From the August 2015 issue

When light strays, part 1

Astroimager Adam Block describes problems in images caused by stray light.
By | Published: August 24, 2015 | Last updated on May 18, 2023
The feeble photons we collect with telescopes and then record with cameras are easily overwhelmed by other photons that find their way into our exclusive astrophotographic party. For the best pictures, imagers must prevent these errant photons from reaching the scope’s focal plane.

In this column, I’ll discuss some common examples of stray photons so you can recognize them in your images. In my next column, I’ll follow up with a technique that takes care of a particular family of scattered light effects.

I have been an imager long enough to collect hundreds of examples, but I can show only a handful here. (More are in the associated video below.)

Image 1
In Image #1, multiple arcs fill the field. It’s even more remarkable when displayed at full resolution because there are as many arcs as there are stars! These are the result of starlight reflecting off the shiny inner surface of the camera adapter nosepiece. Applying black-flocked paper to the inside of the tube instantly solved this issue.
Image 2
The scattered light of Image #2 is equally impressive. All stars, especially the brightest ones, show a many-spoked radial diffraction pattern. A turned down edge (TDE) of the primary mirror scatters light to cause this effect. It’s normal for the edge of a mirror to include some degree of TDE (as long as it is not in the usable diameter that you purchased).

Blackening the mirror’s edge is one way to address unwanted sparkle. A knife-edge-machined circular mask on the 32-inch Schulman Telescope on top of Mount Lemmon hovers over the mirror on standoffs and completely eliminates the scattered light from the TDE.

Image 3
The mysterious light in Image #3 is truly diabolical. The glow would appear in sequential exposures and slowly fade, only to reappear later. The main culprit in this case was the filter wheel. Some use an infrared light to align filters. The light should turn off after a filter moves into position and before an exposure begins, but a software error can keep it on. The slow dimming resulted from a residual image in which charge remaining in thick chips fades. Although not related to the light, it made matters even worse.
Horsehead nebula
Image 4

Images #4 and #5 demonstrate the difficulties of acquiring exposures near bright stars. It’s often best to have bright stars in the field of view rather than just outside it because the latter case causes glows and gradients. In these images, brilliant Alnitak (Zeta [ζ] Orionis) scatters textured rings of light across the Horsehead Nebula (B33) and creates a severe gradient in the field of the Flame Nebula (NGC 2024). In addition, one of Alnitak’s diffraction spikes runs through the field. I chose to remove this from the final image using a content-aware clone-type tool.

Image 5
Image 6

Warning … Image #6 is about as bad as it gets! It shows a nearby bright star ruining the field with this setup. Shiny structures at the chip’s edge cause specular reflections so complex that I have not found a good remedy for this case of scattered light. Sometimes rotating the camera and changing its orientation can help mitigate the reflections, but it isn’t always possible.

And with that I leave you somewhat on a low note by presenting you with an image that would take more time to repair than to create in the first place! However, my next column will look at another interesting example, and I will demonstrate a technique that fixes it as well as other similar problems.