Can you use an iPhone as a telescope finder?
Updated: Feb 18, 2019
If you have a telescope and a phone with the Sky Safari or other similar astronomy application, you might try mounting the phone on your telescope to help aid your observing. I purchased the Celestron Piggyback Mount: https://amzn.to/2DUnumI to use on my Celestron C6 along with a phone holder like this one: https://amzn.to/2DSQpHL in order to mount my iPhone on my scope that is on a manual alt-az mount (no tracking or goto).
While it is a little challenging to get the star map on the Sky Safari app aligned with what you see through the eyepiece (you have to keep adjusting the phone in the mount holder until it lines up), it is possible to get the image on the phone to match your eyepiece view. However, I found that it doesn't remain in alignment if the telescope is pointed at a different portion of the sky. If you swing the scope around to the opposite side of the sky, the image on your phone will be at least several degrees off from your eyepiece view, requiring alignment again. So, it fails as a finder by itself, but it does still prove to be useful as an observing aid by helping you identify objects and stars in the region you are looking at, plus I find that I end up looking at objects I wouldn't otherwise have observed. You might see an interesting object nearby on the phone screen and decide to take a look at that object before going on to your next planned target for the evening.
One downside is that it does place the phone close to your head when observing through the eyepiece so that it can get in the way. I am using a diagonal that threads directly onto the back of the telescope, and using a visual back and standard diagonal would put the eyepiece farther back, eliminating that problem.
The bottom line is that adding your phone to your telescope can aid your observing session, but it won't replace your finder completely.