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CFF Telescopes Classical Cassegrain 300 mm F/20 with ASH ADC BK7

Promotion: CFF Telescopes Classical Cassegrain 300 mm F/20 offered with free Astro Systems Holland atmospheric dispersion corrector BK7 version.

6 790,00 €

-299,00 €

7 089,00 €

One of product is no longer available. This pack cant be purchased

Pack description

Promotion: CFF Telescopes Classical Cassegrain 300 mm F/20 offered with free Astro Systems Holland atmospheric dispersion corrector BK7 version.

Pack content

CFF Telescopes Classical Cassegrain reflector 300 mm F/20 with Supremax33 primary mirror, quartz secondary mirror and Feather Touch 2" Rack&Pinion focuser.

Truss tube OTA, 23% linear obstruction and primary mirror fans. standard delivery includes also Losmandy dovetail and shroud pack (secondary casing shroud, secondary cage shroud, truss shroud).

A fantastic instruments designed for high resolution planetary and lunar imaging or observation.


  • Clear aperture: 300 mm
  • Focal length: 6000 mm
  • Primary substrate: Supremax33
  • Secondary substrate: Quartz
  • Manual adjustments/collimation for both mirrors
  • Primary mirror cage: Carbon Fiber
  • Truss tubes: Carbon Fiber
  • Obstruction: 23%
  • Primary fans included


  • 1160 x 410 x 410 mm including focuser length
  • Weight: 17.8 kgs
  • Back-focus from end of back plate: around 230 mm
  • Focuser: 2'' Feathertouch R&P
  • Focuser travel: 38 mm (FT 2" R&P)

Items included

  • shroud pack (secondary casing shroud, secondary cage shroud, truss shroud);
  • side plates (pair);
  • handle;
  • dovetail.

Data sheet

LeasingAt standard interest

ASH Atmospheric Dispersion Corrector - BK7 multicoated version

What is an atmospheric dispersion corrector and how does it work?

Atmospheric Dispersion Corrector (ADC) significantly improve the quality of images or you live image in eyepiece. Enemy number one in high resolution planetary 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 planetary cameras and a telescope, the image is greatly enlarged and the negative effects of dispersion are then certainly visible. The farther you are 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 is the Atmospheric Dispersion Corrector made?

The secret is in two very quality coated wedge-prisms.

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 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. 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.

How is the Atmospheric Dispersion Corrector used?

Atmospheric Dispersion Corrector (ADC) is easy to use with your planetary camera or with your eyepiece. In just few easy step you will setup ADC for observations or imaging.

  1. 1. Specify Earth horizon in your telescope. For scope where is no right-angle mirror, such as an SCT or refractor (without a star diagonal) the horizontal axis as seen through the focuser is the same orientation as the horizon relative to the observer, and so this makes the task quite straight-forward as the mid-point of the levers is also horizontal. In this case however the mid-point can be horizontally pointing left or horizontally pointing right. In one of these cases adjusting the ADC will make the atmospheric dispersion worse but in the other orientation it will make it better. You need to find which of the two possibilities is correct by experimentation. If you have a right-angled mirror in the system, such as for a Newtonian reflector, the horizontal orientation through the focuser may well not be parallel to the horizon. You'll need to first find the horizontal axis of the sky as viewed through the eyepiece.
  2. Place your ADC in focuser. ADC in rest position (two red arms) must be parallel with horizon. You must update ADC rotation in focuser due telesope rotation.
  3. Place your planetary camera or eyepiece on ADC.
  4. 4. Simultaneously start rotating two red arms for best view. You need to find which possibilities is best for you. You must experimentate!


ADC body is only 25 mm short, it has T2 thread on both sides ( M42 x 0,75 mm). One male and the other side female. Also you can use the corrector in any position. With the T2 male for example in the front or in the back. Make your own combination.


It is possible to add some useful accessories on the ASH ADC, for example you can add a 1.25" nosepiece, a 1.25" eyepiece holder, a T2 male adapter and a C-mount adapter. There is also a kit available with some discount over the purchase of the separate parts.