What happens at night?

One of the most common question is simply: "What happens at night?" What do we do, what does the observatory do, and how much time it take. This page is not intended to describe all the steps in detail, but rather to simply describe the steps of a night of astrophotography

Starting the session

Duration: 1 mn

After checking the weather conditions and the position of the object to be photographed in our observable sky, we begin the session. The first action is to wake up the observatory. Manually, we will turn on the power outlets that control the observatory (Control box), the low-voltage power supplies (12V power box) and the on-board computer (Raspi). If the outside temperature is below 3°C, we usually also switch on the dew heaters.

Automation and control

Duration: 3mn

The computer is now turned on. It is now time to load the automated sequence we already programmed during a preparation night. The sequence defines all the parameters related to the image: the exact position of the object, the cooling temperature of the camera, its gain and offset parameters, the filters used, the exposure times, the number of photos for each filter, etc.

With a quick glance at a few control settings and a click of the mouse, the system boots. From now on, no manual action should be required.

Dome control

Duration 1mn

The two half-roofs of the observatory will now open, and the dust cover will move away from the optical tube. The opening procedure takes about a minute, and the sensors will provide us with information on the roof's position.

Observatory closed - Telesope in sleep position

Observatory open - Telescope slewing

Telescope slewing:

Duration 2mn

Time now for the telescope to slew to the object. Usually, the first mount slew is quite far away from the object, about the diameter of a full moon away. A first photo is taken, and thanks to an astrometry algorithm, the system will synchronize the theoretical position and the actual position of the mount. Once the position is refined, the system will once again attempt to align itself with the target, and will repeat these steps as often as possible, until reaching the desired precision.

Focus

Duration: 3mn

Just like with a camera lens, you need to set the focus on the object to be photographed. In our case, the focus point is obviously at infinity. However, the ideal focus point is only about 40 micrometers wide, and only an automated procedure can achieve this precision. Each step of the focuser corresponds to about 1 micrometer, and the automation will seek to minimize the size of the stars as much as possible while ensuring that the mechanical defects of the assembly are compensated.

As shown in the example below, the algorithm took 1 iteration to reach the point where the stars are as small as possible.

Guiding

Duration: 1mn

The Earth rotates at a rate of 1 rotation per day. By rotating in the opposite direction, the mount compensates for this movement and stabilizes the star in the telescope's field of view. However, no mount is absolutely perfect, and each exhibits a periodic error of varying magnitude depending on its mechanical construction. Therefore, the mount's rotation must be compensated for approximately every second using a second camera mounted on a small refractor to measure the deviation and send small correction pulses. This technique ensures that the stars remain dots, and the image will be more pleasing.

Image acquisition

Duration: 0-7mn

The automatic imaging sequence includes all camera-related parameters. Gain, offset, and exposure times are defined for each filter type. If it was not manually activated at the beginning of the sequence, the sensor will be cooled at this point. This cooling procedure can take several minutes depending on the temperature difference below ambient temperature.

On the image below, only Luminance images are captured. The exotic 80s of exposure time and cropped sensor size correspond to a sequence made for an exoplanet.

Session tracking

Duration: 1 to 5mn if needed

If everything works well, there is no need for any specific monitoring. However, certain conditions require special attention. In the case of an exoplanet, the regularity of the images must be carefully examined. It is a good idea to check the speed of the meridian flip, for example.

Some nights present a meteorological risk such as a snowfall or the arrival of a storm. It is therefore advisable to regularly check the state of the weather and the observatory.

The screenshot above represents the typical control points of a session. At the top is a graphical representation of each step. The two lower lines show the alternation between photos and autofocus.

In this example, we see very frequent filter changes (Luminance, Red, Green, Blue), a sequence that corresponds to a reconnaissance session to test the settings for a new image.

The middle section shows curves for control parameters. The purple curve (number of stars at the guide camera) seems to indicate large fluctuations, while the guiding (red curve) is initially stable, then becomes unstable. This situation corresponds to a passage through tree branches, eventually disrupting the guiding.

The white curve corresponds to the mount's altitude above the horizon. We notice that after passing through the branches, the mount has changed position and therefore the object. This object seems ideally placed, and the guiding is improved.

Sometimes the session is incredibly stable thanks to good weather and optimal settings. It's always a real pleasure to walk past the control screen at 3 a.m. and see this kind of image on the screen.

Even though the monitoring tool presented above works very well, it is always recommended to check a few photos, especially at the beginning and end of the session. In the example below, a satellite crosses the photo near M94, following a trajectory perfectly aligned with the orientation of the sensor!

As we can see, a fully automated session only takes a few minutes to start, typically between 12 and 15 minutes. At the time we had a mobile observatory, the time spent between the arrival on site and the first photo was about 2 hours!

Even though the astronomical evenings are fond memories for us, we could hardly come back to this point today.

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