Post by southernskies on Sept 4, 2019 2:21:36 GMT
Some deep sky objects have such a high dynamic range (HDR) that a camera cannot satisfactorily image the whole of that range in just one exposure. A good example would be the Great Orion Nebula, Messier 42. Imaging the Trapezium cluster within the nebula requires only a very short exposure. Imaging the region surrounding that cluster needs a longer exposure but over-exposes the Trapezium. And imaging the nebula's wide fan needs a yet longer exposure but over-exposes the inner regions. Conventional imagers can blend the different exposures together using post-acquisition HDR techniques so as to reveal the details in each of them all together in one image. Until recently I did not see how this blending could be applied to viewing live EAA images. However, some weeks ago I found an interesting article on the internet that described in principle how to blend long and short EAA exposures into a single screen image in real time. Regrettably I have lost the relevant internet reference and, so far, I have not been able to recover it. So I cannot presently give proper accreditation to the author of the idea. If and when I can resolve that issue I will edit this post accordingly.
The image capture software that that author was describing was basically the same as the one that I use but badged with a different name. I use “ToupSky” which, I understand, is essentially the same as “Rising Sky” and it no doubt comes under other names as well, including the equivalent Mallincan branded software. For exposures longer than 5 seconds it offers a feature called “trigger” that allows you to specify an exposure length of (in my case) up to 15 minutes which can then be repeated indefinitely in a loop until manually stopped. The method suggested by the author uses that trigger mode in conjunction with the software's “live stack” feature. In essence the method involves taking a few exposures to start a live stack, stopping the loop (which pauses but does not cancel the live stack), changing the exposure setting and then restarting the loop which recommences the live stack but now using the altered exposure time. Thus two (or more) different exposure times can be incorporated into the same live stack.
There are four other image capture features in ToupSky that are relevant to mention here :
- the dynamic histogram which allows one to set the dark point of a live stretch;
- the white balance feature that equalises the colour channel curves in the histogram;
- the dark field correction which eliminates hot pixels;
- the flat field correction which corrects any inherent non-flat attributes of the imaging system.
It is useful to be able to select from pre-compiled libraries of dark field and flat field images that cover a range of different exposure settings and histogram peak points. I had already built a library of darks that are named by their exposure lengths. Similarly, I have a library of flats that are named by their histogram peak points. This naming system makes it easy to select the relevant dark and flat as and when needed.
I will not elaborate here on the use of these other features save to say that at the outset the peak positions of the three RGB colour bell curves in the histogram will need to be equalised, probably using the white balance feature. Then each time a different exposure length is used in the stack the other settings will need to be changed accordingly. Thus the appropriate histogram dark point setting and the colour curve peak reading for each different exposure will need to be identified in advance. The choice of dark field correction image will need to match the new exposure setting. And the choice of the flat field correction image will need to match the previously identified histogram peak point produced by that exposure.
Continued in Part 2 ….