One of these stars is actually the JWST
The James Webb Space Telescope (JWST) is in orbit around the Sun, in a stable location known as Lagrange Point L2, a gravitationally stable location in space. JWST is also circling around L2 (as shown by the small white circle), keeping its solar panels pointed at the Sun, while the telescope itself is always shielded from the sunlight. It completes this circle around L2 about every 30 days, and orbits the Sun every 365 days, more or less.
JWST is positioned well beyond the moon’s orbit. Given its one-million-mile distance from us, can this 66x46ft object be seen by my 7-inch refractor telescope? Is it reflecting enough light in my direction?
For a given location and time, where to look? Will it be in the field of view of my telescope during evening hours? How long of an exposure must I take to record this very dim object?
To make this possible, NASA provides us with coordinates (in Right Ascension and Declination) of the JWST for a given earth location and time.
This is what I entered for my telescope’s location, and a time span covering the next few days.
Question: What exactly are Right Ascension (RA) and Declination (Dec)?
One needs to first understand that the spin of the Earth is at an angle relative to its orbit around the Sun. The Celestial Poles are the points in the sky that the stars appear to be circling around. The Celestial Equator is circle in the sky halfway in between the North and South Celestial Poles.
In this time-lapse photo of the North Star, one can see that the North Celestial Pole is the center of all these star circles. Note that the North Star (the bright object in the middle of the circles) is not quite centered on the North Celestial Pole.
Declination is defined as the degrees that an object such as a star is above or below the Celestial Equator. Declination on the Celestial Equator is defined as 0 degree, whereas the North Celestial Pole is at +90 degrees, and the South Celestial Pole is at -90 degrees.
The Earth rotates every 24 hours, so Right Ascension is defined in hours, minutes and seconds, eastward from the Vernal Equinox, which is a fixed line in the sky, connecting the North Celestial Pole to the South Celestial Pole, where RA is defined as 0.
While Ra and Dec seem kind of like Latitude and Longitude, there are some important differences.
One consideration is that RA and Dec are mathematically measured from the center of the Earth, and not too many telescopes are located there. Thus, the need to account for your location on the surface of the earth, especially when viewing objects relatively close to the Earth, as the JWST.
The more important complication is that the Earth wobbles like a spinning top, a phenomenon known as “precession”. In other words, the locations of the North and South Celestial Poles change over time. So, while the Latitude and Longitude of your house is constant over time, the same cannot be said for the Ra & Dec coordinates of a given star.
Astronomers compiled a celestial database (objects matched with a corresponding Ra and Dec pair) for the year 2000. This database is referred to as “Astrometic Ra & Dec”, or “J2000”. If one corrects the Ra and Dec for the present date, it becomes “Apparent Ra & Dec”.
Important Point: One needs to know which Ra & Dec coordinates one is using…
OK, so it turns out that on Feb 26, the skies at my telescope location were forecasted to be clear and dark, and according to Horizon Systems, the JWST would be in my telescopes’ field of view starting around 10:00pm (local time). How do I know this? Because the software I use to control my telescope (TheSkyX) has this cool feature where I can program in a future date & time, and it shows me what part of the night sky will be above my location.
At 9:00pm, when the outside air temp was 7 deg F and dropping, I took a few test images of different exposure lengths to determine how faint of an object my telescope could capture, comparing the test images to star charts.
I settled on a 5-minute exposure, no filters.
BTW, here is a picture of my telescope. An 7-inch TEC180 controlled by a Paramount ME. The camera is a QSI683. A key requirement is that this telescope will very accurately point at a given Ra & Dec location, and will very accurately track that location during long exposures.
I studied my first 5-minute exposure. The circular stars confirm that my telescope is tracking well, and the focus is good. My expectation is that the JWST is moving relative to the stars, thus perhaps I will find an oblong shape.
No such luck.
It is really cold, and even though I have not confirmed that the JWST is in my field of view, I decide to go for it. I program the telescope system to take four hours’ worth of 5-minute exposures.
And go to sleep.
The next morning, I find myself with 49 images, fours hours of image time spanning about 4 ½ hours of time (the pictures are not quite taken back-to-back, they need to be downloaded each time).
I quickly process them with Pixinsight, and using the “Blink” function, I find an object moving relative to the stars. But it is not centered in the image, which makes me question my finding.
I eventually determine that the default RA & Dec coordinates from Horizons are from the “J2000” database, and the coordinates I used to point my telescope are from the “Apparent” database. Oops. The good news is that this nicely explains why the moving object is not centered in my images, by the correct amount and direction.
Next, I manually plot the Ra & Dec coordinates from Horizons over that same 4-hour period, confirming that the non-linear path of the object I observed does in fact correspond with the path predicted by the Horizon’s algorithm.
Last step was to stretch these 49 FIT formatted images, convert them to JPEG, and using Adobe Premier Pro, convert this 4 ½ hours of imaging into an 8 second video, a time compression factor of about 2000.
I am very confident that the moving object you will see in this time lapse is the JWST, almost certainly following a 24-hour elliptical path due to the rotation of the Earth. This 4 ½ hour compressed time-lapse covers about 1/5 of that ellipse.
Note that the JWST period around J2 is about 30 days, meaning that the observed curve path in this time-lapse is not due to that circular path.