What is a Diffraction Spike?

A diffraction spike is the light you see extend from a star in your astro-images. A diffraction spike is caused by how light bends or diffracts around an object or in this case the support beams in your secondary mirror in reflecting telescopes. This spike isn’t present with refractors as it doesn’t have a secondary mirror.

Celestron Schmidt Cassegrain and EdgeHD optical tubes have an optical lens at the front-end where the secondary mirror is held in the middle. With the secondary mirror being held up by the lens it eliminates diffraction spikes.

You can manually create diffraction spikes when using Fastar/ Hyperstar with either your equipment (bulky cameras or Astroimaging cameras) or with strings you place on the front part of your optical tube.  If your DSLR is bulky and is bigger than the secondary mirror, it blocks light coming into your mirrors and therefore creates a diffraction spike in your image. In addition, when you use astroimage cameras, you have cables going in and out of the camera that go across your optical lens preventing light from coming in and creating a diffraction spike. If you want additional diffraction spikes in your reflector or Schmidt Cassegrain, lay string across the optical tube. You can get creative with it.

If you have a reflector telescope or using Fastar/ Hyperstar, you can minimize diffraction spikes by taking multiple images while changing the angle of your telescope and then overlay the images.  This can also work if the diffraction spike is interfering with the object you are trying to image. You can also in post erase the diffraction spike.