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Who has never given in to the temptation to start an image as quickly as possible once night falls, without any preparation? We wrongly think that a new image begins once the first photo is loaded and displayed. However, the image begins well before this phase.
Rigorous preparation is not a guarantee of success, but at least it is a major condition. In this article, we will try to detail the key phases of a good start.
What object to photograph?
What object to photograph?
What to photograph is a matter of taste, but not only. It depends on the season and the position in the sky, the time at which to start and stop the session, but also on additional constraints such as light pollution on certain cardinal points, whether due to a city, public illumination or the neighbor's Christmas garland.
For our part, we like to alternate between complex objects and simpler objects, in other words narrow-band nebulae and LRGB objects that are easier to post-process.
There are also some favorite objects, others that are more classic, and others that are totally innovative with which we have no experience. Alternating allows us to keep the fun, without lingering on a phase of disappointment!
Which field of view
Which field of view
The size of the field is given by the optical solution, but the exact position remains the photographer's choice.
Ask yourself: What story would I like to tell?
Each image tells a story and emphasizes several particular details. Let's be clear: it is illusory to want to show all the details of an image. On the contrary, we must make choices, and accept to minimize one detail in favor of another.
For this reason, we spend about an hour on each new composition, moving the center of the image by a few arc minutes to achieve a result that seems both judicious and aesthetic to us. Very often, we need a dozen iterations before making a reference photo that we like.
Above is an example of the "story to tell".
In this image, we wanted to draw attention to the central tear, while preserving the negative spaces formed by the dark nebulae as well as the lower left corner, in order to soften the visualization. The beauty of the filament-shaped nebulosity located in the center left adds finesse to the composition.
Note here the star on the right edge, whose typical OIII filter halo is cut by the crop. This unsightly aspect could have been improved by preserving a wider edge, generally cropped by a hundred pixels during post-processing.
But after all, we are the Disastronomers, right?
Which exposure time and gain?
Which exposure time and gain?
Our cooled camera could allow us to lower the exposure time to less than a minute, without lowering the quality because of the read noise.
However, most of our photos are taken in 300 seconds. This compromise allows us to optimize the number of photos (and therefore disk space and post-processing time) with the waste due to guiding errors, wind gusts, and other unexpected pleasures.
We then modify the gain in order to maintain the histogram in an optimum, avoiding that the brightest pixels are saturated or on the contrary underexposed.
We generally use 2 types of gains on the ZWO ASI 1600:
139 (1 e-/ADU)
0 (5 e-/ADU).
Minimizing the Gain/exposure combinations simplifies the calibration phase and avoids errors.
For exoplanets, however, the exposure time is much shorter, and will be the subject of a dedicated article.
Know your sensor - Part 1
Which sensor temperature?
Which sensor temperature?
This question is specifically related to our type of setup. Most often, we photograph an object for 10 to 20 nights, most often spread over a period of 2 months.
Our camera hardly allows a temperature delta of more than 28°, which requires taking certain precautions. Thus, it is preferable not to lower the temperature excessively if the image session starts in March and ends in May. The outside temperature will possibly be warmer, and it will be more difficult or even impossible to cool it to the desired temperature.
Usually, the sensor temperature is -15 to -20°C in summer, and -25°C to -35°C in winter.
Know your sensor - Part 2
Don't let the bad photons in!
Don't let the bad photons in!
Very often comes up the usual adage: "Every photo counts".
However, some photons are more desirable than others. To name just a few, those coming from light pollution should rather stay home. For this reason, we prefer to the following version:
"Not all photons are equal.
Don't let the bad ones in and catch the others".
In astrophotography, everything is a matter of compromise, choice, and apportionment. You should not hesitate to delay the start of a session by a few dozen minutes in order to avoid the light pollution of a neighboring city, or to carefully avoid this petty tree who has fun crashing the guiding.
For this, we use several tools:
A precise artificial horizon. It makes easy to know when to start and stop a session
A sky quality map. It helps you to define an optimum, particularly useful in LRGB on fine details
A reference session. It allows you to precisely measure the average sky background value, which we are now trying to keep within a limit of +20% compared to the optimum, located close to the zenith
Artificial horizon - The buildings are represented in overlay
Sky quality map - dark blue color corresponds to mpsas >20.5
Reference session - Median sky background value shown as gray, U-shaped dashed line
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