The focal length of astronomical telescope is very short. Can I use a lens to extend the focal length?

Astronomical telescopes can be equipped with Barlow mirrors (translated as Barlow mirrors or Barlow mirrors) to extend the equivalent focal length. Barlow mirror is equivalent to a concave lens. Used in front of eyepiece and behind zenith lens, it has 2X, 3X and 5X specifications, which can increase the magnification of the system by multiple of the corresponding specifications. In addition, if the working distance of the rear part of the Barlow mirror is increased, for example, before it is inserted into the zenith mirror, an effect larger than its specification magnification can be obtained.

Barlow mirror can also be used for direct focus photography, and its output can be directly connected to the photography interface. A larger image can also be obtained by using a photographic telescopic tube such as CA 1.

Using Barlow to increase magnification should be done according to your own ability, and cannot exceed the ability of the objective lens itself. In addition, the poor Barlow mirror will seriously damage the image quality, so its accuracy should be the same as that of the objective lens.