The optical resolution and the wavelength

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Most observers are used to think that Red light is the best way to achieve high resolution imaging. However, this is not completely true…


We are often told in planetary imaging that the images are best resolved in red light. Indeed, most of our image look best through longer wavelengths (red or near-infrared). However, we are also teached by the laws of optic that in theory, the resolving power of a telescope is, on the contrary, better in the short wavelengths (blue light). 

It is easy to verify this by experimentation. A few years ago I imaged under good seeing the Airy pattern of my telescope (then an excellent Mewlon 210 by Takahashi) through a large panel of filters from UV to IR and here are the results:


These are images of a real star (I don’t remember which one, certainly Vega) that show text-book perfect Airy patterns  with the central disk and a first diffraction ring. The evolution of the size of the pattern is dramatic from the little UV one, to the big IR 780 (I have not made any attempt with my IR 1000 filter  but it would have been almost twice bigger than G).

So why aren’t we seeing more often more detailed blue light images ? 

There are many reasons for this :

  1. First, poor or average seeing will affect more blue light than red light
  2. Second, optical defects of the telescope can also lower the quality of B images more than in red
  3. And finally, the details of a planet can be very different through each colour. So the B image can be more resolved but it will go unnoticed because the details are less contrasted. This is obvious in the case of Mars.

But sometimes things are going well and the B filter beats the R one…

If your telescope is good and if seeing is good, the B image should get out more detailed than the R one. I have seen this many times over the years and it is not a rare situation. But last year during a very steady late summer morning I got a superb example on Jupiter :


jupIR14sept12On this comparison the greater sharpness of the B image is obvious. This is what happens when everything goes right through all the imaging tunnel, from the sky to the camera ! The comparison with IR light at left is even more spectacular, this is a band where the resolution of a telescope is noticeably reduced in comparison with visible light. I did not even resized the image at the same scale than the R and B, the level of resolution was too low ;).

The images are taken with a  250 mm telescope. This is not the higher diameter available today for an amateur. Differences of resolution would be harder to see with a bigger instrument, yet sometimes we do see it.

On the other hand, smaller instruments are going to see this effect quite often.

So when you do planetary imaging, do not forget to give the B filter the place it deserves. Too often we see images where the R layer is more cared than the others.