RE: [ATM] Best webcam/ccd for Foucaultgrams?

[Royston_Jeremy@emc.com]

Hello all,
	It's odd that no-one has mentioned what focal length lens is needed
for looking at the Foucault test with a webcam.
It you want to fill P% of the field for a mirror with an f-ratio of f, and
the smaller of the width and height of the CCD sensor is d, then the focal
length of the lens FL is given by:

FL = 2 * P * d * f / 100 , in the same units as d
Example, d=4mm for an f/6 mirror (25cm, FL=1500mm)
Lens FL = 2 * 75 * 4 * 6 / 100 = 36mm 

You can get the sensor size from the camera datasheet, or multiply the pixel
size by the number of pixels in the width or height, remember that the
pixels are not always square.
Some cheaper cameras have auto exposure only, the image should fill the
field as much as possible, otherwise you get overexposure of the interesting
part.
I use a zoom lens, which makes life easier

Best wishes, Jerry

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Ric Rokosz
Sent: Freitag, 31. Dezember 2004 21:13
To: Dominic-Luc Webb
Cc: atm@atmlist.net
Subject: Re: [ATM] Best webcam/ccd for Foucaultgrams?


--- Dominic-Luc Webb <dlwebb@canit.se> wrote:


> First, regarding the lens, it maybe depends on the
> focal
> ratio you are testing. I am quite happy with my
> Philips
> TouCam webcam for my current projects without any
> modifications. All the images in the pages I have
> been
> able to upload to my webpage are done using this:

Thanks  for the info and link to your web page.I
especialy like the reference to using a 51 Watt
Halogen bulb for the light source ..I'll be probably
going that route aswell as I picked up a really cheap
Alaris weeWebcam (with software) for $7 US for
two,locally,
 
      http://www.factorydirect.ca

that needs lots of light.After getting some helpful
advise from Dale and Juan and Lerch's website,the
Alaris with its wide angle lens will work the easiest
for me (no lathe work to thread custom barrel sections
for new lenses) when placed at the viewfinder of a
35mm camera coupled with a 85-200mm zoom lens.Tried
this out on a 6 inch f/7 uncoated mirror and a
standard 6 volt flashlight bulb,but the weeCam just
barely produced an image on the computer screen.I
subsituted a 14 inch disk at the same location (f/2.7
or so) and the zoom/weeCam combination got the whole
disk in the field,(this was done with the room lights
on)so that problem of imaging fast mirrors is possibly
solved.Given the ultra fast primaries you are doing
the ToUcam has worked since the primary is so close to
the light source.

As for the 51 watt Halogen light bulb,went to the
local car wreckers and pulled out some units.Don't
take the bulb from the outside light housing,only the
inside-the high beams -which aren't used that much as
a rule.Mid '80-90's GM cars are the easiest to get
out,as the housing are very open and no tools are
needed,unlike some of the foreign cars (depending
where you live ) are the hardest.



> I have a Canon A85 digital, which is cheap. This has
> zoom, etc

> Unlike a webcam, the lens does not
> unscrews. Indeed, I
> have taken out the lens at times and connected this
> camera directly in
> front of a 20 mm eyepiece to capture images from an
> 8" F/8.

Haven't tried this yet with the weeCam,hope it works
too..but this weeCam wide angle lens and small cmos
device has its own uniqueness it seems.

Ric 



		
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