Backfocus

[cfranks]

Hi,

I have a Williams FLT132 and a Williams TMB Flattener. Making my own spacer/connectors I am confused in setting the final distance between the TMB and the CCD chip with, in my case, filter and AO glass in between. QSI documentation says subtract 1/3 of the glass thickness, an old SBIG diagram included in IIS thread http://www.iceinspace.com.au/forum/showthread.php?t=106072 also says subtract 1/3 the glass thickness. SX Yahoo Group says to add that thickness! http://tech.groups.yahoo.com/group/starlightxpress/message/98376
So, If i need an adapter of length 45mm with no intervening glass, do I need 40 or 50mm with 15mm glass inserted?

Stars with pointy hats don't look good!

Charles

[Merlin66]

Geee,
I like these occasional easy questions!
The definate answer is that a flat parallel plate (ie a filter) in a converging beam extends the focus by very close to 1/3 the thickness of the plate.
So if you have 15mm of Plain flat glass in the converging beam, the new focal point will be 5mm further from the objective.
You can verify this from Snell's Law or read all about it in Derniak & Derniak's "Geometrical and Trigometrical Optics", p40-48.
( You need a shorter spacer to maintain the 45mm required)

[cfranks]

Thanks Ken,

I knew it was simple but I got mentally cross-eyed!

Charles