<p>You can differentiate between planet (Did you mean asteroid? Planets are quite bright.) and noise quite easily. Just take three pictures of the sky and they will be probably quite noisy. If some spot changes its position on all three pictures, then this is some near object (astronomically near). You can additionally verify it if its trajectory is line or not. If it is not, then it is probably just noise, but that isn't always the case.</p>
<p>Differentiating between a star and noise is harder. You have to take more pictures and hope that the noise will cancel out.</p>
<p>About flux density: this is the measure for light over some surface and it is proportional to the number of photons that hit the sensor.</p>

CC BY-SA 4.0
2021-01-16T04:41:43.343Z
Since, you mentioned number of photons does it mean the measure of energy of respective wavelengths from each object reaching the sensor??   And if yes what signatures are we looking for in particular ? high Gamma intensity or UV or some other part of the spectrum??
user14812745
CC BY-SA 4.0
2021-01-16T04:42:55.913Z
Also if you do find a source which explains this physics please share, that will be a lot of help!
user14812745
CC BY-SA 4.0
2021-01-16T09:36:34.753Z
Electric density is given by electric flux over surface. Similarly, flux density is given by light flux over surface. Unfortunately, stars are very different between each other. Some can be red, some can be yellow, blue, etc. But this is true for noise, too. Some pixel can be red, some can be yellow, blue, etc. In most cases, we take two photos and the noise cancels out. And more photos -> less noise. By the way, such images for detecting stars are usually white and black to get most of the camera. Hubble has a camera with only black or white, for example.
User123
CC BY-SA 4.0
2021-01-15T19:36:34.820Z
2021-01-15T19:36:34.820Z
User123
<p>I understand that differentiating between celestial bodies and noise due to equipment is a fundamental problem in astronomy. I also understand that we solve this problem by looking at something called <strong>flux density</strong> and <strong>continuum and line spectra</strong> but how do I understand these terms ??</p>
<p>Please consider I'm well versed in university level physics so I understand how spectra will help but don't understand which quantity's <strong>flux density</strong> do we look for in astronomy. Please help me understand these terms and hence solve the noise vs star/planet problem.</p>

CC BY-SA 4.0
2021-01-16T00:25:56.180Z
you might enjoy the surprising answers to [Which are stars and which are noise in this comet photo?](https://astronomy.stackexchange.com/q/33681/7982)
uhoh
CC BY-SA 4.0
2021-01-16T12:42:02.717Z
For those interested smartphone can do miracles nowadays.
Alchimista
<star><observational-astronomy><planet><telescope><astrophysics>
How do we differentiate between a star or planet and noise introduced due to equipment?
user14812745
# How do we differentiate between a star or planet and noise introduced due to equipment?

(Asked by: username_0 on 2021-01-14)

I understand that differentiating between celestial bodies and noise due to equipment is a fundamental problem in astronomy. I also understand that we solve this problem by looking at something called **flux density** and **continuum and line spectra** but how do I understand these terms ??

Please consider I'm well versed in university level physics so I understand how spectra will help but don't understand which quantity's **flux density** do we look for in astronomy. Please help me understand these terms and hence solve the noise vs star/planet problem.

**Tags:** star, observational-astronomy, planet, telescope, astrophysics
> **Comments:**
> **username_1**: you might enjoy the surprising answers to [Which are stars and which are noise in this comet photo?](https://astronomy.stackexchange.com/q/33681/7982)  
> **username_2**: For those interested smartphone can do miracles nowadays.  

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## 1 answers

### Answer by username_3 on 2021-01-15. Score: 1

You can differentiate between planet (Did you mean asteroid? Planets are quite bright.) and noise quite easily. Just take three pictures of the sky and they will be probably quite noisy. If some spot changes its position on all three pictures, then this is some near object (astronomically near). You can additionally verify it if its trajectory is line or not. If it is not, then it is probably just noise, but that isn't always the case.

Differentiating between a star and noise is harder. You have to take more pictures and hope that the noise will cancel out.

About flux density: this is the measure for light over some surface and it is proportional to the number of photons that hit the sensor.

> **Comments:**
> **username_0**: Since, you mentioned number of photons does it mean the measure of energy of respective wavelengths from each object reaching the sensor??   And if yes what signatures are we looking for in particular ? high Gamma intensity or UV or some other part of the spectrum??  
> **username_0**: Also if you do find a source which explains this physics please share, that will be a lot of help!  
> **username_3**: Electric density is given by electric flux over surface. Similarly, flux density is given by light flux over surface. Unfortunately, stars are very different between each other. Some can be red, some can be yellow, blue, etc. But this is true for noise, too. Some pixel can be red, some can be yellow, blue, etc. In most cases, we take two photos and the noise cancels out. And more photos -> less noise. By the way, such images for detecting stars are usually white and black to get most of the camera. Hubble has a camera with only black or white, for example.  

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