This was inspired by Adamlamee's iPhone 5 Telescope mount. I had been using a re-mixed version of his until I upgraded to an iPhone 6.
This was designed to fit a 32 mm eye piece. Printed with PLA It fit mine perfectly. If printing with ABS you may need to size it up slightly (something like 100.7%).
The astro picture is Jupiter and four of it's moons. Not perfect, but far better than the morning before when I had to hold my phone to the eyepiece. I can't wait till our moon is full so I can capture some great pictures.
Nothing special with the print settings. If your bottom layer is too flat you might have to clean up the peg holes a little.
I printed this in two pieces so no support would be needed. I highly recommend using some superglue to hold the pieces together. I suspect the pegs could break off pretty easily otherwise. Just a little glue on the surface of the ring that touches the main body should suffice.
I also used a rubber band to hold my phone in. The fit probably doesn't require one, but better safe than sorry.