Here are suggested exposure times for the following two categories of astronomical images:
- Photography of deep sky objects
- The photography of the planets
1- Photography of deep sky objects
For the beginner in deep sky astrophotography, one of the difficulties is to determine the exposure time for each object. In addition, in CCD imaging, it is possible to assemble several images of the same object to form a single image. This technique is called compositing. A single one-hour image requires greater mount tracking accuracy than taking 12 5-minute images and stitching them together. The total exposure time will be the same and the results will be different. The single 60 minute image will show more detail (depth) of the object thus providing a larger gap between signal and noise (S / N). The composite image, with a shorter exposure time per photo, will have a smaller S / N deviation. On the other hand, by combining several images, the noise will be reduced. The composite image (several 5-minute images) will then be less noisy than a single 5-minute image. Comparing the composite image with the single 60 minute image, the stitched image will provide somewhat less (depth) detail on the object. On the other hand, the details revealed (resolved) will be richer. The composite image will be more pleasant to watch because the remaining noise (grain) will be apparent.
To fully understand these 2 concepts, I will detail them:
Unique image
Good news in digital imaging, the signal increases proportionally (or linearly) with the increase in exposure time. This is not the case for noise. It increases by the square root of the increase in signal or exposure time. Increased exposure time by photo will therefore favor the increase of the signal to the detriment of the noise. It will provide a better S/N ratio (a bigger gap between signal and noise). By increasing the exposure time, we will therefore reveal more details of the photographed object. For example we will see more details on the arms of a galaxy.
The compositing of several images
The more images are stitched together, the greater the noise of photons of light decreases. This noise appears during the acquisition of deep sky images. Indeed, in deep sky astrophotography, the photographed objects (galaxies, nebulae, star clusters) have a weak signal. For this reason, it is necessary to use a long exposure time per photo to pick up the weak signal from these distant objects. This weak signal produces significant photon noise in each acquired image.
To explain this photon noise, we can compare it to rain falling on the ground. Only one photo represents a small portion of the downpour, the ground is partially soaked, there are still dry places. Even with a long exposure time, it is as if the rain always fell on the same place without completely covering the ground. With the composite technique, a larger dipped surface is covered. With each image acquired, the water droplets settle in different places on the ground making it possible to capture a completely soaked ground. The result will be a richer and less noisy image. This phenomenon is explained scientifically by the Quantum mechanics (for each image acquired, the photons of light appear in different places).
There is a precise formula to explain this decrease in photon noise:
% remaining noise = 1 / √ number of images
In addition to benefiting from a significant reduction in photon noise, the compositing of several images also makes it possible to increase the S/N ratio. This increase is expressed by the following formula:
Increase in S / N ratio = √ number of images
Based on these two formulas, here is a table that demonstrates the effect of compositing on noise compared to a single image:
| Number of images used | Percentage of remaining noise (compared to a single image) | Increase Signal / Noise Ratio |
| 1 | 100,00% | 1,00 |
| 2 | 70,70% | 1,41 |
| 3 | 57,70% | 1,73 |
| 4 | 50,00% | 2,00 |
| 5 | 44,70% | 2,24 |
| 10 | 31,60% | 3,16 |
| 20 | 22,40% | 4,47 |
| 30 | 18,30% | 5,48 |
| 40 | 15,80% | 6,32 |
| 50 | 14,10% | 7,07 |
| 100 | 10,00% | 10,00 |
| 200 | 7,10% | 14,14 |
| 500 | 4,50% | 22,36 |
| 1000 | 3,20% | 31,62 |
| 10000 | 1,00% | 100,00 |
By stitching only 4 images, we reduce noise by 50% and double the S / N ratio. With 10 frames, the noise is reduced by 68,4% (100 - 31,6) and the S / N ratio is increased by more than 3 times (3,16). With 20 images, noise is reduced by 77,6%. The difference is only 9,2% noise reduction between 10 and 20 frames and the increase in S / N is 1,31 faith (4,47-3,16). So, the table shows that after compositing 10 images, the benefits are less than the first 10 images. Considering that an image can represent more than 15MB in high resolution for the deep sky, and that the exposure time is multiplied by two, one has to judge if it is really worth taking more than 10 images?
To maximize the advantages of the 2 acquisition techniques (exposure time and compositing), the following rules can be retained for the photography of deep sky objects:
- Expose as long as possible for each individual photo. The maximum exposure time will be determined by the tracking quality of the mount, the accuracy of the autoguiding and the light pollution. A longer exposure time will provide more detail to the object (more depth) and will be less noisy than a shorter exposure because the signal / noise will be higher (difference between the signal and the noise larger).
- Take a minimum of 10 single images (68,4% noise reduction and 3,16-fold increase in S / N) having the same exposure time of the object and the assemblies. The compositing of the images will make it possible to further reduce the background noise and thus increase the richness of the details revealed.
Here is an example illustrating the advantages of compositing multiple images:
The image on the left shows a just two minute exposure of the galaxy M101. The image on the right is the composite of several 2-minute images. We can clearly see the grain (noise of photons of light) on the left image. With the compositing technique, the photon noise is much less important in the image on the right. We can also see the richness and depth of the details revealed compared to a single image (reference: The New CCD Astronomy page 110).
Minimum exposure time for photography of deep sky objects
As we have seen previously, for deep sky images, the exposure time rule to adopt is: expose as long as possible. On the other hand, when we have basic equipment, with a mount that has a significant periodic gap, we can ask ourselves: what is the minimum exposure time per photo necessary to reveal enough details on the object to be photographed?
To decrease background noise and increase the richness of detail revealed with these suggested minimum exposure times, in the table below, take 20 composite shots (77,6% noise reduction) and more to best decrease background noise. It should be considered that an exposure time of less than one minute will provide a very noisy individual image for the deep sky because the difference between the signal and the noise (S/N) will be very small. Here are the starting exposure times (minimum) with an f/4 focal length without autoguiding. These exposure times can be achieved, without autoguiding, with the majority of motorized mounts. If you are using a digital camera (Digital Camera) instead of a CCD camera, use ISO 1600 sensitivity.
| Subject | Exhibition time. per photo (sec.) | Number of images | Exhibition time. total (min.) |
|---|---|---|---|
| Open clusters and globular clusters | 45 | 20 | 15 |
| galaxies | 45 | 50 | 38 |
| Nebulae | 45 | 50 | 38 |
To convert the exposure time per photo for other focal lengths, use the following table:
Other focal lengths Conversion
f / 2,5 x 0,4
f / 5 x 1,60
f / 6 x 2,25
f / 8 x 4
f / 10 x 6,25
When the exposure time exceeds 60 seconds per photo, it will probably be necessary to consider autoguiding the telescope. Only high precision mounts can exceed a 60 second exposure time without autoguiding. So, as we can see, for basic equipment, favor a telescope or telescope with a focal aperture of f / 5 and more (ie a larger focal aperture = smaller figure f / 5, f / 4, f / 2,5…). Also, to facilitate the tracking of the object, choose a telescope with a focal length of 1000 mm or less. If your telescope has a focal length of over 1000mm, you can use a focal length reducer to bring it back below 1000mm.
Here is a trick to reduce the exposure time per photo, if it exceeds 60 seconds; use the Bin mode 2 × 2. It allows to acquire 4 times more light than the Bin 1 × 1. We will then divide the exposure time by 4. Here is an example: the minimum exposure time corresponding to f / 8 is 180 seconds (45 ″ x 4). Using Bin 2 × 2 mode, it will be 45 seconds (180 ″ / 4). All monochrome CCD cameras offer Bin 2 × 2 mode. Prefer a wide field matrix to keep a large image, such as the popular matrix Kodak KAF-8300 offered by several CCD camera manufacturers. It should be noted that color CCD cameras no longer allow the 2 × 2 color bin (in the past, there were a few color CCD cameras that offered it, too bad!). Also, not all CMOS matrix cameras offer the 2 × 2 hardware bin which achieves 4 times more signal than the 1 × 1 bin. The 2 × 2 Bin of CMOS matrices provides more frames per second than the 1 × 1 bin. This characteristic is therefore of no use for deep sky imagery. It can be interesting for the imaging of planets by offering more images per second.
With these minimum exposure times, we will be able to photograph all Messiers objects as well as a large number of objects from the NGC catalog. For the latter, choose objects with a magnitude of 9 and less.
2- The photography of the planets
For planets, air turbulence must be taken into account. As these objects are very bright, the exposure time will be very short. To reduce the loss of detail due to air turbulence, we will take several images of the planet and assemble the best images that present the most detail. Indeed the air turbulence is not constant. When you visually look at a planet through the eyepiece, it becomes sharper at times. It is these periods that we want to capture by taking several photos of the planet. With the compositing technique, the details revealed will be richer (and less noisy) than a single good photo acquired at a time when the turbulence is low. This is the advantage of the compositing technique over the single photo.
Here are the suggested starting exposure times for the planets (with a suggested minimum of images):
| Subject | Focal aperture | Exposure time |
|---|---|---|
| Planets | f / 10 | From 0,005 to 0,010 seconds. Select 25% to 50% of the best photos out of 1500 (and more). |
| Planets | f / 20 (Barlow 2x) | From 0,020 to 0,040 seconds. Select 25% to 50% of the best photos out of 1500 (and more). |
| Lune | According to your configuration | Use a polarizing filter and the shortest possible exposure time. Select 25% to 50% of the best photos out of 800 (and more). |
With these initial exposure times, we must avoid overexposing the planet. Prefer a slightly darker image, to be sure not to have areas that are too bright or overexposed.
Time limit before the planets rotate
When photographing planets, there is a maximum time to be observed. This is the time limit before the planet rotates. We use the following formula to determine it:
Time limit in seconds = (3600 x S x R) / (3,1416 x D)
S = Telescope resolution in arc seconds / 2
R = Period of rotation of the planet in hours
D = Apparent diameter of the planet at the time of observation in arc second
Here is an example :
The resolution of the Edge HD 800 telescope is 0,59 ″ of arc. S is therefore equal to 0,295
The rotation period of the planet Jupiter is 9,83 hours
The apparent diameter of the planet at the time of observation is 46,8 ″ of arc (the planet being in opposition)
Time limit = (3600 x 0,295 x 9,83) / (3,1416 x 46,8)
The time limit is therefore 71 seconds or 1,18 minutes
For an image frequency of 30 images / second, we can therefore take a maximum of 2130 images (30 x 71). This is the importance that must be given to the rotation of the planet.
Here are the rotation periods of planets with surface details that require calculating a time limit:
March: 24,61 h
Jupiter: 9,83 hrs
For the planet Saturn, its rotation is 10,23 h. As there are no surface details, we can therefore exceed the time limit according to the above formula.
For the planets Saturn, Mars and Jupiter, favor the periods of opposition ie. the times when they are closer to the earth.
Period of rotation of the moon : 655 h (27,3 days, period of revolution around the Earth)
For the Moon, there is no time limit, because it is always the same surface that faces the Earth. To bring out the details, take the photo near the terminator (part between shadow and light during the first and last quarter of the Moon).
Recommendations for the photography of planets
It is recommended to use a specialized camera for the planetary. These cameras make it possible to produce a film of the REVIEWS ou BEING most of the time. If available, choose the format BEING which is uncompressed and of better quality. Prefer cameras with a frequency of at least 30 images per second. For the beginner and for ease of use, it is also suggested to use a color camera instead of a monochrome camera with RGB color filters, as the challenge in imaging planets is to produce as many images as possible before rotation. of the planet. Indeed, the use of color filters with a filter wheel make it necessary to produce a series of images for each of the red, green and blue filters, thus reducing by three the maximum number of images per filter before the planet rotates ( the time spent rotating the filter wheel must also be taken into account).
Richard Beauregard
Sky Astro - CCD
Revised 2023/08/29