|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Photon Transfer Curve
Hi Jens,
Have you made a linearity curve of the sensor, for example by making different exposure times with the same illumination? The amount of readout noise could also be very high, but this should be seperateable from the photon shot noise in the graph? I have, and there is some residual nonlinearity at low exposure levels, but nothing too great. The readout noise doesn't seem to be too high. My ideas are shrinking, so what else can cause the fact that the shot noise doesnt increase so much as it should (what a lower slope means)? I don't know either!?!? I'm going back to basics here .... evaluating every step of the acquisition process to see if I went wrong somewhere. Best regards, Patrick |
#12
|
|||
|
|||
Photon Transfer Curve
Hi Patrick,
Have you made a linearity curve of the sensor, for example by making different exposure times with the same illumination? The amount of readout noise could also be very high, but this should be seperateable from the photon shot noise in the graph? I have, and there is some residual nonlinearity at low exposure levels, but nothing too great. The readout noise doesn't seem to be too high. But this seems to be important, is the acquisition board designed for imagers, what method is used, CDS (correlated double sampling)? What is the exact sensor name? I think in the time i have all informations needed to get the reason, i can design and built a complete new camera ;-). Is the project an astro camera? Best regards, Jens |
#13
|
|||
|
|||
Photon Transfer Curve
Hi Jens,
Have you made a linearity curve of the sensor, for example by making different exposure times with the same illumination? The amount of readout noise could also be very high, but this should be seperateable from the photon shot noise in the graph? I have, and there is some residual nonlinearity at low exposure levels, but nothing too great. The readout noise doesn't seem to be too high. But this seems to be important, is the acquisition board designed for imagers, what method is used, CDS (correlated double sampling)? What is the exact sensor name? I think in the time i have all informations needed to get the reason, i can design and built a complete new camera ;-). Is the project an astro camera? The acquisition board is a National Instruments LabView ADC. So, It's more of a general purpose low noise ADC. It's a proprietary CMOS sensor, so legally, I can't speak freely about the readout method. I'm sorry. It is not an astro camera, but I do come from the astro community so I'm familiar with some of the specifics of astronomical instrumentation. Best regards, Patrick |
#14
|
|||
|
|||
Photon Transfer Curve
Hi Jens,
Ok. I think I'm on to something here ... The read noise is high: about 280 e rms. At saturation, the noise is 750 e rms. This is only about 2.7 times the read noise. So, I surmise I'm not actually ever reaching the shot-noise limited region! What threw me was that I was able to fit (with low residual error) a line to a major portion of the curve, which I assumed to be the shot-noise limited region. I no longer believe this is the case. However, I still have high dynamic range: saturation = 500ke. So dynamic range is 20*log(500000/280) = 65dB. Is it strange that I can have both high dynamic range and high read noise? I know the two are somewhat decoupled, because the full well is quite high, but something seems a little off here. Best regards, Patrick |
#15
|
|||
|
|||
Photon Transfer Curve
Hi Patrick,
Ok. I think I'm on to something here ... The read noise is high: about 280 e rms. Oh, this explains something. Sensors normally has to be read out by CDS or dual slope integration (weighted dsi is the best), and some sensors can be read without destruction of the charges, so noise can be lowered by multiple readouts. Then there are sensors with charge multiplying processes, but this increases the shot noise. Timing and correct filtering are important for a low noise readout. However, I still have high dynamic range: saturation = 500ke. So dynamic range is 20*log(500000/280) = 65dB. Is it strange that I can have both high dynamic range and high read noise? I know the two are somewhat decoupled, because the full well is quite high, but something seems a little off here. Far off, if you ask me. High dynamic range doesnt increase the readout noise in this amount. Sensors with such a high fullwell have sometimes two different readout modes, one with a higher v/charge output and decreased fullwell - but a slightly lower read-noise, and one with the maximum fullwell and somewhat higher readout-noise. But the values for good CCDs are somewhere about 5 and 8e_rms, dependend on the speed and method of readout, higher than 20e should be avoided. I think you should use a acquisition board, designed or handmade for the sensor. Best regards, Jens |
#16
|
|||
|
|||
Photon Transfer Curve
i forgot:
I think you should use a acquisition board, designed or handmade for the sensor. And with a 16 Bit converter minimum, especially if the output of the sensor is linear. Best regards, Jens |
#17
|
|||
|
|||
Photon Transfer Curve
Hi Jens,
Thank you again for the excellent insight. We are not doing strict CDS, because of the nature of the CMOS device. The reset and video levels are sampled and stored at different times on the device (as in most CMOS imagers). Also, there is only one output on the device. As far as acquisition board goes, I think you are exactly right, the acquisition board may be what is limiting me. I will be moving to a more sophistiated setup soon. But does what I see so far at least advance the theory that I'm never in a shot-noise limited region because at maximum signal, my noise is only 2.7 times the read noise? Best regards, Patrick Jens Dierks wrote: Hi Patrick, Ok. I think I'm on to something here ... The read noise is high: about 280 e rms. Oh, this explains something. Sensors normally has to be read out by CDS or dual slope integration (weighted dsi is the best), and some sensors can be read without destruction of the charges, so noise can be lowered by multiple readouts. Then there are sensors with charge multiplying processes, but this increases the shot noise. Timing and correct filtering are important for a low noise readout. However, I still have high dynamic range: saturation = 500ke. So dynamic range is 20*log(500000/280) = 65dB. Is it strange that I can have both high dynamic range and high read noise? I know the two are somewhat decoupled, because the full well is quite high, but something seems a little off here. Far off, if you ask me. High dynamic range doesnt increase the readout noise in this amount. Sensors with such a high fullwell have sometimes two different readout modes, one with a higher v/charge output and decreased fullwell - but a slightly lower read-noise, and one with the maximum fullwell and somewhat higher readout-noise. But the values for good CCDs are somewhere about 5 and 8e_rms, dependend on the speed and method of readout, higher than 20e should be avoided. I think you should use a acquisition board, designed or handmade for the sensor. Best regards, Jens |
#18
|
|||
|
|||
Photon Transfer Curve
Hi Patrick,
Thank you again for the excellent insight. We are not doing strict CDS, because of the nature of the CMOS device. The reset and video levels are sampled and stored at different times on the device (as in most CMOS imagers). Also, there is only one output on the device. Ok, i am not very familiar with cmos devices, so my insight belongs more to CCDs. Nevertheless (is this a english word?), readout noise should be lower. As far as acquisition board goes, I think you are exactly right, the acquisition board may be what is limiting me. I will be moving to a more sophistiated setup soon. So the readout noise is more a quantization noise. But does what I see so far at least advance the theory that I'm never in a shot-noise limited region because at maximum signal, my noise is only 2.7 times the read noise? My not very professional diagram, depending on 280e readout noise and 500ke fullwell looks like this: http://freenet-homepage.de/JDierks/curve.gif I dont know how much saturation can flatten the slope, but 0.2? But im quite sure that a lower noise level should give you a much better approximization. Best regards, Jens |
#19
|
|||
|
|||
Photon Transfer Curve
Hi Jens,
So the readout noise is more a quantization noise. I believe some of it may be. But does what I see so far at least advance the theory that I'm never in a shot-noise limited region because at maximum signal, my noise is only 2.7 times the read noise? My not very professional diagram, depending on 280e readout noise and 500ke fullwell looks like this: http://freenet-homepage.de/JDierks/curve.gif That drawing IS good. It is quite similar to what I see. So, it looks like I just never reach the shot-noise region. I dont know how much saturation can flatten the slope, but 0.2? I would expect the characteristic steep fall off of the curve at saturation. But im quite sure that a lower noise level should give you a much better approximization. Agreed. Thanks again. Best regards, Patrick |
#20
|
|||
|
|||
Photon Transfer Curve
Hi Patrick,
So the readout noise is more a quantization noise. I believe some of it may be. If it doesnt decrease with another board, you have to rethink the way you gain the data. I dont know how the darklevel of each pixel is sampled in a cmos sensor. But in my understanding, it has to be sampled just close bevor the lightlevel, or has to be integrated for a longer time to get rid of the low frequencies of the amp-noise. That drawing IS good. It is quite similar to what I see. So, it looks like I just never reach the shot-noise region. Yes, but in the case that the read-noise increases with a longer exposure time (and the brighter images were captured with longer exposures), it would maybe pointing to a wrong darklevel acquisition. I dont know how much saturation can flatten the slope, but 0.2? I would expect the characteristic steep fall off of the curve at saturation. Yes, if the converter is saturating bevor the sensor gets close to the fullwell. Otherwise it shouldnt fall off so quick, dependend on the sensors characteristic. Ok, 760e noise_rms is nothing against 500ke fullwell, so it wouldnt flatten the curve a lot, too. Best regards, Jens |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Science and the Curve ball | G=EMC^2 Glazier | Misc | 6 | July 24th 06 01:37 PM |
Milky Way Rotation Curve | Steve Willner | Research | 1 | June 20th 06 09:51 AM |
Milky Way Rotation Curve | Oh No | Research | 9 | June 19th 06 03:17 PM |
The Curve of Binding Energy | Hop David | Policy | 29 | May 19th 06 05:49 PM |
What's the name of that curve? | wavefront | Amateur Astronomy | 8 | April 1st 04 01:50 PM |