Many accessories can improve the quality of astronomical observation. They can be used to facilitate the pointing of the telescope on the observed object, to increase or decrease the focal length of the instrument used to correct some aberrations, or to restore the image in the right direction.
Finderscope ] It is a small telescope mounted in parallel to the lens, which allows pointed a celestial object more easily that with a single telescope. Indeed, when we look through an eyepiece, it has a limited view of the sky which complicates the identification of the object that is to be observed. The finderscope has a wider field of vision, the task will be more simply. As the telescope, it offers a vision reversed (left/right + up/down).
There is another type of finderscope with reflex sight. The sight is created by the projection of a red point or target on a transparent piece of glass or plastic. This kinf of finderscope does not enlarge the images, and unlike conventional researchers, does not reverse vision. Some devices offer the ability to adjust the brightness of the red dot, sometimes too bright.
How to parallel the finderscope with the telescope ?
During the day, targeted as precisely as possible a distant land object (like a chimney) with his telescope. Then do the same with the finderscope. Both instruments are now in parallel.
Why not do it with a star ?
Because it has had time to move in the settings.
Note: Please do not point the telescope toward the sun during this operation.
Polarscope ] If you want to take pictures with long exposure time, or hold to observe a celestial object, it is imperative to point accurately pole north celestial. For this there equatorial mounts feature a polarscope, which is placed in the viewfinder right ascension axis of the frame. Looking through, we see a series of marks indicating the position of the celestial pole in relation to stars that are closest. Simply align its bearings with the corresponding star to the bezel is precisely aligned on the north celestial pole. There is no polarscope for the south celestial pole, because it is not sufficiently visible surrounded by stars.
Tip Tilt Mirror ] Dedicated to astrophotography, this system is designed to compensate for any drift of the guide star during long exposures with a reactivity significantly higher than the conventional guidance system. It allows, among others, to correct polar alignment errors. This is a kind of adaptive optics correcting the first order faults. A true adaptive optics allows also correct for atmospheric turbulence by analysis of the wavefront and deformation of the mirror. But this system is extremely expensive.
Barlow Lens ] Device that is inserted between the focuser and the eyepiece and extends the focal length of a telescope, to multiply its magnification by 2 or 3. With three eyepieces and a barlow lens, so you get six magnifications. This increase comes at the cost of a small loss of light and image quality. The addition of a single lens produces chromatic aberration. It is therefore advisable to choose barlow lenses having two (achromatic) or three lenses (apochromatic) to mitigate this defect. In addition, an anti-reflective coatings reduce light loss. Certain derivative lens extenders of barlow lens include four lenses.
>> Its use is limited to the moon and planets.
Focal Reducers ] This accessory has the opposite effect of the barlow lens. By shortening the focal length, it increases the field and the brightness of the instrument. Used for astrophotography, it reduces the half exposure time and increases the contrast of the image. It is placed between the eyepiece holder and camera case.
Field Corrector (or levellers field) ] Accessory to correct curvature of field and distortion. It is mainly used for astrophotography because these two aberrations are hardly visible visual observation.
Coma Correcto ] Used mainly with newtonian telescopes coma is inherent in its design. This accessory provides a flat image with star point until the field edge.
Star Diagoanl ] In most telescopes, the view is reversed between the top and bottom, and left and right. Many astronomers, bored by this inversion, uses an accessory called star diagonal. This contains a plane mirror or a total reflection prism that restores up/down inversion, but not left/right. It attaches between the focuser and the eyepiece and allows a view at 90 ° to the optical axis of the instrument, making it more comfortable watching, especially when the celestial object is close to the zenith. It is used with glasses and catadioptric telescopes, but rarely with a newtonian telescope whose eye is on the side of the tube.
>> Why not add a lens directly to straighten the image in the design?
Because it would absorb some light, and designers try to minimize light loss.
Erecting Prism ] To restore the meaning of the image both vertically and horizontally, use another type of land called diagonal rectifier. It tilts 45 ° to the optical axis of the instrument to a more comfortable position with telescopes and catadioptric telescopes. However, this accessory has a small disadvantage: it absorbs 10% of light.