TUESDAY, DECEMBER 4, 2012 AT 6:06AM
Discovering Asteroids at iTelescope.Net
<---- Part 2 | Part 4 --->
Collecting Photons
My two previous articles were largely concerned with the rules governing asteroid discovery and the work of the Minor Planet Center (MPC) and the professional surveys.
What I aim to do now is to begin to answer the question: - What do I need to do in order to discover asteroids using itelescope.net? Clearly the first thing you need to do is to select the appropriate telescope so in order to help with the selection process let’s consider what was involved in the discovery of asteroid 316010 which I described in my first article.
On 19th March 2009 when this image was taken, the asteroid was about 175 million miles from the Sun and 87 million miles from Earth. Visible light photons emitted by the Sun took about 16 minutes to travel to just beyond the orbit of Mars where they encountered the half-mile wide asteroid. The majority of the photons were adsorbed by the asteroid but about 15% were reflected, some in the direction of Earth.
After a further eight minutes the Earth-bound photons arrived at their final destination where they illuminated, admittedly rather dimly, the entire dark side of the Earth. A tiny fraction of the photons passed down through the New Mexico sky and onward through the tube of telescope T4 where they scored a direct hit on the 10-inch mirror therein.
Astronomers are in the photon collection business so from that point of view it was mission accomplished but of course it was only the beginning of a story that involves processing the photons to produce images that could be used to detect the asteroid and measure its position accurately.
Asteroid 316010 (above)
Which Telescope should I Use?
The first thing to decide is whether you are going to observe using telescopes based in the Northern or Southern hemisphere. In my second article I mentioned the advantages of observing close to the ecliptic and at altitudes above 60°. You can do both in the Northern Hemisphere between October and March but you are better off in the Southern Hemisphere during the rest of the year.
Let’s assume that you plan to observe between October and March. This leaves the choice from any of the telescopes located at the itelescope.net observatories at Mayhill and Nerpio.
The table below gives selected data for each iTelescope that will enable you to make an initial sort. The data relates to May 2012 and may change as telescopes are added/removed or fitted with different cameras.
You can see that I have highlighted telescopes T3, T14, T16 and T20 together with values in the Aperture and Resolution columns.
The highlighted telescopes are ones that I would not use for asteroid observation. All the rejected telescopes have apertures smaller than 250 mm. In order to discover asteroids you really need a telescope-camera system capable of reaching to at least magnitude 19 and you greatly increase your chances if you can detect objects fainter than magnitude 21.
The larger the aperture the more photons you will collect and for practical purposes apertures smaller than 250 mm will not collect sufficient photons to enable you to detect faint objects.
I have also rejected T14 on the basis of resolution. The lower the value, the smaller the number of arc seconds displayed by each pixel and the better the resolution. Discovery of an asteroid involves not only detection but also the accurate measurement of its position at a given time. The MPC sum up the situation regarding accuracy rather neatly in their statement “Astrometry is a field where bad measures are generally of little or no use”
Their advice is that measurements should be made using equipment with a resolution preferably less than two and never greater than three arc seconds per pixel.
In this table above, I have listed addition data for the seven telescopes that were not rejected in the initial sort. The reason for listing the cameras is that these can change so it is as well to check on the itelescope.net website regarding current usage.
http://www.itelescope.net/telescope-information
The field of view column gives you an idea of the area of sky that will be imaged. If you are trying to discover new asteroids then obviously the larger the field of view the better your chances are. However if you are following up an asteroid and know its position fairly accurately then a smaller field of view is acceptable.
Each entry in the optimum resolution column is a compromise between accuracy and sensitivity. Taking T11 as an example, we saw in the previous table that the highest resolution possible was 0.81 arc seconds per pixel. However we can use the 2 x 2 binning option to combine four pixels into one and thereby reduce the resolution to 1.62 arc seconds per pixel. We degrade the resolution and hence the accuracy of our position measurements but we gain a significant advantage in terms of sensitivity.
In some cases it may be possible to use 3 x 3 binning and still have a resolution of less than 3 arc seconds per pixel. However itelescope.net only runs darks and flats for 1 x 1 and 2 x 2 binning so you would either have to work with uncalibrated images or run your own calibration images. The optimum resolution figures I have listed do not include any 3 x 3 binning options.
Basically we are in the business of collecting photons, dumping them on pixels, generating photoelectrons and using these to produce a CCD image. If we distribute the photons we collect over a large number of pixels then we may end up with a large dim image of the asteroid. If we use 2 x 2 binning to produce fewer but larger pixels then we increase the number of photons per pixel and end up with a smaller but brighter asteroid image which is easier to detect when we eyeball the entire CCD image.
Two things to remember with binning are not to exceed the three arc second per pixel limit referred to above and to check with the itelescope.net website that the binning level you choose is one that is permitted for that particular camera.
The sensitivity factor is my own measure of the efficiency with which a telescope-camera system converts photons into a CCD image. It is calculated using the equation:
Sensitivity Factor = A2 x R x Q
A is the aperture in metres
R is the resolution in arc seconds per pixel
Q is the quantum efficiency of the CCD at 550 nanometers expressed as a percentage (e.g. 50% is entered as 50)
The thinking behind this is that the number of photons collected is proportional to the collection area which for a telescope is proportional to the square of the aperture.
We have already seen that the greater the numerical value of the resolution the brighter the CCD image we obtain.
The higher the quantum efficiency of the CCD the greater the number of photoelectrons produced by a given number of photons and the brighter the CCD image produced. The quantum efficiency of a CCD varies with wavelength so it is necessary to standardise the wavelength at which it is measured. I chose 550 nanometers because it corresponds to the V photometric band which is used by MPC when reporting the apparent magnitude of asteroids. You can obtain quantum efficiency data from the websites of the camera or the chip manufacturers.
Before anybody else points it out, I have to put my hands up to the fact that my equation is a gross simplification of the true situation. In particular the binning advantage is more complex than I have indicated and the “given large enough pixels I can image to the edge of the universe” claim (which is implied in the equation) will not bear close inspection.
What the equation will do is to give you an indication of how the different combinations of aperture size, resolution and CCD sensitivity affect the likelihood that you will detect faint objects. In actual use however the variation in cloud cover and atmospheric turbulence may well mask these differences.
The final column of the table shows the faintest magnitudes that I have measured when using four of the seven telescopes. Most of my work has been carried out using T4 and T11 so the data here is sufficiently large to minimise differences due to atmospheric conditions. I have not made much use of T5 and T17 so the figures here may well underestimate the potential of these instruments. With this in mind I think I can see some evidence of a correlation between sensitivity factor and faintest magnitude detected.
One point that I should make clear that there is all the difference in the world between following up an asteroid whose position, speed and angle of motion are all known and detecting one for the first time.
In the first case it is possible to ensure that the known asteroid is centered in the field of view where optical distortion is at a minimum and to optimise the image stacking conditions to match the asteroid motion.
In contrast an unknown asteroid may appear close to the edge of the field of view where vignetting may reduce the contrast between the object and the sky background.
If we take T11 as an example, my 22.2 magnitude was recorded for a known asteroid under good atmospheric conditions and using optimised image stacking.
Realistically, allowing for average atmospheric conditions and non-optimised image stacking I reckon to lose between 0.5 and 1.0 magnitude advantage. This means that with T11 I can still routinely detect potential new discoveries down to a 21.2 to 21.7 magnitude range.
Pick an iTelescope...Any iTelescope
Now comes the moment of choice. If I am trying to discover new asteroids I tend to favour T11 because it has a large field of view and the highest sensitivity factor. I look forward to evaluating T21 for discovery work since the sensitivity factor and field of view are very similar to those of T11. If weather conditions are such that Mayhill is closed but Nerpio is available then I would use T7 or T17 on the basis that it gives the best compromise in terms of field of view and sensitivity.
Once I have discovered an asteroid and if it is approaching opposition and becoming brighter, I can accept a smaller field of view and/or a lower sensitivity. At this point my choice will be dictated by the weather conditions at Mayhill and Nerpio, the availability of the telescopes and (on the advice of my bank manager) their relative costs. In such a situation T4, T5 or T17 would be equally acceptable.
What Next?
Now that we know which telescopes to use, the next stage is to decide where to point them in order to maximise our chances of asteroid discovery. This will be the subject of my next article.