DSLR Noise
10/22/2015. I've spent most of the day putting cameras in the refrigerator and collecting dark frames to build new master darks. The 180s masters are made from leftovers. Since I intend to standardize on 300s exposures, I've begun gathering long darks to go with them. It's mostly a change to speed up processing in PixInsight. An hour's exposure under the stars takes 20 of the shorter subs or 12 longer ones; more to the point, five hours of exposure takes 100 short subs or 60 longer ones; and even more to the point, it takes 40% less time to process the latter than the former. When you're running PixInsight on a slowish laptop, that matters. In effect, longer subs are a cheap performance boost.
Here is a table of the noise characteristics of the finished files, the better to appreciate how camera model, temperature, time, frame-count, and ISO work together. I followed recipe for dark frame integration on this page.
| Canon 6D |
|
|
|
| time & iso |
temp x subs |
image statistics |
|
| 6D 180s 1600 |
30°F x 32 |
mean 2047
stdDev 12.8
|
Temperature approx. |
| 6D 180s 1600 |
35°F x 24 |
mean 2047
stdDev 33.0
|
Refrigerator sample |
| 6D 180s 1600 |
42°F x 50 |
mean 2047
stdDev 19.1
|
Temperature approx. |
|
|
. . . |
|
| 6D 300s 1600 |
20°F x 33 |
mean 2048
stdDev 15.0
|
Freezer sample |
| 6D 300s 1600 |
35°F x 61 |
mean 2047
stdDev 34.1
|
|
| 6D 300s 1600 |
70°F x 38 |
mean 2028
stdDev 104.4 |
|
| |
|
|
|
| Canon 50D: |
----------- |
---------------- |
--------- |
| 50D 300s 1600 |
20°F x 9 |
mean 1024
stdDev 22.2
|
20°F saps the battery |
| 50D 300s 1600 |
35°F x 34 |
mean 1026
stdDev 40.1
|
|
| 50D 300s 1600 |
50°F x 24 |
mean 1026
stdDev 49.7
|
Temperature approx. |
| 50D 300s 1600 |
60°F x 20 |
mean 1028
stdDev 132
|
Temperature approx. |
| 50D 300s 1600 |
70°F x 31 |
mean 1032
stdDev 249
|
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Interesting that the mean brightness of pixels in the 50D dark frame settles in around 2^10 (1024) while the 6D approaches 2^11 (2048). Both cameras output 14-bit data in raw mode, but the soft- and firmware combined wtih the Digic-4 processor (in the 50D) and the Digic-5 (in the 6D) behave differently. I wonder if the 50D doesn't just lop off the lowest order bit for processing efficiency and noise abatement. In any case, the 6D is the quieter of the two and is less sensitive to temperature. It still benefits from a cool chip, but the 50D really suffers in the warm.
Except where noted, deep-sky photos are made with an SBIG ST2000XM CCD behind a 10-inch Astro-Tech Ritchey-Chretien carried on an Astro-Physics Mach1GTO. The CCD is equipped with Baader wide- and narrow-band filters. The internal guide chip of the CCD most often keeps the OTA pointed in the right direction (I'll let you know when an OAG or guidescope takes its place). Camera control and guiding are handled by Maxim DL 5.12. The stock focuser on the AT10RC has been augmented with Robofocus 3.0.9 using adapters turned on the lathe downstairs. A Canon 6D and a modded 50D find themselves mounted on an Orion 10" F4 Newtonian or carrying widefield glass on an iOptron Skytracker. Beginning in May 2013, PixInsight has taken over more and more of the heavy lifting -- alignment, stacking, gradient removal, noise-reduction, transfer function modification, color calibration, and deconvolution. Photoshop CS4 et seq and the Focus Magic plugin get their licks in, too.
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