If an air spaced objective has too wide an air space, will this contribute to under correction or over correction? Same question if the air space if too narrow. Final question: For a 60mm f/16 objective, does anyone have an educated guess as to what the optimum air space gap would be ? Thanks in advance.
Under/over correction and width of air space
Started by Robert9375, 10/01/2013 10:07PM
Posted 10/01/2013 10:07PM
| Edited 10/01/2013 10:07PM
Opening Post
Posted 10/02/2013 01:57AM
| Edited 10/02/2013 02:01AM
#1
Robert Apruzese said:
If an air spaced objective has too wide an air space, will this contribute to under correction or over correction? Same question if the air space if too narrow. Final question: For a 60mm f/16 objective, does anyone have an educated guess as to what the optimum air space gap would be ? Thanks in advance.
What sort of change are you thinking about? I designed a 60mm f/16 BK7/F2 doublet that had a 0.5mm air space and a Strehl of 1.00 in green light. With the same design and a 1.0mm or 0.0mm gap and refocusing the Strehl was still 0.99 in green.
Most achromats I've seen a very thin foil spacers. Long ago I believe it was common to use pieces of a postage stamp.
Clear skies, Alan
Posted 10/02/2013 08:26PM
| Edited 10/02/2013 08:27PM
#2
A typical Fraunhofer achromat (like you are describing) will go under-corrected if the air space is too wide. Typically these are designed for 0.1 mm airspace. That said, the spherical correction for an F/16 lens changes very slowly as the airspace changes. If the spherical correction is very far off, re-spacing is probably not going to fix it.
Robert Apruzese said:
If an air spaced objective has too wide an air space, will this contribute to under correction or over correction? Same question if the air space if too narrow. Final question: For a 60mm f/16 objective, does anyone have an educated guess as to what the optimum air space gap would be ? Thanks in advance.
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