CubeSat High-Resolution Camera: Why MTF matters

When optical designers compare the optical system of a CubeSat high resolution camera, modulation transfer function (MTF) works as a measure. MTF is typically used for components ranging from something as simple as a single spherical lens to something as complex as a satellite camera.

If you want to understand the components of MTF, it is essential to know what the CubeSat camera is. CubeSat cameras are imaging devices that fit on satellites for earth and space observation. Additionally, you will also have to understand some general principles that define MTF, including its components, importance, and characterization.

What Is MTF?

MTF (Modular Transfer Function) is often used as a quantitative description of the quality of the image from a particular lens, which also includes all aberrations. The lens’s lines (or grids) at different distances (pairs/mm) roughly define MTF. When you look at the MTF graph for each spatial frequency of the CubeSat camera modules, you will see a loss of contrast due to the optical reproduction. For the best resolution of the lens, there need to be more line pairs/mm.

The best lens is considered to provide an image that matches the object perfectly; it would also include all other details like the variations in brightness. Of course, this scenario is not always possible due to the lenses acting as low pass filters. MTF is the amount of attenuation for any detail or frequency, which also indicates the lens’s transfer efficiency.

There is always a point for any lens when the modulation is zero. Also known as the resolution limit, it is often denoted as lp/mm, or lines pair per millimeter. In the case of a macro lens, the minimum line size is denoted by µm.

There are various ways to show the contrast transformation in terms of math. However, the two most popular options include:

  1. The contrast appears on the y-axis in percent, while the location at the sensor (image height) appears on the x-axis. The center of the sensor appears as the zero value.
  2. The contrast appears on the y-axis in percent. The spatial frequency in cycles per mm appears on the x-axis; the contrast at different distances appears on different graphs that can be seen at the sensor’s center.

Spatial Distortion

In a normal case, the image captured by the CubeSat camera will be an exact representation of the object within the field of view. However, there is distortion in various degrees on the lens. In such cases, the images can either come out as compressed or stretched in non-linear ways, making taking accurate measurements very difficult.

Of course, there are various types of CubeSat camera tools that can help eliminate and correct this problem; however, they will still not be able to take the physical depth of the object. Therefore, it is always better to opt for a good-quality low distortion lens rather than buying software and tools to correct these errors.

Generally, short focal length lenses will always have more distortion than long focal length lenses. This happens because the light hits the sensor from a much bigger angle. If you want to keep the distortion low, you can choose a more complex lens design.

Suppose the distorted CubeSat camera resolution will hamper your measurements for your space and earth observation. In that case, it is recommended that you use a longer focal length lens to increase the working distance as you need. However, this will also depend on various other factors like the size of the sensor, etc. At times, this hack may also seem impossible. In such cases, you can use a lens with the same focal length and reduced distortion.

Spectral Transmission And Coating

For most people who wonder how does CubeSat camera work, this is it. Lenses often have various types of coatings that help improve transmissions and reduce surface reflections. The naked eye cannot see these coatings because they are not more than a few microns thick; however, they do wonders in improving the quality of the image considerably.

Of course, the difference in the efficiency of transmitting the wavelength of light will depend on the type of coating and the optical materials. For instance, you can improve the transmission efficiency of the infrared range by utilizing IR-coating. So, what does the CubeSat camera do in this aspect? Special calcium fluoride and quartz lenses need to be used for space-related applications where direct UV is detected.

MTF In The CubeSat Cameras

In the case of CubeSat cameras, the imaging chain acts as a low-pass filter, which will attenuate the higher spatial frequencies more than the lower ones. However, you need to understand that there is a difference in the meaning when referring to either the wavelength or frequency attenuation; you need to know that these variables are inverse of one another. Hence, it is always better to opt for long and short pass filters, mainly if you are working in the domain of the spectral wavelength.

This filtering theory can also be applied in the case of any audio system to characterize the camera’s imaging capabilities. In the imaging chain, you will see that every element has a cut-off frequency. Within the same frequency, the individual responses can be multiplied, which will help determine the overall response. Additionally, the Nyquist sampling theorem will always apply when you use a sensor to sample images at discrete intervals.

Final Thoughts

As you can see, many things happen when a satellite camera is capturing images of the earth from space. So, why use the CubeSat camera? CubeSat cameras are some of the most affordable and popular earth and space observation imaging systems that can help you gather data and information for a wide range of applications like understanding forestry and land cover, providing data related to agriculture like weather conditions, etc.

Let us know what you think in the comments!