Stellar Stars - The Star Of Records Mac OS

The 500 Rule

Stellar Partition manager is a proficient utility in order to implement basic partitioning operations on your Mac device without causing data loss. This software is capable of performing all the hard drive partition related tasks that you can run in your Mac disk utility and even more, but with better options for management and data security. The only Mac OS X only planetarium software is Voyager 4 by Carina Software. While I have not seen it in action, it promises to be a very powerful program that also offers telescope control. Cosmographia - A beautiful Solar System simulator for the Mac available from the App Store. The Yale Bright Star catalog is a good source for stars that have actual names (i.e., 'Polaris' instead of BD+50°1725). Do keep in mind that any star in this catalog is highly unlikely to possess human habitable planets. However, they are useful for naming star sectors, quadrants, or whatever. Here is the catalog, in Gnu Zip format.

One of the first challenges beginners face when getting into astrophotography, is being able to take a photos that is in-focus, with round stars. Because the night sky appears to move from our vantage point on Earth, capturing a long exposure starry sky image on a fixed tripod may reveal star-trailing.

One of the best ways to combat star-trailing when capturing astrophotography images on a stationary (non-tracking) tripod mount, is to use the 500 Rule.

What is the 500 Rule?

The 500 rule is used to measure the maximum exposure time you can shoot before the stars become blurry or before star trails appear. Setting the shutter speed for longer than allowed by this rule will result in images that do not have sharp stars.

The 500 rule can be useful when photographing the night sky on a fixed tripod. The technique works on images of many focal lengths (up to about 200mm), but can be especially effective when photographing the Milky Way with a wide-angle camera lens.

Brian Drourr used the 500 Rule to capture this stunning portrait of the Milky Way above Lake Moxie in Maine.

Whether you consider the 500 rule to be an outdated technique (after all, it was designed for 35mm film grain), or you’ve personally had great success with it, I had to cover this controversial topic on a website that revolves around astrophotography. I believe that the 500 Rule is still very relevant for today’s digital cameras, as long as you treat the guidelines as a rough approximation, not gospel.

Related Post: Looking for an affordable wide-angle camera lens? See my review of the Rokinon 14mm F/2.8.

Introduction

Astrophotography has been gaining in popularity over the past few years thanks to more affordable equipment, and helpful resources available online. Taking a photo of the night sky is a wonderful experience for both the photographer and the ones who get to enjoy the finished product.

However, the process of creating such an incredible photograph is not always easy. It is a very delicate task that requires a specific set of skills and knowledge that must be applied while capturing images in the field.

In the image below, you’ll see how the stars begin to “trail” after just 30-seconds of exposure time using a wide-angle camera lens. This should give you an idea of just how fast the sky appears to move from Earth.

A single, 30-second image captured using a DSLR camera and 18mm lens.

In a hobby like this, there are a variety of things that could go wrong while trying to capture the perfect shot. One of those things is star trails, which could make the stars in your image blurry, and is often not the effect you were hoping to achieve.

Star trails appear in photos due to the natural movement of the Earth, which makes the stars move rather fast, 15 degrees per hour, to be more precise. In order to avoid this, astrophotographers must pay attention to the all aspects of their camera system, such as the sensor size, the exposure time, the image resolution, the star angular speed and so on.

If you are just getting started in astrophotography, you likely do not own an equatorial telescope mount or star tracker yet. Thankfully, there is one simple rule which could replace the countless amount of preparation, the 500 rule.

In this stunning photo by Elizabeth Ford, a closer inspection reveals slight star-trailing after 20 seconds using a full frame camera and 25mm lens.

Why Photographers use The 500 Rule

The 500 rule is more of a guideline, but that does not mean it is not useful. This simple formula can make a big difference in your night sky photography, because (in theory) you’ll be able to create photos with sharp stars to the edges of the field.

It serves to discover the maximum exposure time allowed before the stars become blurry or before star trails appear. Setting the shutter speed for longer than allowed by this rule will result in an unclear photograph (the stars will appear as trails rather than dots).

Using this formula requires the knowledge of math, but you do not have to be a nuclear scientist to understand it and apply it. It is actually very simple but often the smallest changes make the biggest differences.

The following video by Mike Smith shows some great test shots using the 500 Rule with a crop sensor, and full frame cameras at varying focal lengths:

How it Works

The rule is as follows:

SS = 500 / (CF x FL)

If you are perplexed by this formula there is no need to worry, you just need to understand the abbreviations. SS stands for the shutter speed expressed in seconds, CF is the crop factor of your sensor (the ratio between your sensor and a full frame one), while FL refers to the focal length in millimeters.

500 / Crop-Factor x Focal Length = Ideal Shutter Speed

Here is an example of the formula used with my Canon EOS 60Da (APSC-C sensor) camera and a 50mm F/1.8 camera lens:

500 / 1.6 (Crop-Factor) x 50 (focal length of my lens) = 6.25-seconds

That means, using this camera and lens combo on a stationary tripod, I will need to limit my exposures to 6-seconds each if I want to avoid star trailing. To help collect more light in a short exposure, I’ll set the f-stop of the lens to F/2.8 (fast, but a littler sharper than 1.8), and use an ISO setting of 3200.

According to the type of camera you own, you will have to use different crop-factor values. Here is the list you can refer to:

• 1 X – Full frame cameras
• 1.5 (1.6) X – Nikon (Canon) APS-C cameras
• 2 X- Micro 4/3 cameras
• 2.7 X and higher – Compact cameras with a one-inch type sensor (or smaller)

Even though it is widely accepted, the use of the number “500” in this formula does not carry any particular meaning. This number is just something astrophotographers figured out works the best for this type of photos.

This formula works because it automatically calculates the ideal shutter speed for your camera in order to get the clearest possible photograph, and it does it in the shortest amount of time.

Let’s imagine you wanted to take a beautiful photo of the night sky without any previous knowledge of how to avoid star trails. You set your micro 4/3 camera’s shutter speed to 60-seconds using a remote shutter release cable, for example, expecting an amazing result. On the contrary, you end up with a blurry photo which doesn’t represent how nice the sky looks in reality.

The 500 Rule can give you a point of reference for the length of time you should expose the image using your camera system. It’s not an exact science, but it does workwhen capturing images like the one below.

A single exposure at 17mm using the 500 Rule with a crop sensor DSLR on a tripod. Up-close, the stars may trail slightly.

The Science Behind It

The basic idea is providing an easy formula which will guess how long the exposure time can be before the movement of the stars becomes noticeable.

The sky rotates 0.0042 arc degrees per second, or in simple words, 360 degrees in 24 hours. If you own a full frame camera and use a 24mm lens, its horizontal view will be about 73.7 degrees.

Let’s say this camera has a 24 megapixel sensor (6000 x 4000). The before mentioned 73.7 degrees are projected onto 6000 pixels, resulting in 81.4 pixels/degree. With this kind of lens, the exposure time will be about 21 seconds according to the 500 rule (500/24).

The sky will move about 0.09 degrees during these 21 seconds (0.0042*21). 0.1 degrees = 7.3 pixels with this kind of a camera (81.4*0.1).

Exactly this number of pixels (7.3) is the maximum acceptable movement blur before the stars as we see them in the sky become star trails in the picture. But can you really notice this movement in pictures?

Usually, we look at pictures on a computer screen. If you try to zoom in your full-resolution photograph to 100% you will notice that stars are not actually dots – this doesn’t actually matter because you would never notice this with a naked eye.

So, if you’re planning on printing large format versions of your photos, it’s worth thinking about. If you generally share your photos online (Flickr, Instagram etc.), the fine details become much less important.

A reference guide for digital camera sensor sizes. Wikipedia.

For Full Frame Cameras

The calculation is easiest when using a full-frame camera, although you should expect to have to cut down on exposure time a bit. Because you do not need to multiply the focal length by a crop factor, the formula is simply 500 divided by your focal length.

In the case of my Canon EOS 5D Mark II with a 24mm lens attached (a promising camera and lens combination), the formula is 500 / 24 = 20.83 seconds. That means, I could expect to see stars that are mostly still in a 20-second exposure.

20-seconds is probably a little ambitious, so I typically would shoot a few seconds less than that. An 18-second exposure is no slouch, but I’d still recommend using a the lens “wide-open”. For my Canon EF 24-105mm lens, that value is F/4.

Lastly, I’d suggest using ISO 1600 or higher if possible, and using image stacking to help reduce noise afterwards (more on this, soon). If you’re looking into a versatile lens for astrophotography, I can’t say enough about how much I love the 24-105.

The Canon EF 24-105mm F/4L is an excellent lens for astrophotography. (photos shown used a star tracker!)

For Crop Sensor Cameras

There are two types of crop sensor cameras – Canon and Nikon. The Nikon cameras’ crop factor is 1.5 so by using the 500 rule you get this result:

500/ FL / 1.5

So, if you’re using a 50-millimeter lens, the formula will look like this:

500/ 50 / 1.5 = which will result in 7 seconds of exposure.

The crop-factor for Canon cameras is almost the same as Nikon’s – 1.6. The formula should look like this:

500 / focal length / 1.6.

Now, if you use the same 50mm lens, the formula would be:

500 / 50 / 1.6 = resulting in 6 seconds of exposure.

Shutter Speeds

It is important to remember that this rule is not a perfect solution and that slight adjustments will have to be made according to circumstances. Factors such as light pollution, atmospheric haze, and the angle of the stars are something you should keep in mind.

Here’s a chart that could be useful to you regarding shutter speed in general.

Stellar Stars - The Star Of Records Mac Os Catalina

Does the 500 Rule Actually Work?

The above reference chart is a great starting point, but do these shutter speeds actually work in the field? The answer is, sometimes.

I asked this question to the AstroBackyard Facebook group, to see what amateur astrophotographers around the world had to say. There was a mix of responses on the topic of whether the 500 rules actually hold true or not, and the general consensus was that it does a great job most of the time.

Experienced photographer Craig Stocks noted that taking an image free of star-trailing using the 500 rule will depends on the resolution of the camera, and the quality of the lens. The higher the resolution of the system, the more likely you are to see star trailing.

Stellar Stars - The Star Of Records Mac Os 7

In the example below, he used a Sony A7R2 and Sigma 14mm f/1.8 Art lens at ISO 2500 for just 20-seconds. Even 20-seconds is actually a shorter exposure time than the 500 rule would allow, star trailing is visible in the image when viewed at 100%.

In a lower pixel resolution camera, this effect is not as pronounced, and the 500 rule produces a better result.

It’s also important to mention that capturing images close to the horizon will pronounce the effect, as the stars will appear to be moving slightly faster from your vantage point.

The bottom line is, the 500 rule should be considered to be a great starting point, but it is not a precise measurement, and you will need to experiment using your specific camera and lens system.

The NPF Rule

While searching for a definitive answer as to whether the 500 rule (or 600 rule) is still relevant with today’s cameras, I was pointed towards this article discussing the NPF Rule. The NPF Rule has been added to the PhotoPills app to provide a real-time shutter speed recommendation.

They call it “Spot Stars” which is essentially an NPF Rule calculator.

The Spot Stars Calculator found in the PhotoPills app.

The author notes that larger sensor cameras such as the Nikon D810 do not produce acceptable results using the 500 Rule, especially if you plan on printing the images in large format. The original NPF Rule was developed by Frédéric Michaud, and the formula is as follows:

(35 x aperture + 30 x pixel pitch) ÷ focal length = shutter speed in seconds.

If you don’t know what the pixel pitch is (measurement in microns), just divide the sensor’s physical width in millimeters by the number of pixels in width, and multiple by 1000. For example, the sensor in my Canon EOS 60Da has physical a width of 22.3 mm, and a sensor resolution of 5196 x 3464 (4.29 µm).

So, the formula for my Rokinon 14mm F/2.8 lens would be 35 x 2.8 / 30 x 4.29 = 14.01 seconds. That’s a lot shorter than the recommended exposure length of 22 seconds using the 500 Rule!

The Power of Stacking

If your goal is to improve the quality of your photos, you should use always RAW format instead of JPEG. This simple change will provide you with flexibility which will come in handy when editing your photos. If you are not using a tracking head to track to the movement of the sky, the stars will never be in the same position.

The amount of light you collect for one pixel depends on how long a star stays in one place. Increasing your ISO is not always a good idea either, because it can significantly increase the amount of noise present in your image.

The best solution to improve your image quality in the departments of greater signal (light), smoother details, and overall less noise, is image stacking. Stacking your astrophotography images is one of the most powerful things you can do to reduce noise, and it’s really not that hard to do.

In the following video, I use Adobe Photoshop to manually stack a set of 30-second images to improve the quality of my image.

The first step is to take a set of photos at a relatively low ISO, following the 500 rule. The next step would be combining or stacking all of these photos afterwards in order to drastically enhance all of the little details in the final photograph.

This procedure requires you to align the sky according to all the exposures, and optionally calibrate the photo, but certain software such as DeepSkyStacker, Starry Landscape Stacker, and Sequator can facilitate this and make the process faster and easier.

As time consuming as the process may be to manually stack images in Adobe Photoshop as shown in the video above, I enjoy the experience very much. There is something about seeing your image improve slightly over time that is very satisfying!

Image stacking is a central step when it comes to editing in astrophotography. We would not be able to witness the beauty of night sky through pictures in this quality we have to today without utilizing this technique. Increasing the ratio of signal to noise gives a much cleaner photo.

For foreground details, you may need to mask this area and blend in a separate exposure highlighting this area. In the example below, a single image of the terrestrial landscape was blended in to replace the blurry result from registering the stars in the image.

If you plan on stacking your images, be aware that the ground will be blurry in the image unless you blend in a stationary version.

Useful Tools

Here are some of the many stacking software options available to you. I tend to use Adobe Photoshop to stack my un-tracked images, or DeepSkyStacker. Adobe Photoshop has an image stacking script option to automate the process, and it is worth checking out.

⦁ Sequator – It is great to use if you are a Windows user because it is free. It makes the process of image stacking extremely easy and you will end up with a beautiful photograph in no time. It also supports RAW format which is important when trying to achieve the best quality.

⦁ Starry Landscape Stacker – This software is available only for Mac OS X users and there is a free trial period. You won’t have to worry that your pictures will turn out anything less than great when using this software. The only con about the software is that it doesn’t support RAW files, so you have to convert them to TIFF using Adobe Camera RAW, or similar software.

⦁ DeepSkyStacker – Another software that is supported on Windows. It simplifies the process of stacking by doing registering, simple post stacking processes, and saving the final result to a TIFF file.

Star Trail Images

What about times when you want to show star trails? Star trail photography can result in beautiful portraits of the apparent movement of stars in the night sky like the image below. The process involves capturing images with much longer exposure times than you would use for the 500 rule.

In this star trial image by Quentin De Meur, the photographer intentionally allowed the stars to trail and stacked several photos together.

Photographers will use the longest exposure time possible (without blowing out the highlights), to capture the most amount of movement in the sky as possible. In many cases, this is a 30-second exposure on a fixed tripod. The images are then blended together in Adobe Photoshop using the “lighten” blend mode to create ultra-long star trails over time.

Stellar Stars - The Star Of Records Mac Os Download

The 600 Rule

This is another variation of the 500 rule formula, The only difference is that 500 is being replaced by 600 – everything else stays the same and the formula functions in the same way. The result is a slightly longer exposure time.

If you are using a full frame camera then the equation will be : 600/ FL = SS

However, with slight alterations and taking the crop sensor sizes into consideration, this formula can be adjusted in order to fit all the other types of frames.

If you are using a crop sensor camera such as Nikon or Canon you need to add the crop sensor value into the equation: 600/ FL / CF = SS

Again, this is just a rule of thumb and it might not work in all situations, but it could be worth giving a shot.

Conclusion

The 500 Rule is not the be all end all solution to capturing the perfect image of the night sky, but it is an extremely useful point of reference. Many beginners (myself included), start with a crop-sensor, entry level DSLR camera and kit lens (usually 18-55mm or similar).

For users in this situation, I believe the 500 Rule is an excellent formula to try out the next time you set up your tripod. If you have a high-resolution sensor with 30 MP +, you may want to dial back the 500 Rule suggestion, or look into the NPF Rule.

By simply by understanding how this formula works, you will have a better understanding of the process of capturing photos of the stars at night, which are essentially moving targets.

Whether you are using a full frame camera, or a micro 4/3, the 500 rule situations. is a practical benchmark in many situations. When used in combination with image stacking and different software tools, capturing a beautiful image of your favorite constellation or the Milky Way is within your reach.

This astronomy software page contains links to sites with PC software of interest to the astronomy hobbyist. These programs include useful astronomy utilities and programs that are educational and fun. Some of this software is free and some is commercial or shareware. There are also a few demo versions of some commercial software products.

Each entry contains a description of the software type as well as the last known price. Please read the information on each publisher's site for more detailed information. These astronomy software links are provided for informational purposes only. Sea and Sky does not endorse these products.

Free Software Shareware Commercial Software

• Astroviewer
  Platform: Windows 98 and above Cost: Free Demo: Download available
  AstroViewer is a planetarium software that helps you to find your way in the night sky quickly and easily. Due to its intuitive and easy-to-use graphical user interface, it fits well to the demands of astronomy beginners.

• Celestia
  Platform: Windows, Linux, Mac OS X Cost: Free Demo: N/A
  Celestia is a very unique 3-dimensional universe simulator. With it you can travel throughout the Solar System, to any of over 100,000 stars, or even beyond the galaxy. Celestia comes with a large catalog of stars, planets, moons, asteroids, comets, and spacecraft. If that's not enough, you can download dozens of easy to install add-ons with more objects. The program is highly configurable and expandable. A library of add-ons and expansions is available including everything from additional stars & galaxies to science fiction worlds and spacecraft. For those with fast processors, you can even download and install super high resolution images of the planets and moons enabling you to zoom in close and explore every subtle detail. You can even take screen shots of your favorite scenes. Celestia is highly recommended by Sea and Sky!

• Home Planet
  Platform: Windows 95 and above Cost: Free Demo: N/A
  Home Planet is a comprehensive astronomy, space, and satellite-tracking package for Microsoft Windows 95 and Windows NT 4.0 and above. Features include an Earth map showing day and night regions, position of selected satellites, positions of the planets, positions and phases of the Sun and Moon, sky map based on either the Yale Bright Star Catalogue or the 256,000 star SAO catalogue, including rendering of spectral types, planets, Earth satellites, asteroids and comets, and much more.

• Stella 2000
  Platform: Windows 95 and above Cost: Free Demo: N/A
  Complete astronomy software suite exploring realistic skies in real time, with observing log, Sky Quiz, Live Orbits, telescope support, spoken pronunciation guide, a half-million word Encyclopedia Astronomica, and concentrated searches embracing planets, comets, asteroids, DSOs, and over 300,000 stars. An integral HTML guide to the solar system, a 1000-term astronomical dictionary, and the 2nd revised and enlarged edition of Aspects of Astronomy--a book-length primer covering topics such as 'What are the Stars?', 'Choosing a Telescope', 'Cosmology', 'Dark Matter', 'Eclipses', and 'The History of Astronomy', to name but a few--are closely coordinated with the sky display and picture windows.

• Stellarium
  Platform: Windows, Linux, Mac OS X Cost: Free Demo: N/A
  Stellarium is a free GPL software program which renders realistic skies in real time with openGL. It is available for Linux/Unix, Windows and MacOSX. With Stellarium, you really see what you can see with your eyes, binoculars or a small telescope. Loaded with advanced features, this incredible software will turn your PC into a virtual planetarium! This is definitely one of the best free astronomy programs currently available for download. Stellarium is highly recommended by Sea and Sky!

• WinOrbit
  Platform: Windows 3.1 or later Cost: Free Demo: N/A
  WinOrbit is a free software package for Microsoft Windows (3.1 or later), which will compute and display the position of artificial Earth satellites. The principal feature of WinOrbit is a series of Map Windows, which display the current position of satellites and observers on a simple world map, together with information such as bearing (azimuth), distance, and elevation above the observer's horizon. The maps can be updated in real time, or in simulated time, or manually set to show the situation at any time past or future. An additional Table Window displays much more-detailed information about one or more satellites in a tabular form.

• Cybersky
  Platform: Windows XP and above Cost: $34.95 Demo: 30-day trial download available
  CyberSky is an exciting, entertaining, and educational astronomy program that transforms your personal computer into your personal planetarium. CyberSky provides an excellent way to learn about astronomy and to explore the wonders of the sky visible in the distant past, the present, and the far-off future. CyberSky displays accurate charts of the sky as seen from any location on the Earth. Sky charts can include stars, constellations, deep sky objects, and solar system objects, and can be enhanced by the addition of labels, coordinate system grids, and reference lines. CyberSky's user-friendly interface allows you to easily change your view of the sky, search for celestial objects, and display data about those objects. CyberSky also prints attractive sky charts that you can take outside with you.

• StarStrider
  Platform: Windows XP and above Cost: $22.00 Demo: 30-day trial download available
  StarStrider is a three-dimensional star plotting program that allows the user to see the stars and constellations from distant points in the sky. With the help of StarStrider you will be able to travel to the stars and watch their alien skies. Travel to Vega and see how the Sun pales to a faint star, a star among thousands of others. Look at the Pleiades from behind! With ordinary red/blue 3D glasses your experience will be even greater. Now you can see the variable distances to the stars - without even leaving our solar system if you don't want to! Cassiopeia looks really different when you realize that the constellation aren't flat. You'll also learn to appreciate the fact that the closest stars are not always the brightest.

Stellar Stars - The Star Of Records Mac Os X

• MegaStar Sky Atlas
  Platform: Windows 95 and above Cost: $129.95 Demo: N/A
  MegaStar is the first software to integrate the Hubble Guide Star Catalog, and combine it with a massive deep sky database of 84,000 objects. It is also the first to implement the 'eyepiece view,' with deep sky objects plotted to scale and galaxies rotated to show position angle. This is an extremely detailed visual sky atlas program with too many features to mention here. Check out their site for more information. A demo version of the software is available for download.

• Starry Night
  Platform: Windows XP & above, Mac OS X 10.4 & above Cost: $24.95 - $239.95 Demo: N/A
  Starry Night is the most visually stunning and realistic astronomy program in its class. A powerful tool for both serious observers and casual stargazers, Starry Night lets you view the universe from anywhere in the Solar System. Explore over 19 million celestial objects and travel across 14,700 years of night skies. Features include a 19 million object Hubble Guide Star catalog and the ability to add new objects and databases. An evaluation version of the Basic version is available for download.

Glossary of Terms Astronomy FAQ Messier Catalog Astronomical Calendar
Events & Star Parties Clubs & Organizations Equipment Guide Astronomy Software
Astrophoto Gallery Astronomy Suppliers Astronomy Resources My Sky

Stellar Stars - The Star Of Records Mac OS