<p>You can estimate the pixel size just from the number of pixels if you know the scale of your image or certain object in the image, but really you should already know this and the beamsize (which you could take a guess at but can't be sure). This information should be given with the image by whoever actually made it. If you have a data file it may be in the headers, or if you got the image from a website check the caption for details or an original source.</p>

CC BY-SA 4.0
2018-11-08T14:10:44.093Z
2018-11-08T14:10:44.093Z
FJC
<p>Pixel size is usually (beamsize/3). Based on the axes and number of pixels, you can find out pixel size by doing
pixel_size = image size(in arcsec) / number_of_pixels.</p>

<p>Usually, the image is done by first finding the beamwidth of the telescope and then putting 3-5 pixels inside a beamwidth in any axis.</p>

CC BY-SA 4.0
2018-11-05T07:27:35.803Z
2018-11-05T07:27:35.803Z
Pranshu Mandal
<p>I have an image with some sources in it. I need to get the pixel size and beam size of that image. How can I calculate it? </p>

CC BY-SA 3.0
2016-05-04T10:34:58.780Z
Are you saying you already have a picture of a known part of the sky, and want to know the image size in degrees, or similar?
Andy
CC BY-SA 3.0
2016-05-04T10:47:00.917Z
Infact I have a synthetic image with a source of known flux. Inorder to give it to a source extraction software as input, it require that the pixel size should be equal to one third of the beam size. And I don't know how to calculate the beam size in this case
Rian
CC BY-SA 4.0
2018-11-05T12:43:38.727Z
From your description, it's impossible to know. You need to ask whoever prepared the image what the beam size is in pixels; there's no way to know otherwise.
Peter Erwin
CC BY-SA 4.0
2020-02-03T08:55:49.363Z
Is it a FITS file? The beam size will likely be in the header
usernumber
<data-analysis><photometry>
How to calculate the pixel size and beam size of an image?
Rian
# How to calculate the pixel size and beam size of an image?

(Asked by: username_0 on 2016-05-04)

I have an image with some sources in it. I need to get the pixel size and beam size of that image. How can I calculate it?

**Tags:** data-analysis, photometry
> **Comments:**
> **username_1**: Are you saying you already have a picture of a known part of the sky, and want to know the image size in degrees, or similar?  
> **username_0**: Infact I have a synthetic image with a source of known flux. Inorder to give it to a source extraction software as input, it require that the pixel size should be equal to one third of the beam size. And I don't know how to calculate the beam size in this case  
> **username_2**: From your description, it's impossible to know. You need to ask whoever prepared the image what the beam size is in pixels; there's no way to know otherwise.  
> **username_3**: Is it a FITS file? The beam size will likely be in the header  

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## 2 answers

### Answer by username_4 on 2018-11-08. Score: 0

You can estimate the pixel size just from the number of pixels if you know the scale of your image or certain object in the image, but really you should already know this and the beamsize (which you could take a guess at but can't be sure). This information should be given with the image by whoever actually made it. If you have a data file it may be in the headers, or if you got the image from a website check the caption for details or an original source.


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### Answer by username_5 on 2018-11-05. Score: 0

Pixel size is usually (beamsize/3). Based on the axes and number of pixels, you can find out pixel size by doing
pixel\_size = image size(in arcsec) / number\_of\_pixels.

Usually, the image is done by first finding the beamwidth of the telescope and then putting 3-5 pixels inside a beamwidth in any axis.


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