Asteroid Metis Multiband Flux Comparison

Contents

  1. Asteroid Metis

  2. Multiband Imaging

  3. Asteroid Light Flux

  4. Image Acquisition

  5. Exposure Decision

  6. 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)

 

Figure 1. Asteroid 9 Metis “Lightcurve based 3D-model” Wikipedia

 

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

A NEO (Near Earth Object) observed from the Observatory

Contents:

  1. Amor Asteroids

  2. NEO Target Selection

  3. 2001 MK3 Asteroid

  4. Imaging Concerns

  5. Slewing the Telescope

  6. Image Acquisition

  7. Calibration

  8. Stacking

  9. Astrometrica to the Rescue

  10. Near Earth Asteroid Detected!

AMOR ASTEROIDS

Amor class is a group of asteroids that cross the orbit of Mars.. also known as, Earth-grazing asteroids. Close encounter to Earth or Mars can turn Amors into Earth-crossers (Apollo Group). Amor members show a broad variety of compositional types, evidently having originated from several sources. (Oxford Astronomy Dictionary).

NEO Target Selection

On 30 April, 2020, i checked on Minor Planet Center website for targets (which are desirable for the night) for Astrometry (precise positioning of asteroids or comets) from my observatory in Lahore. The following list came up:

MPC website says the observations for this asteroid are desirable now.

2001 MK3 Asteroid

2001 MK3 is an Amor class asteroid which is less than 2 km in size and was 1.1 AU ( 164,600,000 Km) away on April 30, 2020. It orbits the sun every 2.2 years at an average distance of 1.7 AU (Sky Safari Pro Mobile App). One Astronomical Unit is the mean distance of Earth and Sun.

Asteroid 2001 MK3 orbit around the sun. It crosses the path of Mars but not with Earth. (Images from Sky X)

Imaging Concerns

Imaging and detecting faint asteroids is not an easy task from a light polluted city sky. Stars and Deep Sky Object virtually stay where they are in the sky and we can take multiple exposures for hours long and can keep on collecting their light on a few pixels. On the other hand asteroids (and comets) move noticeably in the sky and hence their light is continuously dispersing on pixels across the CCD. The outcome: a single pixels has far less light of a moving asteroid than a ‘fixed’ star in the sky.

Slewing the Telescope

But why not try it and give it a go so i downloaded and updated 2001 MK3 asteroid in The Sky X software.. This is the software i use to control all my equipment at my observatory. The asteroid is in Herculus Constellation and the following image would be my field of view (FOV) according to the software.

 

2001 MK3 Position in The Sky X Software

 

The mark of red circle with three dots, is the position of the asteroid in the sky. I slewed the telescope to the target location.. Now i could see the yellow circle (telescope pointing position in Sky X), moving from the home position to the target position on the Sky X software’s planetarium window. I always use ‘Closed Loop Slew’ in Sky X.. it does three things Automatically: Slews to the target, Plate Solves & Notices to error in pointing and then Slews again with the target in dead center. This feature is such a blessing!

Eden Observatory

Remotely controlled Observatory

Celestron C14 Telescope

Losmandy Titan Mount

SBIG Adaptive Optics

SBIG CCD

Astrodon Filters

Optec Rotator Optec Focuser

Primaluce Eagle

System Focal Length: 4400 mm

Telescope at Eden Astronomical Observatory

 
 

Plate solved in the Sky X shows the correct position of the telescope view in the Sky X software. Since my Field of View is small (8 x 8 arcminutes), one of my usual concern is: Do i have enough stars in the field which will give me plate solving for a successful Astrometry? Sometimes there are not enough stars and my only choice is to wait till the asteroid crosses another patch of the sky which has more stars.

This time i was lucky and could see many stars..

Imaging initiated!

Image Acquisition

I kept the sub exposure time at 30 seconds so i could keep the asteroid at one spot. Following is the first image from the telescope.

 

Single 30 seconds exposure in Luminance filter

 

Calibration

I took some 65 images. Now need to ‘clean’ these images.. a process which is called Calibrating the Raw. There are various sources of noise in the raw images coming from the camera. Calibration helps a lot in clearing up this noise. There are many sources on the web where this cleaning process is explained so i wont go in these details here.

Calibration Frames: BIAS, DARK and FLAT

 
 

Calibrated Image

 

After calibration, we can see some dim stars as well and an even background.

Stacking

Next step is to Stack or combine all images and let’s find out if the asteroid is visible in the stacked image. Stacked image improves Signal to Noise ratio a lot, which simply means the dim objects become brighter.

 
All Combined 2001 MK3.png

All 65 images combined, created this image.

 

For those who do not know, telescopes not only need accurate tracking of the sky (due to planet Earth’s rotation around its axis), telescopes also need guiding as well because no tracking is perfect. Here in this imaging run i was not guiding the telescope.. hence the stacking error on the left side.

The combined image showed nothing at the location of the asteroid. Not good!

But i knew the asteroid is very dim against my bright background of city sky and the photons coming out from it will be there in my image somewhere. Since it is moving, i would be need another technique.. a very clever technique indeed!

Astrometrica to the Rescue

Astrometrica is a great software for asteroid trackers. It is simple yet quite powerful to locate an asteroid and find its exact position in the sky.

One of the great feature it has is, it will download an asteroid’s speed, position, the angle in the sky in which it is moving and then it will stack the whole set of images keeping that moving asteroid at one point. This looks pure magic to me!

Following is what Astrometrica retrieved. You can see 2001 MK3 was moving at 0.710 arcseconds per min and with 91 degree position angle. My single exposure was 30 seconds and my image scale is almost 1 arcseconds (0.94 to be exact).

 

2001 MK3 speed and position (Astrometrica Software)

 

Near Earth Asteroid Detected!

Ladies and Gents.. 2001 MK3 finally detected :) and right where it is supposed to be! The red box is the position where Astrometrica shows this asteroid should be. The stars trail as expected. Signal to noise Ratio (SNR) is 14 which is not that bad since i only have to know the exact position.

2001 MK3 exact position in the final image (Astrometrica Software)

 

The 3D flux graph shows the raised position where this asteroid is. Rising above the background noise.

This is my first Near Earth Asteroid detection.. many more would be coming! As far as i know, this is also the first detection of NEO from Pakistan.