Institute of Astronomy

 

Ask an Astronomer - Telescopes and Instruments

Adaptive Optics - Tip/Tilt correction for amateur astronomy

Published on 14/01/2014 
Question: 

In our local astronomy club we are debating the effectiveness of available amateur tip-tilt adaptive optics at removing the effects of seeing. The promotional literature shows tighter stars with higher peaks using the AO. Could it be that the AO is only compensating for errors in the telescope mount? Or is it possible that AO can partially compensate for bad seeing? They advertise 5 to 10 corrections per second.

Adaptive Optics (AO) systems measure and correct any distortions coming into your telescope/detector. It does this by measuring in real-time the distortion on the incoming 'wavefront' and applying a correction to it, flattening out the distortions so you obtain a sharper image on your detector. Typically AO is used to measure and correct the atmospheric effects (i.e. bad seeing) on a wavefront coming into a telescope system but it can be used to correct systematic problems. As such, you could use it to correct for errors in the mount providing they are not too severe although as these effects are most likely repeatable, mounts can either be trained or the use of an auto-guider may be as effective and cheaper.

If you are trying to remove the majority of the atmospheric distortions you would need a system running at many hundred times a second and this is what many of the professional large telescopes around the world do today. It is however possible to get a correction with a much simpler system by thinking about turbulence being made up of different scales of distortion. You have larger scales i.e. tip-tilt, defocus etc. which will affect the whole of your image while much smaller scales will only effect a smaller part. This means you can think of these large scale distortions causing most of the spread of the PSF and if you can remove these effects from the system, you can improve your image. For tip-tilt, typically you will get a factor of two improvement in image quality but this improvement will only be as good as the measurements and corrections of the AO system - think of it more as a 'theoretical' limit. If you can also remove the defocus term, by that point you'll have image quality by about a factor of 5. The more corrections you do, the greater this factor becomes but to do these extra corrections adds complexity, cost etc.

Regarding timescales and seeing - the timescale of correction is set by how fast the atmospheric turbulence changes. This is principally set by the 'wind crossing time' of a telescope (i.e. how long it takes cell of air (think of it as a cubic metre) to move from one side of the telescope entrance aperture to the other) and how many 'turbulent cells' you have across an aperture. Turbulent cells of air are typically 10-20cm across in size (this is seeing dependent - on a clear night it could be bigger) and so across the diameter of a 4 metre telescope we have approximately 20. As such, you need to correct at least 20 times a second on a 4m telescope and for smaller telescopes, you can work slower than that. The slower you can go, the easier the system is as the correcting mirror can move more slowing and you can collect more photons from your reference star before needing to make a measurement i.e. improving sensitivity. There is however a limit to the level of corrections you can make - the correcting mirror will have a limited 'stroke' on each element which is the maximum movement possible. As such, if the tip-tilt value is greater than this (at the edge of the mirror) you will be unable to correct the distortion. This is why typically AO systems on large telescopes do not work well with seeing greater than 1-1.5" (there is additional complexity) although on smaller telescopes some level of correction should still be possible. 

Using astronomy to date historical events

Published on 03/01/2013 
Question: 

I cannot get any clear answers to what should be a simple question. "What percentage accuracy do the ancient astronomers have in fixing dates for the first millennium BCE?"

I am researching the period 1000BCE to 00BCE.
There is a wealth of information about Sumerian and Babylonian astronomical dating.
The "Kings lists" are constantly mentioned.
I simply want to have a reasoned answer to the following:

Allowing for human error, scribal miscopying etc - did the ancients in 1000BCE to 00 accurately observe and plot the planets so that we can retrospectively today recognise that which they recorded and afford it absolutely certain dates (to the nearest five years or so)
Do we know, from the positions that they give a planet in the sky that they called X, that it was that which we call, say, Mars.

Are there examples which clearly establish this e.g.
Carved in stone –
“In the 4th year of King X reign Planet V appeared in the west (whatever) and then moved behind the full moon to disappear in the northeast" - this today our computers tell us is exactly a description of Venus on 4th December 560BCE
Carved in stone –
In the 56th year of the reign of King W planet H did rise in the east and set at midnight one step into the quarter west(whatever) - this is an exact description of Mars on 4th January 500BCE"
Carved in stone –
King X was replaced by King W in his eighth year.

That is - we now know these astronomical events were indeed 60 years apart, they could be seen exactly like that from Babylon and it is clear that this was indeed the 60 years from King X 's 4th year to King W's 56th year.

There is much discussion of accuracy and more of eclipses. The only solid facts must surely come from verifying planet observation and then tracking those dates and comparing them to those given by the ancients.

Or as usual am I being simple minded?

As you have found astronomical observations can indeed be used to help provide support for chronologies of the ancient world, no method is perfect however and there are problems with this.

All astronomical observations one can make related to the Solar system contain many layers of cycles.  Everything of course varies on the cycle of a year as the Earth orbits the Sun, however due to the motion of other bodies and slow periodic changes in Earth's orbit there are also longer period cycles.  For example the times at which Venus rises and sets has a cycle of about 8 years (as well as other longer ones).  In general the shorter cycles are the most prominent while distguishing where an observation falls in a longer term cycle is more difficult.

The end result is that for your example stone carving there might be a match with 560BCE, 552BCE, 544BCE, 536BCE or 528BCE.  Sometimes knowing that it must be one of those dates might be sufficient to combine with other data and pin down an exact match, and sometimes even being able to pin it down within 5 cycles or whatever the case may be might be superior to what was known before, but gaps and inaccuracies in ancient records make it difficult to pin down exact dates from historical astronomical observations.

The most accurate records tend to be those of eclipses (because they are hard to miss and have a short duration), which helps with placing individual eclipses within the longest eclipse cycles and so providing more precise dates.  There are still problems with gaps simply because the further back you go the fewer records have survived whether or not they were originally taken, and scribal errors accumulate over time and are difficult to account for.

An additional problem is that in early history (and even comparatively recently) the calendars were not constant.  Many ancient calendars, such as that used by the Babylonians, were lunisolar with months based on lunar phases plus leap months as required to keep reasonable synchrony with the solar year.  That is fine provided that one knows how the scheme on which the leap months are added, but therein lies the difficulty in that they tend to be adjusted on a more ad-hoc basis or systematically over the medium term but with unknown larger jumps in the long term (think about the break that occurred in England in 1752 with the switch from the Julian to Gregorian calendars).  In fact even today this problem exists in the form of 'leap seconds' added by the international body which supervises global time standards, as they are irregular and unpredictable.

Sorry I can't really give you a simple answer, but I hope this goes some way to at least explaining why it isn't a simple question!

Observing the early universe

Published on 24/09/2012 
Question: 

How do astronomers observe the beginning of the universe: what wavelengths do we use, and what instruments are best for these measurements?

The early universe is of great interest to astronomers and cosmologists. It is also a popular topic amongst the general public, as we like to know where we came from. Astronomy has typically relied on light, that is electromagnetic radiation, for observing the universe. Electromagnetic radiation comes in a range of wavelengths, or equivalently frequencies, of which familiar visible light is just a small part. The spectrum goes from gamma rays at the highest energies, smallest wavelengths and highest frequencies, through X-rays, ultraviolet light, visible light and infrared to radio waves at the lowest energies, longest wavelengths and lowest frequencies.

The most useful for observing the early universe are microwaves (part of the spectrum of radio waves). We can measure the cosmic microwave background (CMB), which is the oldest light still in existence. Before this was emitted, the universe was effectively opaque: light couldn't travel freely because it was so hot and dense. Our cosmological models predict that the light that makes up the CMB was emitted about 400,000 years after the Big Bang. So we can't quite see the beginning of the universe, but it is close!

The CMB has been observed using many instruments. It was first discovered using radio receivers on the ground by American astronomers Penzias and Wilson in 1964. They would win the 1978 Nobel Prize for this. Lots of exciting science has been done more recently following a number of space-based missions. There was COBE launched in 1989 (the team won the 2006 Nobel Prize), then WMAP launched in 2001. The most recent mission is Planck, from which the data is still to be released (due January 2013). This will produce the most detailed map of the CMB sky yet.

While the space based missions are best for entire sky surveys, it is possible to measure parts of the sky from Earth. There are a number of ground-based and even balloon-based detectors. One telescope, the Arcminute Microkelvin Imager (AMI) is based here in Cambridge.

The CMB is currently our best tool, but it may be possible in the future to see further back in time. For this we would need to shift from using electromagnetic waves to gravitational waves. Gravitational waves are predicted by Einstein's theory of general relativity, they are tiny ripples in spacetime. We know they exist from careful observations of binary pulsars (for which Hulse and Taylor won the 1993 Nobel Prize). However, we have still to detect them directly.

We have ground-based detectors (LIGO in America, and Virgo in Europe), that are currently being upgraded. Japan is also in the process of building a detector, KAGRA. It is hoped will make the first direct detection in the next few years. However, these are unlikely to see gravitational waves from the early universe - just like electromagnetic radiation, gravitational radiation comes in a range of frequencies. We might be able to detect the gravitational background using pulsar timing arrays. Radio telescopes watch the signals emitted from a network of pulsars across the sky and look for correlations in deviations from their regular signals. The difficulty with these measurements, apart from having to make very careful timings, is that the frequencies are so low, you have to wait for years to see an entire wave. It is hoped that the planned Square kilometre Array will make these measurements easier.

Another possibility would be a space-based detector. This would be sensitive to frequencies between pulsar timing arrays and the ground-based detectors. Amongst many other sources, it could detect a gravitational wave background. Unfortunately, there is no funded space based detector mission at the moment. There may be eventually a detector based upon the design of the  Laser Interferometer Space Antenna (LISA). The European Space Agency is launching LISA Pathfinder in 2014 to test the technology needed, and hopefully a full mission shall follow that.
 

J1950 and J2000 Epochs

Published on 23/08/2011 
Question: 

What are the epochs J1950 and J2000 when looking at objects in the sky? And what do I have to put in my telescope program?

Astronomers use different epochs to give coordinates of objects in the sky due to changes in motion due to primarily the precession of the Earth on its rotation axis. Much like a spinning top, as the Earth rotates, it's rotation axis gradually rotates as well although much slower than the daily rotation we see. Because of this precession, the positions of the stars change over time with a small motion every day. On small timescales this motion isn't noticeable however over decades it is. For this reason, astronomers update their coordinates every 50 years to make it simpler when finding objects.

Although for an object you can find coordinates in either J1950 or J2000 (the two most recent epochs), this won't actually be the correct location in the sky today. However, what your telescope program will do will take the coordinates from those epochs and then calculate what they should be today i.e. J2011 and then move your telescope there. This saves people doing things by hand and so means you can find objects with the standard coordinates quickly and easily.

As such, all you should need to do is find the coordinates of the object in either J2000 or J1950 and can then put those into your program. The program will then do the rest.

Amateur detection of Near Earth Objects

Published on 04/04/2011 
Question: 

Is it possible for an amateur astronomer to assist with detecting near-Earth asteroids or comets? If so, what would be the minimum telescopic aperture and type of photographic equipment required to conduct this kind of research?

Amateur astronomers can and do play an important part in detecting near-Earth objects. Today more than 5% of all near-Earth objects are discovered by amateurs and this proportion is on the increase. The Minor Planets Centre (http://www.minorplanetcenter.net/) based at Harvard University is the organisation responsible for cataloguing and archiving all discoveries of small bodies in the solar system and have a wealth of information to help potential amateur astronomers. The professional search programs typically use telescopes with a diameter of ~1m; LINEAR, one of the longest running and most successful programs currently has two 1m telescopes and a 0.5m telescope. When conducting searches smaller telescopes are to some extent preferred since they have a larger field of view and can image a larger area of the sky at once.

When you have your telescope, discovering asteroids is still not trivial! Because these objects are particularly faint (due to their size), the detectors being used (usually a CCD) need to be sensitive enough to be able to distinguish them from the background noise from the device. For CCDs, when they are cooled the background noise is reduced and because of this, most of the CCD detectors you can purchase for amateur astronomy today use fans to cool the CCD well below the ambient temperature.

Even with both of these, the main requirement to successfully discover asteroids and other small (and faint) objects is a dark site with good seeing (clear, still skies). Combining all of these things together means you could potentially discover some new asteroids!