GALLERIES
Photos without telescope and without equatorial mount
Due to the rotation of the Earth, the exposure time is limited when an equatorial mount is not used. Without tracking, the pose is limited to a maximum of 20 seconds with a focal length of 11 mm and 2 seconds with 200 mm. That is why it is better to be content with quite bright objects.
Photos without telescope with equatorial mount
To make wide field photos I use my 16-50 or 70-200 f2.8 lenses and the defiltered Sony, mounted on an equatorial mount. The great opening of these objectives allows revealing the gas clouds or the nebulae of our Milky Way. The treatment of the images is identical to that used for the photos with telescope (see below).
Photos with telescope
Acquisition
I essentially use the Sony Alpha 77 to take pictures of the deep sky. The information on how to use this device in astrophotography is practically nonexistent, for which I have determined the optimal ISO sensitivity by means of successive tests. The choice of 800 iso seems a good solution between noise and acquisition speed.
The selected exposure time is also a compromise between the numbers of photons captured, that is, the ability to capture the fainter parts of the target, and the background noise due to the brightness of the sky. In my observation post, with few pollution (4.5 on the scale of Bortle), five minutes seems to me a correct value. The exposure time can be greatly reduced for certain bright objects such as the heart of M42 or certain globular clusters.
The mount, even perfectly adjusted, can’t do a sufficiently precise tracking for several minutes for photography. For this it is necessary to resort to self-guiding. This consists in following a star with the help of a camera that, via a program interface, will give the correction orders to the mount at the moment in which the star deviates from its axis. In my case I chose the Lacerta MGEN 2 system: it consists of a B & W camera (3.6x2.7 mm sensor, pixel size 4.7mm) and a hand controller with screen. The camera is installed on an Altaïr telescope with a diameter of 60mm and 225mm focal length. This effective system is completely autonomous and you do not need to use a computer.
Processing
To increase the signal-to-noise ratio, it is necessary to make a large number of pictures. In my beginnings I was tempted to try several goals the same night and therefore make few poses for each goal. It was not a good idea and it is better to concentrate on a single object every night or several nights in a row.
Each photo generates parasitic signals that are added to the signal of the desired photo. The signal related to the electronics of the device, the thermal signal due to the heating of the sensor, which depends on the ambient temperature, the exposure time and the iso sensitivity, and finally the heterogeneity of the sensitivity of the sensor, the spots or particles in the optical path, etc.
To eliminate these parasitic signals we will make offset, dark and flat pictures. About twenty darks are enough and about fifty flats and offsets, that are quick to perform, is the most desirable.
The offset pictures are made only once, with the lens cap at high speed (1/4000 or 1/8000 of a second).
The darks should be made (also with the lens or telescope cap) precisely at the same temperature, the same exposure time and iso sensitivity as the photos of the object. The exposure time are generally high (15-20 times 5 minutes for example), the temperature evolves during the night and it is a pity to lose hours of clear sky to realize this type of pictures. For all this I have chosen to make a library of darks.
The flats must be made exactly with the same optical path and the same set-up as those used in the photos. The quality of the flats is essential to obtain a uniform sky background without any particles.
It is possible to find large online information on the realization of these three types of pictures. I will not comment here.
I use IRIS to pre-process the photos. This program combines the images of offset, dark and flat with the raw images, aligning them and adding them. This is the minimum in a pre-procesing, IRIS is capable of executing numerous other tasks. Then I do the post-processing with Photoshop.
Since early 2019, I use Siril for image processing. Recently developed and faster, it can be a substitute for Iris.
I essentially use the Sony Alpha 77 to take pictures of the deep sky. The information on how to use this device in astrophotography is practically nonexistent, for which I have determined the optimal ISO sensitivity by means of successive tests. The choice of 800 iso seems a good solution between noise and acquisition speed.
The selected exposure time is also a compromise between the numbers of photons captured, that is, the ability to capture the fainter parts of the target, and the background noise due to the brightness of the sky. In my observation post, with few pollution (4.5 on the scale of Bortle), five minutes seems to me a correct value. The exposure time can be greatly reduced for certain bright objects such as the heart of M42 or certain globular clusters.
The mount, even perfectly adjusted, can’t do a sufficiently precise tracking for several minutes for photography. For this it is necessary to resort to self-guiding. This consists in following a star with the help of a camera that, via a program interface, will give the correction orders to the mount at the moment in which the star deviates from its axis. In my case I chose the Lacerta MGEN 2 system: it consists of a B & W camera (3.6x2.7 mm sensor, pixel size 4.7mm) and a hand controller with screen. The camera is installed on an Altaïr telescope with a diameter of 60mm and 225mm focal length. This effective system is completely autonomous and you do not need to use a computer.
Processing
To increase the signal-to-noise ratio, it is necessary to make a large number of pictures. In my beginnings I was tempted to try several goals the same night and therefore make few poses for each goal. It was not a good idea and it is better to concentrate on a single object every night or several nights in a row.
Each photo generates parasitic signals that are added to the signal of the desired photo. The signal related to the electronics of the device, the thermal signal due to the heating of the sensor, which depends on the ambient temperature, the exposure time and the iso sensitivity, and finally the heterogeneity of the sensitivity of the sensor, the spots or particles in the optical path, etc.
To eliminate these parasitic signals we will make offset, dark and flat pictures. About twenty darks are enough and about fifty flats and offsets, that are quick to perform, is the most desirable.
The offset pictures are made only once, with the lens cap at high speed (1/4000 or 1/8000 of a second).
The darks should be made (also with the lens or telescope cap) precisely at the same temperature, the same exposure time and iso sensitivity as the photos of the object. The exposure time are generally high (15-20 times 5 minutes for example), the temperature evolves during the night and it is a pity to lose hours of clear sky to realize this type of pictures. For all this I have chosen to make a library of darks.
The flats must be made exactly with the same optical path and the same set-up as those used in the photos. The quality of the flats is essential to obtain a uniform sky background without any particles.
It is possible to find large online information on the realization of these three types of pictures. I will not comment here.
I use IRIS to pre-process the photos. This program combines the images of offset, dark and flat with the raw images, aligning them and adding them. This is the minimum in a pre-procesing, IRIS is capable of executing numerous other tasks. Then I do the post-processing with Photoshop.
Since early 2019, I use Siril for image processing. Recently developed and faster, it can be a substitute for Iris.
Photo gallery of galaxies
Photo gallery of nebulae
Photo gallery of planetary nebulae
Photo gallery of clusters
If the digital camera is well adapted to deep-sky photography, it is not recommended to use it for pictures of planets. Indeed, for this it is necessary to make a large number of photos without compression very quickly. A planetary camera (historically a webcam) makes videos of several thousand images per minute which allows the turbulence to freeze. In addition, its small sensor is well suited to planets with a very small apparent diameter (40 seconds of arc for Jupiter).
I have selected a planetary camera Altaïr GPCAM 2 equipped with a Sony IMX 224C sensor (1280x960 pixels of 3.75um, sensor size 4.8mm x 3.6mm). I have added a Barlow lens that increases the focal length of the Newton by 2.7 times.
The acquisition of films must be done with the help of a computer. After having used the program supplied with the camera (Altaïr Capture) simple and effective, I opted for FireCapture, more elaborate.
The process is guaranteed by AutoStakkert! 3 and Registax 6.
I have selected a planetary camera Altaïr GPCAM 2 equipped with a Sony IMX 224C sensor (1280x960 pixels of 3.75um, sensor size 4.8mm x 3.6mm). I have added a Barlow lens that increases the focal length of the Newton by 2.7 times.
The acquisition of films must be done with the help of a computer. After having used the program supplied with the camera (Altaïr Capture) simple and effective, I opted for FireCapture, more elaborate.
The process is guaranteed by AutoStakkert! 3 and Registax 6.
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