Slashdot Mirror

As already mentioned, the Bayer filter is part of the CMOS sensor itself. It's not a separate part that's tacked on near the end of the manufacturing process.

There is, however, a separate filter in front of the sensor on pretty much every DSLR. This is a IR cut-off filter. Naked CMOS sensors are very sensitive into the IR spectrum. This high-pass IR filter prevents deep red and IR from overwhelming the resulting image, producing a balanced red against the green and blue end of the visible spectrum.

There are several cases where one would want to modify their DSLR and have this filter removed. The primary users of this method are astrophotographers who wish to use a much cheaper DSLR on their telescopes vs. a very expensive purpose-made camera. There are a few small companies such as Hutech which can perform this service under warranty.

Why?

Nebulas and stars in particular emit light (human-visible and not) in a variety of specific wavelengths. These particular wavelengths are produced by ionized elements in the star or nebula complex. In your run-of-the-mill nebula, copious amounts of Hydrogen-alpha and doubly-ionized Oxygen tend to produce much of the light. H-alpha's emission line is deep in to the red spectrum, which the IR cut filter on DSLRs dutifully blocks from reaching the sensor. Removing this filter lets the DSLR capture additional light and detail from the nebula... stuff you wouldn't get with a stock DSLR.

If you take a stock DSLR and try to image (for example) the Horsehead Nebula, you're not going to get far because the thing emits almost entirely in the H-alpha band. Put on a camera that doesn't cut the deep red, and you'll get a result that's closer to what you'd expect.

There is a trade-off to doing this mod, of course... in that you're effectively turning your DSLR into a IR camera, and if you want it to be close to normal again, you'll need to put a IR filter on your lens.

photographer

Both are employed pretty heavily by advanced "Amateur" astronomers. I put amateur in quotes because people at the high end of the hobby may have setups costing $50,000-$100,000+ dollars, going up to as much as people are willing to spend. There are several companies (http://www.sbig.com/ for example) that specialize in producing imaging equipment and software for these setups. It's pretty amazing what these people are able to do.

It may be at its limit for the number of megapixels but, there's still a lot of things to improve like the maximum color range a digital camera can record. With 16 bits color channel, we would be able to record a lot more informations so we wouldn't be limited as much when we try to capture a high dynamic range picture. There's tools like in Photoshop CS2 to give you the abilities to have high dynamic range but it would be a lot better to have it directly in the camera.

The CCD cameras used by astronomers routinely produce 16 bits per pixel. Most of these are monochrome devices: to shoot a colour picture you must shoot pictures through red, green and blue filters, then combine them.

The key advantages for astronomy are zero reciprocity failure (film loses sensitivity in long exposures; CCDs don't), high quantum efficiency (almost all the photons intercepted by the sensor are noticed) and excellent linearity (you can digitally subtract extraneous light, like city lights).

However, even in astronomy, there is a hard core who still do film. There are many reasons: some people just like the look, others enjoy the craft of wet darkroom work, and so on.

My favourite camera is a 4x5 press camera, a Crown Graphic. It takes perfect 1950s newspaper photographer pictures. And I develop and print them myself.

...laura

I don't mean a camera with a color sensor that just gets desaturated. I want a dSLR camera with a sensor designed strictly for black and white.

CCDs are inherently monochrome devices: it's only by adding little filters to each cell that they can take colour pictures. Each RGB pixel in the output image is then interpolated from the individual red, green and blue-sensitive pixels on the CCD.

Astronomers use monochrome CCDs with filters and cooling hardware to reduce noise. See, for example, SBIG.

I amused myself once by taking a junker 35mm SLR body and adapting a black and white webcam to it. Interesting results...

...laura

But I will try:

There are two solutions to this problem. The first, is to cool your sensor like you asked. A peltier cooler in between the sensor and the heatsink is the only way to do this. A heatsink will reduce you to ambient temperature at best. If you want to go below that, you need a peltier cooler at least, or something much more exotic like a compressor-based refrigeration unit, evaporative cooler, or liquid nitrogen cooling.

Alternately, you could use a CCD with a cooler already built in, such as those from Santa Barbara Instrument Group.

Finally, you could simply use a sensor that isn't so noisy. All the digital SLR cameras nowadays use CMOS sensor technology, because it's bigger than CCD primarily, but it also has a lot less noise since it isn't crammed into such a small space.

Another approach often used by astronomers is to take the noisy CCD, do an exposure of a given length, then cover it so no light can reach it, and take another exposure of the same length to create a "dark frame" that contains nothing but noise. The noise in both images will be approximately equal, and can be subtracted out using photoshop or similar software, resulting in a very clean image.

SBIG has an adaptive optics device for amateurs called the AO-7. It doesn't correct for everything that professional systems do, but it does improve the image considerably.

6.5 meter main mirror, .64 secondary deformable mirror. This is a big scope. Polomar is 5 meters.

They hope to image extrasolar planets, for example, to get spectra. HST has already gotten spectra for an extrasolar planet - even though it has not imaged such an object. Step one: get the spectra for the parent star, step two: get the spectra during a transit, step three: compare.

I'm not sure why they think that a deformable secondary is better than AO afterwords.

Adaptive Optics are available for the ameteur astronomer. For example, SBIG

Film still rules for taking pictures in low-light.

So that's why the shuttle keeps visiting the Hubble Space Telescope, to pick up the film!

The is also a company called SBIG that makes a line of digital imagers for amatuer astronomers.

Steve M

I can sympathize with what you are trying to do, as I've tried both uses with my Nikon Coolpix 950. The short answer is both are possible, but it isn't likely to be easy to get what you want out of the setup. Once drivers are available, you should be able to use your digital camera as a webcam fairly effectively, though you may have issues with autofocus, flash, and other camera-only adjustments. For astrophotography, it isn't likely you'll be able to get worthwhile images with the Elph, but I've included a few links on how to get started.

Digial Camera Webcams

Digital cameras defintely produce a much more compelling image than a typical webcam. I have a 3Com HomeConnect (a pretty good quality webcam) and it looks just awful compared to my Nikon N950 (not just resolution, but also trueness of color, CCD noise, and sensitivity). The main limitations are that you can't usually take mini-movies or fast sequences, some key functions are often only controllable on camera (for instance auto-focus and flash), and you'll need a power cord for the camera if you don't want to drain the batteries very quickly.

The easiest way to control the camera from a linux box is with Scott Fritzinger's GPL'd gphoto program. gphoto allows basic control of a variety of cameras through serial or USB connection (and supports both interactive and commmand line modes - add a bit of perl and cron and you can do all sorts of fun things). Its still under development, however, and unfortunately doesn't currently support the Digital Elph (PowerShot S100) to my knowledge. I'm not sure how involved it would be to write a USB Elph driver for it, but you can check out the site if you feel up to it.

Digital Cameras and CCD Astrophotography

With astrophotography, you are getting into a rather specialized and involved use of CCD devices and generally speaking, it takes a good bit of expertise and dollars to get good results. You don't mention what you are looking to capture or what existing equipment you have, so I'll point out some of the basics and you can research further from these. FWIW, I'm not by any means an expert here, but I've been looking to jump in, so I'm seeing the same issues.

While there are limited exceptions, CCD astrophotography generally requires the use of specialized equipment. Your Canon Digital Elph doesn't have the required sensitivity (its equivalent to ISO 100 film), ability to take long exposures, long and fast enough lenses, or adapters for telescope mounting. While its possible to use a barn door tracker or equitorial tracking camera mount with the Elph, the results aren't likely to be worth the effort.

If you really get interested in astrophotography, you'll probably want to pony up for a specialized system like those built by Celestron and SBIG. These are highly sensitive, small array CCD cameras with specialized cooling and software for high gain operation. Add a high quality telescope, equitorial tracking mount, and related accessories, and you are talking about no small dollar commitment. Also, you'll need a lot of time and patience to find and capture accurately really good photos. I'd like to try CCD astrophotography out, but will be playing with 35mm (add a T mount and a Meade ETX and you can get started for under $1000) until I decide I'm really committed and move to a less light polluted neighborhood.

Sky & Telescope has a pretty good guide on where to start. Some good introductions to astrophotgraphy are:

• Sky and Telescope Imaging Resources
• Amateur Astrophotography links
• CCD Astrophotgraphy (annoying sounds)
• Santa Barbara Imaging Group (SBIG), a leading astro CCD maker
• Pin's Astronomy Page

Have fun, RJS

I can sympathize with what you are trying to do, as I've tried both uses with my Nikon Coolpix 950. The short answer is both are possible, but it isn't likely to be easy to get what you want out of the setup. Once drivers are available, you should be able to use your digital camera as a webcam fairly effectively, though you may have issues with autofocus, flash, and other camera-only adjustments. For astrophotography, it isn't likely you'll be able to get worthwhile images with the Elph, but I've included a few links on how to get started.

Digial Camera Webcams

Digital cameras defintely produce a much more compelling image than a typical webcam. I have a 3Com HomeConnect (a pretty good quality webcam) and it looks just awful compared to my Nikon N950 (not just resolution, but also trueness of color, CCD noise, and sensitivity). The main limitations are that you can't usually take mini-movies or fast sequences, some key functions are often only controllable on camera (for instance auto-focus and flash), and you'll need a power cord for the camera if you don't want to drain the batteries very quickly.

The easiest way to control the camera from a linux box is with Scott Fritzinger's GPL'd gphoto program. gphoto allows basic control of a variety of cameras through serial or USB connection (and supports both interactive and commmand line modes - add a bit of perl and cron and you can do all sorts of fun things). Its still under development, however, and unfortunately doesn't currently support the Digital Elph (PowerShot S100) to my knowledge. I'm not sure how involved it would be to write a USB Elph driver for it, but you can check out the site if you feel up to it.

Digital Cameras and CCD Astrophotography

With astrophotography, you are getting into a rather specialized and involved use of CCD devices and generally speaking, it takes a good bit of expertise and dollars to get good results. You don't mention what you are looking to capture or what existing equipment you have, so I'll point out some of the basics and you can research further from these. FWIW, I'm not by any means an expert here, but I've been looking to jump in, so I'm seeing the same issues.

While there are limited exceptions, CCD astrophotography generally requires the use of specialized equipment. Your Canon Digital Elph doesn't have the required sensitivity (its equivalent to ISO 100 film), ability to take long exposures, long and fast enough lenses, or adapters for telescope mounting. While its possible to use a barn door tracker or equitorial tracking camera mount with the Elph, the results aren't likely to be worth the effort.

If you really get interested in astrophotography, you'll probably want to pony up for a specialized system like those built by Celestron and SBIG. These are highly sensitive, small array CCD cameras with specialized cooling and software for high gain operation. Add a high quality telescope, equitorial tracking mount, and related accessories, and you are talking about no small dollar commitment. Also, you'll need a lot of time and patience to find and capture accurately really good photos. I'd like to try CCD astrophotography out, but will be playing with 35mm (add a T mount and a Meade ETX and you can get started for under $1000) until I decide I'm really committed and move to a less light polluted neighborhood.

Sky & Telescope has a pretty good guide on where to start. Some good introductions to astrophotgraphy are:

• Sky and Telescope Imaging Resources
• Amateur Astrophotography links
• CCD Astrophotgraphy (annoying sounds)
• Santa Barbara Imaging Group (SBIG), a leading astro CCD maker
• Pin's Astronomy Page

Have fun, RJS

For using it for astro photography you will need to be able to control the shutter speed. Many of the cheeper digital camers don't allow this. I don't know if yours allows shutter speed control or not. I own a Nikon 990 which allown me to do long exposures, up to 60 seconds if I hold the shutter button down. Unfortunatly it starts to show significant dark current problem on any photograph over 10 seconds long. It even shows some on 8 second shots. Another problem you will run into is the CCD chip in the camera has a color mask built in. This gives it color ability, but it also reduces the amount of light going to each resolveable pixel, typically 1/3rd to 1/4th. This means image times are atleast 3 to 4 times longer than with a BW CCD for a color shot. It also causes color effects as the image is sampled at different spots for each color. One can't get by the time issue for color astro work. However one can get around the color sampling at differnt spots. Astronomers use BW CCD cameras and color wheels, then take one shot each for red, gree, and blue. For good measure most astro photographers will also take a straight BW shot to. Another thing is the resolution the image's pizels are sampled at. Most consumer digital camers use 8 to 12 bit analog to digital converters in them. For astronomy you need as many bits as possible Many astro cameras are now at 16 bits.

There are cameras specially made for astronomy. Unfortunatly their prices are astronimical in relation to regular consumer cameras. SBIG is a major maker of astro camers for amatures and profesionals. It's a volume issue, plus these cameras have special coolers to help control and reduce dark current. The increased sampling resolution also helps jack the price up.

You can do some limited astro work with consumer digital cameras. It's mainly limited to very bright objects like the moon and sun. Note if photographing the sun, only use proper sun filters made for looking at the sun. Any telescope or camera lense can and likely will intensify the light into dangerous eye dammaging levels.