Tumbleweed Observatory's

Astronomy Hints



Use Star Pointer to Find Messier and Other Star Targets

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About Star Pointer

Star Pointer

What Is It?

Welcome to Star Pointer, the site that tells you where to point your telescope for any Messier, Caldwell, or Herschel 400 object that is currently visible. In addition, it can present a collection of double stars that make good targets for small telescopes. Star Pointer displays a page like the page segment shown above. It presents visible all targets in a chosen catalog that are currently at an elevation of 25 deg or more. You'll need a telescope with setting circles, like the capable Orion 09007 SpaceProbe 130ST Equatorial Reflector Telescope. Or, you can take your simple Altazimuth telescope and add setting circles, as illustrated on the Pipe Fitting Tripod.

Star Pointer is a handy observing utility, especially for your next Star Party, as it gives you ready access to multiple star-object catalogs on your Smart Phone, Tablet, ChromeBook, or Laptop. With a bit of luck and a well aligned telescope, it lets you dispense with charts, at least when looking at objects contained in any of the selectable catalogs.

How Do You Use it

To use the utility, simply select the catalog and type of coordinate frame you wish to see, and the current list and position of all visible items in the selected catalog will be listed for your current observing location.. The objects are listed in azimuth order, starting with the Southwest. That way setting objects that might drop off of the chart are shown first, and moving from item to item causes the least telescope movement.

Even if you use a telescope that already has a computer driven mount, the Star Pointer utility is handy because it shows at a glance what's currently up for any of the selected target lists. I use it to select targets when using my Celestron NexStar 5 SE Telescope. First, it lists what is visible, and if I enter them into my NexStar hand controller in the order shown, the telescope can move from object to object quickly, as they are in azimuth order.

The utility displays on a plain black screen with red lettering so as to have the least impact on your night vision. Not fancy, but hopefully more useful that way. Messier and Caldwell objects are listed just by their catalog sequence numbers. At least some computer driven telescopes, like my NexStar 5SE have both the Messier and Caldwell lists in their memory, and entering just the respective list's target number is sufficient. But at least for my NexStar, the Herschel 400 list is not specifically included, though all of the objects are by other names. That's why the Herschel 400 list includes the Herschel 400 sequence number, and each object's Messier or NGC id.

Location Is Necessary

The utility does need to know your Latitude and Longitude in order to make the necessary object position calculations. If coordinates are entered manually, they are saved in year-long cookies so that the next time you use the utility on the same computer or device, it will already know your observing location. You can select the Enter Lat/Lon button at any time to enter new coordinates. You can enter either degrees, like 45.52, or hours:minutes:seconds, like 45:30:10. The displayed position is always in degrees.

Star Pointer's Location Method Sequence:

  • Use Cookies if set by manual entry on a prior use.
  • If no Cookies, use Geolocation if user allows it. *
  • If error on Geolocation or not allowed, ask user, then save in cookies.
  • * - Note, if your device doesn't have GPS, it's probably better to block auto-geolocation and enter a manual value. It avoids Google's geolocation routines, which often aren't that accurate anyway.

    Works For Both Alt/Az And Equatorial Mounts

    If using an Altazimuth mount, just select your desired catalog and begin picking objects. Point your leveled and North-aligned telescope to the indicated coordinates and enjoy. North aligned for an altazimuth mount basically means that the azimuth setting circle reads zero when pointing at Polaris. If not in the telescope FOV, the object should be just outside of the FOV. A little searching may be required, depending upon the accuracy of your alignment. When ready for the next object, just select it from the displayed list, as the object coordinates are automatically updated about every 30 seconds or so.

    If using an Equatorial mounted telescope, first use the Init button to see a list of calibration stars. Find one that is visible, point your North-aligned telescope at it, and position your setting circles to read the values displayed for that star. Right Ascension is displayed in hours:minutes:seconds format, and Declination is displayed in Degrees, which is most convenient for most small telescopes. Once you've set your telescope setting circles as per the Init table, proceed to selecting targets of your choice for telescope pointing coordinates.

    I've found that some users can't easily adjust their telescope's Right Ascension indicator. For those users, the Init selection provides another option. You can point your telescope to one of the listed guide stars, and insteading of trying to adjust your Right Ascension indicater to match the chosen star, you can select the Set RA Reading button that is provided.

    This allows you to select the index of the selected start (0 thru 10), and then enter your Right Ascension indicators reading. Enter it in hours-colon-minutes, like 13:25. The app will then calculate a bias between the proper reading and your telescope's RA reading. It will add this bias to the displayed Right Ascension values for each presented object.

    Note that when listing RA/DEC coordinates, the Right Ascension values are biased values, not static star chart values. That is, the Right Ascension values are adjusted to reflect the amount of Earth rotation since the Init button was pressed. This allows you to set your RA setting circle only once, and from that time on you can point your telescope by the recently updated coordinate list and be pointed at or near the object of your choice.

    If you have trouble, be sure that your location is entered properly. Also note that the computer time must be correct for the Altazimuth coordinates, but is less important for the RA/DEC coordinates. What's important with the RA/DEC coordinates is to set your setting circles to the right position according to the guide star that you pick, and that you do that shortly after using the Init button.

    Hopefully the periodically updated star lists will work for you, though I cannot guarantee that they will. I've used the results of the equations in other forms however, and have had good luck with them.

    Use Star Pointer as an Observing Planner

    While the original intent of the utility was to give real-time pointing coordinates for favorite star targets, by using the Time Bias feature, one can force the utility to advance the time so you can see what might be up later in the evening. For example, say it's only 4 in the afternoon and you want to see what objects will be observable at 9 in the evening (21:00). You can select the Set TmBias button and then select 5 to bias the local time by 5 hours. That way the charts will be presented for a time 5 hours from your current time, letting you see what's up around your expected viewing time.

    Just remember to reset the time bias back to zero when trying to use the coordinates real time. It always defalts to a time bias of zero when started, so you'll generally be just fine.


    Trouble Shooting

    If the positions listed by Star Pointer don't seem to be accurate, there's a few things you can check.

    First, is your location correct? It's listed under the control buttons. If it's not correct, you can select the Enter Lat/Lon button and re-enter it.

    Next, is your computer time correct? If you're using Equatorial coordinates, then the computer time isn't important, but it's very important for Altazimuth coordinates.

    Finally, is your telescope properly aligned?

    For equatorial mounts, the polar axis must be aligned with Polaris (for Northern Hemisphere observers). Be sure to use the Init EQ Mount list of alignment stars to pick one, and set your setting circles to the reference star values. This only needs to be done at the beginning of your observing session.

    For Altazimuth mounts, the vertical axis must be plumb (or the base level). Also, the azimuth setting circle should read zero when pointing at Polaris.