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Tell doublet from triplet?

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#1 BigC

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Posted 06 August 2014 - 06:35 PM

Apologies if  duplicating but wonder :can a person tell if an objective is doublet or triplet by looking at it without taking anything apart ?

Would be nice to verify scope from estate and so on.



#2 Scott in NC

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Posted 06 August 2014 - 07:03 PM

You could possibly do this by shining a laser through it and counting the reflections as it passes through each element.



#3 jrbarnett

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Posted 06 August 2014 - 08:23 PM

You could possibly do this by shining a laser through it and counting the reflections as it passes through each element.

Unless it uses a fluorite element, which will be invisible to the laser.  :grin:

 

- Jim



#4 mblack

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Posted 06 August 2014 - 08:25 PM

You could possibly do this by shining a laser through it and counting the reflections as it passes through each element.

 

+1

 

Now, looking straight into the objective, with no other information available? That I could not do. Without a light to shine on the objective, I would have to look at the wall of the lens cell. That is an easy visual exam that anyone can do in daylight.


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#5 Scott in NC

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Posted 06 August 2014 - 08:32 PM

 

You could possibly do this by shining a laser through it and counting the reflections as it passes through each element.

Unless it uses a fluorite element, which will be invisible to the laser.   :grin:

 

- Jim

 

Yeah, I thought about that, Jim.  But doesn't the laser still make a reflection when it crosses the coated surface on each side of the fluorite element?  I thought the laser was just invisible as it crosses through the fluorite element. 



#6 mblack

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Posted 06 August 2014 - 08:41 PM

 

 

You could possibly do this by shining a laser through it and counting the reflections as it passes through each element.

Unless it uses a fluorite element, which will be invisible to the laser.   :grin:

 

- Jim

 

Yeah, I thought about that, Jim.  But doesn't the laser still make a reflection when it crosses the coated surface on each side of the fluorite element?  I thought the laser was just invisible as it crosses through the fluorite element. 

 

 

Scott, I do believe Jim had his tongue firmly implanted in cheek.  ;)



#7 Binojunky

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Posted 07 August 2014 - 01:35 PM

I would be more concerned about its optical performance than the number of elements in its objective, I bought a fairly high end 80mm  triplet a few years ago from a store going out of business, it was an optical dog, images fell apart at anything over x100, lost my shirt on that thing, fast forward to one of the clearance Celestron ED 80,s being offered for  $350 from AWB, this thing is terrific, everyone is wrapped up on triplets but a good doublet isn,t to be sneezed at.

  Regarding imaging, I one saw some of the best images ever taken from a lowly achromat, expertise is just as or more important than the gear, DA.



#8 PowellAstro

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Posted 07 August 2014 - 03:45 PM

I have a new ES102 triplet but it only shows the reflections of a doublet with a light or laser but it is a triplet as I have had the lens apart. You also could not tell by looking into the front at the lens cell. The middle element does not seem to show reflections of any kind. The color correction is super as there is no CA seen on the moons limb or anywhere else. I also see no color/CA on bright stars or planets even at 100x per inch, and past that the instrument is just gorgeous!



#9 Scott in NC

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Posted 07 August 2014 - 06:22 PM

 

Scott, I do believe Jim had his tongue firmly implanted in cheek.  ;)

 

 

Gotcha. :foreheadslap:



#10 Mark Costello

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Posted 08 August 2014 - 11:51 AM

The telescopes I've had have come with name plates or decals on the optical tube assemblies identifying the model number.  If one of these is on this telescope, you could look it up....



#11 SpooPoker

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Posted 08 August 2014 - 11:36 PM

Measure the mass of the OTA less the finderscope and dew shield.  Remove the focuser and measure the width of the tube.  Calculate the area (area of big circle less the area of the smaller circle) and multiply by the length of the tube.  Multiply this by the known density of aluminum to get a rough measure of the mass of the tube.  Subtract this from the OTA mass and subtract the mass of the focuser.  Now you have an estimate of the mass of the lenses plus cell.  Compare with mass of a typical doublet and infer from there :p

 

More seriously, one can work out the focal ratio of the scope.  Compare with focal ratio's of more commonly designed doublets or triplets.  Put the scope under a bright star test and work out how much false color you can see.  If there is a moderate amount of false color at high magnification and the focal ratio is reasonably fast by refractor standards (i.e. f/6 or f/7), it will likely be a doublet.  If the focal ratio is reasonably fast (i.e. f/6 or f/7) and the false color is negligible, it will likely be a triplet.  If the focal ratio is on the slower side, i.e. f/9 plus, it could well be a doublet.  This will not be 100%, but it could give an indication of the design, particularly if the aperture is 4" or more.  80mm or less, it will be a tougher tell.


Edited by SpooPoker, 08 August 2014 - 11:38 PM.


#12 Scott in NC

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Posted 09 August 2014 - 08:50 AM

The telescopes I've had have come with name plates or decals on the optical tube assemblies identifying the model number.  If one of these is on this telescope, you could look it up....

And with the miracle of the modern-day internet, this is probably the easiest way of all to tell! :grin:








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