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Tuesday, November 17, 2015

Finding Celestial Targets -- With Your Cell Phone

How To Locate Celestial Targets

Hint -- near the end of this page you'll find an easy way to find targets with your telescope, but to start, let's cover the familiar star object finding methods.

Finding Stars -- The Common Way



Above is a clip of a star chart from the Xephem planetary program. The red labels have been added. It's a chart of the constellation Hercules, and shows a couple of favorite dropping off points for galactic stargazers: M92 and M13. These favorite Messier objects are Globular Clusters, amazingly compact cluster of stars visible in amateur astronomer telescopes.

Charts like this one have often been used to help find these particular targets, and similar charts to find other targets. Buy large numbers, amateur astronomers in the past located such targets using star charts and the Star Hopping technique. With the advent of so many affordable computerized telescopes nowadays, like the popular Celestron 60LCM Computerized Telescope and the Celestron NexStar 5 SE Telescope, star hopping isn't as popular as it once was.

But users of Dobsonians, like the model at left, and a number of portable 60mm to 70mm refractors and some 90mm Maksutov table-top telescopes still adhere mostly to the star hopping method of finding targets. It involves locating some guide stars near the targets of interest, and  using them as a reference. If the guide stars are naked-eye visible, so much the better.

The technique involves peeking through the finder and locating the guide stars, which may or may not quite all be in the field at the same time. Then, by studying the chart, an estimate is made of where the target is with  reference to these guide stars. In the Hercules diagram above, one might use the reference stars labeled R1 and R2 for example, and notice that M13 is about 1/4 of the way from R1 to R2, almost on a line between the stars.

Thus, one would use their finder telescope or sight to position the view at the best estimate of the target point, then finish finding the target by looking through the main instrument, perhaps  making necessary small sweeping moves to locate the target.

This works rather well, and some accomplished amateurs do so much observing that they know the reference stars and target positions without charts for most of their favorite celestial objects. But for less experienced observers, it can take some time to find what they're after, which may limit them to finding only a few targets in an evening of observing.

A Seldom Used Alternative



Most telescopes purchased with Equatorial Mounts have setting circles on the mounts. The setting circles are easily seen on the image of my ETX 90 above. The setting circle on the polar axis is called the right ascension setting (RA) circle, and the one on the declination axis is the declination setting circle. The RA circle is marked in hours and minutes, like the RA coordinate of stars. The declination setting circle is marked in degrees like the declination coordinate of stars.

Using setting circles can greatly reduce the time it takes to find targets, yet they are not so commonly used. Why is that?

Partly because it can be easy to misread the coordinates of a target from a chart, or on the setting circles. Plus, one must be always looking up coordinates from a star chart or some type of device a couple of times each in order to keep the RA setting current. Finally, the nature of the readings and taking into account Earth's rotation can confuse users.

A main issue is that most amateur telescopes do not have "powered" setting circles. That means the RA setting circle doesn't turn with time to compensate for the rotation of the Earth. If the RA setting circle isn't powered, then the technique for using the setting circles is easy to mess up.

A user with a typical non-powered RA setting circle must start by pointing their telescope (which must be on a properly north-aligned mount) at an easy to find star target, looking up that star's coordinates, and adjusting their telescope setting circles to read the star's chart values. Usually only the RA setting needs to be adjusted.

Then one can look up the coordinates of a desired target that may not be so easily found and use the setting circles to position the telescope for the new target. With luck, the target can be quickly located.

But after observing the target for some minutes. the user has to be cognizant that the Earth has rotated some in the meantime, at about 15 degrees per hour. So a person can't just look up another target's chart coordinates and re-position the telescope expecting to find the new target. The RA setting circle will be off by however many minutes the observer spent on the last target.

The proper procedure is to re-set the RA setting circle to the current target's chart coordinates before moving on. So with updated settings, the cycle is: find a target, view the target, and then re-set the RA setting circle before moving to next target.

Too often, users forget to update the RA setting circle before moving. When this happens, users have to go back to square one, locate an easily found star and use its chart coordinates to update the setting circles.

For many people, this repeated act of reading chart values and updating setting circles, just to read the chart again for a new target and then read setting circles to position the telescope, is just too laborious. So, it's back to star hopping.

Altazimuth Mount Setting Circles

This is where setting circles on an altazimuth mount can be advantageous. Many of us old timers have been taught about the many advantages of the equatorial mount, and certainly for time-lapse astro-photography, an equatorial mount is a necessity. But for observing, nothing is as simple as a basic altazimuth mount. I use my old Pipe Fitting Tripod tripod all the time with a long-focus
60mm refractor. It's the easiest instrument for stargazing that I have.

It turns out that setting circles on an altazimuth mount can make it a very handy instrument for finding star targets. Of course, one can use the star hopping method, but one can also use any of a number of computer tools to get current altazimuth coordinates for targets. Xephm is a good tool for this. Click on a target within the Xephem display and the current azimuth and elevation are displayed in the upper right-hand corner of the Xephem window.

The situation with altazimuth setting circles is a bit different than with equatorial setting circles. Equatorial setting circles are based on a coordinate system where stars have fixed locations on the celestial sphere, but the star coordinate frame moves with respect to the Earth coordinate frame. Thus the need to keep updating the RA setting circle to re-connect the Earth frame to the celestial frame.

The altazimuth frame is a system where each star's coordinates are a function of time, but the setting circles only need to be adjusted at the time one aligns their telescope. No repetitive adjusting of setting circles, but star coordinates are momentary.

I like the system better. For some years I've used a calculator program for pointing my altazimuth telescopes. The calculator program contains a few favorite target lists, like the Messier List. It lets me choose a target, and it then shows me the current azimuth and elevation for the chosen target. A very simple system.

Ah, but you don't happen to have such a calculator program you say, and using a laptop and running a computer planetarium program is too inconvenient.

Then I have a deal for you!

Star Pointer -- The Easy Way

How about a tool that presents all of the targets that are "up" in your location from a selected popular target list, and even tells you where to point your telescope. And that's whether you use an altazimuth mount or an equatorial mount. It also shows the magnitude of the object, and what it is (galaxy, cluster, etc.).

A tool that will run on your laptop, your Chromebook, or even your tablet or smart phone. What would you pay for a tool like that?

How about -- nothing!

Just load the Star Pointer web page into your smart phone or tablet browser, set up and align your telescope, and get started. Pick from the Messier, Caldwell, or Herschel 400 target lists, and start observing, not hunting. Make any telescope you have into a computer assisted telescope by running the Star Pointer web utility on a browser on your laptop, Chromebook, tablet, or smart phone. How handy is that?

All Star Pointer needs is an accurate time setting on your computer device, and your location, within about an arc-minute. With that, it can compute all objects, from any of three available target collections, that are at an elevation greater then 25 degrees. It also displays updated pointing coordinates about every 30 seconds. If you're browser running device has GPS, Star Pointer will get your location automatically.

I've used the program as a pointing aid successfully with my old homemade pipe fitting mount and its altazimuth setting circles, my Long Focus DOB with it's added setting circles, and my equatorial mounted 6 Inch Rich Field telescope. Star Pointer has performed well in every instance.

The altazimuth mode is simple enough. Just be sure your mount's vertical axis is indeed vertical (or the base level), and that the azimuth axis reads zero when the telescope is pointed at Polaris. That's about it. Pick targets from the Star Pointer lists and point to the indicated coordinates.

The equatorial mode is just as simple, but given the previous discussion about RA setting circles and Earth rotation, it needs a little explanation. To use an equatorial mounted telescope, first make sure the Eqatorial mount has a polar axis that is properly aligned. Then select the Init EQ Mount button on Star Pointer, which lists (for the Northern hemisphere) a list of about a dozen bright reference stars. Pick one, and point your telescope at it, which is easy enough given how bright they are. Then adjust your setting circles to read the coordinates of your chosen target.

Unless you happen to leave the Star Pointer web page and later reload it, you're done fiddling with the setting circles. Just pick a target  (they'll now be listed with equatorial coordinates), and point your telescope. As you observe and move to other targets, the Star Pointer program adjusts the presented RA values to reflect the Earth's rotation. So you don't have to keep re-adjusting your setting circles. Just use them.

Can it get easier than this?

A little, perhaps. You can use a computer driven telescope, but many of these are only movable by their computerized motors. Gone is the convenience of just moving a telescope quickly by hand to a new target.

Yet to their credit, computerized telescopes have no real need of a cell phone or tablet web app to know where a star is. Or is that statement a bit premature?

I have a NexStar 5 SE, and to use it I always first looked at my copy of Xephem to see what targets were up. I'd make a list of them, then go out to my NexStar. I'd align it (of course), then start selecting objects from the list. The NexStar doesn't need help with coordinates, it finds the targets on its own. But still -- which targets were "up?" I needed my list.

Not anymore. Star Pointer shows me what's up. It also shows me what's up in azimuth order, starting with the South West. So using Star Pointer, not for coordinates but for what's visible, I can glance at the Star Pointer list and select desired targets from the NexStar controller. By having the Star Pointer display targets in azimuth order, entering from the list facilitates the NexStar in moving from target to target with minimal movement.

So, even though the NexStar doesn't need to know where any given target is, it still needs to be told which one to look at next. Star Pointer gives that information in a useful order.

If you have a simple altazimuth mounted telescope, make some setting circles (get a copy of the Setting Circle PDF), and start using Star Pointer. Already have a telescope with setting circles?, then next time out let Star Pointer help you out.