One day in the orbit of seven dwarf planets
Observing some of the most famous objects in our galaxy with a 25” inches telescope is certainly a rewarding experience. The list of galaxies, nebulae, and other objects is endless. However, there are many other objects which are not usually noticed by amateur astronomers: maybe they are not well known or simply because they require minimal aperture to access them. This is the case of trans-Neptunian objects (TNOs).
These small bodies are responsible for the restructuration of the hierarchy of the Solar System. The so-called dwarf planets can tell us how the Solar System was four billion years ago. The problem, however, is that they are very difficult to image. Its brightness its around the 20th magnitude and they are located between 30 AU and 960 AU. Moreover, they usually have a low albedo and its size is typically between 200 km and 2300 km.
In the summer of 2012, we began to plan how to capture the maximum number of dwarf planets on two consecutive nights to detect their motion. To achieve this goal we needed very specific conditions that were only met some days ago:
- New Moon was needed (because they are extremely faint objects).
- We needed two consecutive good weather nights, because an isolated night would not allow us to record the TNOs movement with respect to the starry background.
- The chosen nights should allow us to capture a sufficient amount of those objects.
- It should be done in winter, when the nights last more than 12 hours. That would give us enough time to locate each object and have enough exposure time.
Only considering the Moon variable limited us to 12 opportunities throughout the year. Adding the other limitations did make it even more difficult to find the right time to do it.
On December 5th, we decided it was the right time to execute the project . After a careful selection of the available objects, we chose eight dwarf planets which were visible that night. At sunset we found Salacia in Pegasus. It was followed by Eris in Cetus and Sedna and Chaos in Taurus (although the latter was the only one that was not possible to image due to an error in the prediction of its position). At midnight we got Varuna in Gemini, Orcus in Sextant, Makemake in Coma Berenices and nearly at dawn, we were able to capture Haumea in Bootes . Prior to the observation sessions, we obtained five images (with different zoom) of the field where each of the TNOs were located from the Digitized Sky Survey of Monte Palomar.
Example of the images from the Digitalized Sky Survey thet we used to manually target Eris. It is interesting to remark the small field of view that we were aiming at (the rectangle roughly corresponds to the captured CCD field). Many of the fainter stars shown were not visible through the eyepiece, making it very difficult to locate the desired fields. Click on the images to enlarge them.
After leaving the city, we look for a place that was as far away from the annoying light outbursts of the surrounding wind generators. We set up the telescope and performed a very precise polar alignment (in fact it has been the best aligment we have achieved with the 3 meter focal telescope). When the objects are so faint, perfect alignment suddenly becomes crucial. The telescope is mounted on a high precision Osypowsky equatorial platform. Despite of its great performance, it does not allow us to use any GOTO function and thus, all pointing was manually done.
Firstly, the target area was located with a Telrad followed by a careful inspection of the area with a wide-field eyepiece, comparing it with the previosly loaded images in an Android tablet. After a few minutes of detailed scrutiny we changed to a zoomed and more detailed image in order to recognize different asterisms until the exact spot we looked for was located. Once in position, we carefully exchanged the eyepiece for the CCD. When we were sure to be pointing to the right place we started with a sequence of very short exposures between 1 and 5 seconds (to avoid the effects of seeing and wind, so obvious at this focal) . This procedure (some kind of slow "lucky imaging") was repeated for each of the objects during both nights.
During the whole two nights the temperature was close to zero degrees and even less as dawn approached. In fact, after the first night, all instruments were covered by a quite evident crust of frost. We were worried about the weather conditions because even a soft wind or dew on the mirrors, could spoil the whole project (specially if it happened on the second night). However, we were quite lucky with this regard.
After all the shots were taken, all the raw data were processed in the following days. We were primarily searching for the detection of each TNO, although this would lead to noisy images. The shots have some parasitic light that was impossible to eliminate and some other problems, but we have chosen not to over process the images (and even less for an aesthetic purposes).
The 7 TNOs that were imaged during the two sucessive nights. Click the image to see a larger version.
Perhaps this is the work which we are most satisfied with due to the amount of time we needed to carry it out and the technical challenge that it involved. Once it has been successfully finished, it does nothing but encourage us to think about future challenges.
Finally, but not least, we have to thank Gustavo Román for his amazing work when designing and constructing the Obsession Rover. Without it, this kind of projects would not be possible. And, of course, many many thanks to Paloma and Noelia for their support and encouragement to spend some whole nights outside the warmth and tranquility of our homes.