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AstroScan "Test of the optics"

ATM beginner clubs DIY dob equipment mirror making observing optics reflector
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#1 Oregon-raybender

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Posted 28 October 2020 - 07:52 PM

Two years ago, I was asked to test the optics of an AstroScan. There was no requirement, just curious to see how good it was.

 

I took the scope apart to test the flatness of the window (how good would it be in transmission) and is the primary mirror

polished to a asphere ( at least -1cc) 

 

So the results were: the window was a few fringes flat, convex one side and concave on the other. The total gives zero power

and a fine transmission. (the black dot is my reference dot) The primary was tested with 75 line screen, it did have an aspheric shape

(I did not do a KE test)  The secondary was 1/8 wave, the fringes were hard to image and not shown.

 

Over all the scope was better than expected and performed wonderfully, a very nice scope, even at high power.

I make no judgement of the rest of the scopes out there, but this was good to see as just one data point.

 

Attached are images of the testing. The window was tested with a 1/10 flat, under a sodium light box (Shop model) and the

primary was just a simple R screen setup. I think this scope was made very well. My niece was very happy to receive it. 

 

Starry Nightswaytogo.gif

 

 

 

 

Attached Thumbnails

  • Front side of window copy.jpg
  • backside of window copy.jpg
  • Primary Mirror r test copy.jpg

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#2 jelloptic

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Posted 28 October 2020 - 08:18 PM

Thanks for the report on the AstroScan performance.

Recognized the "VK" logo on your light box; used many Van Keuren products years ago, they made quality stuff.

 

kbl



#3 dogbiscuit

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Posted 28 October 2020 - 08:54 PM

I once tested the primary mirror of a friend's AstroScan (Ronchi and Foucault with and without a mask) and it was perfect.  Perfect edge, perfect correction, a very smooth surface, and no hint of a kink in the Ronchi.

 

Would like to test that mirror again with a Bath interferometer.

 

Wonder if other Edmund telescope mirrors are as good?


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#4 ed_turco

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Posted 29 October 2020 - 10:22 AM

Unfortunately, I've tested more than a few Astroscans and some were horrific.  Edmund had a reputation for 1/4 wave optics that were probably better than that.  Good decent images!  

 

Some Astroscans were a disappointment.  But then again, mirror at f/4 weren't exactly easy to figure.  



#5 Alan French

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Posted 29 October 2020 - 11:01 AM

IIRC correctly, the Astroscan primary mirror was not figured, the "figuring" was done when applying the aluminum coating, which varied in thickness such that it made the correction. 

 

Clear skies, Alan 



#6 Jeff B

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Posted 29 October 2020 - 11:11 AM

IIRC correctly, the Astroscan primary mirror was not figured, the "figuring" was done when applying the aluminum coating, which varied in thickness such that it made the correction. 

 

Clear skies, Alan 

Ok, now that's interesting.

 

Thanks!

 

Jeff



#7 Alan French

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Posted 29 October 2020 - 11:21 AM

Here's a discussion about this "figuring."

 

https://www.cloudyni...need-some-help/

 

Clear skies, Alan



#8 DAVIDG

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Posted 29 October 2020 - 11:29 AM

IIRC correctly, the Astroscan primary mirror was not figured, the "figuring" was done when applying the aluminum coating, which varied in thickness such that it made the correction. 

 

Clear skies, Alan 

 Alan,

   That was correct for the some of the first ones made when the demand was great and Edmund was trying to keep up. The process was to take a spherical mirror and using a special mask when the mirror was coated, apply the coating so it was thicker in some  areas then others. The process was developed by Strong who invented the aluminumization  process and is discussed in his book "Procedures in Experimental Physics"  Here is a link were the method is outlined starting on page 180  The method was only used for a short while. I happen to own a mirror that was made this way. When I stripped it to have recoated, I discovered that under the aluminum it was coated with chrome which did not come off when stripped. The mirror was done this way so it could be coated over chrome and maintain the parabolic shape. I heard that they only did this on a few Astroscan mirrors then went to directly coating them with aluminum so if stripped the figure would revert back to a sphere.

 

                       - Dave  



#9 Alan French

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Posted 29 October 2020 - 01:02 PM

 Alan,

   That was correct for the some of the first ones made when the demand was great and Edmund was trying to keep up. The process was to take a spherical mirror and using a special mask when the mirror was coated, apply the coating so it was thicker in some  areas then others. The process was developed by Strong who invented the aluminumization  process and is discussed in his book "Procedures in Experimental Physics"  Here is a link were the method is outlined starting on page 180  The method was only used for a short while. I happen to own a mirror that was made this way. When I stripped it to have recoated, I discovered that under the aluminum it was coated with chrome which did not come off when stripped. The mirror was done this way so it could be coated over chrome and maintain the parabolic shape. I heard that they only did this on a few Astroscan mirrors then went to directly coating them with aluminum so if stripped the figure would revert back to a sphere.

 

                       - Dave  

I wonder if Sue had one of these. She bought one very early, even before the introductory sale. It now has a new home, so there's nothing I can check, but it came with a different eyepiece, not the 28mm RKE, at least as later made, marked 1-1/8". 

 

Clear skies, Alan



#10 Oregon-raybender

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Posted 29 October 2020 - 02:55 PM

I find it interesting that one would go with coating verses polishing to make an asphere for a simple 4 inch mirror. It would require several, if not more chambers with motorized spinners to make the shape. As a optician, I ran a slurry polisher that could make simple asphere shape all day long.  The F/4 is not hard to do. Perhaps the coating was cost savings verses polishing. Oh well. I would have to say, this mirror tested as one that was polished to shape.

Looking at the R test, the asphere was not much to change from a sphere.

 

I asked a friend of mine who was interested in making the mirror. His cost was

was much lower than expected and well within a production cost to sell. 1/8 wave

 

Starry Nightswaytogo.gif

Attached Thumbnails

  • Astroscan 75 line.jpg

Edited by Oregon-raybender, 29 October 2020 - 02:59 PM.


#11 Steve Dodds

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Posted 29 October 2020 - 03:00 PM

I find it interesting that one would go with coating verses polishing to make an asphere for a simple 4 inch mirror. It would require several, if not more chambers with motorized spinners to make the shape. As a optician, I ran a slurry polisher that could make simple asphere shape all day long.  The F/4 is not hard to do. Perhaps the coating was cost savings verses polishing. Oh well. I would have to say, this mirror tested as one that was polished to shape.

Looking at the R test, the asphere was not much to change from a sphere.

 

Starry Nightswaytogo.gif

Yes but to parabolize it it needs to be hand figured by an experienced optician, (1/2 hour to an hour each), and they were cranking these out by the thousands.




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