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Meade ED APO 178mm front lens question

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21 replies to this topic

#1 deanbrown3d

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Posted 22 September 2022 - 11:35 PM

Hi all,

 

Does anyone remember the 178mm ED APO and its radial alignment screws? I want to know - do these grub screws push onto a metal lens holder, or onto the actual side of the glass (possibly with a little tape between)? I have read the fab article on the alignment, but I can't tell from the drawing exactly what's going on.

 

I only ask because I tend to align it quite often, and I want to know just how 'gentle' I have to be. Some of the grub screws get fairly tight, and I'm not sure if it's just the weight of the glass pushing back or something is sticking.

 

Thanks!

 

 



#2 CHASLX200

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Posted 23 September 2022 - 05:58 AM

I never fooled with mine. Mine went back to Meade 3 times and never was right and got my money back. This was in 1999 before the cell fix.


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#3 jgraham

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Posted 23 September 2022 - 07:14 AM

If I recall right the screws press against a ring. Do a web search on "Meade ED Collimation" should give helpful results. I zeroed in the collimation and centering of my 152ED and it has held pretty well.

#4 jgraham

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Posted 23 September 2022 - 07:15 AM

Give this a try...

https://www.google.c...nBOemVE1RBh0KCg

#5 deanbrown3d

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Posted 23 September 2022 - 08:16 AM

Right that is the pic I don't understand, and exactly why I am asking. It looks like the 'centering screws' just head into a white space - the interior space of the telescope.



#6 deanbrown3d

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Posted 23 September 2022 - 09:12 AM

Looking at another pic in that link, attached here, you're right it does push on a spacer ring. I don't see how that can move just one of the lenses though. Any thoughts?

Attached Thumbnails

  • ED-APO-178c.jpg


#7 ccwemyss

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Posted 23 September 2022 - 09:24 AM

It looks like pushing on the spacer ring differentially would change the angle of spacing between the elements by a very small amount.

 

Chip W. 


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#8 deanbrown3d

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Posted 23 September 2022 - 11:41 AM

It looks like pushing on the spacer ring differentially would change the angle of spacing between the elements by a very small amount.

 

Chip W. 

Are you thinking it changes the angle between the lenses, rather than the sideways offset? I had always assumed the opposite.



#9 The Planetman

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Posted 23 September 2022 - 04:12 PM

The radial screw pushes against an aluminum spacer ring.

#10 ccwemyss

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Posted 23 September 2022 - 04:33 PM

If the screws push against the spacer ring, rather than an element of the lens, it would seem that moving the ring could change the gap (e.g., increase it on the side pushed in, decrease it on the opposite side).

Chip W. 



#11 The Planetman

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Posted 23 September 2022 - 05:56 PM

The lenses and spacer are taped together and move as one unit. 


Edited by The Planetman, 23 September 2022 - 05:57 PM.


#12 ccwemyss

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Posted 23 September 2022 - 06:00 PM

So the tape is something rigid? Then the screws would indeed move the whole assembly. My early 152ED doesn't have them. 

 

Chip W. 



#13 jgraham

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Posted 23 September 2022 - 07:15 PM

Read page 10. The front element is fixed in the cell, the rear element floats. The screws bear against the ring, not the glass.


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#14 deanbrown3d

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Posted 24 September 2022 - 07:57 AM

So the tape is something rigid? Then the screws would indeed move the whole assembly. My early 152ED doesn't have them. 

 

Chip W. 

I'm assuming there is a separate tape on the rear element + spacer, and that is what moves laterally. The document does show in a pic the ED element + spacer taped up together. (Drawings suggest there is tape on the front element too. Must be a separate piece. Otherwise both glasses would be moving with the grub screws.)



#15 The Planetman

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Posted 24 September 2022 - 08:37 AM

Here is a good reference for the lens.

https://avt-astro.co...e-reattachment/

#16 CHASLX200

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Posted 24 September 2022 - 09:29 AM

The cell was a train wreck on many of these.


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#17 Terra Nova

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Posted 24 September 2022 - 09:49 AM

The cell was a train wreck on many of these.

I agree. We had one (the 7”) here at the Cincinnati Observatory for a number of years that was almost impossible to collimate and couldn’t hold collimation once the feat was accompanied. When they decided to cell it, they offered it to me and I passed without hesitation. (It was also much to large for me to handle, a five inch refractor being my limit.)



#18 deanbrown3d

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Posted 24 September 2022 - 11:41 AM

The struggle is real - what with the mount AND the tube needing loads of attention. But when I see 15 cloud bands and numerous white spots and other features on Jupiter, it's (almost) worth it..confused1.gif


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#19 jgraham

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Posted 24 September 2022 - 12:40 PM

Exactly. I enjoy the challenge of getting these problem children to behave and it’s wonderful when they blossom into a fine instrument. It’s amazing how the last little tweak usually does it. My 152ED put up a fight, but it finally came around. :)

 

I went through similar pains with my Unitron 155c. The scope needed to be collimated but it had a fixed cell. It turns out there was just enough play in the mounting screws to nudge the cell into position. It now gives an incredibly sharp image and is one of my favorite double star scopes. The 152ED is _almost_ there. It’s collimated and I’m in the last stages of centering the rear element.

 

So much to do, so few clear nights…

 

Enjoy!


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#20 deanbrown3d

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Posted 24 September 2022 - 05:11 PM

Yeah, it's' soooo good to have this forum to get help! It was such a big rush for me all summer to get this thing ready for the planet show that is currently upon us. (Apparently the best Jupiter for 50 years!) I put in a new RA drive motor - a stepper bolted to the outside. I built a triple set of dollies to actually move the thing on gravel. I built the coffin. And I worked on the alignment process and now have drill-holes in the dew shield to make that easier. Probably some more stuff but that's all I can remember for now. Oh and on cloudy nights I have been building a javascript simulation model to fly through a galaxy.


Edited by deanbrown3d, 24 September 2022 - 05:13 PM.

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#21 Terra Nova

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Posted 24 September 2022 - 06:24 PM

Didn’t Meade send out replacement cells for some of them?



#22 CHASLX200

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Posted 24 September 2022 - 06:41 PM

Didn’t Meade send out replacement cells for some of them?

You had to send it to Meade. Not in 1999 when i had mine. It was a total hunk of junk. Focuser had image shift as well. Mine went back to Meade 5 times. One time before i was sucked into buying it and 3 times when i had it and one time after.  It was sold again to another sucker that put the OTA on the mount with no weight and BAM into the tripod leg and not once but twice. I hope that scope was sunk deep offshore to save another poor person from buying it.




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