Star Spikes Pro

[Ailene Goldhirsh]

Diffraction spikes are lines radiating from bright light sources, causing what is known as the starburst effect[1] or sunstars[2] in photographs and in vision. They are artifacts caused by light diffracting around the support vanes of the secondary mirror in reflecting telescopes, or edges of non-circular camera apertures, and around eyelashes and eyelids in the eye.

In the vast majority of reflecting telescope designs, the secondary mirror has to be positioned at the central axis of the telescope and so has to be held by struts within the telescope tube. No matter how fine these support rods are they diffract the incoming light from a subject star and this appears as diffraction spikes which are the Fourier transform of the support struts. The spikes represent a loss of light that could have been used to image the star.[3][4]

A small number of reflecting telescopes designs avoid diffraction spikes by placing the secondary mirror off-axis. Early off-axis designs such as the Herschelian and the Schiefspiegler telescopes have serious limitations such as astigmatism and long focal ratios, which make them useless for research. The brachymedial design by Ludwig Schupmann, which uses a combination of mirrors and lenses, is able to correct chromatic aberration perfectly over a small area and designs based on the Schupmann brachymedial are currently used for research of double stars.

Images from telescopes with segmented mirrors also exhibit diffraction spikes due to diffraction from the mirrors' edges. As before, two spikes are perpendicular to each edge orientation, resulting in six spikes (plus two fainter ones due to the spider supporting the secondary mirror) in photographs taken by the James Webb Space Telescope.[9]

An improperly cleaned lens or cover glass, or one with a fingerprint may have parallel lines which diffract light similarly to support vanes.[10] They can be distinguished from spikes due to non-circular aperture as they form a prominent smear in a single direction, and from CCD bloom by their oblique angle.

A cross screen filter, also known as a star filter, creates a star pattern using a very fine diffraction grating embedded in the filter, or sometimes by the use of prisms in the filter. The number of stars varies by the construction of the filter, as does the number of points each star has.

A similar effect is achieved by photographing bright lights through a window screen with vertical and horizontal wires. The angles of the bars of the cross depend on the orientation of the screen relative to the camera.[7]

In amateur astrophotography, a Bahtinov mask can be used to focus small astronomical telescopes accurately. Light from a bright point such as an isolated bright star reaching different quadrants of the primary mirror or lens is first passed through grilles at three different orientations. Half of the mask generates a narrow "X" shape from four diffraction spikes (blue and green in the illustration); the other half generates a straight line from two spikes (red). Changing the focus causes the shapes to move with respect to each other. When the line passes exactly through the middle of the "X", the telescope is in focus and the mask can be removed.

Shown to the left in this exposure of the bright star Antares, diffraction spikes are artifacts that show themselves on brighter stars in our images when the beams of light entering the objective end (the business end) of your lens run into an obstacle and are interfered with and bent, causing the light to spread out.

During my testing, I took some test exposures to see how well it worked before I committed. Needless to say, I was happy with the results. Most of these images (except for Sadr) are very unprocessed, and all were taken with the exact method and setup shown here.

Co-founder of PhotographingSpace.com, co-owner of several telescopes and mounts, too many cameras, and not enough hard drives, Cory is an American expat living in South Africa with his wife, Tanja Schmitz.

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Our Advanced Stacker Plus has two built-in ways to increase star brightness. We call those Bump Up and Pump Up the stars. Bump Up creates a small blur by literally duplicating the shot , nudging the duplicate(s) and recombining . Pump Up is more sophisticated and tries to find the stars so it can then apply enhancements to just the stars. But there is a new tool in the arsenal that I have begun using: Star Spikes Pro from ProDigital Software. Version 4 is the latest as of this writing.

You can use the Star Spikes Pro plugin to add diffraction spikes and diffusion. The most common diffraction spikes you see with stars are due to obstructions in the telescope used to photograph them and many people come to think of the spikes as evidence of astrophotography. You can create diffraction spikes easily on your own.- just stop down your aperture; however stopping down to make stars create those spikes will not work well.

The final operation was to use an Adjustment Layer (8) to increase the contrast and restrict that adjustment to the sky (where you see white) and tone the adjustment down a little with a low-flow back brush on one area that looked a little too dark.

With some experimentation, and some coaching from the plugin author I discovered that Star Spikes Pro has several features that make the process easier than I imagined. Instead of creating the transparency (deleting the moon and landscape) I only needed to select the area I wanted Star Spikes Pro to operate on.

The net is that you can get that nice diffusion effect for your stars without having to compromise by shooting through a diffusion filter. However if you DO want to try a diffusion filter, I recommend you take two shots quickly. One with the filter off, one with the filter on. You can then place the diffused shot over the normal shot. Set the diffused shot to Lighten and mask in (or out) the areas where you want the diffusion to show through.

I am not affiliated with ProDigital Sofware. I am a happy customer of Star Spikes Pro (and another product called Astronomy Tools). I was not paid, or encouraged to write about the product. I chose to because it is that good. Rogelio Bernal Andreo author of Hasta La Vista Green and purveyor of DeepSkyColors is a friend and a multi-multi award-winning astrophotographer. He has a Kickstarter Project that I recommend you look into called Notes From the Stars

As incoming light waves diffract (bend slightly) around the edges of the struts, the waves begin to overlap each other, causing an interference pattern like ripples overlapping in a pond. This interference pattern appears in the shape of the orientation of the struts; these are the diffraction spikes we see. For example, in Hubble Space Telescope images, we typically see four spikes at right angles to each other because Hubble has four metal arms holding its secondary in place.

You can get many types of diffraction spikes, depending on the shape of your mirror (or aperture) and the number and orientation of secondary mirror struts. All point sources (i.e., stars) are affected by diffraction spikes, but brighter sources have larger spikes, which is why diffraction spikes appear obvious around the brightest stars but are unnoticeable in fainter ones.

I was reading this question about the JWST's diffraction spikes, and I was rather surprised by the magnitude of the 4 sets of diffraction spikes.The large hexagonal spike pattern I believe is formed from the honeycomb shape of the primary mirrors, while the small horizontal spikes are formed from the one supporting truss that isn't aligned with the hexagonal axes.

Furthermore, how would I go about calculating the diffraction spike pattern for an arbitrary 2d secondary mirror support structure shape observing a point source? My intuition tells me that this will be some kind of integral transform. There at least ought to be a way of approximating it without having to calculate the full wave equation propagating through the lenses.

Diffraction is easily (in the sense that your college professor will give it as a homework problem :-) ) calculated given the structure of any aperture. If you start with any optics textbook and read about Fresnell and Fraunhofer zones, you'll get the basic idea. For complicated structures, the solution is basically a superposition of the diffraction pattern from each aperture (for example, a single-slit pattern applied repeatedly for a row of identical slit apertures).

Photos of open star clusters always appear to be more pleasant when stars have diffraction spikes. But if your telescope does not have support vanes from a secondary mirror you are out of luck. One solution is to simply tape in a cross pattern some string or fishing line over the dew shield. Or you can turn to digital enhancement. Below is a procedure to enhance your photos by digitally adding diffraction spikes using GIMP 2.8. in 8 easy steps! No special plugin or filter required.

Finally, using the Ellipse Selection Tool, select the cross and Copy to clipboard. This will automatically assign it to the Clipboard Brush (red arrow and box below). Note that I have kept the screenshot of my previous version with the black background in the snapshot below to make it easier to see.

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