Project #7 Neptune and Pluto
The purpose of this exercise is to observe the motion of Neptune against the background of stars. Pluto is more difficult so we will just take a picture of it.
Preparation
First you need to find your target. You can use any place that lists planetary positions such as http://www.heavens-above.com. If you are looking up the position before your observing night (and you should be) just make sure you have a location for the time and date of your observation. Make sure your target is up! You can confirm your object is up for two hours by using the object seasonal observability link at Astronomy Tools Pick ‘other’ for your observatory and put in 5 33 31 west longitude and 34 56 45 north latitude. Be sure to use epoch 2000.
Get a chart of the region around your target at the Naval Observatory (Flagstaff Station)
You can use the defaults except you have to enter the coordinates, of course, and choose 12 mag (Neptune) or 15 (Pluto) for the R2 mag limit. (You can use I, for infrared, instead of R2, for red, if you want. It will not make much difference. Hit retrieve data and you will get a picture in the upper window, which will be what you are looking for (except for your target). When you take your CCD picture is will look something like this [maybe rotated] but with the target in it. Some of the fainter stars you will not see in the eyepiece and may not see in the CCD picture depending on how faint you expose.
Mark where the target will be on the USNO chart. You can also list the stars on the USNO chart by clicking the USNO A2 Star List link to see if they will be bright enough to see.
While Neptune is about 8 mag (fairly bright) and should show up in the eyepiece it may not show up in the finder. You need to find a bright offset star as you had in Exercise #1. Open our list of bright stars and pick one close to your target. Pluto is VERY faint (13.5 mag). It will not show up in the finder NOR the eyepiece and will only be seen in the CCD in a dark sky. You will need to have an offset star AND a good finder chart of its field.
Observations
Neptune
Neptune should be visible in the eyepiece, maybe as the brightest object in the field. Check the magnitudes of the other objects in the field
Once you think you have Neptune you need to take two pictures about 1 hour apart. They must have a good exposure of Neptune and at least two stars. You may have to try several exposure times to get all your objects. Your goal is to have two good pictures from which you can measure the position of Neptune and thus measure its motion. The field may drift a bit. You can abort the sequence, recenter a bit, and then start up the procedure. Don't be too picky. You would like to keep the Neptune and two stars in the frame.
Pluto
Pluto moves much more slowly in the sky than Neptune, so measuring the motion would take two separate nights. Just getting the picture is the challenge. You will not be able to see Pluto in the eyepiece. You will need to take a long exposure in the region in which Pluto is supposed to be and see if it shows up. When you move from your offset star make sure that you have the correct field. Take longer and longer exposure pictures until Pluto shows up. Make sure your focus is good. You may even use no filter to get more light. Your finder should show all stars down to 15mag. Pluto should be about as bright as the 14 mag stars. If the exposures get too long you may need to adjust the track rate a few 0.01 above or below 15.000 to keep the stars from trailing.
Reduction and analysis
Neptune:
Measure the motion:
Choose a good exposure at the beginning and the end of the observing run.
Measure the x, y coordinates of at least two stars on either side of Neptune in
an NS and EW direction. Measure the x,y coordinates of the Neptune. Interpolate
the Neptune’s position in RA and DEC by using the RA and DEC of the stars using
your list you printed with the finder chart. [If you did not print it out, do
so now.] You should write the beginning and end RA and DEC in your report with
their start and end times.
Calculate the distance traveled in pixels and then, using the scale of the telescope, in arc seconds. How many arc seconds did Neptune move in what amount of time? Explain how this could be converted to the rate of motion in Neptune’s orbit. Be clear about what other information you would need.
Without motion to confirm which object in your frame is Pluto you need to make a strong case that that what you mark as Pluto really is Pluto. Do so.
Write-up
For Neptune you need to have printouts of the beginning and ending images you used to show the motion as well as your calculations. Show the motion on the printouts. Which direction is it? For Pluto just indicate your candidate. Also include the finder chart with Neptune or Pluto sketched on it.