A sub-exposure calculator
After reading some more about optimal sub-exposure time (Lodriguss, Kharusi, Berry) specifically for DSLR's and after doing some tests with SQM-values, back-of-camera histograms and analysis of my RAW data I made myself a spreadsheet that advises about optimal sub-exposures and ISO values. It is based on the rule that the total noise in the exposure should be much higher (20-50x) than the read noise of the camera.
http://zalmstra.nl/astro/sub-exposure%20calculator.xls
The spreadsheet uses measured SQM-L (Sky Quality) values if you have them, but you can do without.
This is just a first usable version. It needs some more calibration and I want to include filter support.
To put the discussion about optimal sub-exposure in perspective, here is a quote from an email I received from Kharusi about this subject: "Most beginners under-estimate how much impact itegration time makes. Worrying about the length of subs is trivial compared to the impact that shooting more integration time will make."
His advises:
- do not use longer sub-exposures than advised from this calculator. It might lead to worse results caused by seeing, sattelites, planes, guiding errors, saturation.
- Do not overestimate the advantage of high ISO's. In general 1600 is ok, but go back to 800 or even 400 to prevent overexposure. (Lodriguss advises to use 800 en 400 at low temperatures)
- maximize total integration time (number of frames x sub-exposure time). Consider to combine sub-exposures from 2 or more nights.
Optimal sub-exposure length
Deepsky astrophotograhy is all about signal-to-noise-ratio (SNR). The higher the better. There is a lot of literature about this subject. I can recommend the five SNR articles from Craig Stark.
In (very short) summary: One can improve the SNR by using longer exposures and by using more exposures.
I want to find out what is the optimal sub-exposure time (=time per frame) in my situation, using a DSLR with different optics under fairly good Dutch skies. To me it seems the longer the better, up to a certain limit, and I want to find this limit. I think there are two limits on the maximum exposure time:
- Guiding.
- Sky fog
This blog is about the 2nd limit. Long perfect guiding is not easy, especially with long focal length. But when imaging with a short 135mm lens I have mastered this and I can autoguide for any length of time using PHD and a 200mm lens with a DSI guiding camera.
The max exposure time due to skyfog is affected by ISO value and by focal ratio. Doubling ISO from 800 to 1600 halves the max exposure time. Doubling the focal ratio from f/3.5 to f/7 quadruples (4x) the max exposure time. So when I know the max exposure time for one combination of ISO value and focal ratio I can calculate the max exposure time for any other combination.
I tried 5 and 8 min ISO 1600 exposures of M45, The Pleiades, with my 135mm f/3.5 lens on the EOS20d and hope to find out if I reached the maximum exposure.
To begin with: I need much more knowledge of the theory behind deepsky astrophotography. Apart from the above mentioned article series of Stark Labs I have another source that will help my knowledge: optimum exposures from Starizona. In it, there is a link to another in-depth and instructive discussion from Hiddenloft about optimal exposure times. Both sources mention the characteristics of popular CCD camera's. Many recommendations depend on these camera characteristics, but they are not well known for DSLR camera's. Here I found the characteristics of my EOS 20D camera.
There is soo much information on the internet about deepsky astrophotography and not all of it is equally true or instructive. I am a bit overwhelmed by the complexity of the subject. And sometimes even experienced astrophotographers that made a big study of this, seem to concur about essential conclusions. Just Google up "optimal sub-exposure" and start reading....
One important finding up to now after reading some of the theory: apart from a maximum exposure time determined by sky-fog (which is quite high, as I found out), there is the more important minimum exposure time. This minimum is determined by the read-out noise of the camera and it is affected by the amount of sky fog reaching your sensor, so it depends on skyfog itself (more skyfog, lower minimum), the ISO value of my DSLR and the focal ratio of my optics (faster optics, lower minimum).
Several astrophotographers agree that, as long as this minimum exposure Tmin is met, there is not a big SNR difference between N frames of Tmin and one frame of N*Tmin exposure wich will take equal amount of valuable imaging time. Apart from that, using one or few long exposures has a few disadvantages over many short ones:
- it is harder to guide error-free for a long time.
- Seeing-effects on the atmosphere may incur the sharpness of the image when using longer sub-exposures
- there is more chance of a problem with an sattellite or clouds ruining your subframe.
- the minimum pixel value will be higher so you will lose some dynamic range.
- There is the risk of clipping in bright parts of the objects. For instance you can lose usefull colour information in bright stars.
- In addition, DSLR's compared to (cooled) CCD camera's can add another source of noise (dark current) during long exposures, getting more problemous when the camera is heated during long exposures.
This all favors short exposures as long as the mimimum Tmin is met.
After reading the excellent weblog of Samir Kharusi on this subject (he states that, given the total imaging time, the minimum sub-exposure Tmin is as good as any longer sub-exposure time http://www.samirkharusi.net/sub-exposures.html) and after some email discussion with him about this SSL-regime I decided to skip searching for maximum sub-exposure time and instead:
- to do tests to determine this minimum exposure time following Mr Kharusi SSL regime, depending on local conditions and the camera&optics used. His weblog entry http://www.pbase.com/samirkharusi/image/37608572 shows how to use an SQM-meter (wich I have) or short testframes to determine this minimum.
- investigate if my results improve when using longer sub-exposures without changing total imaging time. If mr Kharusi is right, they will not!
- I will do tests with and without my CLS clip-in filter. This skyfog discussion has wetted my appetite to retry this sky-fog CLS filter!
I will report results from these test, which will take several weeks (considering local cloud conditions) in another post.