Star Hopping with PS Align Pro Push-To

PS Align Pro can now add Push-To capability to any telescope with just your compatible phone and with a simple easy to make adapter. A star hopping feature makes the most of your phone's electronic compass accuracy, navigating reliably to dim DSO targets via nearby bright stars.

PS Align Pro is familiar tool that I use as a polar alignment aid. Recently I discovered that it has many new capabilities that I'd been ignoring. Green has been added as a built in night mode color. Included are 20 useful calculators for the astronomer and astrophotographer. The extensive astronomical object databases also caught my eye. I realized with the interface from the daytime polar alignment function, PS Align Pro had the pieces for an electronic Push-To device. I contacted the developer, Dimitrios Kechagias, who already had Push-To on his to-do list. He volunteered to proceed, if I would help test the capability for him on the iPhone.

Push-To

A push-to aiming system on a telescope allows you to navigate directly to any celestial object. Your own muscles and a simple move left/right, up/down indicator substitute for power hungry slewing motors and heavy batteries. Precision time, GPS location, and automatic conversion of object location from celestial coordinates to horizon based azimuth and altitude take care of the mental work. Precision encoders measure the required altitude and azimuth angles of the telescope. The required electronics and encoders cost about half the price of a medium size amateur Dobsonian telescope.

Modern smart-phones with GPS, electronic compass and level sensors have the potential to replace these sensors and electronics with an app, but are they accurate enough for practical use? If the push-to not accurate enough to place a dim deep sky object reliably in the initial eyepiece field of view, a painful manual search of near-by locations may be needed.

Smartphone Push-To Mount

The Push-To function in PS Align Pro requires that the long axis of the phone be parallel to the optical axis of the telescope. I built a simple adapter to mount my phone in an Orion-Synta finder dovetail which works for all of my telescopes. Construction requires 4 parts:

  • a commercial aluminum Orion/Synta dovetail bar (mine came from Svbony on eBay)
  • a riser block to provide clearnace for the dovetail clamp screws
  • a plywood platform to support the phone
  • a flexible TPU case for the phone.

The plywood platform is about an inch short of the phone case size. This gap is placed at the top to allow the TPU case to flex for phone insertion and removal. The case, platform, and riser are all glued together and the aluminum dovetail is screwed into the riser. The final adapter is shown below

I tested with my Mak on an equatorial wedge as well as an Alt-Az refractor and Dobsonian with my phone mounted in the finder dovetail clamp using the adapter above.

Using Push-To

The electronic Go-To function is found behind the spiral galaxy button on the main PS Align Pro screen. Here you can select the databases to be searched, filters for types of objects, distance from horizon, total and surface brighness, and size. You can also configure units, thumbnails, and coordinates. You can also select how objects will be sorted, including the ability sort by distance from a specific location in the sky. Once these persistent settings are made you proceed to the object search screen by selecting "Show Data".

The Data/Search screen filters results as you type the name or catalog number that you are interested in. Once it is visible, select it to see more information and functions.

The expanded object data panel allows you to resort the results list by distance from the selected object and to Push-To the object location.

Electronic compass locations are often several degrees off. PS Align Pro has two functions that mitigate this error. The first allows you to correct for compass offset any time you Go-To an object. Make your first target be a bright star. If the star isn't centered in your field of views, center it and press the "Sync To Target" button. This will store the offset for future Go-To targets. You have to repeat this occasionally because periodic phone calibrations can invalidate synced offsets and offsets vary between targets far apart in the sky.

Hop-From

If your target isn't close by your current location, you are offered a choice of "Push to Direct" or "Hop from Star". Hop from Star automatically gives you a few options of a bright star near your target to Hop-to first. This gives you the opportunity to sync any local offset at a bright intermediate target. Once the sync is performed you Go-To the nearby final target. I found that this intermediate step makes the final target location much more accurate, almost always in a 1 degree eyepiece field of view.

Using Hop-From I was able to locate dim targets in a bright suburban sky like the Whirlpool Galaxy and the Ring Nebula. The image below shows the offset of the Whirlpool after a Hop-From Mizar and Ring Nebula after a Hop-From Vega

Equatorial mounts

Push-To is most intuitive with an Alt-Az mount. An equatorial fork mount can put my mounted phone at any angle around the telescope axis including upside down. The key is to ignore the arrows on the phone and remember that Left/Right and Up/Down are the directions relative to a person standing behind the scope. You may not be able to see your phone screen standing behind an equatorially mounted telescope. For this, PS Align Pro includes an Apple Watch app which conveniently shows the Push-To Screen on an Apple watch.

Sources of error

PS Align Pro gives an indication of compass calibration accuracy on the Compass and Push-To screens. If it isn't good, try swinging the phone in wide arcs to trigger a re-calibration. Avoid nearby magnets and ferro-magnetic material.

I expected my best results would be with my Explore Scientific AR102 refractor and Questar Maksutov catadioptric scopes which are all alloy and glass. The Vixen Porta II mount I used for my refractor, has a couple of powerful magnets in a hidden compartment to retain Allen wrenches. I had poor results with this mount until I added a steel magnetic shield over the wrench compartment. For my magnetic shield, I cut some steel sheet to match the shape of the rubber tool compartment cover and painted it to match.

I was pleasantly surprised to find Push-To with my steel tube 200mm Orion Dobsonian worked just fine without any special steps. Everyone has different equipment and locations. If you have compass accuracy problems, simple steps may be able to solve them. Make sure to calibrate your compass by swinging your phone in wide arcs. Consider steps like replacing steel hardware with alloy or stainless steel, magnetic shields, or relocating away from structures with ferromagnetic material like an iron railing.

Conclusion

The PS Align Pro app Push-To function with Hop-From adds useable Push-To capability to almost any scope with little cost and effort.

Content created: 2020-06-23

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