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Long ED doublets at last !

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11 replies to this topic

#1 Frank Theys

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Posted 07 December 2017 - 04:37 AM

Logo2red.jpg

 

Well, it took nearly 2 years for the first 120 mm F12.5 ED doublets to become reality at last. As some of you already know these are designed and produced by Lichtenknecker Optics, Hasselt-Belgium and have very good color correction besides being very lightweight (only 1.15 kg!) so with very fast cooling characteristics. 

 

spotdiagramFrT120EDredb.jpg

 

They are available lens only or as a complete telescope tube, the FrT120dED (weight : 7.9 kg).

 

FrTdED120_02a.jpg

 

FrT120lens01a.jpg

 

 

First observations on the moon show a lens that has no color that I can see and that accepts high magnifications with ease retaining high contrast and brightness.

 

This is a run of only 10 pieces, so if you have for a long time wanted such a long focus ED lens this is your chance.

 

Best regards,

 

Frank Theys

FrTelescopes

 

http://www.frtelescopes.com/frt120ded

 

 

 

 

 


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#2 dothead

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Posted 07 December 2017 - 05:37 AM

very good news, indeed!

This is a nice alternative for those for whom the 150 mm F/15 ED doublet, available from AOK Swiss (lens also made by Lichtenknecker) is a bit too large a telescope.

Could you please disclose which glass types are used in your 120 mm F/12.5 ?  FPL 51 ?



#3 Redbetter

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Posted 07 December 2017 - 07:17 AM

While anything but cheap, these are interesting and hopefully they will be useful to some regulars.  I am somewhat confused.  The link says "7.9 kg" for "telescope weight" (OTA?) vs. 1.15 kg above?  I like the option, even if am not waiting for an ~5" diameter, long ratio ED refractor.

 

I assume the focuser in the original link is 2", vs. 1.25"?  This sort of info should be in the description.

 

Glass type should be listed.   FPL-51 might be respectable at this ratio.  FPL-53 doesn't require any pencil sharpening.



#4 Frank Theys

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Posted 07 December 2017 - 06:17 PM

To clarify some things :

 

"telescope weight" is indeed "OTA weight" which is 7.9 kg  and the 1.15 kg is the weight of the lens only (lens with lenscell).

 

It is a 2" focuser (Moonlite) as explained in the "general specifications".

 

The precise glass types used are proprietary to Lichtenknecker Optics who prefer not to divulge them, there is certainly no FPL-53 used. What is of importance is the end result in the spotdiagram : all colors come to focus inside the Airy disc, performance was shown to be better than a classic Steinheil FPL-51 doublet of the same dimensions. The goal is NOT to minimise CA to the max but to reduce it significantly and at the same time reduce other second order abberations often present in short focal ratio lenses, certainly doublets.

 

 

Frank Theys


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#5 SeattleScott

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Posted 09 December 2017 - 10:23 AM

To me it would seem like a waste to use FPL53 in an F12.5. If I am going to pay up for FPL53 I want a shorter tube.


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#6 dothead

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Posted 09 December 2017 - 06:22 PM

Well, at least some people thought/think different - the very reason of the APOMAX's (130 mm F/12 ED oiled triplet) existence was its superior color correction & image quality.

FPL53 or CaF2 is mandatory in a 10 inch F/12 doublet, for instance. A 10 inch F/12 FPL 51 doublet would not exhibit APO color correction - neither would a 10 inch F/8 FPL 53 doublet...


Edited by dothead, 09 December 2017 - 06:35 PM.


#7 Richard Whalen

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Posted 12 December 2017 - 08:51 PM

Any word on optical quality? Do they come with test report?


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#8 Daniel Mounsey

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Posted 13 December 2017 - 08:49 PM

Very Nice!



#9 Passerine

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Posted 14 December 2017 - 10:47 PM

Just curious:  Will there ever be an FrT100dED?

 

Dave



#10 Frank Theys

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Posted 15 December 2017 - 06:00 AM

Dave,

 

It is very unlikely there will ever be a FrT100dED, the difference in manufacturing cost is too little to justify that : the glass has to be cut from the same glass blocks as for the 120 mm lens so the raw material used will be nearly the same and the grinding itself of a 100 mm lens is not so much different than for a 120 mm lens, the total cost will hardly be 10-15% lower. And before someone asks wink.gif  : I also thought about a 80 mm lens but even there despite the lower quantity of glass needed the cost for grinding would be too much to justify its production, 120 mm is really the optimum size for the available glass blocks. Remember : we are talking about (very) small scale manufacturing. 

 

Now, if there are enough people (at least 6?) presenting themselves and providing a partial prepayment : I would gladly oblige ...  

 

Frank Theys



#11 Frank Theys

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Posted 16 December 2017 - 05:41 AM

Richard,

 

The minimum garantueed optical quality is : PTV 1/4 lambda  and RMS 1/26 lambda at 523 ┬Ám.

No test report will be provided.

 

Frank Theys



#12 dothead

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Posted 16 January 2018 - 06:00 AM

it would be very interesting to compare the FrT 120 mm F/12.5 FPL-51 doublet APO to two other high quality refractors, namely:

 

- Takahashi TSA 120 mm F/7.5 FPL-53 triplet APO

- Agema Optics 120 mm F/8.7 CaF2 doublet APO

 

The Lichtenknecker 150 mm F/15 FPL-51 doublet APO (sold by AOK Swiss) has sensational optics - for instance, M 57's central star has been glimpsed with this telescope from a very dark sky in the high Swiss Alps !

 

Ralph


Edited by dothead, 16 January 2018 - 06:03 AM.



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