Wednesday, March 09, 2005

Pentax Takumar lens

I bought a 55mm f/1.8 Pentax screw mount lens off of Ebay last week to use as a wide-field imager in front of the Starlight Express SXV-H9 CCD camera. The total cost was $11. What I didn't expect was that the rear element of this lens was made with a thorium-rich glass. The radiation might preclude use of this lens. I get counts of 6000 microrem/hr with the counter at the surface of the element. I did some counting last night:

With the detector 2 cm away and 10 samples of 10 seconds each:
Bare: 2743
Single sheet of paper: 2466
Aluminum (1/16 inch thick): 577
Aluminum and paper: 563
With geiger counter reversed: 80
With geiger counter reversed and Al: 60
From side of geiger counter: 96
From side with Al: 87
Background count was 12.

Then, I did some distance counting.
Distance Count rate
2cm 2721
4cm 1145
6cm 647
8cm 391
10cm 282
12cm 203
14cm 165
16cm 130
18cm 98
20cm 87
22cm 80
24cm 57
26cm 59
28cm 47
30cm 38
32cm 42
...
40cm 26

I also measured the rates with the 1/16 inch thick Aluminum sheet in the way--this way, I was hoping for only the photonic component of the radiation, so no air absorption.
6cm 156
8cm 88
10cm 64
12cm 48
16cm 30
20cm 18

Graphs are needed, I know. I just wanted to post something. Everyone else is busy with end of quarter work.

1 comment:

edwardolive said...

what about the other lenses. i have a whole collection of super takumars in my bedroom. maybe i should store them somewhere else.

tommyoshima told me about this blog on flickr.

does anyone else know about this?

info@edwardolive.com

http://www.edwardolive.info/ (Photos)
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many thanks

ciao