AY 203 - Finding objects

This exercise sets up a scheme for finding faint objects in obscure places, which you can then try at the telescope. Each of you should pick a different object from the following list for your project.

M27 planetary nebula NGC 7662 planetary nebula
M92 globular cluster NGC 7789 open star cluster
NGC 7331 spiral galaxy NGC 253 spiral galaxy
M74 spiral galaxy NGC 1068 spiral galaxy
M2 globular cluster

These objects are selected as being easy enough to see in the 16-inch telescope, but located in places which are not quite trivial to find in the sky. This rules out things such as the Ring Nebula, which sits halfway between two naked-eye stars a couple of degrees apart in Lyra.

You need to start by retrieving the celestial coordinates of your object. For these (anything but individual stars, in fact) you van use the NASA Extragalactic Database (NED). You can go directly here to search by the object's name. Note that these coordinates specify the year 2000. Due to precession of the Earth's axis, which is the reference for our celestial coordinate system, the location of a distant object in this system slowly changes without it having to move. Always be sure that the charts you use and the positions you are plotting are from matching epochs; using 2000 coordinates and an older 1950 chart will put objects a degree off (and you won't see them through the telescope).

Now you need to produce a set of charts starting with the whole visible sky (somewhere around 9 p.m. daylight time for this date) and zooming in, marking your coordinates on each one. The final ones can be based on images from Skyview, marking star patterns that will help you go from bright stars to where you want to be. You'll actually have to do this, so keep in mind that you may not be able to count on seeing all the stars on a map. Plan on using a field of view of 1/2 degree or less with the telescope, and about 5 degrees with the finder telescope. You want to be able to navigate from the whole sky down to the telescopic field of your object; the final proof will be when we get to Moundville, and you call me over to the telescope, I either will or will not recognize the target in the eyepiece.

A sequence that should work starts with the all-sky map at yoursky. Find a version of this map which you like as regards labelling stars, grid lines, and so on, then print it. Mark your object's coordinates on it, perhaps assisted by the cursor readout in Skyglobe. Then zoom in on the region in yoursky to a field of perhaps 10 degrees with the virtual telescope function, finding and marking sets of recognizable stars to navigate there from bright stars in the finder telescope. Try not to use the "find in catalog" feature yet. Print this chart.

Finally, you can use a set of sky images to help pin down the star field around your object. Warning: lomg-exposure photographs or CCD images make faint stars, galaxies, and nebulae much more prominent relative to bright stars than you will see them at the eyepiece! One way to do this is with the Skyview "virtual observatory. Be sure to fill in the field of view you need and enter the coordinates at the top of the form. First try selecting NEAT/Skymorph under the Optical survey group, with a field of 1 degree or so. The B&W linear color table probably prints best. This survey uses fairly short exposures to look for asteroids, but does not cover the whole sky; if it comes back unavailable for your coordinates, use the long-exposure photographs of the Digitized Sky Survey. Remember, these won't be as easy as they look from the photos...

Your end product is a set of charts that you are confident will get you to your object using bright naked-eye stars, patterns you can see in the finderscope, and finally the main telescope itself. This technique represents the venerable art of starhopping.

AY203 home page | UA Astronomy Home Page | Bill Keel's home page

Last changes: 9/2002