The artificial lighting of our cities greatly affects the darkness of the sky. It is increasingly difficult to find observation sites without light pollution. We can thus consider that the majority of astrophotographers are concerned by light pollution at different levels.
The following discussion concerns the photography of deep sky objects in a light pollution environment. It should be noted that for the observation and the photography of the planets, one can do it in full city without constraints, because these objects are very luminous.
Before evaluating the maximum exposure time per photo according to the light pollution, it is necessary to know the importance of the light pollution of its observation site. Also, it is necessary to determine the interest of using a light pollution filter. Then, we can undertake the evaluation of the maximum exposure time in order to avoid overexposing the sky background.
Assessment of the light pollution of its observation site
Visual magnitude of deep sky objects
Astronomy books and planetariums present the visual magnitude or apparent magnitude (whose acronym is mv or V) deep sky objects. It is a measurement that defines the brightness of the object seen from Earth. For example, the Andromeda galaxy (M31) has a visual magnitude of 3,4 (the smaller the number, the brighter the object). It is the brightest galaxy in the boreal sky. A difference of one magnitude increases or decreases the brightness of the object by 2,512 times. The evaluation which will be presented below will present the limit magnitude (minimum) observable according to the light pollution of the observation site. Also, it is necessary to know the visual limiting magnitude of his telescope or telescope. It is calculated by the following formula:
2,1 + 5 x Log D
D being the diameter of the telescope in millimeters. You will therefore need a scientific calculator to do the calculation for your observation instrument. It should be noted that it is this formula which is used by the manufacturers of telescopes or glasses when they mention the visual limit magnitude of their observation instruments.
Here is the visual limit magnitude according to the diameter of the following telescopes or glasses:
| Diameter | magnitute visual limit 2,1 + 5 x Log D |
Adding Astrophoto |
Magnitude astrophoto limit 7,1 + 5 x Log D |
| 500 mm (20 inches) | 15,60 | 5 | 20,60 |
| 432 mm (17 inches) | 15,28 | 5 | 20,28 |
| 356 mm (14 inches) | 14,86 | 5 | 19,86 |
| 320 mm (12,5 inches) | 14,63 | 5 | 19,63 |
| 280 mm (11 inches) | 14,34 | 5 | 19,34 |
| 254 mm (10 inches) | 14,12 | 5 | 19,12 |
| 203,2 mm (8 inches) | 13,64 | 5 | 18,64 |
| 150 mm (5,9 inches) | 12,98 | 5 | 17,98 |
| 102 mm (4 inches) | 12,14 | 5 | 17,14 |
| 80 mm (3,15 inches) | 11,62 | 5 | 16,62 |
The visual limiting magnitude of the human eye (without telescope) is 6, considering the maximum pupil diameter of 6 mm. This decreases with the age of the observer (the pupil dilates less). Note that the visual limit magnitudes of the above instruments are those obtained when the human eye looks through the telescope. The visual limit magnitudes of the instruments were therefore calculated with an eyepiece which gives an exit pupil of 6 mm, ie the same maximum diameter as the pupil of the human eye, thus offering maximum brightness of the object observed visually. It should be noted that an upper limit magnitude can be obtained by using a magnification that will darken the sky background without changing the point brightness of the stars, but this does not apply to large objects such as galaxies and nebulae. (they will also be darkened with magnification). Also, magnification is irrelevant in deep sky astrophotography since the imaging camera is installed directly at the focus of the telescope or telescope most of the time.
In astrophotography, the limiting magnitude will be higher. But, there is no precise formula to determine it, because it depends (in addition to the diameter of the telescope) on the sensitivity of the CCD or CMOS matrices, on the focal length and on the exposure time. According to Denis Bergeron's analysis concerning the limiting magnitude of an instrument in astrophotography on a star, he was able to access the magnitude of 19,5 with his 254 mm telescope with an exposure time of 10 minutes at the focal length f / 7 in a site of low light pollution (blue zone). He used a monochrome CCD camera in luminance (clear filter). We can therefore consider that the limiting magnitude of the instrument used in astrophotography offers at least five additional magnitudes (or 7,1 + 5 x Log D). This analysis shows that the use of a telescope or telescope of small diameter, for example of 80 mm in the table above, one can reach the magnitude of 16,62 and even a little more in astrophotography in a site low light pollution. It should also be noted that the maximum limiting magnitude on earth is around 21, regardless of the diameter of the telescope, considering the surface brightness of 22 (mag / arcsec2) from the sky in a site without light pollution.
If we increase the exposure time per photo or if we use a more open focal length, see note 1 at the end of the presentation to know the gain in limiting magnitude.
Light pollution assessment
Here is the global light pollution map that will allow you to display the assessment for your observation site:
https://www.lightpollutionmap.info/
The visual limit magnitudes at the telescope are calibrated on a star, therefore on a point source, with a 320 mm (12,5 inch) telescope and a magnification of 250x. For the photography of deep sky objects, it is also necessary to take into account the surface gloss extended objects. Here is the description of the color chart:
| Colour | Bortle | Situation | Magnitude Limit visual telescope | Magnitude estimated limit astrophoto | A unique, new property built in the traditional French style, with many energy saving features. |
 | 1 | Sky without light pollution | > 17 | 21 | It takes a telescope of 500 mm (20 inches) and more to reach the limiting magnitude of 21 in astrophoto. |
 | 2 | Very little light pollution | 16,5 | > 19 | Telescopes or scopes with a diameter of 254 mm (10 inches) and larger can access magnitude greater than 19 in astrophoto. |
 | 3 | Rural sky | 16 | > 19 | Light pollution present near the horizon. |
 | 4 | Transition between the rural sky and the suburbs | 15,5 | 17,5 | Low light pollution. |
 | 5 | In the suburbs | 14,5–15 | 16,5 | Light pollution all around. We must start to limit the exposure time in astrophotography so as not to overexpose the sky background. A telescope with a diameter of 80 mm can reach the limiting magnitude of 16,5 in astrophoto. |
 | 6 | In the suburbs with a brighter sky | 14–14,5 | 15,5 | The Milky Way is still visible to the naked eye (without a telescope) from 30 ° above the horizon. |
 | 7 | Transition between the suburbs and the city | 14 | 15 | The Milky Way is no longer visible to the naked eye. |
 | 8–9 | In the city | - | 14 and less | The limiting magnitude in astrophoto was established according to my personal analysis. |
For the Bortle scale see this link.
The color chart is valid from a good transparency of the sky. Local conditions could cause the limit magnitude to vary downward, such as light pollution at the observation site (lampposts, neighbors' lights), high humidity, smoke caused by forest fires, pollution atmospheric, haze at ground level, etc.
The interest of consulting the color chart above is for deep sky objects with a continuous light spectrum (galaxies, star clusters and reflection nebulae). We use the surface gloss of these objects to determine if the object can be photographed at its observation site. For example, the surface brightness of the Andromeda galaxy M31 is 13,35 mag / arcmin2 depending on the software Stellarium. It can therefore be photographed in a white zone (in town) according to the color chart above (see the column Estimated limit magnitude astrophoto which is 14 and less for the white area)! We will see in the section Maximum exposure time per photo depending on light pollution below how to go about it. For the skeptics, by consulting this section, there is the presentation of an image that I took of M31 in my observation site which is in white zone!
For emission nebulae and planetary nebulae, it is not useful to consult the color chart above, as narrow band filters will be used. Details in the section Use of emission filters for color cameras below. For monochrome cameras see the following link on the use of narrow band filters to combat light pollution.
Use of emission filters for color cameras
Two major categories of pollution control filters exist on the market for color cameras: general filters for continuous light spectrum objects and specialized filters for emission nebulae.
General filters for objects with a continuous light spectrum
These filters are used for star clusters, galaxies and reflection nebulae. Since these objects emit a signal across the full width of the bandwidth visible to the human eye (including light pollution), they perform poorly in combating light pollution. Here is an example of transmission of the IDAS LPS-P2 filter from Hutech (also newly named IDAS LPS-D1):
The blue line shows the transmission lines of the IDAS LPS-P2 filter. The other colored lines show the main emission lines of city lights (Hg = mercury vapor, Na = sodium vapor). It should be noted that today, with the tendency to use white LED lights for city lighting, it can be considered that light pollution increasingly covers the entire visible light spectrum decreasing even further the light pollution. effectiveness of these filters for objects with a continuous light spectrum.
In addition to the current observation concerning the use of these general antipollution filters for objects with a continuous light spectrum (star clusters, galaxies and reflection nebulae), the main drawback of these filters is that they require doubling or tripling the exposure time to capture the same amount of light as photography without a light pollution filter. Also, the color balance is difficult to achieve, there remains a dominant blue color in the image. Also, after several searches I have done on the web, several experienced astrographers recommend against the use of emission filters dedicated to objects with a continuous light spectrum.
For all of these reasons, I do not recommend using these filters. My personal strategy is to use short exposure times and take a lot of images to benefit from the following formula: Increase in S / N ratio = √ number of images. I got better results with this technique.
Specialized filters for emission nebulae
Here is an example of a specialized color camera filter, which I have used before, for emission nebulae:
IDAS LPS-V4 filter from Hutech : This filter is designed to provide maximum contrast in the light spectrum of emission nebulae while effectively filtering light pollution. The spectra covered are: H-Beta (486.1 nm), OIII (500.7 nm) and H-Alpha (656.3 nm). It allows more than 95% of the light to pass through these light spectra. In these, the passband is narrower than the IDAS LPS-P2 filter mentioned above. It is therefore more effective against light pollution. It is not recommended for photography of galaxies, star clusters and reflection nebulae, as it will filter the luminance of these objects too much while not preserving the color balance. This filter should therefore be used only for the photography of emission nebulae. I recommend producing a synthetic luminance image using the Red channel, which makes it possible to obtain an H-Alpha luminance image with a color camera. Then, it is superimposed on the RGB image produced with this filter. For more details on producing a synthetic luminance image, click on this link. It should be noted that the color balance of emission nebulae is well preserved with this filter. Also, to promote the transmission of the narrow H-Alpha bandwidth as much as possible, it is recommended to have your color camera filtered.
Here is the transmission graph of this filter:
By comparing the transmission lines of the IDAS LPS-V4 filter with the IDAS LPS-P2 filter above, we see that the passbands of the filter are much narrower than the IDAS LPS-P2 filter, which makes it much more efficient. to fight light pollution. A very interesting feature of the IDAS LPS-V4 filter is the narrow bandwidth of only 19 nm (at 50% of the filter transmission) for the emission line Ha located at 656,3 nm in the graph. The two pass bands of the filter are located in the transmission lines of emission nebulae. It is therefore an ideal choice for photographing such objects with a color camera.
Here is an image taken with this filter in a white area:
Here is another example of an emission nebula filter that appeared on the market during the year 2018:
STC Astro Duo-Narrowband Filter : Two light spectra are covered; H-Alpha (656.3 nm) and OIII (500.7 nm). It lets more than 90% of the light pass through them. The narrow band is only 10nm for OIII (at 50% of the filter transmission) and just over 10nm for H-Apha. Thanks to these very narrow bandwidths, it would be even more effective in combating light pollution than the IDAS LPS-V4 filter above. Although I have never used this filter, many astrophotographers have very positive comments about experimenting with this filter.
Here is the transmission graph of this filter from the manufacturer's website Cyclops Optics :
Maximum exposure time per photo depending on light pollution
Here are two methods for determining the maximum exposure time per photo as a function of light pollution for objects with a continuous light spectrum (star clusters, galaxies and reflection nebulae). The first method is to use a table quickly estimating the exposure time per photo. The second asks to perform calculations in order to more precisely determine the exposure time.
Table of maximum exposure times per photo according to light pollution
You will find below a table allowing to determine an exposure time according to the conditions of the sky background. To use this table, all you have to do is start from the sensitivity of your digital camera (left column), then go to the right until you reach the F / D ratio you are using (or the closest whichever you are using). Then go down in the column towards the bottom of the table. We will then have the exposure times (without an antipollution filter) and the number of images recommended for the white, red, orange and yellow areas.
This table is established for a good transparency of the sky and for objects being more than 30o of the horizon. Exposure times are calculated to avoid overexposing the sky background. In deep sky photography, the rule to follow is to expose as long as possible. On the other hand, in an environment of light pollution, it is necessary to avoid overexposing the sky background. These exposure times therefore provide an indicator (starting point) of exposure time for the different areas of light pollution. The data in the table comes from my own exposure time tests and is updated as needed.
| ISO sensitivity (ASA) | F / D ratio | F / D ratio | F / D ratio | F / D ratio | F / D ratio |
| 400 | - | 1,4 | 2 | 2,8 | 4 |
| 800 | 1,4 | 2 | 2,8 | 4 | 5,6 |
| 1600 | 2 | 2,8 | 4 | 5,6 | 8 |
| RGB or CCD color (*) | 2,8 | 4 | 5,6 | 8 | 11 |
| CCD Luminance | 4 | 5,6 | 8 | 11 | |
| Sky background conditions | Exposure time per photo | Exposure time per photo | Exposure time per photo | Exposure time per photo | Exposure time per photo |
| In town (white zone) | |||||
| Number of images | 50 and more | 50 and more | 50 and more | 50 and more | 50 and more |
| Exposure time per photo | 2 min | 4 min | 8 min | 16 min | 32 min |
| Monochrome CCD bin 2 × 2 | - | 1 min | 2 min | 4 min | 8 min |
| Between the suburbs and the city (red zone) | |||||
| Number of images | 30 and more | 30 and more | 30 and more | 30 and more | 30 and more |
| Exposure time per photo | 3 min | 6 min | 12 min | 24 min | 48 min |
| Monochrome CCD bin 2 × 2 | - | 1,5 min | 3 min | 6 min | 12 min |
| Milky Way still visible (orange zone) | |||||
| Number of images | 20 and more | 20 and more | 20 and more | 20 and more | 20 and more |
| Exposure time per photo | 4 min | 8 min | 16 min | 32 min | 64 min |
| Monochrome CCD bin 2 × 2 | 1 min | 2 min | 4 min | 8 min | 16 min |
| Sky may polluted (yellow zone) | |||||
| Number of images | 10 and more | 10 and more | 10 and more | 10 and more | 10 and more |
| Exposure time per photo | 8 min | 16 min | 32 min | 64 min | 128 min |
| Monochrome CCD bin 2 × 2 | 2 min | 4 min | 8 min | 16 min | 32 min |
It should be noted, for the new cameras with a cooled color and monochrome CMOS sensor for the deep sky, that the calculations detailed below will have to be carried out to know the maximum exposure time. The reason is that these use the gain function to increase the camera's sensitivity to light and not the ISO adjustment which is used by digital cameras (APNs or DSLRs) in the consumer camera market. Although ISO and gain are the same thing, there is no equivalence between gain and ISO, ISO being a universal standard for light sensitivity between cameras. It is therefore impossible to include these cameras in the table above.
In the table, I suggest in some situations to use Bin 2 × 2 mode. This reduces exposure time by a factor of 4, providing much shorter exposure times per photo and total. To learn more about this mode, click on this link.
Of course, we are not obliged to use these maximum times when they exceed 15 minutes per photo. Very few astrophotographers exceed this exposure time. For example, the exposure time in the yellow zone (sky can polluted) for a luminance image at f / 8 is 32 minutes in a 1 × 1 bin. I suggest then to use an exposure time of 10 or 15 minutes per photo.
For monochrome CCD cameras in use with narrow band filters for emission nebulae (Ha, OIII and SII) as well as the IDAS LPS-V4 filter or the STC Astro Duo-Narrowband filter for color cameras above, it is not useful to consult the table, because these filters fight effectively light pollution, even in white area. The rule to follow is therefore to expose as long as possible.
In the table, we can see that the more light pollution there is, the more we increase the number of exposures. Instead of using a general pollution filter for star clusters, galaxies, and reflection nebulae, this strategy will work better. It should be understood here that in a site of significant light pollution, the difference between the signal and the noise (S / N) is much smaller than in a site with little light pollution. By taking a lot of images, we increase the gap between the S / N while reducing the remaining noise. For more on this, see the detailed explanation of the suggested exposure times on point The compositing of several images. To make it easier to understand the importance of taking lots of images, I will reproduce here the 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 |
In a site with little pollution (yellow zone), I recommend taking at least 10 images, the percentage of noise remaining will be 31,60% and the increase in the S / N ratio will be 3,16. By taking 50 images for the white area, the percentage of noise remaining will be only 14,10% and the S / N increase will increase to 7,07, more than double that of taking 10 images. So that's the importance of taking a lot of images in a site of extreme light pollution (white area).
Calculation of the maximum exposure time per photo taking into account light pollution
The table of exposure time per photo as a function of light pollution provides a general estimate of the exposure time per photo in a light pollution environment. For more exatitude, here is a calculation which will allow to know precisely the maximum exposure time per photo, taking into account its observation site and its personal equipment.
The calculation will take into account all the personal variables which are:
- The camera used
- Image format (e.g. 16 bit)
- The Bin 1 × 1 or other
- His telescope
- The focal length used
- The object photographed
- His place of observation
- The blackness of the sky: The light pollution zone
- The transparency of the sky
- Humidity
- The height of the photographed object (preferably 30o and more above the horizon)
- For cameras with a CMOS sensor, the ISO sensitivity or gain used to take the photo
- Image calibration (Bias, Black, PLU)
- The exposure time test
- The image used: RGB, Luminance, H-Alpha, Red, Green, Blue and others
- The pollution filter used (not recommended for star clusters, galaxies and reflection nebulae)
- The number of composite images
We use a calibrated and composite image of an object that has been photographed according to the indications in the table of maximum exposure times for your observation site. The object photographed must be at least 30o above the horizon. The image must not be processed (stretching). It is important to use a calibrated image so as not to consider in the evaluation the noise included in the Bias and the Black (Dark). Also, it must be assembled (composite) in order to benefit from the increase in the S / N ratio and the reduction in background noise, as presented in the table above.
For new cameras with a cooled deep sky CMOS sensor, start your test using a gain of 15 dB (150 in units of 0,1 dB). For color CMOS, use the RGB or color CCD line in the table of maximum exposure times per photo above. For monochrome CMOS with clear filter, use the CCD Luminance line. Please note that these cameras with CMOS sensor do not support the hardware 2 × 2 Bin which can reduce exposure time by a factor of 4.
After several readings that I have done on the subject, the best course of action is to determine the maximum acceptable ADU (the sky background average exposure test) value for the sky background in a light pollution environment. The maximum value I determined is as follows for a 16-bit image (65536 possible shades):
10% of 65536 = 6600 shades affected by light pollution
For more information on establishing this standard, see note 2 at the end of the discussion. For comparison, the ADU value of the sky background in an environment without light pollution is less than 1000 ADU for exposure times of 10 minutes per photo in Bin 1 × 1 and a composite of 10 photos for the image of luminance at an f / 4,5 focal length (therefore very open telescope). For example, the ADU value of my image of galaxy M101, taken in New Mexico in a sky without light pollution (gray area), is only 841 for the luminance image (light filter).
We can adapt the calculation for others image formats (for example for a 12-bit image = 10% of 4096 possible shades, for a 14-bit image = 10% of 16 384 possible shades). By using this standard, the dynamics of the image will therefore be distributed over 90% of the remaining shades, or 59000 shades devoted to the deep sky object (for a 16-bit image).
So I will proceed using a real example in an environment of extreme light pollution (white area):
- The camera used: Atix 383L + monochrome
- Image format: 16 bit (65536 possible shades)
- Bin 2 × 2
- Telescope: Celestron Edge HD, 203,2 mm (8 ″) diameter
- Focal length (with focal reducer): f / 6,3
- Photographed object: M92
- Observation site: My permanent observatory in Longueuil
- Darkness of the sky: White area
- Sky Transparency: Above Average and Transparent
- Humidity rate: 53% both sessions
- Object altitude: 60 to 82 degrees
- ISO sensitivity or gain: so, this is a CCD camera
- Image calibration: Black, PLU and Bias
- The exposure time test: 60 seconds
- The image used: Luminance (light filter)
- Emission filter used: None
- The number of composite images: 49
Calculation of the ADU value of the sky background
With the mouse, point to at least six places that represent the sky background and for each note the value. Then take the average of the values. Here is the information to take note of:
- With the Nebulosity software: the information appears at the bottom of the image on the right
- With Maxim DL software: Idem. If necessary, take the information next to the letter i: (which is the luminance)
The sky background ADU value in this example is: 4915
Calculation of maximum exposure time
Formula :
Maximum exposure time = The exposure time test / The sky background ADU value for the test * 6600
So for this example: 60 sec / 4915 * 6600 = 81 seconds or 1,3 minutes
Compare this result with the table of exposure times. The focal length used is f / 6,3. In the table, take the close focal length f / 5,6 of the line CCD Luminance and get off the line In town (white zone). The exposure time is one minute for the f / 5,6 focal length in Bin 2 × 2 with a recommendation to take 50 images (49 were taken in this example). The maximum exposure time of 1,3 minutes in the example in Bin 2 × 2 is therefore very close to the exposure time mentioned in the table above which is one minute in Bin 2 × 2. Note that in this example, the maximum exposure time according to the detailed calculation is similar to that in the table, but not always. The reason is that there are a lot of variables as mentioned in the detailed calculation above. It is therefore preferable to carry out this calculation which will take into account the actual conditions in the field and the equipment used.
Here is the image of the globular cluster M92 produced with these recommended exposure times for the white area:
Click on the image to view it full size
Here is another example of an image, the galaxy M31 "The great Andromeda galaxy" taken in a sky of extreme light pollution (white area):
Click on the image to view it full size
The visual magnitude of the galaxy is 3,4 and its surface gloss 13,5 (mag / arcmin2), or below the limiting magnitude of 14 of my observation site (white area, see above).
Here is a final example of a galaxy (NGC891) taken in a white area with the same exposure times indicated in the table of maximum exposure times:
Click on the image to view it full size
The visual magnitude of the galaxy is 9,9 and its surface brightness 13,6 (mag / arcmin2), or below the limiting magnitude of 14 of my observation site (white area, see above).
I attempted to photograph galaxy IC342 without success. I had no signal in the arms of the galaxy. Its visual magnitude is 9,1 and its surface brightness is 14,9 (mag / arcmin2), or above the limiting magnitude of 14 of my observation site (white area, see above). This is the main reason for the lack of signal in the arms of the galaxy. It was one of my tests to validate the data from the table of light pollution zones.
In conclusion, when you go to a new observation site, start by using the values from the exposure time table per photo. Then, I recommend that you perform the detailed calculations to have more precision on the maximum exposure time. Also, you must respect the limit magnitudes in astrophotography of light pollution areas as well as that of your observation instrument.
Note 1:
If we increase the exposure time per photo or if we use a more open focal length, here are the calculations necessary to know the gain in limiting magnitude. As mentioned above, in the section Visual magnitude of deep sky objects, a difference of one magnitude increases or decreases the brightness of the object by 2,512 times.
To bring out stars of very low magnitudes against the sky background, we must use the formula concerning the increase in the Signal / noise ratio: it increases by the square root of the increase in exposure time. So, to access an additional limiting magnitude, it will be necessary to increase the exposure time by 2,5122 or 6,31 times (the square root of 6,31 being 2,512). To go from the limiting magnitude of 19,5 to 20,5, the exposure time per photo should be 63,1 minutes (10 x 6,31). It should be noted that no astrophotographer uses this exposure time per photo. If we double the exposure time i.e. at 20 minutes (2 x 10 minutes per photo), the gain in limiting magnitude will therefore be only 0,32 (2 / 6,31). The limit magnitude in astrophoto will drop from 19,50 to 19,82.
Now if we use a more open focal length, for example f / 5 instead of f / 7, we use the formula shown in the website's web page titled Astronomical calculations in section theFocal aperture and exposure time. So, to compare the exposure time of one focal aperture versus another, we take the focal aperture number and squared it. Here are the calculations:
f / 7: 7 x 7 = 49
f / 5: 5 x 5 = 25
49/25 = 1,96 times brighter or brighter at f / 5 than at f / 7
The gain in magnitude limit at f / 5 will therefore be only 0,31 (1,96 / 6,31). The limit magnitude in astrophoto will drop from 19,50 to 19,81.
This analysis therefore makes it possible to conclude that if we use a longer exposure time per photo or a more open focal length, the gain in limiting magnitude will be a little higher. It will be very demanding to add an additional limiting magnitude.
Note 2: The ADU value of the sky background at 6600
For a 16-bit image (65536 possible shades), the ADU value of the sky background is set to 6600. It is determined by the following formula:
10% of 65536 shades possible = 6600 shades affected by light pollution
The maximum sky background exposure time is therefore established so as not to exceed the value of 6600 for a 16-bit image. This standard is based on the balance and color balance of the image using levels or the Black and White Point method (see process # 2 in the processing of deep sky images). The black point representing the sky background is set to the value 25 (10% of 256 shades for an 8-bit image). This therefore consists, during the acquisition of the images, in exposing the background of the sky so as not to exceed the value of 25 for an 8-bit image or 6600 for a 16-bit image. Exceeding this value, the sky background is overexposed. For example, if the ADU value of the sky background is 12000, all the information about the deep sky object between 6600 and 12000 will be lost (for example the weak signal of the arms of a galaxy between 6600 and 12000 will be lost ). Indeed, when the black and white point technique is applied during image processing, the sky background at 12000 will be reduced to 6600, which explains the loss of the weak signal of the object between 6600 and 12000 .
Also, it should be considered that the weak signal of an object which is lower than the signal of the sky background is impossible to reproduce since it is drowned in the brightness or the luminosity of the sky background. By bringing the background of the sky to the maximum value of 6600 when taking the photo, we will thus obtain the longest possible exposure time per photo for the light pollution zone of the observation site, which will give the best report signal / noise per photo (the largest difference between the object's signal and the background noise of the sky). So that's the importance of respecting this standard.
Here is the maximum ADU value of the sky background for the different image formats :
- 8 bits: 10% of 256 possible shades = 25
- 10 bits: 10% of 1024 possible shades = 100
- 12 bits: 10% of 4096 possible shades = 400
- 14 bits: 10% of 16384 possible shades = 1600
- 16 bits: 10% of 65536 possible shades = 6600
- Etc
Richard Beauregard
The Sky Astro-CCD
Revised 2023/08/21