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Posted: 14 October 2010

Slow Shutter
Tomoki Kobayashi

The iPhone camera has no shutter speed selection capability so the user has no control over the length of the exposure. The "Slow Shutter" app is an attempt to overcome that limitation for low-light situations. The app has "shutter speed" settings of 1/15, 1/8, 1/4, 1/2, 1, 2, 4, 8, 15, 30 seconds and "Exposure Value" settings of -2.0 to +2.0 in 0.25 increments, and an Auto setting. If you use Slow Shutter on an iPhone 3, 3GS, or 4, there is also a HDR setting. You set these by using dials on the Settings display. You can also enable a timer to avoid image shake from touching the "shutter button".


Slow Shutter accomplishes the affect of long exposures by taking several images and "blending" (as the author calls it) the images into a single image. This is very similar to "stacking" of multiple astronomical images, except that it is done in-camera. The process actually works very well on some types of scenes. The images are reduced in size from the standard iPhone image size to make the multiple exposures for blending.


If you have been following my "Cassiopeia Observatory" reports, then you have seen my iPhone afocal astrophotography adapter and images of the moon, sun, some planets, some bright stars, and even the International Space Station. Sometimes, the results have been amazing. If you haven't checked out the reports, visit my "Cassipeia Observatory" web site and look through the iPhone reports on the "Reports" page.

As of result of my previous iPhone astrophotography, I was curious about using the app for some types of astrophotogaphy, like bright Deep Sky Objects (DSO). Although the author was dubious about the app working for astronomical imaging, he provided a promotional code and I purchased the app from the iTunes Store. I did initial testing on my Meade 8" LX200-ACF, but this report should be applicable to almost any size telescope. The tests were done using an iPhone 4. For my astrophotography testing, I used settings of 30 seconds and EV +2.0 (except as noted).

I first took some images of the inside of the observatory as a test of non-astrophotography use of Slow Shutter. This first (unedited, other than reduced scale) image is with the Apple "Camera" app:


Here is the same view but with the iPhone flash:


The same view taken with "Slow Shutter" using "30 seconds" and "EV +1.25":


As you can see in the Slow Shutter image, even minimal light can be used and provide a better image in low light situations than the Camera app (without flash). You can tell that mutiple exposures were "blended" as the image is blurred from handholding the iPhone during the exposures.

My first test DSO was M45, the Pleiades. Here is M45, afocal with 26mm eyepiece, taken with the Camera app:


And the same view taken with Slow Shutter, 30 seconds, EV +1.25:


Obviously, the star images were brighter with Slow Shutter. So, there was some hope that Slow Shutter might actually work for astrophotography.

The next night I did some further testing. My target was M13, the Great Globular Cluster in Hercules, using the 26mm eyepiece. Here I stumbled across a deficiency in my homemade iPhone afocal adapter. The current design does not have precise optical alignment when the iPhone is attached to the adapter. On bright objects like the moon and planets, this has not been a problem as I could easily see the object and reposition the camera over the eyepiece. However, on faint objects like DSOs, unless there is a bright star in the field-of-view, since the object can not be seen on the phone display, it is impossible to get a good optical alignment. I will have to redesign my afocal adapter. But that is being left for another day.

In attempt to increase the DSO brightness for imaging, I added the focal reducer to the telescope. I still couldn't see M13 on the display to know that I had good optical alignment. I then decided to slew the telescope to the bright star Vega, which is nearby M13. I could see Vega on the display and so I was able to align the camera lens properly over the eyepiece. I then slewed back to M13. I took an image using the Apple Camera app; post-processing showed no image of M13 was captured. I then took an image using Slow Shutter set for 30 seconds and EV +2.0. After Slow Shutter finished the in-app processing, I could see a faint "smudge" on the display that was M13. Here is the image (reduced in scale, with some post-processing editing done in Apple "Aperture" software on the Mac):


There is a star visible in the upper lefthand corner. M13 is the very faint small fuzzy patch just above the center of the frame and slightly to the left. Not a great image certainly but Slow Shutter did allow M13 to be captured!

I then tried to capture M57, the Ring Nebula, with the 26mm eyepiece + focal reducer. Nothing was visible on the display before or after the imaging. Post-processing also revealed that M57 was not captured. Too faint.

Next, was M45, the Pleiades. This was an easy target with its bright stars. Plus I knew I had captured it on the previous night. I decided to use a 40mm eyepiece + focal reducer to get the largest field-of-view and the brightest image. The stars were easy to see on the phone display. The image done by Slow Shutter showed several bright stars. Post-processing brought out some fainter stars:


My last target was the bright M42, Great Nebula in Orion. Here is an image captured by the Apple Camera app, 40mm eyepiece + focal reducer:


Some nebulosity is just visible around the Trapezium stars left of center.

And here is the Slow Shutter version:


Although the images are very similar, nebulosity is more evident in the Slow Shutter version. But neither image are of the same quality as what can be done when using a digital camera with full exposure controls.

This ended my initial testing of Slow Shutter. The concept of making long exposures on the iPhone by "blending" (or stacking) is a good one. However, since the exposures are in reality still short, only a minimum amount of light is actually captured. For bright objects like the moon and planets, short exposures with the Apple Camera app work great. For faint Deep Sky Objects, the Camera app will not work at all. Slow Shutter can capture bright DSOs but only in a very limited way. Using the iPhone for DSO astrophotography is just a curiosity at this point. Perhaps over time, improvements in iOS and Slow Shutter will increase the iPhone's astrophotography capability. But for now, we will need to limit our iPhone imaging to the sun (with proper protection), the moon, planets, and brighter stars.

Thanks to the author, Tomoki Kobayashi, for allowing me to try Slow Shutter for astrophotography. It was worth the effort. I will update this report when improvements in iOS and/or Slow Shutter allow for better astrophotography.

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