You aren't even close to having to worry about dark currents in the sensor.
Using a slit made by putting a business card into a slice in a rubber plumbing
cap, at a distance of over a foot, and using only a 20 watt fluorescent light bulb
as the light source, my camera was taking shots at ISO 800 using exposure times
between 1/8 second and 1 second.
Those results allowed me to experiment with the camera stopped way down, to
apertures as small as F32, and ISO ratings of 100, using 30 second exposures.
You won't be stopping down at all, letting all of the light hit your sensor.
And the $5 sensors we are talking about can be used at ISOs well above 3200
before noise becomes a problem for a spectrogram.
Given that you have been talking about using 500 watt light sources, I don't think
you will be worrying about dark currents or UV absorption in thin films any time soon.
You don't need a sketch to visualize a lens focusing at infinity.
Just picture a kid burning ants with a magnifying glass.
The ant is the slit. The sun is effectively at infinity, and the light from it is parallel.
You don't need a cylindrical lens. A regular lens will focus a slit onto a plane just fine.
All we are talking about here is a simple cheap toy magnifying glass, since we don't
care about aberrations. Those are taken care of in the software calibration phase.
-----
Get a free science project every week! "http://scitoys.com/newsletter.html"On Tue, Nov 22, 2011 at 7:27 AM, John Griessen <john@industromatic.com> wrote:
On 11/22/2011 06:59 AM, Nathan McCorkle wrote:The way to think of random noise integration is the dark current is
With that in mind, consider very narrow slits, and compensate for theWith longer integration times I feel that dark currents would start to
> dimming by
> integrating over time. You can also do HDR by taking several images and
> combining
blur the image to where it couldn't be ignored. Maybe not
an amplitude of light per time just like the signal you are viewing.
Integrating makes both larger and smooths out low frequency noise ( 1/f noise).
So you do an integration long enough to get more light collected at your
imager pixel so it's within the manufacturer's recommended range.
That won't help for low levels similar to the dark current.
A narrow slit with strong light won't be that bad.
Only the astronomers wait for hours staring at regions of cosmos with lN2
cooling so the dark current is lower. You'll be integrating for milliseconds.What does the sketch of that look like? A cylinder lens and correcting
On 11/22/2011 06:59 AM, Nathan McCorkle wrote:
> First, you could simply eliminate focusing altogether, by having the optics
>> focus on infinity. The slit then looks like a point source at infinity, and
>> is
>> always in focus.
lens close to the detector? Close to the slit seems like a good place
to stop spread...
Oh, here's an idea -- Some optical fiber has different refractive index per radial distance
from center and the effect is to funnel an input cone of light into smaller
bounce angles as it travels down the pipe, and exits with a narrower cone angle.
That's what to use for input fiber.
John
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