<p>Simple - as you say, in order to get lots of dynamic range in deep astronomical images, you generally need to split your exposure time down into a number of sub-exposures.</p>

<p>When you combine these images, you don't just add them up, you "median stack" them (take the median on a pixel by pixel basis) or perform a slightly more sophisticated flux-weighted averaging that kicks out the outliers.</p>

<p>This process gets rid of any moving objects in the final image as well as cleaning up all the cosmic rays.</p>

<p>The problem with this reflector that has been launched is that it will be the equivalent of a very bright star. It will totally saturate any single image it appears in and might actually be bright enough to damage, or at least leave a persistent image, in some infrared detectors.</p>

CC BY-SA 3.0
2018-01-27T16:51:17.480Z
Is there somewhere where I could read more about median-stacking and outlier-filtering? I can certainly imagine how one might do it, but is there a standardized flow written up somewhere to make sure people don't get too creative or even do it wrong? Also, you wouldn't want to artificially force a steady state upon the data, from time to time real astronomical time-dependent events happen! There are also asteroids - would they be flagged by this process, or suppressed? There must be some SOP that's evolved over the years and become standardized.
uhoh
CC BY-SA 3.0
2018-01-27T16:53:19.663Z
If you think there is something to expand upon with regard to intentional highly reflective spacecraft and the problems they might cause, I could certainly split that off and ask it as a separate question.
uhoh
CC BY-SA 3.0
2018-01-28T00:31:19.333Z
@uhoh No, there is no standardised process. What I have described deals with asteroids too.
ProfRob
CC BY-SA 3.0
2018-01-28T03:21:43.887Z
OK, I'll try to do some more reading then, thanks!
uhoh
CC BY-SA 3.0
2018-01-27T09:28:03.050Z
2018-01-27T14:30:01.440Z
2018-01-27T14:30:01.440Z
ProfRob
<p>There are thousands of satellites and spent rocket parts in low earth orbit (LEO). Mostly near the lines of dawn and dusk they can be in sunlight while observatories below are still viewing the night sky, as are the observatories like Hubble (also in LEO).</p>

<p>They are moving roughly 7 km/sec and as seen from Earth that motion will be roughly a half-degree per second, depending on distance.</p>

<p>Considering the number of observatories imaging the sky at any moment and the number of objects in LEO, streaks across images must be happening quite regularly.</p>

<p>While that would strongly impact a one-hour exposure on emulsion, these days solid state imaging is used and these are probably regularly buffered to provide the dynamic range necessary to capture dim objects with very bright images in the same field. </p>

<p>While a fraction of a second streak by a satellite will not normally completely obliterate a long exposure, it is still a problem and it seems would have to be managed in an accepted and systematic way.</p>

<p><strong>Question:</strong> How do observational astronomers manage streaks and other artifacts from objects in LEO? Since these things are (usually) carefully tracked and their trajectories predictable, do observatories, or at least observers, ever plan ahead for these events, or schedule to avoid them? Or is it just handled automatically in post-processing?</p>

<hr>

<p>For subtext, see astronomer Alex Parker's <a href="https://twitter.com/Alex_Parker/status/956629225695514625" rel="nofollow noreferrer">Tweet</a> (image shown below) and then astronomer Benjamin Pope's more recent <a href="https://twitter.com/fringetracker/status/956970885486993409" rel="nofollow noreferrer">Tweet</a> about the Humanity Star, as well as his response "Oh god why would you do this to us astronomers"? response <a href="https://twitter.com/RocketLab/status/956247341991739392" rel="nofollow noreferrer">here</a>.</p>

<p>Also see: </p>

<ul>
<li><p>The Washington Post: <a href="https://www.washingtonpost.com/news/speaking-of-science/wp/2018/01/26/company-shoots-shiny-orb-into-orbit-astronomers-irked-over-space-graffiti/?utm_term=.008265c5398d" rel="nofollow noreferrer">Company shoots shiny orb into orbit and angers astronomers over ‘space graffiti.’</a></p></li>
<li><p>New York Times: <a href="https://www.nytimes.com/2018/01/28/science/rocket-lab-humanity-star.html" rel="nofollow noreferrer">Is This Shiny Satellite Sky Art or ‘Space Graffiti’?</a></p></li>
<li><p>Scientific American: <a href="https://blogs.scientificamerican.com/life-unbounded/twinkle-twinkle-satellite-vermin/" rel="nofollow noreferrer">Twinkle, Twinkle, Satellite Vermin</a></p></li>
</ul>

<p>Within Stack Exchange:</p>

<ul>
<li><p><a href="https://space.stackexchange.com/a/21582/12102">How will several thousand Low Earth Orbit satellites affect the night sky?</a></p></li>
<li><p><a href="https://math.stackexchange.com/a/2620121/284619">What is the name of the polyhedral shape of the Humanity Star?</a></p></li>
<li><p><a href="https://space.stackexchange.com/q/24616/12102">Does Humanity Star have non-reflective triangular panels? If so, what are their characteristics?</a></p></li>
<li><p><a href="https://space.stackexchange.com/q/22476/12102">Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy “ruined”?</a></p></li>
</ul>

<hr>

<p>From Alex Parker's <a href="https://twitter.com/Alex_Parker/status/956629225695514625" rel="nofollow noreferrer">Tweet</a>:</p>

<p><a href="https://i.sstatic.net/2G1vr.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2G1vr.jpg" alt="enter image description here"></a></p>

<observational-astronomy><artificial-satellite>
How do observational astronomers manage streaks and other artifacts from objects in Earth Orbit?
uhoh
# How do observational astronomers manage streaks and other artifacts from objects in Earth Orbit?

(Asked by: username_0 on 2018-01-27)

There are thousands of satellites and spent rocket parts in low earth orbit (LEO). Mostly near the lines of dawn and dusk they can be in sunlight while observatories below are still viewing the night sky, as are the observatories like Hubble (also in LEO).

They are moving roughly 7 km/sec and as seen from Earth that motion will be roughly a half-degree per second, depending on distance.

Considering the number of observatories imaging the sky at any moment and the number of objects in LEO, streaks across images must be happening quite regularly.

While that would strongly impact a one-hour exposure on emulsion, these days solid state imaging is used and these are probably regularly buffered to provide the dynamic range necessary to capture dim objects with very bright images in the same field.

While a fraction of a second streak by a satellite will not normally completely obliterate a long exposure, it is still a problem and it seems would have to be managed in an accepted and systematic way.

**Question:** How do observational astronomers manage streaks and other artifacts from objects in LEO? Since these things are (usually) carefully tracked and their trajectories predictable, do observatories, or at least observers, ever plan ahead for these events, or schedule to avoid them? Or is it just handled automatically in post-processing?

---

For subtext, see astronomer Alex Parker's [Tweet](https://twitter.com/Alex_Parker/status/956629225695514625) (image shown below) and then astronomer Benjamin Pope's more recent [Tweet](https://twitter.com/fringetracker/status/956970885486993409) about the Humanity Star, as well as his response "Oh god why would you do this to us astronomers"? response [here](https://twitter.com/RocketLab/status/956247341991739392).

Also see:

* The Washington Post: [Company shoots shiny orb into orbit and angers astronomers over ‘space graffiti.’](https://www.washingtonpost.com/news/speaking-of-science/wp/2018/01/26/company-shoots-shiny-orb-into-orbit-astronomers-irked-over-space-graffiti/?utm_term=.008265c5398d)
* New York Times: [Is This Shiny Satellite Sky Art or ‘Space Graffiti’?](https://www.nytimes.com/2018/01/28/science/rocket-lab-humanity-star.html)
* Scientific American: [Twinkle, Twinkle, Satellite Vermin](https://blogs.scientificamerican.com/life-unbounded/twinkle-twinkle-satellite-vermin/)

Within Stack Exchange:

* [How will several thousand Low Earth Orbit satellites affect the night sky?](https://space.stackexchange.com/a/21582/12102)
* [What is the name of the polyhedral shape of the Humanity Star?](https://math.stackexchange.com/a/2620121/284619)
* [Does Humanity Star have non-reflective triangular panels? If so, what are their characteristics?](https://space.stackexchange.com/q/24616/12102)
* [Two week mark; has Mayak (Маяк) been spotted yet? Reflector deployed? Astronomy “ruined”?](https://space.stackexchange.com/q/22476/12102)

---

From Alex Parker's [Tweet](https://twitter.com/Alex_Parker/status/956629225695514625):

[![enter image description here](https://i.sstatic.net/2G1vr.jpg)](https://i.sstatic.net/2G1vr.jpg)

**Tags:** observational-astronomy, artificial-satellite


---

## 1 answers

### Answer by username_1 on 2018-01-27. Score: 6 | Accepted answer

Simple - as you say, in order to get lots of dynamic range in deep astronomical images, you generally need to split your exposure time down into a number of sub-exposures.

When you combine these images, you don't just add them up, you "median stack" them (take the median on a pixel by pixel basis) or perform a slightly more sophisticated flux-weighted averaging that kicks out the outliers.

This process gets rid of any moving objects in the final image as well as cleaning up all the cosmic rays.

The problem with this reflector that has been launched is that it will be the equivalent of a very bright star. It will totally saturate any single image it appears in and might actually be bright enough to damage, or at least leave a persistent image, in some infrared detectors.

> **Comments:**
> **username_0**: Is there somewhere where I could read more about median-stacking and outlier-filtering? I can certainly imagine how one might do it, but is there a standardized flow written up somewhere to make sure people don't get too creative or even do it wrong? Also, you wouldn't want to artificially force a steady state upon the data, from time to time real astronomical time-dependent events happen! There are also asteroids - would they be flagged by this process, or suppressed? There must be some SOP that's evolved over the years and become standardized.  
> **username_0**: If you think there is something to expand upon with regard to intentional highly reflective spacecraft and the problems they might cause, I could certainly split that off and ask it as a separate question.  
> **username_1**: @username_0 No, there is no standardised process. What I have described deals with asteroids too.  
> **username_0**: OK, I'll try to do some more reading then, thanks!  

---

