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achromat radii calculation

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#1 Old Will

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Posted 06 April 2013 - 08:09 PM

Hello, Can someone give me a set of radii multipliers for a 4" achromat using SK51 & DF2? Is there a difference between DF2 & F2? I don't have the melt data as these three sets were a gift. Thanks, Will

#2 Mike I. Jones

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Posted 07 April 2013 - 04:47 PM

All set up to run. Are you aware that SK51 and DF2 are not the greatest glass pairings for a refractor? They give significant spherical aberration unless the internal radii R2 and R3 can be made to different values, or you're willing to aspherize.

Mike

#3 Old Will

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Posted 07 April 2013 - 04:53 PM

yes, and thanks a bunch

#4 Old Will

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Posted 07 April 2013 - 09:22 PM

ok. I have three sets (gifts) go ahead and calculate for the best possible and I will make the necessary adjustments, coatings, R2 - R3 spacers. I will also make them long focal length. I appreciate the time & effort

#5 Old Will

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Posted 07 April 2013 - 09:24 PM

by the way is
df2 & F2 significantly different?

#6 Mike I. Jones

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Posted 07 April 2013 - 10:08 PM

Do you see any markings on the blanks anywhere? Per the table in Smith, "Modern Optical Engineering", p. 152, B&L DF-2 and Schott F4 are actually a match. F2 is 620364, while F4 and DF-2 are both 617366. I fitted the index values from Smith in ZEMAX and will run your lens with SK51 and DF-2.
Mike

#7 Old Will

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Posted 07 April 2013 - 10:16 PM

no, hand written ; DF2 & SK51 I'm hoping that longer than F/15 will make up for the other faults, what do you think?

#8 Mike I. Jones

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Posted 08 April 2013 - 02:17 PM

Here you go, Will. It's in Excel, so let me know if you can open it OK. The multipliers are not just single numbers, but vary with focal length. I ran the 4" SK51/DF-2 combination from 40" to 65" focal lengths in 1/2" increments. You can see how the radius and focal length multipliers change as the focal length increases - they are all curves.

I set up the second worksheet so you can just enter the focal length you want, and it uses the curve fits on the other worksheet to calculate your radii and thicknesses. These require very little post-spreadsheet optimization, if any.

Mike

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#9 Old Will

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Posted 08 April 2013 - 03:48 PM

I really appreciate this Mike, thank you, Will
Do you attend the Texas star party? I can't this year but I'll for next year. If so, perhaps you'll get a look-see thru your calculations ;-) Will

#10 Mike I. Jones

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Posted 08 April 2013 - 04:56 PM

Cool! As you progress we can do like I did with Jim Egger, in that as you finish up on one lens, and measure its radii and thickness very carefully, we can re-optimize the remaining element specifically for the finished element. This will generally change the radii, thickness and airspace a little and bring the performance back. Then when the second lens is finished, the only variables left to optimize are the airspace and the BFL.

Here if you need,
Mike






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