Astrophotography camera for deep-sky: what to choose
Choosing from various astrophotography camera for the deep-sky may seem complicated. In this article, through a series of practical examples, we will see how to choose astrophotography camera to record also the weakest details of galaxies, nebulae and star clusters through with long exposures (these camera are different from the ones used for planetary and lunar photography that record short videos). Astrophotography camera for the deep-sky are usually cooled, ie have a system with Peltier cells which cools and stabilises the sensor at low temperature in order to reduce the noise of the image and therefore improve quality.
In this article we will meet some technical terms which must be taken into consideration when choosing an astrophotography camera for deep-sky astrophotography:
– resolution capability: it’s the minimum distance in arc seconds, with which two objects may be identified as separate and it’s a function, closely related to the telescope diameter, calculated with the following formula: a= 120 / D where “a” is the resolution capability in arc seconds, “120” is a constant and “D” is the telescope diameter in millimeters.
– seeing: it’s the set of phenomena that contribute to the deterioration of the image quality of both in visual and photographic application, such as atmospheric turbulence, humidity and light pollution.
– sampling: this is the concept more difficult to understand but it is essential for choosing the right camera as a function of the telescope. Its value indicates how many arc seconds a single pixel of our sensor can record.
After this introduction you may understand that, if you already have a telescope, the choice of astrophotography camera depends also on the sensor pixels size.
Let’s start from a practical example with the Borg fluorite apochromatic refractor 107FL f3.9 with ESATTO 3″, 107mm diameter and 417mm focal length. Its resolution capability is calculated with the formula a=120/D that is 120/107=1.12 arc seconds. According to the Nyquist principle, the pixel size of the sensor must cover more than half of the smallest detail that our telescopes can resolve. According to this, the astrophotography camera pixels will have to record the signal of an area of sky equal to 1.15/2 = 0.56 arc seconds.
Starting by the formula
C = (206265 x d) / L
where: L = telescope focal length in millimeters d = sensor pixel dimensions in millimeters C = sampling value in arc/sec 206265 = conversion factor from radiant to arc seconds, we can now calculate what we need to know to choose an astrophotography camera.
We start from the analysis of a camera example, QHY247C: by the technical specifications we see that the pixel size is 3.9 microns. According to the formula we will have a sample of (206265 x 0.0054) / 650 = 1.93 arc seconds. The result is quite far from the theoretical value that we calculated and tells us that we are undersampling the image, that is, our camera is not capable of recording all the details that the optics is able to record. So what?
We are no considering the seeing and more precisely the effects of atmospheric turbulence, responsible for the distortion of the light that pass through the Earth’s atmosphere before reaching our instrument. In areas where the seeing is medium, seeing allows us to record the finer details on 3-4 pixels. Therefore, by applying the Nyquist principle, our sampling will be around 1.5 to 2 arc seconds. Often it is not necessary to go below these values, as would we upsample the image that is the maximum detail would be reported by a number of pixels greater than two. So the 1.93 arc seconds value obtained with QHY247C is perfectly in the range dictated by seeing conditions and the match with Borg fluorite apochromatic refractor 107FL f3.9 with ESATTO 3″ is therefore perfect. Do we want to look for something else that will work for our example? If we use the formula as a constant, setting the min and max value of our ideal sampling, the value of the sensor pixel size with our Borg fluorite apochromatic refractor 107FL f3.9 with ESATTO 3″ will be from 3 to 4 microns.
Let’s make another example with SkyWatcher Newton QUATTRO 200/800 f4 with SESTO SENSO that has 200mm diameter and 800mm focal length. We calculate the min and max value of the pixel size of our hypothetical sensor, on the basis of theoretical sampling values from 1.5 to 2 arc seconds:
d = (L x C) / 206265
1) with 1,5 arc seconds: (800 x 1,5) / 206265 = 5,8 micron
2) with 2,0 arc seconds: (800 x 2) / 206265 = 7,8 micron
It is inside this pixel size range that we must focus our search. What is the astrophotography camera that meets these requirements? An example would be the QHY128, which uses a sensor with 6.0 microns pixels.
OTHER FEATURES: TYPE AND SIZE OF SENSOR
By having calculated the ideal size of the sensor pixels, we can now select an astrophotography camera based on other parameters including:
– Sensor type: sensors for astrophotography camera for deep-sky may be color or monochrome. The first, of course, allow you to get directly a color image and thus are these camera are easier to use than monochrome ones (that need a series of filters, such as a LRGB set, to allow us to reconstruct the image in color using special processing techniques). Monochrome cameras, however, have an important advantage: they are, given the same sensor, more sensitive than the corresponding color one. This not only allows us to record weaker details with the same exposition time (or you can also say, you can record the same stellar magnitudes with shorter exposition time than the one required by a color camera) and, more importantly, allow to use narrow-band filters against light pollution (such as H-alpha, OIII and SII filters) that increase a lot the nebula contrast against the sky background (also reducing the size of the stars letting you point out better the framed nebula) and allow you to perform astrophotography also from areas with a lot of light pollution.
So the choice of the sensor type also depends on the place from you want to make astrophotography. If you can access a site with little light pollution, you can choose a color astrophotography camera (perhaps using a broadband nebula filter to slightly reduce light pollution – in order to use the filter with a color camera it will not have too narrow band to let more light pass to the camera and to avoid altered colors). If you record pictures from a place with evident light pollution, choose a monochrome camera. Obviously monochrome camera is always recommended (in fact, is almost always chosen by experts astrophotographers) given the added sensitivity but they’re also the most complex to use.
– Sensor dimension: at the same telescope focal length, a larger sensor allows to frame a greater area and therefore is to be preferred. But astrophotography camera with larger sensors also have a higher cost, which of course is a parameter to be considered in the choice of an astrophotography camera for deep-sky
In this respect, we must consider also the corrected area provided by the telescope: for example it is useless to buy a Full Frame sensor with 43mm diagonal and then use it with a telescope that provides the corrected area of 20mm (that indicates the diameter of the corrected image circle generated by the telescope in which the stars are point): the result is in fact an image with elongated stars toward the edges of the image. In our example with Borg fluorite apochromatic refractor 107FL f3.9 with ESATTO 3″, this telescope with field flattener has a 55mm diameter flat field so we can choose a very large sensor. If we use a cheaper telescope, such as the SkyWatcher EVOSTAR 72 ED apochromatic refractor with SESTO SENSO, we can choose astrophotography camera with APS-C sensors (with a 28mm diagonal sensor).