has helped me setup both my Orion Mak 127 and C8, and correct tilted baffle/primary mirror issues.

It's easy to set this up, and view at a few different distances, whereby you can determine some anomolies quite readily withoutopening the scope, if one is careful and systematic. ~12-15ft of space is required, typically.

Just a thought,

Ed

#19 T1R2

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Posted 23 August 2013 - 12:43 AM

I see a little bit of collimation error and a little astigmatism, #5 pic. is to far inside and outside of focus to reveal anything but a false well collimated scope, go with a smaller diffraction pattern.

If this scope is under warranty, send it back and get a new one. severe astigmatism will show far inside and outside of focus, a slight case will reveal itself right before best focus but should still resolve into a tiny point.

#20 Cliff Hipsher

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Posted 23 August 2013 - 08:39 AM

Which part of the process is giving you trouble?

#21 KerryR

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Posted 23 August 2013 - 08:52 AM

I read somewhere that astigmatism that disappears at about the point where 4 or 5 rings are visible, as you move away from focus, will have very, very little impact on the image at focus. No idea how accurate a statement that is. Moot anyway, because it looks to me that this scope is worse than that.

The ovals are not smooth ovals, which suggests something more than just 'simple' astigmatism.

I get the feeling this is in the glass. But, if it were me, and the scope couldn't be returned for a refund or different ota, I'd take it apart and look for things that might be pressing too tightly on the mirror(s), such as the rear plate that holds the primary mirror on the carrier, the shims that absorb the space between the bore in the mirror and the carrier, or perhaps a deformed gasket between the front of the mirror and the carrier, or etc.

You have to be prepared for the possibility that you might damage the scope and be left with nothing. Personally, I enjoy this sort of trouble shooting and disassembly, it's part of the hobby for me.

#22 MrJones

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Posted 23 August 2013 - 10:02 AM

Thanks again everyone.

The scope was unused but not from Meade and Meade's warranties are non-transferable. I tried! I also did not use the scope for a few weeks after I got it and it took several months to convince myself that I had a problem. The vendor has a 30 day return policy of course.

I have done the front view thing. It does really look like there is a problem with everything lining up. I have also peered up the baffle many times in different light and am pretty convinced that the secondary is not quite centered with the baffle aperture. I'm not really sure any of this means anything but I guess it all points towards a tilted baffle tube.

Since it really does seem to be reasonably bad astigmatism (bright stars are never nice little points like in my C9.25) I'm going to try to get the mirror out again. Maybe I just wasn't aggressive enough with it. Also, I still remarkably get not terrible views with the scope so I have hopes for great things if it survives my surgery.

One more question before the anesthesia ... are ACFs normally more difficult to collimate then non-ACF or Edge SCTs? I seem to get slightly different results with different amounts of intra- and extra- focus and I usually just have to pick some compromise whereas it's pretty easy to home in on collimation with my C9.25.

#23 TG

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Posted 23 August 2013 - 01:12 PM

Thanks again everyone.

The scope was unused but not from Meade and Meade's warranties are non-transferable. I tried! I also did not use the scope for a few weeks after I got it and it took several months to convince myself that I had a problem. The vendor has a 30 day return policy of course.

I have done the front view thing. It does really look like there is a problem with everything lining up. I have also peered up the baffle many times in different light and am pretty convinced that the secondary is not quite centered with the baffle aperture. I'm not really sure any of this means anything but I guess it all points towards a tilted baffle tube.

Since it really does seem to be reasonably bad astigmatism (bright stars are never nice little points like in my C9.25) I'm going to try to get the mirror out again. Maybe I just wasn't aggressive enough with it. Also, I still remarkably get not terrible views with the scope so I have hopes for great things if it survives my surgery.

One more question before the anesthesia ... are ACFs normally more difficult to collimate then non-ACF or Edge SCTs? I seem to get slightly different results with different amounts of intra- and extra- focus and I usually just have to pick some compromise whereas it's pretty easy to home in on collimation with my C9.25.


In an SCT with a spherical secondary, astigmatism is usually from a mis-centered corrector. (A mis-centered secondary can be adjusted for simply by tilting it till collimated.) However, you seem to have an ACF so its secondary is aspherical, IIRC. This means it has to be dead center with the primary, in addition to the corrector being centered as well. Unless there is some stress on the primary which is causing the astigmatism or the baffle tube is out of whack, I would start with the corrector/secondary.

There have been methods published by folks on how to center correctors and/or secondaries on the web. Unfortunately, I can't remember any off-hand right now. A simple method to check for the corrector's centering would be use a sight tube (the kind used for Newt collimation). Alternately, you can pull the secondary and stick a collimation laser in the visual back and then check if the spot comes out centered.

In any two-mirror optical system, the collimation steps are to center the secondary, tilt the secondary and then tilt the primary, with perhaps another iteration or two of this process. With SCTs there's an added complication in the form of the corrector and the primary being fixed in tilt but the basic principle still applies.

Finally, I should say that it's very unlikely that the astigmatism is ground into the glass due to the mass production methods used so you have hope of getting it out of the system. I recently bought a 4" JSO SCT that also showed astigmatism (stars were crosses at focus). A little measurement showed that the corrector wasn't centered. Fixing it and recollimating removed the astigmatism entirely.

Tanveer.

#24 MrJones

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Posted 23 August 2013 - 03:45 PM

Thanks, more to think about. I have centered the corrector + secondary with calibers as close as I could and it didn't seem to be off much if any and didn't affect the astigmatism. The Meade is different than my C9.25 in that the secondary holder fits very snugly in the corrector hole - so no play to adjust the secondary relative to the corrector. I didn't mention it but I have also done the artificial star tests previously, rotating the corrector and secondary separately to see if that helped and also removing the cork spacers and trying the corrector in NSEW type positions. I got a little variety in results but the factory marked positions seemed as good as anything else so I kept them there. I also have spent a lot of time looking up the baffle tube with sight tube and my laser. It does seem like the secondary is not quite centered but there is not enough play in the corrector even after removing the cork spacers to get it to a centered position through the sight tube, and I have no idea if this is the correct way to assess the secondary position.

It's a little embarrassing to admit, but I tried the mirror again since I have the day off and it slid right out one I perfectly aligned it with the slots as per KerryR and the 14" disassembly guide. In fact this scope is pretty much the same as that 14" minus the mirror lock. Thumbs up to Meade engineering for the construction at least - very solid.

Without the mirror I now feel safe to stick the calipers down there and it sure does look like the baffle tube is tilted about 0.5 mm as I measure a 1 mm difference between 2 opposite sides.

So it's down to just the main tube and baffle tube and I apparently have to bake this to loosen the thread lock and be able to unscrew the baffle tube. I'll probably go for it, center the tube somehow if I can remove it, reassemble and test before I fool around with the mirror assembly.

#25 orion61

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Posted 24 August 2013 - 03:12 AM

I agree it looks more like the secondary is a bit off center






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