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CCD sensitivity and resolution
How does one figure out the arc second resolution potential for a CCD chip?
Is there a way to know what magnitude limit a CCD imager has? |
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CCD sensitivity and resolution
On Fri, 25 Jan 2008 20:49:27 GMT, "TMA" wrote:
How does one figure out the arc second resolution potential for a CCD chip? Is there a way to know what magnitude limit a CCD imager has? Calculate the resolution A (in arcsec per pixel) as A = 206265 * d / F where d is the size of the pixel, and F is the focal length of the telescope, both in the same units. Calculating the magnitude limit is quite complex. It depends primarily on the QE (as a function of wavelength) of the sensor, the exposure time, and on the aperture of the telescope. You also need to consider the various noise sources: readout, dark current, sky, and the statistical noise on the signal itself. Some of the noise sources are themselves dependent on the exposure time and the focal ratio of the telescope. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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CCD sensitivity and resolution
Chris L Peterson wrote:
On Fri, 25 Jan 2008 20:49:27 GMT, "TMA" wrote: How does one figure out the arc second resolution potential for a CCD chip? Is there a way to know what magnitude limit a CCD imager has? Calculate the resolution A (in arcsec per pixel) as A = 206265 * d / F where d is the size of the pixel, and F is the focal length of the telescope, both in the same units. Calculating the magnitude limit is quite complex. It depends primarily on the QE (as a function of wavelength) of the sensor, the exposure time, and on the aperture of the telescope. You also need to consider the various noise sources: readout, dark current, sky, and the statistical noise on the signal itself. Some of the noise sources are themselves dependent on the exposure time and the focal ratio of the telescope. Chris is quite right on both counts. I'd like to add a little bit to his discussion about magnitude limit. The bottom line is that you want the signal in the brightest pixel of the star image to be at least 2 or 3 times the noise. (Higher if you want to do decent astrometry.) The amount of signal you get depends on the QE, the exposure time, the aperture, *and on the seeing*. If the seeing is excellent, or if your pixels subtend a lot of sky, most of the starlight will fall in one pixel -- ideal for detection, not so good for precision astrometry. As the pixel size shrinks, or as the seeing gets worse, the same number of photons will now be spread over more pixels, and that lessens the peak signal and hurts detection. The other half of it is noise. Shot noise (because light comes in discrete photons) goes as the sqrt of the number of electrons (NOT the sqrt of the data number!). The same calculation also applies to other sources of electrons -- stray light, dark current, skyglow. There's also read noise in the on-chip amplifier, which is generally independent of the signal level. There are other noise sources too, but read noise and shot noise are the two biggies. Add them in quadrature (square root of the sum of their squares) to get the total. If your camera has a lot of read noise but a very dark background, all you have to do is to get the signal 2 or 3 times as much as the read noise and you're OK. Doubling the exposure would get you stars 50% fainter. But if shot noise is the bigger concern -- as is usually the case -- you must quadruple the exposure to gain a factor of 2 in sensitivity. The signal goes up by a factor of 4 but the noise by a factor of 2. Hope this helps. -- Bill Owen |
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CCD sensitivity and resolution
On Fri, 25 Jan 2008 20:49:27 GMT, "TMA" wrote:
How does one figure out the arc second resolution potential for a CCD chip? Is there a way to know what magnitude limit a CCD imager has? The resolution of the CCD chip in arc seconds would be 206,265 times the pixel pitch divided by the focal length of the telescope. Use the same units for pixel pitch and focal length. The limiting magnitude of the CCD chip depends on the aperture of the telescope, the length of the exposure, conditions at your site, etc. To learn what the limting magnitude is you would have make images and see what you get. Bud |
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CCD sensitivity and resolution
Chris L Peterson:
Calculate the resolution A (in arcsec per pixel) as A = 206265 * d / F where d is the size of the pixel, and F is the focal length of the telescope, both in the same units. Calculating the magnitude limit is quite complex. It depends primarily on the QE (as a function of wavelength) of the sensor, the exposure time, and on the aperture of the telescope. You also need to consider the various noise sources: readout, dark current, sky, and the statistical noise on the signal itself. Some of the noise sources are themselves dependent on the exposure time and the focal ratio of the telescope. Damned near impossible. Add air/light pollution to the imponderables. I think that it would be a lot less trouble to measure the magnitude limit over a number of exposures under ideal skies. If (and I recognize that's a big if) Hubble GSC 2.2 is accurate, I have captured mag 21 stars with my Tak 180mm f2.8 astrograph and SBIG STL-11000 from my suburban location. There is a mag 19 star (according to GSC 2.2) in the images that I captured with the a 76mm 'scope and Canon 20D DSLR in a four-minute exposure and posted at http://www.davidillig.com/ast-73p_s-w060429.shtml. Davoud -- usenet *at* davidillig dawt com |
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CCD sensitivity and resolution
On Fri, 25 Jan 2008 23:12:26 GMT, Davoud wrote:
Damned near impossible. Add air/light pollution to the imponderables. I wouldn't go that far. I have a fairly simple flux calculator in a spreadsheet that does a pretty accurate job of estimating limiting magnitude for a given sensor, exposure time, sky background, aperture, and desired S/N. But... I think that it would be a lot less trouble to measure the magnitude limit over a number of exposures under ideal skies. .... you're right, it's easy to measure, and you're going to get the most realistic idea of your camera's performance that way. If (and I recognize that's a big if) Hubble GSC 2.2 is accurate, I have captured mag 21 stars with my Tak 180mm f2.8 astrograph and SBIG STL-11000 from my suburban location. That sounds about right. Do you recall what kind of exposure time was required? _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#7
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CCD sensitivity and resolution
Is the pixel pitch just the width of a pixel?
The resolution of the CCD chip in arc seconds would be 206,265 times the pixel pitch divided by the focal length of the telescope. Use the same units for pixel pitch and focal length. The limiting magnitude of the CCD chip depends on the aperture of the telescope, the length of the exposure, conditions at your site, etc. To learn what the limting magnitude is you would have make images and see what you get. Bud |
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CCD sensitivity and resolution
On Sat, 26 Jan 2008 00:21:47 GMT, "TMA" wrote:
Is the pixel pitch just the width of a pixel? It depends on the sensor. Some have dead space between the pixels, so the active area isn't as wide as the spacing between them. A lot of newer sensors use little lens arrays over the top to try and get the sensitive area larger. In any case, when calculating resolution, the pitch is the distance between adjacent pixels, which isn't necessarily the actual pixel width. Also, some sensors have different pitches horizontally and vertically, and therefore different resolutions. If so, you can calculate them separately. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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CCD sensitivity and resolution
Davoud:
If (and I recognize that's a big if) Hubble GSC 2.2 is accurate, I have captured mag 21 stars with my Tak 180mm f2.8 astrograph and SBIG STL-11000 from my suburban location. Chris L Peterson: That sounds about right. Do you recall what kind of exposure time was required? About three minutes. The magnitudes (21--or less) are correct, but I made a misstatement. I see that GSC 2.2 has a magnitude limit of 19.5. Sure enough, using XEPhem with field stars set to mag 30, the dimmest stars displayed are in the mag 19 range. I have stacks of FITS images that show stars dimmer than shown in XEPhem, though they don't always show up in my reduced images because I try to process for aesthetics. Davoud -- usenet *at* davidillig dawt com |
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CCD sensitivity and resolution
On Sat, 26 Jan 2008 16:45:30 GMT, Davoud wrote:
About three minutes. The magnitudes (21--or less) are correct, but I made a misstatement. I see that GSC 2.2 has a magnitude limit of 19.5. Sure enough, using XEPhem with field stars set to mag 30, the dimmest stars displayed are in the mag 19 range. Yes, mag 19 sounds reasonable for three minute exposures. I've got images showing stars around mag 21, but with exposure times more in the hour range (and with less sky background). I have stacks of FITS images that show stars dimmer than shown in XEPhem, though they don't always show up in my reduced images because I try to process for aesthetics. Makes sense. The dimmest stars are going to be right up against the noise floor, and when you process for aesthetics, you usually pull up your black level to hide that noise. It's a fun exercise to see just how deep you can get- something you can try if you ever get a long enough stretch of good conditions that you can "waste" some time on non-aesthetic imaging g. From your comments over the last couple of years (since you got some top-notch equipment on line, naturally) it sounds like your good imaging opportunities have been depressingly rare. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
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