Tag Archives: diffraction spikes

Adding Special Touches to Your Astro Landscape

1000 ISO, f/2, 3 minute exposure with some augmented stars

Because stars are pinpoints of light, the camera does not capture them as our eyes see them. To our eyes, brighter stars stand out more noticeably than dimmer ones. At a workshop in Alabama Hills, one of the participants, Julian Köpke, was using a diffusion filter so the stars captured would look more like you see with the naked eye. Sometimes nature provides its own diffusion filter in the form of high, thin cirrus clouds as shown below. The large bright orb is the star Sirius in the constellation Canus Major (Big Dog). The orange star near the top of the frame is Betelgeuse in the constellation Orion. One nice thing about the blur that the clouds added is the star color is more noticeable. But the diffusion here is not uniform because the belt stars (Alnitak, Alnilam and Mintaka) and “corner” stars (Bellatrix, Rigel, Saiph) in Orion are all noticeably brighter than the surrounding stars while in this photo only Betelgeuse and Rigel stand out.

Dog Star [C_065586]

You can create a make-shift diffusion filter by shooting through a nylon stocking – or buy a diffusion filter. The disadvantages of using a filter are that everything is blurred – including the foreground and you reduce the amount of light collected. Most night sky photographers try to avoid clouds and you will get an image like this:

The moon and Teapot Asterism in Sagittarius – over Lone Pine Peak – as shot.

When what you had in mind is something like this:

Same Photo as above, but with the Teapot Asterism in Sagittarius enhanced.

How to Bring Out Star Color And Enhance The Apparent Star Size

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.

NOTE: Star Spikes Pro and HLVG described later are currently only available on Windows machines.

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 first time I tried to use Star Spikes Pro it did not quite work as I expected.

Look hard. Star Spikes Pro decided the moon was a huge star outclassing all others.

Indeed it took me a bit to realize what was going on. The good news is it was easy to work around. The huge moon looks like a huge star to Star Spikes Pro – and that makes perfect sense since the plugin is usually used with Astrophotography that does not involve landscapes.

Here is how I made it work as I wanted and limited the effect to just the desired stars.

Layer Palette and Steps to Enhance The Teapot Asterism

Above left is the layer palette. Look carefully and you may spot the fix. After loading the image (1) I first duplicated the original and called the new layer Heal (2). I then did minor contrast adjustments, used the healing brush to remove hot pixels and other offenses (short satellite trail). Next I duplicated the Heal to another layer (3) and fed it into Hasta La Vista Green – a free plugin written by Rogelio Bernal Andreo of DeepSkyColors. HLVG removes green which is an unnatural sky color usually caused by RGB artifacts. HLVG operates on the entire layer and does not know the difference between land and sky. To leave the natural green in my landscape I used the quick selection tool, dragged it across the sky followed by Select -> Modify -> Expand 4 pixels. Then I created a Layer Mask using “Reveal Selection” (4). That made the foreground come back to its normal state. If you look carefully you will notice I also used a white brush to add some of that green removal back onto the mountain by painting on the HLVG layer mask (4).

The next operation was a finger twisting sequence that has no menu equivalent: Ctrl-Alt-Shift-E (on Mac that’s Command-Option-Shift-E). What that sequence does is “flatten” all the visible layers and create a NEW layer in the process (5). That layer I called Input to SSP.  Since I had discovered that Star Spikes Pro was confused by the moon (and could be confused by the foreground), I used the quick selection tool again and brushed it across the foreground. By default using the quick select tool again ADDs to the current selection so I brushed it around inside the moon and its halo. At this point I did not need to create another layer (Ctrl-J/Command-J or Duplicate Layer) but I did so that it was easy to see what happens next. After creating the new layer I selected it and used the delete key. Delete removes the selection making it transparent – that is the foreground and moon were now gone (6).

Next up: let Star Spikes Pro loose on the image. First deselect (Ctrl-D Command-D) or Select -> Deselect), and feed the sky layer to Star Spikes Pro via Filter -> ProDigital Software -> Star Spikes Pro.  The defaults for SSP produced the image below (I’ve zoomed in on the teapot asterism)

I felt the color was a bit too strong, and I did not want the diffraction spikes. The next step was to select “Advanced” – just below Settings, set the Primary quantity to zero. Next was the Secondary tab where I reduced the quantity to 44, the intensity I bumped up to 23. Soft flare I set quantity to 12, bumped up the intensity, dialed down the size a little and dialed down the Hue to -21. These adjustments were all based on eyeballing the image and were made for aesthetic appeal.  After all the adjustments looked about right, I saved the settings as a new adjustment I called “DiffusionOnly”. Finally I clicked OK and my layer was all nicely done by the SSP filter.

The filter processed a few more stars than I intended to augment. The simple solution was to create a “Reveal All Layer Mask”, select a brush, the color black and paint out all the effects I did not want on the layer mask (7).

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.

The topmost layer in the layer palette is my watermark.

There Is An Easier Way!

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.

Also, instead of masking off the stars I did not want affected after the fact, Star Spikes Pro has two tools to greatly simplify things the: “Hide” tool to turn off any effect that I did not want, and the “Show” tool to turn the effect on.

 

Star Spikes Pro limited to specific section of the sky via a selection and using the Hide tool to turn off an effect.

 

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.

If you’re wondering whether there is a way to get the diffusion effect on a Mac or without purchasing Star Spikes Pro, there is, but it requires a lot of Photoshop twiddling and it is not anywhere near as pleasant as using ProDigital Software’s Star Spikes Pro.

Disclaimer and Book

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

Notes from The Stars: 10 Award Winning Authors

Tip: Star Bursts

An artifact sometimes called a star burst or star filter and sometimes incorrectly identified as lens flare consists of spikes of light that radiate from bright light source(s) as in Photo 1, below. These sometimes pleasing spikes are not that difficult to achieve without using tools, or filters, or image manipulation.  Here we explain how to get these potential eye-pleasers, and also how to avoid them.

Driven to Diffraction [5_020199]

Photo 1: Diffraction Spikes in a Night Scene. Taken with a 7-blade aperture at f/14

First, let’s call these spikes what they are. The lines radiating from a bright light source – e.g. the streetlamps in Photo 1 – are diffraction spikes. When light encounters an edge it warps around the edge just as a wave in the ocean can flow around a boat. Where are these edges in my camera? Should I fear for my safety? The sharp edges are inside your lens! The mechanism, sometimes called an iris,  that controls your aperture is made from a set of 5 to 15 blades that open and close to change the size of the opening in the lens. This opening controls how much light is allowed to pass through your lens onto to your sensor or film.  Usually photographers refer to this opening as their f/stop.

 

f-stops or f-numbers refer to the size of the opening in the iris as a proportion of the lens diameter and focal length. It is not all that important to know – or even understand what this means, but if you want to dig in deeply, I suggest Matthew Cole’s “A Tedious Explanation of the f/stop“.

If you, like me, are wondering how reflection, refraction and diffraction are related – here is a succinct definition from PhysicsClassroom:

Reflection involves a change in direction of waves when they bounce off a barrier; refraction of waves involves a change in the direction of waves as they pass from one medium to another; and diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path.

So in short, refraction is what your lens is designed to do: pull in light and focus it using multiple lenses. Internal reflection causes flare and is mostly undesirable. Diffraction is also an unintended consequence of lens design – unless you want that star burst. When “stopped down” (larger f/stop numbers) diffraction produces noticeable spikes. To make spikes more prominent increase the f/stop to f/11 or higher. How much depends on the construction of the lens.

The number of spikes created is unique to each lens and depends on the number and shape of the blades in the lens. Spikes always appear in pairs. An even number of blades produces an equal number of spikes. In Photo 2 below you can count 8 spikes. Many lenses have six blades and thus produce six spikes. Lenses with an odd number of blades produce twice as many spikes as blades – so Photo 1 may have been taken with a lens having 14 blades (unlikely), or a lens with 7 blades which is correct for the the Canon 16-35mm 2.8 L II lens.

Of course you can also buy star filters, if you wish. But stopping down is sufficient. There is one more simple way to produce diffraction spikes: place tiny dark threads (or hairs) over your lens.  Two threads at right angles will produce 4 spikes.

Note that when you “stop down” (use large f/numbers) the dirt and dust on your sensor will become more apparent as small dark dots or lines. It is also true that the stronger the diffraction the less sharp your image will be overall.

GEO ism [5_030572]  + TRIVIA Contest!

Photo 2: Diffraction spikes from a lighthouse. Notice how many spikes?

What if you do not want those spikes? Answer: keep the aperture as open (wide) as possible. Photo 3,  shot at f/5.6, shows almost no diffraction spikes.  Do not be confused by the radiating beams from the top of the lighthouse. Those beams are from the Fresnel lens in the Pigeon Point Lighthouse which throws out focused light in 24 directions simultaneously.  Notice in Photo 3 that the moon has some vague spikes but the bright lights in the windows and doors show almost no sign of diffraction spikes unlike Photo 2.

 

 

 

Photo 3: This f/5.6 photo reveals no noticeable spikes from the windows and the moon has barely noticeable spikes. The spikes from the lighthouse are from the 24-beam Fresnel lens - not diffraction.

The Contest Results

Here was the two-part question we asked in the Trivia Contest.

  1. What caused the starburst effect seen (in photo 2)?
  2. Without resorting to photo editing is it possible to get a different number of spikes? If so, how?

More than seven contestants provided answers to the trivia challenge. We scored each answer as follows: Full credit was awarded if both questions were answered completely. a score of 50% was achieved by clearly identifying stopping down and diffraction as the cause of the starburst effect.  An additional 50% was awarded if the answer mentioned changing lenses having a different blade configuration as the means for changing the number of spikes.   Partial answers got partial credit, so for example one answer to “How do you get a different number of spikes” was “stop down further” – that scored zero points – stopping down further may make the spikes more pronounced but does not change the number of spikes. The answer “The number of spikes is based on the number of blades in the lens” we scored at full credit even though it doesn’t mention swapping lenses as we felt that switching lenses was implied.  Final scores ranged from 40% to 90% correct.  For the purpose of this trivia contest any score over 65% was deemed correct.

Those answers scoring 66% or higher in the order they answered were:

Congratulations to Brian who is signed up for the November Star Circle Workshop in Lone Pine, CA. If Jack or Deborah attend they will receive a $25 rebate – and that is in addition to the current early sign up discount.

Discount registration expires on April 30, 2011Sign up soon to save yourself $100.