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Cave Astrola secondary installation and adjustment

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#1 compt1ga

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Posted 17 February 2019 - 04:44 PM

Can someone please tell me how to adjust the height (distance from the primary) so that the secondary is centered in the focuser? It seems as though I should be able to rotate the long center bolt without also rotating secondary mirror - it should be height adjusted by the threaded central bolt/acorn nut assy while keeping the mirror facing the focuser.

Anyway I'm struggling and can't figure out how to get everything tightened down one I have it where I want it.

Thanks,

 

reference picture...  The center bolt threads into the spider's central cylinder with the vanes which would be between right 2 nuts in the picture. Those two nuts appear to be used to lock the height against the central cylinder. Is the acorn nut on the left supposed to be hard against the top of the mirror holder?

http://www.djlewis.u...adjKnobs8x6.jpg


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#2 TOMDEY

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Posted 17 February 2019 - 05:42 PM

I had a 12.5-inch Astrola, probably built like yours there. The two plain hex nuts to the right should squeeze the spider hub. The acorn to the right is redundant. I would probably get "fresh" new brass nuts and a compression washer, just to make the thing more serviceable and stable. Ye ole Astrolas were/are wonderful instruments... but pretty deficient is some of the mechanicals. Brute-force coarse hardware... just the way things were built. back then.

 

PS: I modified mine so that the folding flat there is balanced about the center of the spider. That prevents it from "drooping" as the scope is pointed around.    Tom

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#3 TOMDEY

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Posted 17 February 2019 - 07:36 PM

Can someone please tell me how to adjust the height (distance from the primary) so that the secondary is centered in the focuser? It seems as though I should be able to rotate the long center bolt without also rotating secondary mirror - it should be height adjusted by the threaded central bolt/acorn nut assy while keeping the mirror facing the focuser.

Anyway I'm struggling and can't figure out how to get everything tightened down one I have it where I want it.

Thanks,

 

reference picture...  The center bolt threads into the spider's central cylinder with the vanes which would be between right 2 nuts in the picture. Those two nuts appear to be used to lock the height against the central cylinder. Is the acorn nut on the left supposed to be hard against the top of the mirror holder?

http://www.djlewis.u...adjKnobs8x6.jpg

Oh, just noticed... the center bolt does not thread into the spider hub... that is a pretty loose cylindrical thru-hole in the hub. So, you don't have to rotate the stem to get it to advance or retract. Just use those two hex nuts and everything will work just fine!    Tom


Edited by TOMDEY, 17 February 2019 - 07:36 PM.


#4 compt1ga

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Posted 17 February 2019 - 07:42 PM

Thanks Tom. I'm trying to understand...

 

My center hub of the spider is threaded.

 

The rounded head of the acorn nut is setting in a chamfered hole in the SM holder like a ball joint. That makes me think that the acorn nut should be torqued tight as possible on the center bolt.

 

Should the SM (with pull bolts and plate) be rigidly tightened onto the center bolt by the hex nut?  If so, then you would have to turn the whole thing in the spider to set the height. Or, should the SM, plate and 3 pull screws be free to rotate while setting the height? Then the force of the 3 pull screws are needed to keep the SM from rotating in the final adjustment by squeezing the plate against the acorn nut. Hopefully that make sense. 


Edited by compt1ga, 17 February 2019 - 08:01 PM.


#5 TOMDEY

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Posted 18 February 2019 - 12:25 AM

OK... no, don't mess with that forward acorn nut etc. Well, if it were me, I would just march over to the drill press and drill out the spider core to yield a slip-fit thru-hole for that threaded stem! Otherwise, the design will only accommodate full turns of the stem, longitudinal increments equal to the thread pitch... which might be good enough, come to think of it?! [what is it... 1/4-20, 5/16-18, 3/8-16...?]

 

Regarding the three pull screws for tip-tilting the folding flat: "as tight as possible" well... just reasonably thumb-tight... don't get too carried away!    Tom



#6 compt1ga

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Posted 18 February 2019 - 10:45 AM

Thanks Tom,

I'm glad you understood the issue resulting from the threaded center core: Weird design for sure. Maybe one thread height isn't an issue but it bugs me. I don't have access to a drill press so I will have to live with it. I'm going to get some wing nuts for locking the rod to the core. 



#7 KentTolley

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Posted 18 February 2019 - 01:45 PM

1/4-20 center-bolt so one thread height equals 1.2 mm.

So what if the focus point is 1.2mm up or down the focuser?

You want to center secondary under the focuser.   Use a sight tube.

Your primary's  collimation screws will  also increase or decrease distance from primary to secondary.

And once you lock down the secondary holder center bolt in the spider hub you may also alter the distance between primary and secondary by adjusting the 3 screws on secondary holder.  Check that the secondary  mirror is still centered under the focusser with a sight tube after you center the primary image on the face of the secondary.  You may have to turn the center bolt once or twice to re-center the secondary u der the focusser.   And then once again have to re-center primary on the face of secondary.

No need to drill out the threads of the spider hub.  Recommend Cheshire ep and sight tube.  


Edited by KentTolley, 19 February 2019 - 10:34 AM.

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#8 compt1ga

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Posted 26 February 2019 - 09:27 AM

Thanks everybody. I feel as I've got it all back together and collimated. My laser says its spot on and everything looks well centered.




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