TUESDAY, DECEMBER 4, 2012 AT 6:00AM
Discovering Asteroids at iTelescope.Net
This NASA webpage provides a good starting point for those wanting an introduction to the different types of asteroids and comets that are classed as Near Earth Objects (NEOs).
No article dealing with the measurement of NEOs by amateurs would be complete without mentioning the work of Peter Birtwhistle at his UK observatory:-Over the last 10 years Peter has made over 12,000 NEO position measurements and his website provides a wealth of information on the techniques he uses.I have also been fortunate in drawing upon the experience of Professor Jaime Nomen who represents the La Sagra Observatory and the La Sagra Sky Survey (LSSS). The LSSS is the most prolific European NEO survey of all time and their recent discovery, 2012 DA14, is predicted to make an extremely close approach to Earth in 2013.I am grateful for the information that Peter and Jaime have provided. Any errors or omissions in what follows are however entirely down to me.
What are the Chances of Discovering an NEO?
In August 2012 of the 590,000 known asteroids approximately 9,000 were NEOs so at first sight it would appear that about 1.5% of asteroids are NEOs. However these figures are distorted by the fact that in order to become “known” an asteroid has to be observed on two or more linked nights and be given a provisional designation. The major surveys will follow up any asteroid that appears to be an NEO but will not deliberately seek to obtain a second night on non-NEO objects. The best estimate that I have is that in the summer months in the Northern hemisphere (when the North American monsoon limits competition by the surveys) you can expect to find one potential new NEO in every 100 square degrees of sky that you image.
You will not receive an NEO discovery credit for most of these because either they turn out not to be NEOs or they are NEOs which have been discovered previously and then lost. A very rough estimate is that 10% of the potential NEOs will your NEO discoveries.
As you can see our chances of discovering an NEO are not great but we have to balance this against the fact that they are important objects whose discovery is the prime objective of the major asteroid surveys. What I aim to do in this article, should you be lucky enough to image one, is to give you the best chance of producing measures that will enable the surveys to follow it up.
How do I know it’s an NEO?
In my previous article I mentioned the use of the MPC NEO Checker
If when you blink your images you see an object which is moving faster than and/or in a very different direction to the other asteroids in the field of view then it is certainly worth running through the checker.
I did point out however that not all NEOs can be recognised simply by their speed and direction so I routinely check all potential new discoveries to see how the rate on the checker.
Astrometry of NEOs
The rapid motion of NEOs means that they are challenging targets that require special techniques in order to make accurate position measurements. The techniques vary depending on whether you are measuring an NEO whose speed is known or one that you have discovered by chance. In order to describe the techniques I have divided NEOs into three distinct groups:-
- NEOs that have been observed previously. These range from numbered objects to ones that have only been observed on one or two nights.
- Potential new NEOs that appear as essentially circular images.
- Potential new NEOs where the image is non-circular.
In the case of Group 1, you can optimise the imaging conditions to cope with most NEOs. In the case of Groups 2 and 3, we will have discovered these by chance and we work with the images we have rather than the ones we would like.
Group 1: NEOs That Have Been Observed Previously
In April 2012 NASA announced the start of a new citizen science project called “Target Asteroids!”
Amateur astronomers are invited to observe selected NEOs and to report any combination of astrometric, photometric and spectroscopic data. In this announcement NASA acknowledge the important contribution that amateur astronomers have made to the refinement of orbits of newly discovered NEOs.
The Target Asteroids! website includes a list of NEOs and in order to support the project and to provide data for this article I chose NEO 141018 for observation.
I planned to observe 141018 on 12 June 2012 using T11 with 2 x 2 binning. A check using the MPC Ephemeris page showed that at that time the NEO was moving at 1.88 arc seconds per minute. As we saw in Part 6, the 2 x 2 binning level gives a resolution of 1.62 arcseconds per pixel and if we want to avoid trailing we need an exposure time short enough to prevent the asteroid moving more than 1 pixel i.e. more than 1.62 arcseconds. Dividing the resolution by the speed gives us the exposure time that will result in a movement of one pixel i.e. 1.62 / 1.88 = 0.86 minutes or about 52 seconds.
iTelescope provides darks and flats calibration date for 60 second exposures so I chose this as the exposure time.
I aimed to observe the NEO five times over the course of an hour during which time it would move nearly two minutes of arc. Any object moving this distance is quite likely to pass in front of a star at some point. In theory it should be possible to position five exposures over an hour each of which is free from stellar interference. In practice however this depends on the orbit being known very accurately and having planetarium software that displays stars bright enough to interfere. I chose the easier option of collecting sixty 1-minute images with the aim of selecting five spaced out along the arc which were free from interference.
This animation shows images 1, 15, 30, 45 and 60. As you can see there are two instances of potential stellar interference. Between the first and second images the NEO appears to skim past a magnitude 16 star while between the third and fourth images it passes directly in front of a magnitude 18 star. My choice of images, more by luck than judgement, avoided any interference but, had it occurred, the fact that I had 60 images to play with means that I would have no problem in selecting five that were interference-free.This method described above gave fairly accurate position measurements for 141018 with the MPC quoting my residuals at between 0.0 and 0.4 arc seconds.If 141018 had been fainter I would simply have produced as many 60-second images as necessary and stacked them in three sets according to its speed and direction. The method is exactly the same as described in Part 7 of this series. The fainter the asteroid the more images I would have to collect and stack. This task can best be described as tedious but possible and in this situation a large aperture scope fitted with a sensitive camera can be a real advantage.
If 141018 had been moving faster I would have been faced with three problems:-
- Obtaining non-trailed images.
- Obtaining images that were bright enough to measure sufficiently accurately.
- Measuring the time sufficiently accurately.
Obtaining non-trailed images is the easiest problem to solve since I would have simply used the exposure time calculated as described above. Although iTelescope does not recommend exposure shorter than 60 seconds you can go as low as 0.1 seconds provided you run whatever calibration frames are required.
Obtaining bright enough images is simply a matter of stacking the required number of exposures. It has to be remembered however that the shorter the exposure time the fewer photons we collect and the fainter the image becomes. Clearly there can be situations where the asteroid is so faint that the number of images required is too large to be practical.
The real problem with fast moving asteroids is that you need to measure the mid-point time of each image very accurately. CCD cameras invariably record the start time of an exposure to the nearest second and Astrometrica uses this value to calculate the mid-point time. Although this is more than adequate for the most asteroid astrometry you need the time measured to a few tenths of a second when imaging really fast-moving NEOs.
Peter Birtwhistle describes the method that he uses to obtain this level of accuracy, but as far as iTelescope users are concerned we do not as yet have this option. The only advice I can give if you should wish to image a very fast moving known object is to do your best to get accurate position measurements (as described below) and hope that the time error is not excessive.
One comforting fact is that many NEOs only tend to be very fast-movers for relatively brief periods in their orbit. For example on 24 August 2012 the MPC listed 24 newly discovered NEOs with speeds ranging from 0.5 to 35 arcseconds per minute. Of these only five were moving faster than five arcseconds per minute.
Group 2: Potential New NEOs That Appear as Essentially Circular Images
This is the easiest group to deal with. It involves a situation where, during routine imaging, we come across a potential new discovery which checks out as an NEO and which is bright enough and sufficiently slow moving for us to treat it as a normal asteroid.
In such a case the method used is as described in Part 6 or Part 7 depending on whether we are working with single or stacked images.
Group 3: Potential new NEOs Where the Image is Non-Circular
This is the most challenging group to deal with where we come across an NEO where the image is trailed.
I did not have a real example so for demonstration purposes I constructed this animation using the 141018 1-minute images to produce three sets each made up of 5 images. The stacks were not corrected for the NEO motion. When you include the download time, each stacked image is equivalent to a single image with an exposure time of about eight minutes.
Another way of looking at this is to regard each image as a 60 second exposure of an asteroid moving eight times as fast as 141018 in other words at about 15 arcseconds per minute.
As you can see the NEO image is now elliptical rather than circular so in order to measure its position I increased the aperture radius from four to eight pixels. This larger diameter circle then contains the trailed NEO image but at the cost of some loss of accuracy.
I also held down the Control keyboard button while clicking on the NEO image. When you do this Astrometrica calculates a simple centroid. I find this method is more accurate that simply clicking on the image.
I could have checked the accuracy of my measurements by reporting them to MPC but I did not want to clutter their 141018 orbit solution with dubious measures so I opted instead for this residual calculator developed by Dr. Jure Skvarc of the Crni Vrh Observatory, Slovenia.
This correctly identified the asteroid and gave residuals ranging from 0.15 to 1.12 which although not as good as those obtained using 60 second images would be acceptable for follow-up work and as a discovery observation.
If we were able to accelerate the asteroid we would see the image elongate progressively to a longer and longer ellipse and then to a trail. Although you can increase the aperture radius up to a maximum of 30 pixels there will obviously come a point where this method can no longer give acceptable results.
Another problem with trailed images concerns the apparent magnitude of the trail. NEO 141018 for example had an apparent magnitude of 17.2 and as you can see from the animation shown earlier it was a very easy target for T11
The circular image is concentrated over a relatively small number of pixels but if we were able to accelerate the asteroid to a point where the circle became a trail, the same number of photons would be spread over a much larger number of pixels and each pixel would appear dimmer. As the trail becomes longer we reach a point where it is too faint to be detected.
Assuming we do come across an NEO which is both fast enough and bright enough to produce a detectable trail, a method of determining its position is described by the UK based amateur astronomer Roger Dymock in his excellent book “Asteroids and Dwarf Planets and How to Observe Them”
The method involves measuring the position at the beginning of each trail. Astrometrica will automatically report the time for this position as the mid-point time of the exposure and in order to correct this to the beginning of the exposure it is necessary to subtract half the exposure time and edit the Astrometrica report accordingly before sending it to the MPC.
Logic dictates that an article titled Near Earth Objects should be followed by one dealing with Far Earth Objects. Consequently in my next article we will travel to Jupiter and beyond and assess the discovery opportunities that await us.