Astrophotography 101 : Capture Planets using a Smartphone. Part 1

Photography is accessible to everyone now with a decent smartphone as they are all equipped with Cameras that are more powerful than the dedicated cameras of yesteryears. Today, everyone wants to try Astrophotography as it’s an exciting thing to see the images of planets and deep sky objects in detail, captured by us humans, using cameras. So let’s see how to capture and process planets using a smartphone on a small telescope. 

Yes! You need a small telescope as the Cameras do not have enough optical magnification to capture planets in detail. But using this guide, you can capture planets and obtain the details without spending a fortune on Planetary Cameras. 

Let’s list what we need first…

Tools:

The Method

Now that you’ve got all the necessary ingredients, let’s begin…

We begin, first, by setting up the smartphone onto the telescope. Attach the smartphone to the scope using an adaptor bracket.

Set the camera to Video capturing mode. Yes! Video. While we can capture several images and process them, video capture does the same with 24/60 frames shot within a span of one second, giving us the images we require without disturbing the setup. 

You can also go FULL DIY and skip buying the adaptor bracket and make one yourself,  like Bharat Achar did for his phone.

The Bahtinov Mask

Fix the Bahtinov mask in front of the telescope tube opening and ensure that it is a good fit. Now, point the scope to some bright star in the sky. Any star will do as long as it is bright enough.

When you look at the ‘live-view’ on the phone camera app, you’ll see a 6 pointed star. These are the diffraction spikes for the star you are pointing your scope to. 

Adjust the focuser so that the seeking spike is at the exact center of the spikes that are shaped like the letter ‘X’. This point is the point of best focus.

Why Stars when capturing planets?

The planets appear way bigger than stars when looked at through a telescope; and also since at this magnification, the atmospheric turbulence is so much that it is quite literally like looking at a pebble through the flowing water. This makes it difficult to focus planets to picture perfection and be sure about it. Bahtinov mask allows you to verify focus at micron-level precision using diffraction spikes from a point source (a star).

Focus.. Check. Next?

Now that the setup is at its finest focus, point the scope to the desired planetary object without disturbing the focuser.

Capture a high resolution, high frame rate video of the planetary object. We recommend you change these settings before we begin. If  your phone records at 60fps at 720p, or 24fps at 1080p… set the highest option.

As mentioned above, due to atmospheric turbulence, every other photograph taken of a planet, unless from a space telescope, is going to be distorted. Depending on various conditions, you can expect only 1 to 5 out of every 10 photographs to be distortion-free. This is the reason we capture a video instead of a photograph.

At 24fps, a 10 second long video will capture 240 images of the planet with just two taps on the app. With 60fps, that 10 seconds long video captures 600 images, thus capturing thousands of photographs in form of its frames in a short video. This method is called Lucky Imaging.

But the Sky Moves!

No! You need not have a tracker for this project.

To manually chase the object throughout the video: Move the scope so that the object is in the boundary of the field of view (FOV); wait till the object moves across the FOV with time; Move the scope again to place the object in the boundary like before.

Do NOT attempt to move the scope continuously. It can likely make your entire footage shaky.

The frames of video captured while you wait in between movements are essential data here.

Capture Capture Capture!

We need around 1000 such frames for good results. Given a smartphone captures at least 30 fps at full HD, or whatever is the frame rate you’ve set the camera to, you can calculate and figure out how long a video do you require. 

That’s just  the minimum. You can record longer videos, and the longer, the better. 

While most  smartphones capture videos in MPEG-4 ( .mp4 ) format, an Audio Video Interleave ( .avi ) format video is preferred. A .mp4 video will still do the job. So just set the camera and capture. 

Bahtinov Mask

The shape of aperture and the components of a telescope have various effects on images formed by an optical instrument.

A Newtonian reflector telescope, will have its secondary mirror inside the tube supported by an arrangement we typically call a Spider. This arrangement may be a 3-leg spider or a 4-leg spider or even a 3-leg curved spider. These arrangements are designed to not interfere with the light from the star, but they do, although minimally, obstruct the path of the light. 

This minute obstruction causes what’s called star spikes. You can also see in the image, that the secondary mirror itself obstructs incoming light. This causes low contrast images due to wavelet diffractions (detail is beyond the scope of this project). But, a Russian amateur astronomer Pavel Bahtinov exploits the principles of aperture shapes, and diffraction patterns caused by them to make something very useful.

The Bahtinov Mask is a very simple tool, which you can make yourself. the design varies depending upon the focal ratio  of your telescope, or so it seems.  Astrojargon has simplified the rigorous calculations to be put into designing it. You can simply download the latest release of the Mask Generator and input your telescope’s focal length and aperture and it will generate the mask for you. Use the app, generate the mask and print it  out on a sheet of white paper to proceed..

Using printed sheet, trace and cut some thick opaque sheet of material. A thick cardboard or foamboard will do the job. For small telescopes of 4″ aperture, a sheet of black paper can also be used conveniently.

Once you cut this mask, place it at the opening of your telescope. Point the scope to a very bright star, and you will see diffraction patterns around it. When focuser is varied, this pattern shifts. Refer the following gif to see the best focus point, where the spikes are all aligned.

Clear Skies!

So head-out and capture the planets visible in the night sky and save the footage onto your computer. Capture as many videos, and as long, as possible and come back for the Part 2 of the post on Planetary imaging using a Smartphone.