Enter the pixel size (um) and the focal length (mm) into the Calculator. The calculator will evaluate the Arc Seconds Per Pixel. 

Arc Seconds Per Pixel Formula

ASPP = (PS/FL) * 206.265


  • ASPP is the Arc Seconds Per Pixel (arc-sec/pixel)
  • PS is the pixel size (um)
  • FL is the focal length (mm)

To calculate Arc Seconds Per Pixel, divide the pixel size by the focal length, then multiply by 206.265.

How to Calculate Arc Seconds Per Pixel?

The following steps outline how to calculate the Arc Seconds Per Pixel.

  1. First, determine the pixel size (um). 
  2. Next, determine the focal length (mm). 
  3. Next, gather the formula from above = ASPP = (PS/FL) * 206.265.
  4. Finally, calculate the Arc Seconds Per Pixel.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

pixel size (um) = 75

focal length (mm) = 90

Frequently Asked Questions

What is an Arc Second?

An arc second is a unit of angular measurement equal to 1/3600th of a degree. It is commonly used in astronomy to describe the apparent sizes of celestial objects and the precision of telescopic observations.

Why is the Arc Seconds Per Pixel (ASPP) important in astronomy?

ASPP is crucial for determining the resolution of an image captured by a telescope. It helps astronomers understand the smallest detail that can be resolved by the telescope’s camera, which is essential for studying distant celestial objects.

Can the Arc Seconds Per Pixel value affect the quality of astronomical images?

Yes, the ASPP value directly affects the quality of astronomical images. A lower ASPP value means higher resolution, allowing more detailed observations. Conversely, a higher ASPP value results in less detail being captured.

How does changing the focal length of a telescope affect the ASPP?

Increasing the focal length of a telescope increases the ASPP value, leading to lower resolution images. Reducing the focal length decreases the ASPP value, resulting in higher resolution images. Adjusting the focal length is one way to optimize the balance between field of view and image detail.