Thursday, March 28, 2013

Astrophotography - time is priceless

In action
Typical astrophotography setup contains typical elements: camera, telescope and the mount. The importance of components is hovewer of different order (especially assuming some budget). The most important is the mount - we should spent not less than 50% of total money for this component. The telescope focal ratio (diameter to focal length ratio) is quite important - it determines the total exposure time required. The camera is also important, however as long as we are not in a scientific grade instrument class, all amateur CCD cameras will provide decent and comparable results.
This time we will say a little more about how different aspects of our setup will affect the total exposure time required to achieve some measurable results. Let's define our goal first: we need to picture Owl Nebula (M97) that has apparent magnitude 9.9mag and angular diameter 3.3 arcmin, that gives us surface brightness value of 21.2 mag/arcsec2. We want to have signal to noise ratio (SNR) equals 40 on our final picture. Now we can calculate how different parameters affect total exposure time.
Note - we don't exposure single 300 minutes photo. We do 60 five minutes exposures and stack them. And the result is very similar to one 300 minutes photo.

(Geek mode on)

Camera dark current
This parameter is often overrated. Modern thermoelectrically cooled astrocameras has dark current value very low. Even modern DSLR cameras has this parameter under control, unless we do our sessions in the ambient temperature that exceeds 20C. Let's take a look into the graph:
Horizontal axis reflects sky background flux - the 20mag/arcsec2 means quite decent environment we can see some of Milky Way. 21-21.5 means dark, rural sky. 19-19.5 means city suburbs. Vertical axis is calculated total exposure time. We can see that camera dark current does not affect total exposure time much. All other noise sources are more important in this scenario.

Camera read out noise
Read out noise is the constant value of noise added to each frame taken by camera. It is added always. Another graph:
Here we can see some more significant differences. Camera read out noise affects total exposure time, especially when our camera has this parameter higher than 5e. For decent sky conditions and two cameras with 5 and 10e read out noise for the latter one we need to make total exposure time 50% longer to achieve our goal.

Camera quantum efficiency
WTH is quantum efficiency? It is amount of photons that we caught with camera pixel that will be converted to the actual electrons released that will product measurable signal. So if 100 photons will come to the pixel, and they will release 50 electrons we have QE=50%. Take a look at another graph:

Yeah, that is not surprise - the more effective our detector is the more signal we got. It is linear. DSLR cameras have QE between 30 and 50% (however Bayer matrix makes it not fully usable). Amateur CCD cameras QE varies between 40 and 65%. And last but not least...

Telescope focal ratio
Focal ratio (f stop) is the ratio between telescope diameter and its focal length. Fast scope (or camera lens) is the one that has large diameter and short focal length. It also means it is fast - you can achieve our goal quicker. Last graph:
Yeah, pretty impressive. Popular astrophoto refractors with ED lens have often focal ratio f/6 - f/8, that means they are slow. With dedicated astrophoto newtonian scope with f/4 ratio you can get the same result 2-4 times faster. ED refractors are more compact, do not require collimation, but it comes for a price. 

Summary
Dark sky - priceless. Each graph shows that suburbian sky with 19mag/arcsec2 sky glow will require four times longer total exposure time than under rural, dark sky. Even after this four times longer exposure the image will not be the same - the most faint picture regions will suffer more from light pollution. Image dynamic range will be compromised.
Focal ratio - often underestimated. Fast scope gives good results fast. Slow scope gives them later, but more comfortable.
Quantum efficiency - important, but available within limited range. Scientific grade CCD cameras cooled with liquid nitrogen can have QE 80% and more. Amateur ones are in 40-60% range.
Readout noise - quite important, especially for small pixel size cameras when full well depth is low and we need to collect many short frames to avoid oversaturation. Also more important under dark skies or when imaging with narrowband filters. 
Dark noise - could be important, but its value is so low comparing to readout noise and sky flux noise that it can be really forgotten. Is visible when picturing with DSLR during hot nights. More important with narrowband filter exposures.

Clear (and dark) skies!

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