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Equipment Discussions >> Reflectors

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Jeff Porter
super member
*****

Reged: 09/03/10

Loc: Utah
Re: Cautious Tale for Anyone Respotting a Primary new [Re: FirstSight]
      #5770528 - 04/01/13 11:19 AM

Chris,

Thanks for the update. Most of my experience has been with mirrors whose coatings were only a few years old or so. I was curious to see if there was some other issue with older mirrors.

Congrats on the new focuser for your NP-101. That is a great scope from what I have read.

Jeff P


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Cautious Tale for Anyone Respotting a Primary new [Re: FirstSight]
      #5771023 - 04/01/13 03:11 PM

". . the phenomena is entirely one of the inherent mechanical flex in the compression ring mechanism, and not one of loose registration in the focuser or imprecision of the collimation tool. ."

I think the problem is inherent in the cylindrical slip-fit system with inward radial clamping that we use. Because there's space in between that allows movement between the cylinders, the inner member can easily slide sideways a little at the line of contact where the surfaces are asymptotically flat. The maximum skew can be dramatically reduced by decreasing the space between cylinders to about 0.001" with tighter manufacturing tolerances.
So called "compression rings" make the problem worse. Because of the groove machined in the drawtube, there is no drawtube surface directly opposite (180 degrees from) the end of the clamp screw – there is only empty space. This increases the tendency for the accessory to tip when the clamp screw is tightened. The reason for this is that a machined hole is almost always slightly larger in diameter just at the opening.The only good thing compression rings do is to protect the accessory barrel from marks, but this can also be done with a well-designed plastic tip on the clamp screw.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Cautious Tale for Anyone Respotting a Primary new [Re: Howie Glatter]
      #5771285 - 04/01/13 05:23 PM

Quote:

". . the phenomena is entirely one of the inherent mechanical flex in the compression ring mechanism, and not one of loose registration in the focuser or imprecision of the collimation tool. ."

I think the problem is inherent in the cylindrical slip-fit system with inward radial clamping that we use. Because there's space in between that allows movement between the cylinders, the inner member can easily slide sideways a little at the line of contact where the surfaces are asymptotically flat. The maximum skew can be dramatically reduced by decreasing the space between cylinders to about 0.001" with tighter manufacturing tolerances.
So called "compression rings" make the problem worse. Because of the groove machined in the drawtube, there is no drawtube surface directly opposite (180 degrees from) the end of the clamp screw – there is only empty space. This increases the tendency for the accessory to tip when the clamp screw is tightened. The reason for this is that a machined hole is almost always slightly larger in diameter just at the opening.The only good thing compression rings do is to protect the accessory barrel from marks, but this can also be done with a well-designed plastic tip on the clamp screw.




From your lips to the ears of focuser and adapter makers.
I had a Moonlight focuser without a brass protection ring (I call it that since it isn't really a compression ring, or collet) and the fit was so snug I had to rotate my collimation tools into place in the focuser. And the 1/4" aluminum thumbscrew had a tip wide enough not to tip the eyepieces with "safety grooves".

Now I have a Moonlite focuser with a brass ring and though the fit is tighter than most I've encountered, it's still sloppier than the prior focuser's drawtube without one, and for just the reasons you mention.
Fortunately, I use a Paracorr in that scope, and it fits snugly, along several inches of its length, below the machined out brass ring housing.

I'm glad to know there are those who value tight tolerances and good machining over gimmicks like brass rings and "safety" grooves.


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bob irvin
sage


Reged: 03/22/11

Loc: Greater Los Angeles, CA
Re: Cautious Tale for Anyone Respotting a Primary new [Re: FirstSight]
      #5773145 - 04/02/13 03:28 PM

Good thread & good "heads up" to those of us on the road to doing this but
haven't yet.

thanks Chris.

bob


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FirstSight
Duke of Deneb
*****

Reged: 12/26/05

Loc: Raleigh, NC
Re: Cautious Tale for Anyone Respotting a Primary new [Re: bob irvin]
      #5843833 - 05/06/13 03:36 PM Attachment (6 downloads)

GOOD NEWS: Today, my 12" Primary was delivered back to me from recoating, and the result indicated that the scratches near the center were only in the old (now replaced) coating, and not into the mirror surface itself. Had the scratches been permanently etched into the surface, true they would have been under the shadow of the secondary but nevertheless would have annoyingly interfered with accurate collimation using the Catseye tools.

ALSO ON THE UPSIDE is how obviously apparent the difference is between the old replaced coatings and the new ones: formerly, many sleeks and pits were apparent (completely aside from any dust) on the old coatings, and there were some definite pinholes beginning to show when seen from behind with a bright light shining on the surface. The new coating is...well, pristine, and no pinholes whatever appear when seen from behind with a bright light shining on the surface. This could potentially result in reduced light scatter and improved contrast, though I won't be able to tell whether there's any visible improvement until I get the mirror back out under the stars.

I re-spotted the mirror this time with a Catseye Hotspot (and Catseye hotspot-adapted template), reinstalled it in the cell and OTA, and re-collimated it. I will vouch that the hotspot is indeed significantly easier to precisely center using either Glatter or Catseye tools (I use both) than even the Catseye red triangle was. The only downside is that although P itself is easily visible in the autocollimator, the three other reflections are marginally less easy to see than they were with the red triangle, but not so much as to be truly at all problematic, and may be mainly due to the changeover in color from red to white.

Edited by FirstSight (05/06/13 05:04 PM)


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FirstSight
Duke of Deneb
*****

Reged: 12/26/05

Loc: Raleigh, NC
Re: Cautious Tale for Anyone Respotting a Primary new [Re: FirstSight]
      #5843835 - 05/06/13 03:37 PM Attachment (7 downloads)

So what else is new today, the day I got my primary back from recoating? SURPRISE SURPRISE, SURPRISE, Sergeant!

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Cautious Tale for Anyone Respotting a Primary new [Re: FirstSight]
      #5843854 - 05/06/13 03:46 PM

Chris, the 4 images of the center marker should be EASIER with the white marker. It makes the fainter reflections easier to see.

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