Here we are at the stage of acquiring images of deep sky objects. To achieve this crucial step, everything that has been described in the previous sections will be used. It is therefore important to understand and master these sections before starting to produce the images of the object itself.

  1. Determine the equipment to be set up for the photograph of the object to be captured by referring to the section Astronomical calculations
  2. Carry out the equatorial stationing of the telescope with the help of the section Setting up the telescope and the search forPolar axis of rotation 
  3. It is recommended to make a PEC
  4. Determine the exposure time by referring to the section Suggested exposure time. The rule of thumb for deep sky photos is to expose for as long as possible
  5. For deep sky images, it is suggested toself-guide Frame
  6. Focus on the object using the section of focus from this site
  7. Make sure to acquire the images that will be used for pretreatment
  8. Make composite images of the object to be photographed

Make composite images of the object to be photographed

It is advisable to use the same software that will be used for the acquisition and preprocessing of deep sky images. The results will thus be maximized. It simply consists of taking the photos of the deep sky according to the chosen exposure time. If you have a camera that can be cooled to a constant temperature, use the lowest temperature possible. This is because the background noise in the image halves with each lowering of the temperature by six degrees Celsius. This will therefore make it possible to maximize the signal / noise ratio of the image.

Several software programs offer the option of Dithering when capturing photos. I recommend you to use it, as this feature will later remove noise such as hot and cold pixels left over by using some compositing methods eg Sigma-clip. For more information see the section The different compositing methods of this website.

Center the object in the field of view of the CCD camera or the digital camera

It is not easy to make the object appear in the field of view of the CCD or APN (digital camera) camera, especially if the camera sensor is very small like in entry-level models. Most of the time, the deep sky object is very dark so it does not appear in the finder. If you do not have a "GOTO" computerized telescope, you must gain experience in finding objects by guiding yourself on marker stars near the object. A good knowledge of the sky and the constellations will be of great help to you in this task. However, it will be very difficult to make the object appear in the field of view of the camera. During the search process, consideration should be given to removing the camera, placing an eyepiece to visually center the object in the center of the telescope's field of view. (At assuming the object is visible through the eyepiece, which is not always the case!), put the camera back on, do a one minute exposure test to see if the object is visible in the field of view. vision of the camera, refocus ...

Even if you have a GOTO mount, it is not easy to bring the object into the field of view of the CCD camera. It is good to know that many GOTO mounts offer high precision aiming, allowing the object to appear in the camera's field of view. I therefore invite you to check if this function exists for your frame by consulting the user manual. Here is the procedure with my CGEM mount:

Note: Before starting, make sure that the finderscope is perfectly aligned with the field of view of the telescope or telescope used to image. I always do this synchronization when setting up the telescope. It essentially consists of centering the same star in the field of view of the two devices (the researcher and the telescope or telescope).

  • Return to the main menu using the mount's control lever by pressing the button. undo Many times
  • Push the button Menu
  • Select Precise GoTo
  • Select Database
  • Select the catalog: for example Messier
  • Enter the number of the object to be imaged and press Enter
  • The telescope points to a bright star near the object to be imaged
  • Center the star in the finder
  • To push on Enter
  • Center the star on the imaging camera screen
  • To push on Enter
  • The telescope moves towards the object to be imaged
  • Start with exposure sequences lasting from a few seconds to several seconds to see the object on the screen
  • If necessary, center the object on the screen using the joystick

With the mount's high precision aiming, the object to be imaged will always appear on the imaging screen. Indeed, the software of the joystick will calculate the distance between the bright star and the object to be imaged, ensuring a very precise pointing, because the difference between the two is very small.

The procedure is much the same for mounts which have this very useful function for centering the object in the field of view of the imaging camera.

Center the object in the field of view of the camera using an astrometric calibration software (in English Plate Solving)

There is another way to precisely center the object to be imaged in the camera's field of view.We use a software which allows the astrometric calibration (Plate Solve in English). To achieve this centering, it is necessary to dedicate the control of the mount to a software other than that included in the joystick of this one. Here is a non-exhaustive list of software that allows you to perform this centering:

Using these software, it is possible to precisely center the deep sky object remotely using the software TeamViewer for example.

Analyze the histogram

When acquiring images, most astronomical software offers the possibility of adjusting the histogram on the screen. It is important to note that the adjustment only affects the displayed image. It does not modify the image that will be saved on the hard drive of the computer. On the other hand, this histogram makes it possible to visualize the information which is captured by the camera according to the chosen exposure time. Here is an example of a histogram:

The alt attribute of this image is empty, its file name is histogram.jpg.

Suppose the chosen exposure time is 5 minutes per frame and you want to take 10 for a total exposure time of 50 minutes. Before you begin, take a single 5-minute test photo. The histogram will display the resolved values ​​of the image similar to the histogram above. The information in white represents the resolution of the image starting from the left for the darkest values ​​(including the sky background which is never opaque black) going to the right for the lightest values ​​(which mostly represent the stars). By moving the red cursor to the right and the green to the left, we can analyze the information captured and validate that the exposure time chosen is long enough to reveal enough details on the object.

If we look at the image without manipulating the histogram on the screen, it appears dark. This should not be worried, because the treatments that we will see in the next section will resolve the image and reveal all the details from the darkest to the lightest. 

Richard Beauregard
Sky Astro - CCD

Revised 2021/01/27