Astro Systems Holland

What is atmospheric dispersion and what is the cause of this:

Enemy number 1 in high resolution plantery imaging is our atmosphere. However, the seeing effects are not the only causes of smearing and fading of fine details.

At large distances from the Zenith (near to the horizon) there is another cause: Atmospheric Dispersion. This effect is also known as wavelength dependent refraction (blue light is dispersed more than red light).This wavelength dependent refraction causes blue and orange-red sides around objects while imaging them at relatively low declination.

Normally this effect is not quite visible, but during imaging with modern webcams etc. and a telescope, the image is greatly enlarged and the negative effects of dispersion are then certainly visible.The further you are away from Zenith (closer to the horizon), the worser this effect becomes because of the larger dispersion. As a result of this the quality of the image becomes worse.

The position of the planets will be quite low in the coming years. This will cause problems while imaging and observing them, not only because the seeing effects are more visible at lower declination, but also because of the higher dispersion.

During planetary imaging, the seeing effects can be corrected by making movies with a high number of frames per second, in this small movie there will always be images where the seeing is frozen. The best images (with frozen seeing) can then be added together with software (e.g. Autostakkert, Registax). The result of this is a sharper planetary image, but the atmospheric dispersion still causes red and blue edges around the image. Stacking software cannot compensate for this (yet).

The angle of refraction in the atmosphere is dependent on the wavelenght of the incoming light. Because of this we need to apply different corrections to enable us to put the colors correctly "on top of each other". Only then we can see the image without refraction effects. There is no software yet known to us that can compensate for this (except professional software used by professional astronomers). Not even the RGB shift functionality in Registax can correct this sufficiently.

0 Degrees - Zenith 90 degrees above the horizon, 0.00 arc seconds of dispersion

30 Degrees - 60 degrees above the horizon, 0.35 arc seconds of dispersion

45 Degrees - 45 degrees above the horizon, 0.60 arc seconds of dispersion

60 Degrees - 30 degrees above the horizon, 1.04 arc seconds dispersion

75 Degrees - 15 degrees above the horizon, 2.24 arc seconds dispersion

How to solve this:

Here the Atmospheric Dispersion Corrector (ADC) offers a solution. The corrector consists of a main body. In this main body there are two coated wedge-prisms that can be adjusted individually against each other to compensate for the atmospheric dispersion.The wedge prisms are made from 1/10th lambda Schott N-BK7 or fused silica and are of the highest quality.

The prisms have a broadband coating on both sides (see graph) and as you can see in the graph for N-BK7, this coating blocks the shorter wavelengths (near UV) below 380nm. This causes problems while imaging Venus (for example). The cheapest solution is to use N-BK7 prisms without coating, then the light passes up to 320nm. The best solution however (but unfortunately also the most expensive), is to use the fused silica (quartz) version with a broadband coating, then the light will pass up to 250nm.

Both prisms can rotated against each other in a continously adjustable way. By rotating the two prisms the "broken" lightrays are corrected for the different colors and are refracted in such a way that they come together in the same spot.

At 0 degrees adjustment of the prisms between each other there is no effect, at 180 degrees there is the maximum effect. With this the amount of dispersion can be corrected depending on the height of the object. The result will be a good image, without chromatic abberation caused by the atmosphere.

We assume that the used optics are in good order and of good quality and that they don't cause possible other chromatic abberations! A real APO refractor or Newtonian reflector have of course less (or no) chromatic abberation than an ordinary achromat, or similar system.

When the seeing is bad, or the relative humidity is high, the atmospheric dispersion can be higher than normal. In this case there is a chance that even the atmospheric dispersion corrector cannot fully neutralise this.

Adapters to fit the ADC to your system:

The basic ADC has on both sides T2 thread, one size has male and the other side female. The ADC can be used in both directions so you can use the thread you need on your system in the way that it suits you best. The only important thing is that the handles of the corrector are perpendicular to the horizon in the neutral position. Please refer to the manual for the operation of the handles of the ADC.

There are four adapters availiable to fit the ADC to your telescope system and your camera (or eyepiece).

1) At the telescope side the 1.25" nosepiece adapter enables rotation in the focuser of the image (easy for rotating the ADC parallel to the horizon).

2) At the other side there is a 1.25" connection for an eyepiece/camera.

3) If your camera has a female T2 connection (e.g. ASI camera's) you can order a male T2 male adapter (to convert the female to a male).

4) A C-mount adapter for camera's with a C-mount connection (e.g. Basler/DMK/Watec/Pointgrey).

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