Tuesday, 12 August 2014

A simple Push-To controller for your telescope using an Android phone

I have two telescope mounts - a Celestron SLT Go-To Alt-Az, and a powered EQ2 basic German Equatorial Mount (GEM), which has no computer control capability. However, I have added Push-To capability to my EQ2 using my Android Smartphone, an app called SkEye https://play.google.com/store/apps/details?id=com.lavadip.skeye , and a means of attaching my phone to my telescope.

You are probably familiar with the "planetarium" apps which use your phone's sensors to show the night sky in which ever direction you are facing and pointing your phone. SkEye works like this, except it allows you to use what it calls "Indirect Mode", in which you attach your phone or tablet to the telescope, point the telescope at a target, and then tell the app what you are targeting, so it can build an angular offset into its sky view. The app then uses your device's sensors (compass, accelerometers and gyroscopes) to recognise all movements of the device, and thus the telescope that it is attached to.

What is very clever is that the app allows you to perform multiple alignments, and to re-align as the evening's viewing proceeds. The first alignment will work very well for targets within a few degrees, but when you slew right round the sky, chances are that you could be a few degrees off when you re-point the telescope. This is easily fixed - simply target and align a prominent star or planet in the general vicinity of the target you are seeking, align it (which adds it to your list of alignments, rather than erasing any previous alignments), and you should then have surprisingly accurate alignment and "Push-To" functionality in that region of the sky. In this way, the app then only needs to keep track of small phone / telescope movements over a small area of sky, rather than having to rely on a fully-aligned sky hemisphere. If the app's precision starts to "drift", just do another local alignment, and you're good to go again.

There are two versions of SkEye - a free Basic version and a paid Pro version. I think the Push-To functionality of SkEye is well worth the purchase price, so I have gone for the Pro version - 5-Stars! (Pun intended)

I fix my phone to my telescope using a very cheap "Gorilla Pod" type flexible tripod that I bought on eBay. It grips quite well on its own, but by adding a couple of rubber bands to really grip the legs around the OTA, it is really solid and secure. (It wouldn't seem like such a cheap option if your expensive phone falls of the telescope and smashes on the ground!)

The app developers point out that the phone should be mounted well away from any magnetic materials, to ensure the compass works properly. My OTA is aluminium, so that's not a problem, but the phone holder is mostly plastic, but with a bit of wire in the phone bracket. The compass can detect this when it is very close, but a bit of trial and error revealed that my phone's compass sensor is mounted near the top of the phone, and the app works well if I mount the phone in "landscape" orientation, offset to keep the top of the phone as far as possible from the mounting bracket, as shown in these photos.

You might need a bit of experimentation to find an arrangement that works with your phone and telescope. Other options for securing your phone / tablet could involve permanently fixing a cheap case to your telescope, using rubber bands or Velcro, etc.

Add this to my simple way to achieve a good polar alignment on an equatorial mount http://julianh72.blogspot.com.au/2014/08/handy-tool-for-polar-aligning-your.html and you have a very capable Push-To telescope mount, at next to no cost!