Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Mike Loffland
Web Guru (Astronomics)
Reged: 09/03/04
Posts: 2080
Loc: Norman, Oklahoma
|
|
Signal to Noise: Part 3 - Measuring your Camera
|
RHK
newbie
Reged: 03/25/08
Posts: 2
|
|
Craig, I've been trying to (slowly) follow your signal to noise articles because I'm tempted to try astrophotography with a DSLR camera so that I don't need a laptop with me at the scope for the duration of the sessions. I assumed that I could do post-processing of the images from the flash card with s/w on my iMac later. I was therefore surprised to see this statement in your most recent article: "Now, fire up the camera and connect in your capture software." This was just before taking a set of at least 30 dark frames. How does "capture software" play at this point? If we were using a dedicated CCD imager I could understand because it can't function without s/w running on a laptop, but with a DSLR I'm clueless. How and why does a DSLR need to interface with capture software? I do have a USB cable for the camera of course, but I've only used it for downloading photos directly to a laptop or desktop PC or connecting with photo editing software, like iPhoto or Photoshop Elements, but I haven't done that frequently. I usually download photos from the flash card through a flash card reader. Could you please explain your statement? Sorry if this is all obvious to the vast majority of you readers, but I can't move on if I don't understand. Thanks.
|
Mike Clemens
Post Laureate
Reged: 11/26/05
Posts: 4268
Loc: Wasilla, Alaska 61N
|
|
Most people probably sequence their shots with an attached computer. If you can produce the requested flats, bias and darks of the appropriate duration without capture software, thats fine.
|
Craig
Vendor (Stark Labs)
Reged: 09/16/03
Posts: 583
Loc: Irvine, CA
|
|
Mike is correct. I made the assumption that you'll be using capture software to do this. But, if you've got an interval timer for your DSLR, that will work just fine. I'd note that whatever loads those raw files should ideally load the raw bayer matrix directly and show you a monochrome image on screen and not a color image. Your sensor is really a mono sensor with an array of color filters on it and software is used to make it a color image. If you do your tests based on the color image, you're also testing whatever the software did to make the color image ("debayering" or "demosaicing").
Some of the software out there will load it as the raw data and others won't. FWIW, one can always use "dcraw" to get this pure sensor data.
Craig
--------------------
Stark Labs Astrophotography software
Borg 101 ED f/4, C8, and too many cameras to mention
|
Agnotio
sage
Reged: 08/29/08
Posts: 230
Loc: Ottawa, Canada
|
|
Hi Craig,
I'd like to say thanks for the series. I followed the whole guide, and even joined up AstroPhoto Insight so I could read your articles analyzing other cameras.
Some of the results I got are that the system gain of my Canon Xsi (450D) is 0.30 e-/ADU at ISO 800, with a full-well capacity of roughly 4850 electrons with the 14-bit ADC. The read noise was 4.6e- at ISO 800.
However, I couldn't measure the dark current because of the problem you mentioned in other articles: the scaling that Canon applies to the dark frames. The mean of my subframes decreased with increased exposure time while the standard deviation increased, just like you observed. This is really a shame because it means dark subtraction won't work optimally on Canon DSLRs with automatic scaling. I wish there was an option to turn it off. Although, I have to say I haven't noticed this effect from my suburban area; subtracting dark frames gets rid of hot pixels, which is all I'm really looking for as photon noise from skyglow is a much bigger problem than dark current for me. It looks like I will have to revise my habit of taking the read noise out of my image using dark frames though, as the scaling will throw a wrench in this process as well. I wonder if there's any way around it?
One result I got that was anomalous was that the log-transformed histogram of my read noise frame had shoulders almost like you got for the Canon 5D Mk II. Apparently, there is non-repeatable horizontal pattern noise there, because the FFT of the read noise frame still showed a vertical line instead of just random noise. Some disappointing results there, maybe you just put me in the market for a CCD 
Anyway, Cheers.
--------------------
Sky-Watcher Equinox 80ED
EQ6 Pro with EQMOD
Canon 450D
Sky-Watcher 10" f/4.7 Dob
|
bill w
Postmaster
Reged: 03/26/05
Posts: 6590
Loc: southern california
|
|
finally made it through 
love the series
thanks for writing it up
--------------------
-bill w
nexstar 8 GPS
canon 300D, Toucam Pro II
SXV-H9C, H9, SX Exview autoguider, SX-AO
FS 102 (OLV), FS60 CSV, 8" LX200R, G 11
http://astro.whwiii.net/
image processing monitor calibrated to just differentiate darkest boxes:
|
Craig
Vendor (Stark Labs)
Reged: 09/16/03
Posts: 583
Loc: Irvine, CA
|
|
Glad to hear some are working their way through it! Yes, you'll find that the Canon DSLRs aren't "textbook" in a number of ways. But, you do get a huge chip for a reasonable price and can take shots of the kids with that. Dedicated astro-CCDs tend to fall short in these ways (although I have seen shots of Alan Friedman's kids taken with his astro-cams).
Craig
--------------------
Stark Labs Astrophotography software
Borg 101 ED f/4, C8, and too many cameras to mention
|
Barry E.
journeyman
Reged: 12/27/08
Posts: 6
|
|
Great article, Craig. I finally had the time to sit down and go through this analysis with my modified Canon 40D and here's what I got:
System Gain: 0.049 e-/ADU
Full-Well Capacity: 3162 e-
Read Noise: 4.255 e-
Dark Current: 0.010 e-
Dark Stability: About 25 minutes
Bias: Stack of 100 frames FFT, showed 3 faint vertical lines/frequencies.
I'm looking forward to your next installment and some advice on how to take this analysis and do something with it.
--------------------
Barry E.
Mocksville, NC
|
themos
journeyman
Reged: 04/01/08
Posts: 9
|
|
For my Canon 450D at ISO400 (nominal)
gain is 0.568 e-/ADU
read error is 6.3 e-
full well capacity is 9143 electrons
dynamic range is 1449 or 32db
dark stability: unknown, std dev of 1-minute darks climbed from 13.9 to 17.4 and had not levelled off after 30 dark frames.
|
|
1 registered and 1 anonymous users are browsing this forum.
Moderator: spaceydee
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 700
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
