<p>As you took your pictures at N 60° and assuming either on Nov 1st or Dec 1st 2024 (last two new moons), Andromeda was at roughly 15° Altitude at around 18:00 and 20° Altitude at 20:00 (assuming MEZ as timezone). This is rather low in the sky and light sources from human settlements will have a high influence, if they are „below“ your target and create gradients in your pictures. But as you can see there‘s no apparent gradient visible in your pictures (e.g. from left to right or top to bottom), that could stem from a street lamp or other human light source. The gradients that you see in the pictures, which are rotational symmetric from center to the edge is vignetting, these are controllable by flats and averaging, such as you do in your summary picture. That the background changes from the first to the last picture is consistent with airmass and light pollution in the form of a „glow“ of the atmosphere. I would also not rule out scattered light from the sun due to clouds that are behind the horizon for you. Such a change in background is often present in pictures, due to moon, sun, airmass and light pollution.</p>
<p>These are the factors determining image quality for an astronomy image with regard to sun, seeing and light pollution (I am excluding equipment here):</p>
<ol>
<li><p>Imaging is usually done during astronomical night, which is defined as having the sun 18 degrees below horizon, because at that point <a href="https://en.wikipedia.org/wiki/Night#:%7E:text=During%20nautical%20twilight%2C%20the%20horizon,horizon%20is%20called%20astronomical%20night." rel="nofollow noreferrer">refracted light from the  sun is no longer visible</a>.</p>
</li>
<li><p>Seeing gets worse, the more the light travels through the atmosphere (turbulence of the atmosphere). The relevant concept is <a href="https://en.wikipedia.org/wiki/Air_mass_(astronomy)" rel="nofollow noreferrer">Airmass</a>. You want the light to go through as less Airmass as possible. Due to this many astrophotographers avoid taking pictures near the horizon. Instead they usually restrict themselves to objects that are +/-30° or so of the meridian and high-enough over the horizon (if looking south or north). Take as a rule of thumb, that a 60° zenith angle means you're looking through double the atmosphere, while it is 1 if you look straight up. In addition gradients from light pollution from ground sources is minimized.</p>
</li>
</ol>
<p>Light pollution comes in a variety of flavors, most of which lead to various gradients in your pictures. These here are the most important sources:</p>
<ol start="3">
<li><p>the moon. If it is nearly full moon make sure you point away your telescope from it as far as possible.</p>
</li>
<li><p>Bright stars just outside your field of view. Try making a picture of the flame nebula, which is right next to Alnitak, a star of magnitude 2 in <a href="https://en.wikipedia.org/wiki/Orion%27s_Belt" rel="nofollow noreferrer">Orion's belt</a> (see the picture there).</p>
</li>
<li><p>Human sources like street lamps, the garden lights your neighbors, the city a couple of kilometers away, etc. etc. etc.</p>
</li>
<li><p>clouds moving through your field of view, that are illuminated from below (see 5).</p>
</li>
<li><p>Note that 5+6 can get worse with airmass (#2). There’s simply more atmosphere that scatters light back to you.</p>
</li>
<li><p>Light from satellites &amp; planes. Those make streaks and other artifacts in your pictures, not gradients.</p>
</li>
</ol>
<p>Serious imaging of faint objects is usually done in</p>
<ul>
<li>clear skies (good weather)</li>
<li>dark skies (regions that are still unaffected by light pollution. Bortle 1 at best)</li>
<li>near new moon</li>
<li>at or near the meridian</li>
<li>away from the horizon</li>
</ul>

CC BY-SA 4.0
2024-12-12T17:34:24.053Z
In your first paragraph, did you mean Alt where you have azimuth?
Greg Miller
CC BY-SA 4.0
2024-12-14T08:40:55.287Z
Yes, fixed it. @GregMiller
Grimaldi
CC BY-SA 4.0
2024-12-12T17:15:25.640Z
2024-12-14T12:45:33.127Z
2024-12-14T12:45:33.127Z
Grimaldi
<p>I have a series of images taken between about 18:50 and 20:15, of Andromeda galaxy. They show progressive darkening of the sky around the galaxy. During that day, night started 18:00 so it was exactly 18:00 when sun is 18 degrees below the horizon.</p>
<p>I am wondering whether the progressive darkening of the sky around Andromeda galaxy was caused by Andromeda moving higher up in the sky (where there is presumably less light pollution), or further away from streetlight lamps (so less streetlight gets into the telescope tube), or if 18:50 simply was a too early time of day to start photography.</p>
<p>Is it enough for astrophotography that the sun is 18 degrees below the horizon? Or does waiting for the sun to go even lower improve my chances of avoiding light pollution?</p>
<p>The sun was 23 degrees below the horizon when I started photography in this location. Moon was below the horizon too during that time, very close to the sun in fact.</p>
<p><b>Edit: added two exposures</b></p>
<p>These were taken on 3.12.2024.</p>
<p>First exposure:</p>
<p><a href="https://i.sstatic.net/lQwskD29.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/lQwskD29.jpg" alt="first exposure" /></a></p>
<p>Last exposure:</p>
<p><a href="https://i.sstatic.net/z1rWWe65.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/z1rWWe65.jpg" alt="last exposure" /></a></p>
<p>...so the sky is getting darker as time passes. Maybe it's the streetlights then. It's 60 degrees north latitude. ISO 1000 / f/5 / 30sec exposure.</p>
<p>Here's the final image: <a href="https://i.sstatic.net/mCBSXXDs.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mCBSXXDs.jpg" alt="final image" /></a></p>
<p><b>Edit2:</b></p>
<p>The <a href="https://www.lightpollutionmap.info/" rel="nofollow noreferrer">light pollution map</a> says Bortle 6, but I suspect the local streetlights may make it more like Bortle 7 unless the object is so high in the sky that absolutely no streetlights get into the telescope tube. But I have seen pictures taken in Bortle 8 areas, which made me interested in astrophotography, since if someone can do it in Bortle 8, why couldn't I do it in Bortle 6? The sky quality meter I ordered hasn't arrived yet, so I can't say any specific magnitude/arcsec<sup>2</sup> figures.</p>

CC BY-SA 4.0
2024-12-10T20:12:44.167Z
18 degrees should have been more than enough considering Andromeda would have been on the opposite side of the sky.  You're most certainly seeing light pollution issues.
Greg Miller
CC BY-SA 4.0
2024-12-12T12:23:48.060Z
A picture or two would help us in answering your question. If you opted to acquire them in a RAW format, it is of high importance to use the same transfer function when displaying these on a computer screen. There are different ways how to achieve this, which depend on the software you chose to process your raw images. If you didn't acquire raw images and you downloaded jpegs from your camera, the algos in the camera doing that can interfere. Hopefully you switched your camera to manual mode and used the same settings.
Grimaldi
<observational-astronomy><photography><light-pollution><twilight><dark-night-sky>
Is it enough that it's night (sun 18 degrees below horizon) for astrophotography?
juhist
# Is it enough that it's night (sun 18 degrees below horizon) for astrophotography?

(Asked by: username_0 on 2024-12-10)

I have a series of images taken between about 18:50 and 20:15, of Andromeda galaxy. They show progressive darkening of the sky around the galaxy. During that day, night started 18:00 so it was exactly 18:00 when sun is 18 degrees below the horizon.

I am wondering whether the progressive darkening of the sky around Andromeda galaxy was caused by Andromeda moving higher up in the sky (where there is presumably less light pollution), or further away from streetlight lamps (so less streetlight gets into the telescope tube), or if 18:50 simply was a too early time of day to start photography.

Is it enough for astrophotography that the sun is 18 degrees below the horizon? Or does waiting for the sun to go even lower improve my chances of avoiding light pollution?

The sun was 23 degrees below the horizon when I started photography in this location. Moon was below the horizon too during that time, very close to the sun in fact.

**Edit: added two exposures**

These were taken on 3.12.2024.

First exposure:

[![first exposure](https://i.sstatic.net/lQwskD29.jpg)](https://i.sstatic.net/lQwskD29.jpg)

Last exposure:

[![last exposure](https://i.sstatic.net/z1rWWe65.jpg)](https://i.sstatic.net/z1rWWe65.jpg)

...so the sky is getting darker as time passes. Maybe it's the streetlights then. It's 60 degrees north latitude. ISO 1000 / f/5 / 30sec exposure.

Here's the final image: [![final image](https://i.sstatic.net/mCBSXXDs.jpg)](https://i.sstatic.net/mCBSXXDs.jpg)

**Edit2:**

The [light pollution map](https://www.lightpollutionmap.info/) says Bortle 6, but I suspect the local streetlights may make it more like Bortle 7 unless the object is so high in the sky that absolutely no streetlights get into the telescope tube. But I have seen pictures taken in Bortle 8 areas, which made me interested in astrophotography, since if someone can do it in Bortle 8, why couldn't I do it in Bortle 6? The sky quality meter I ordered hasn't arrived yet, so I can't say any specific magnitude/arcsec2 figures.

**Tags:** observational-astronomy, photography, light-pollution, twilight, dark-night-sky

> **Comments:**
> **username_1**: 18 degrees should have been more than enough considering Andromeda would have been on the opposite side of the sky.  You're most certainly seeing light pollution issues.  
> **username_2**: A picture or two would help us in answering your question. If you opted to acquire them in a RAW format, it is of high importance to use the same transfer function when displaying these on a computer screen. There are different ways how to achieve this, which depend on the software you chose to process your raw images. If you didn't acquire raw images and you downloaded jpegs from your camera, the algos in the camera doing that can interfere. Hopefully you switched your camera to manual mode and used the same settings.  

---

## 1 answers

### Answer by username_2 on 2024-12-12. Score: 3

As you took your pictures at N 60° and assuming either on Nov 1st or Dec 1st 2024 (last two new moons), Andromeda was at roughly 15° Altitude at around 18:00 and 20° Altitude at 20:00 (assuming MEZ as timezone). This is rather low in the sky and light sources from human settlements will have a high influence, if they are „below“ your target and create gradients in your pictures. But as you can see there‘s no apparent gradient visible in your pictures (e.g. from left to right or top to bottom), that could stem from a street lamp or other human light source. The gradients that you see in the pictures, which are rotational symmetric from center to the edge is vignetting, these are controllable by flats and averaging, such as you do in your summary picture. That the background changes from the first to the last picture is consistent with airmass and light pollution in the form of a „glow“ of the atmosphere. I would also not rule out scattered light from the sun due to clouds that are behind the horizon for you. Such a change in background is often present in pictures, due to moon, sun, airmass and light pollution.

These are the factors determining image quality for an astronomy image with regard to sun, seeing and light pollution (I am excluding equipment here):

1. Imaging is usually done during astronomical night, which is defined as having the sun 18 degrees below horizon, because at that point [refracted light from the sun is no longer visible](https://en.wikipedia.org/wiki/Night#:%7E:text=During%20nautical%20twilight%2C%20the%20horizon,horizon%20is%20called%20astronomical%20night.).
2. Seeing gets worse, the more the light travels through the atmosphere (turbulence of the atmosphere). The relevant concept is [Airmass](https://en.wikipedia.org/wiki/Air_mass_(astronomy)). You want the light to go through as less Airmass as possible. Due to this many astrophotographers avoid taking pictures near the horizon. Instead they usually restrict themselves to objects that are +/-30° or so of the meridian and high-enough over the horizon (if looking south or north). Take as a rule of thumb, that a 60° zenith angle means you're looking through double the atmosphere, while it is 1 if you look straight up. In addition gradients from light pollution from ground sources is minimized.

Light pollution comes in a variety of flavors, most of which lead to various gradients in your pictures. These here are the most important sources:

3. the moon. If it is nearly full moon make sure you point away your telescope from it as far as possible.
4. Bright stars just outside your field of view. Try making a picture of the flame nebula, which is right next to Alnitak, a star of magnitude 2 in [Orion's belt](https://en.wikipedia.org/wiki/Orion%27s_Belt) (see the picture there).
5. Human sources like street lamps, the garden lights your neighbors, the city a couple of kilometers away, etc. etc. etc.
6. clouds moving through your field of view, that are illuminated from below (see 5).
7. Note that 5+6 can get worse with airmass (#2). There’s simply more atmosphere that scatters light back to you.
8. Light from satellites & planes. Those make streaks and other artifacts in your pictures, not gradients.

Serious imaging of faint objects is usually done in

* clear skies (good weather)
* dark skies (regions that are still unaffected by light pollution. Bortle 1 at best)
* near new moon
* at or near the meridian
* away from the horizon

> **Comments:**
> **username_1**: In your first paragraph, did you mean Alt where you have azimuth?  
> **username_2**: Yes, fixed it. @GregMiller  

---

