Optical Accessories

The role of optical accessories in astrophotography

Optical accessories are the elements that directly influence the light path before it reaches the sensor. In astrophotography, these components are used to correct inherent image defects of telescopes or to adjust the focal length. Without proper optical correction, many telescopes exhibit aberrations such as coma or field curvature, especially when using larger camera sensors.

Image field correctionFlatteners & Coma corrector)

Most telescopes do not naturally project a flat image, resulting in blurry or distorted stars at the edges of the picture. A field flattener corrects this curvature without changing the focal length. For Newton telescopes, a comacorrect necessary to eliminate the characteristic ‘tailing’ of stars at the edge of the field. These accessories are crucial for photographers working with APS-C or full-frame sensors.

Focal length adjustmentReducers & Barlow lenses)

With a focal reducer shortens the focal length of the telescope. This increases the field of view and lowers the aperture ratio (f-number), thus decreasing the required exposure time. This is a common method with Schmidt-Cassegrains and refractors for deep-sky photography. Conversely, an Barlow lens the focal length, which is necessary for capturing small details on planets and the moon.

Quality of coatings and glass

When choosing optical accessories, the quality of the glass and anti-reflection coatings are decisive. High-quality coatings minimize internal reflections and ‘halos’ around bright stars. Additionally, the clear aperture of the optical element must be large enough to fully illuminate the sensor without causing mechanical vignetting.