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TEC 200ED - Long Term Assessment

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#26 Yu Gu

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Posted 06 November 2019 - 08:49 AM

I've been thinking about the same thing recently as well since this center zone showed up in DPAC of several top brand APOs. I don't think it's aspherization. If you look at the design of TEC140ED for example:

http://www.astrosurf...c-apo-140f7.gif

Although it's nulled at green, the green line is not straight. By design, it actually should have a zone in the middle with all spherical surfaces. It's part of the design optimization to achieve the best balance at different wavelength (If I understood correctly).

Gu



#27 peleuba

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Posted 06 November 2019 - 11:55 AM

I was never a fan of the rubber tube ring liner TEC uses. I always applied a strip of felt to my TEC rings.

 

Exactly.   On both my 160's I used painters tape on the OTA precisely where the rings gripped.  It prevented "ring-rash".


Edited by peleuba, 06 November 2019 - 11:55 AM.


#28 peleuba

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Posted 06 November 2019 - 12:04 PM

So there appears to be a uniform aspheric on the lens and, if you look carefully at the previous DPAC images, you can see it in the outer portions of the bands.  What is also interesting is how the shape and "strength" of the aspheric changes with the color of the light.  I had trouble getting a decent image at focus in red as red is always a difficult color for me to capture with lower contrast and the aspheric appears just not as strong in red to boot.

 

Jeff - great images, as always!!!  

 

I also find red to be the hardest color to capture on camera when DPAC testing.  

 

Interesting on the "aspheric" artifact that you captured.  This is probably there by accident, perhaps even an error in figuring.  I am reasonably certain that all of the TEC designs are spherical.  But, would want to hear your thoughts about it and any knowledge you've gleaned from TEC over the years on this telescope.


Edited by peleuba, 06 November 2019 - 12:19 PM.


#29 Jeff B

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Posted 06 November 2019 - 02:31 PM

Thanks everyone.  I like doing and sharing this stuff and I'm glad others find it useful.

 

Just about every single lens system I've had a gander at in DPAC has some sort of small center zone smack dab in the middle, covering spans ranging from ~10% to 20% of the aperture.  This is where they have the least affect on the "performance" of the lens as it covers so little area and the depth of field is huge (meaning the glass does little "work" on the light beam as it passes straight through the lens with little bending, unlike out towards the edge).  Also some designers will indeed allow a little, tiny bit of SA  in green in order to mitigate it a little in the red and blue to give a more "balanced" design...at least on paper anyway.  Now, I suspect what I see at focus is not really just the physical surface profile of the lens, but the surface profile at some focal point of the wave front at that color as the light is passing through the lens elements.   If an aspheric profile is applied to one surface to null out SA in say green for whatever reason, I should not see it in green.....but I may see it in blue and red.

 

Thanks Moshen and yes, getting this round of null images was indeed time consuming compared to how I used to do it.  I had to get everything just right.  The attached images show the old "before" process and the "improved" (actually, more careful) process with the 200ED.  A big difference.

 

And thanks sensei Paul.  Yeah, I don't think the "aspheric" was applied on purpose either and have no further intel on the lens or its figuring process other than Yuri dropped the 160ED & 200ED lines because sales were low and he had trouble getting good quality matching elements.  I also don't know what was "bad" about the matching glass supply.  Also, as you know and have related here, there have been changes in opticians over the years so I suspect any more specific info regarding how specific lenses were figured is gone and they do/did not use DPAC, rather, they used test plates for the individual elements.  But you never know and would be cool to find out such stuff.

 

Jeff

 

 

 

Attached Thumbnails

  • At focus in green old way.jpg
  • 200ED DPAC Green At Focus.jpg


#30 Jeff B

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Posted 07 November 2019 - 02:55 PM

And now the final installment to this thread.

 

One of the first things I do with an OTA is to check the focuser axis alignment to the center of the lens with my Glatter laser as well as the alignment of the lens assembly with the focuser axis with a high quality Cheshire eyepiece.

 

So here are a couple of pictures showing the laser spot on the back side of a mask carefully positioned over the objective.  The mask has a small hole (~1/16" diameter) precisely in the center of the mask.  The hole was precisely centered on the objective using multiple measuring points from the cell face plate I.D to the center of the mask center hole.  I got to with +/- 1/64".  

 

The results speak for themselves.  The result was the same over the entire FT focuser travel and if I rotated the laser.  This is one of, if not the best, of this type of measurement I've taken, especially impressive as this OTA is over 10 years old, has been through multiple owners, and outside over four years cycling through temperature extremes.  Remarkable!

 

The Cheshire showed one single, well defined reflection spot right in the middle too.

 

Mechanical construction and long term durability does not get any better than this!  Ok, except for maybe the paint...but still, that's over 10 years of harsh environment.

 

Well done Yuri!

 

Jeff

Attached Thumbnails

  • Focuser Alignment.jpg
  • Focuser Alignment - Close Up.jpg

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