Is the Hubble better than ground based telescopes?

Many people think that the Hubble Space Telescope is our most powerful telescope. The Hubbel's location in space gives it important advantages including freedom from atmospheric distortion and no daylight to interrupt operation. The Hubble has a small size for an astronomical telescope, with an objective mirror size of 2.4 meters. The size of a single mirror for a space telescope is limited by the diameter of the rocket used to place it in orbit. The largest ground based telescopes now have mirrors of about 10 meters and 30 meter telescopes are under construction. The light that a telescope captures increases with the square of telescope size giving these telescopes sensitivities of 10 to 100 times that of the Hubble.

At the time the Hubble was launched, ground based telescopes were limited by atmospheric turbulence (astronomical seeing) which blurs images. New technology has largely removed this barrier to resolution of ground based telescopes. Adaptive optics technology warps a mirror in the light path of a telescope to dynamically correct for atmospheric distortion. It uses a reference star image to calculate the distortion of the air in real time. When there isn't a good reference star near the target, an artificial reference star can be created by a ground based laser exciting sodium high in the atmosphere.

Adaptive optics allow large ground based telescopes to capture clearer images than the smaller Hubble telescope. This is why there are no plans for a direct replacement for the HST. New space telescopes all operate in frequency bands outside the visual range of the human eye where signals are blocked by the atmosphere. Space telescopes are now the best alternative for images in the deep infrared, ultraviolet, and other bands where the atmosphere is optic. On the ground, there is a building boom in large telescopes for visual band images.

These technologies, including adaptive optics and lucky imaging, are easily retrofitted onto existing telescopes. Here is one example Palomar Observatory: PALM-3000: Exoplanet Adaptive Optics for the 5-meter Hale Telescope by R. Dekany et al in the Astrophysical Journal. This image compares an image of Jupiter's moon Ganymede take with the 70 year old 5 meter telescope on Mt. Palomar, retrofitted with adaptive objects, to a similar image taken with the Hubble Space Telescope. The ground based image is far superior to the HST image:

Adaptive optics cannot correct atmospheric distortion over very wide fields of view. A survey telescope like the Vera Rubin (LSST) 8m telescope with 3.2 GigaPixel camera and f/1.23 focal ratio would be better located in space. We may see this in the future when we have the technology to assemble multiple mirror telescopes in space.

The James Webb infrared space telescope will test our technology to deploy a self-deploying multiple mirror telescope with a 6m aperture in space. If successful this could pave the way for large visual band telescopes in space. It has proven difficult and expensive to develop this technology. The James Webb IR telescope has more than a decade of delays and billions of dollars in cost overruns. Until this becomes proven and less costly technology, ground based telescopes in the visual range will capture better images at much lower cost.

Content created: 2020-02-10




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