Astronomy with a cellphone — better pictures

Add a lens to improve results and do more things

Comparing pictures taken with just the Pixel 3a (top) and a Pixel 3a (bottom) with an added 60mm lens. Comet Neowise C/2020 F3, 15-Jul-21. Bright sky from local light pollution. Unprocessed pictures.

At public observing events, if I ask attendees what the most important function/feature of a telescope is, invariably the top answer is magnify. Those confident answer turn to quizzical looks if I respond “microscopes magnify hundreds of times or more and your view of (some object) was only magnified by 90, and microscopes are certainly much smaller than my telescope.”

If a microscope can magnify many more times than a telescope, why are telescopes so big? (Image: composite created from openclipart.org CC0 1.0 license)

Eventually someone figures out that the ability to collect light is the key aspect of a telescope. Deep space objects, being far away, are very dim. We need to collect a lot of photons emitted or reflected by those objects to make them visible to the human eye, or in the case of a camera, keep the exposure times reasonable.

It is the light collection issue that ultimately dooms a cellphone only approach to many astronomy activities. The cell phone lens is very tiny. Picture a rainstorm where each drop is a photon. If I want to collect water quickly do I place a thimble outside or a large bucket?

Collect more photons with a bigger bucket. (Image by user congerdesign from Pixabay CC0 1.0)

Unlike DSLR type cameras where different lenses can be attached, cell phones don’t offer any standardized method for attachment of lenses. Luckily some companies have stepped in to try and fill that void, though with the lack of standardization good options exist for only a few models of cell phones.

The most universal approach is the clip on lens; generally these are two parts, the clip and a lens that attaches to it with a bayonet type flange.

Typical cell phone lens clip to provide a way to attach a lens to the phone.

While the clip on lens may work for general use, experimentation has shown that they aren’t the best solution for astronomy use:

• A phone case can cause the clip to not keep the lens perpendicular to the phone. A thin or thick phone, even without the case, may not quite match the spacing of a “one size fits all” clip. Theoretically the side of the clip holding the lens would stay parallel to the phone, in real life the phone floats a bit between the two clip pieces.
• Getting the add-on lens optical axis and the phone’s camera optical axis perfectly centered is difficult, typically you need good light to see what you’re doing.
• A slight misalignment might not be noticed in pictures taken in daylight; astrophotographs might show annoying vignetting, coma, and other optical aberrations.
• When out taking pictures in the dark it’s easy to bump the clip.
• The clip covers part of the display, which might not be a problem when trying to manually adjust app settings.

Clip not holding lens optical axis perpendicular to the phone (exaggerated for illustrative purposes). Removing the phone case can solve this problem.

Clip covering part of the display

Luckily at least one company has come up with a solution: a phone case that is made to hold the lens aligned and centered. The downside is those cases are only available for specific models of phones.

The setup I use is a Sirui 60mm lens and a Moment phone case. Initially I had tried using the Sirui with the just the universal clip and hit all of the problems listed above. Unfortunately Sirui didn’t make a case for the Pixel but Moment did, and they both use the same mounting system.

60mm focal length add on lens example to compare diameter with the cell phone lens

No contest about which one will collect more photons from a galaxy 10 million light years away.

No endorsement of specific brands or products intended, the products used here are what I decided to purchase.