Contents
Asteroid Metis
Multiband Imaging
Asteroid Light Flux
Image Acquisition
Exposure Decision
Results and Analysis
Asteroid Metis
Discovered by Andrew Graham on 25 April 1848, “Metis is one of the larger main belt asteroid. It is composed of silicates and metallic nickel-iron, and may be the core remnant of a large asteroid that was destroyed by an ancient collision. Metis is estimated to contain just under half a percent of the total mass of the asteroid belt.” (Wikipedia)
“It has the dimensions of 222*182*130 km. Hubble Space Telescope’s images and lightcurve analysis are in agreement that Metis has an irregular elongated shape with one pointed and one broad end. It appears to be more dense than most other asteroids with a diameter close to 200 km. “ (Sky Safari Pro)
Multiband Imaging
A few months ago, i imaged this asteroid at my observatory and since it is very bright, i thought to try a multiband imaging with photometric filters i have in my filter wheel. The filters are from Astrodon Company and these are B, V and I (Blue, Visual, Infrared).
Figure 2. Astrodon Photometric Filters Transmittance
Figure 2 a. SBIG ST9XE Quantum Efficiency VS Wavelengths
You can see Astrodon filters bandpass here. There is an overlap of some 25% at 490nm of B and V filter. I filter is nicely separated from B and V but has an overlap of about 40% of 150 nm wavelengths. My CCD has low QE (CCD response for incident photons) in B and I band but high in V band. Which simply means, i would be requiring more exposure for equal amount of flux sources.
Asteroid Light Flux
Since asteroids are much cooler objects in the solar system, their light peaks in Infrared band. Somewhere around 2500-5000 nm. Now i cannot possibly imagine to image at these wavelengths because my filter transmission ends sharply at 900 nm.
But i was hoping to see the brighter asteroid in I band among all of my three photometric bands.. but had no idea how it will actually turn out to be.. needed to experiment on this and see it with my own eyes.
Image Acquisition
Sky X software found the asteroid in no time.. and in the software i found quite a bright star near the target asteroid. The red circle is the asteroid’s position and it is very near to a bright star.
Since the asteroid is very bright, at just over 9 magnitude, i decided to do the unguided imaging with my telescope.
Next step: Deciding the exposure time.
Figure 3. Asteroid Metis Position in Sky X Image
Exposure decision
ADU value is the number which represents the QE of the photon count per pixel. I needed to keep it in 30k range so it does not become non linear. By exposing the camera with different time and keeping an eye on the maximum ADU count for all three BVI filters, i was ready to start my imaging run.
Results and Analysis
So have a look at the following images and their respective graphs.. what do you make out of these? You can comment down there.. Let’s analyze it together :)
Figure 4. Metis (B band)
Figure 4 a. Flux Graph (B band)
Figure 5. Metis (V band)
Figure 5 a. Flux Graph (V band)
Figure 6. Metis (I band)
Figure 6 a. Flux Graph (I band)
Figure 7. Metis (L filter 400-700 nm)
Figure 7 a. Flux graph (L band)
A comparison was made between the maximum flux of the asteroid Metis and the closest bright star.. here is the result. What will be your favorite band to image an asteroid?
Figure 8. Intensity Comparison Graph