Besides poor telescope collimation, the turbulence is the second source of bad astronomical images as it directly influences the instrument development quality. The astronomical image quality, visual or photographic, whether film or CCD, is dependent on the atmosphere condition. When the sky is covered in the middle night (it happens !!!), you have to wait ... But a clear sky is not always favorable to the observation and/or imaging.

It is important to have stable images.

An experienced observer knows quickly enjoy the atmospheric turbulence rating. Successive observation of bright stars at different heights (at the zenith, then at 40 or 50° and 20°) is sufficient to give an idea of the quality sky. If the stars twinkle overhead, the turbulence is intense, collimation and a correct focus is not possible. This technically impossible, it is impossible to perform the CCD image and even less on film. Significant and rapid brightness variations are the result of strong turbulence. Instead, slow brightness variations are used to expect good results in CCD imaging and correct focusing.

The ideal is to install an observatory top of a hill where the air flow is laminar. The French astronomer Eugène Antoniadi had established a scale that I personally use the notation in my drawings :

Focusing a telescope

The difficulty of focusing strongly differs if it concerns a visual or photographic observation.

Visually, the focus behind the eye is almost always easy, if the turbulence is not strong because the eye tends to accommodate defects and show a clear image, even if it is not quite. Our eye vision is very rough and is therefore too imprecise to CCD and film imaging.

When it comes to imaging, the focus immediately becomes more complicated, especially if it comes to telescopes Schmidt-Cassegrain or Maksutov-Cassegrain with primary mirrors very open because the perfect focus is played in an interval a hundredth of a millimeter. Newton telescopes (like the refractor telescopes) have a tolerance of up to widest point and therefore easier to adjust because their mirror is less open than compact competitors. If the collimation of your telescope is good and that your images remain poor, this is final focus in your instrument it will incriminate.

Scale turbulence - Eugène Antoniadi

CCD camera assistance with a measure FWHM

LEVEL

COMMENTS

V
Perfect Airy disk visibility and details of the observed planets. No tremor. Easy focusing, even at very long lens. Ability to achieve level A in collimation.

IV
Slight undulations, with lulls of few seconds. Focus remains fairly easy, with medium focal lengths. Collimation Level A or Level B

III
Poor visibility, with large atmospheric tub. Focus delicate spot with large focal lengths. Inability to achieve the level A in collimation.

II
Poor visibility, with large atmospheric tub. Turning to the critical point. Abandonment of large focal lengths greater than 3 meters. Film imaging prohibited.

I
Very poor visibility to barely make rough sketches. Focusing failed in CCD imaging. Collimation is not possible.

For deep sky imaging, some software directly driving a CCD camera offer assistance called a measure FWHM. In this area, it actually produces miracles because for planetary imaging, better still rely on our own sensibility. For me having used many times for practicing deep sky with a small monochrome CCD, this measurement tool FWHM quickly becomes indispensable. The audience of the most advanced focus to track the quality focus in real time using a chart.

Correct focus

Defective focus

Turbulence and focus

05/15

 

OBSERVATION AND IMAGERY

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Arnaud FIOCRET
 

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