The Etna Astros Astronomy Club

What is "Seeing"?

Observing planets, planetary nebulae or any celestial object with details at high power requires excellent seeing conditions. The seeing is the term used in astronomy to quantify the steadiness or the turbulence of the atmosphere. Seeing should not be confused with sky transparency, which is the terminology used to qualify the darkness of the sky. When we look at planets, we need high power to see all the fine details but most of the time we are limited by turbulence occurring in the telescope (local seeing) and/or in the atmosphere. During a night of bad seeing we are usually limited to see only two bands on the Jupiter disc and we can hardly use power over 100-150x. On excellent seeing conditions we can use high power and see many bands, white spots, festoons and details in the great red spot. Excellent seeing with high quality telescopes can also show details on the largest moon of Jupiter, Ganymede. What we are seeking is the best nights where we can boost our telescopes to their limits… which reach as high as 50X per inch diameter for quality telescopes… which means 500x for a quality 10-inch (25cm) instrument.

The seeing can be rated through astro-amateur telescopes with the following guidance….


Perfect motionless diffraction pattern.


Light undulations across diffraction rings.


Central disc deformations. Broken diffraction rings.


Important eddy streams in the central disc. Missing or partly missing diffraction rings.


Boiling image without any sign of diffraction pattern.

Of course, the diffraction pattern diameter is related to the aperture of the telescope. The diffraction pattern of a 4 inch telescope is twice as large as for an 8 inch instrument. So the seeing rating with this method will depend of the diameter of the telescope. An astro-amateur rating the seeing at 4/5 with a 6 inch telescope will certainly appear as a 3/5 with a 12-14 inch optical instrument. So it is important to understand or be aware of this difference. This forecast is based on the data accumulated with 11-14 inch telescopes during a four year period, so this study was done with the average modern astro-amateur telescope diameter. Astro-amateurs owning a smaller telescope may find the following (see the sky clock) forecast a bit pessimistic but you can adjust the colour index to your observations. Amateurs owning an 8-20 inch telescope should find this product quite useful and when the forecast shows a seeing 5/5 over an area… it should be the best planetary conditions for for any telescope diameter.