Here’s an interesting subject for the budding astronomer – how do you note down what you’ve seen during an observing session. In many cases the answer will be ‘not at all’ – plenty of people enjoy having a look through the eyepiece, maybe take a picture or two and leave it at that. However, there are those who might want a more permanent record that they can refer back to and maybe share with others. In this internet era the entire thing may be, or may be later transferred to, a version that is done electronically. The records are known as logbooks and whilst there must be hundreds of ways of doing this, there are a few things that are considered normal practise.
When planning an observing session, the best results can be obtained through planning the session. Take a look at something like Stellarium to get an idea of what will be out in the night sky in the area you can see. Use metcheck or a weather satellite to get an idea of how long the clouds will give you. If you’ve selected a few targets for your telescope, now might be the time to work out how to set up your log book.
Remember to identify yourself, and any co-observers, your observing site (latitude, longitude) and the full date on which you are observing. If you observe both sides of midnight, include both dates on which your observing sessions occurred This might best be done by writing down your start time and date and end time, plus new date if required.
Make a note of the weather conditions. Is it hazy? Is there much light pollution (reflected glow from streetlights or the glow from a nearby or full Moon)? How much of the sky is covered in clouds? These can be important in deciding whether or not you really had a chance of seeing those fainter stars or nebulae.
Always remember to write down the tools of your trade. Which telescope (diameter, focal length) was used with which eyepiece (focal length). If you divide the telescope focal length by the eyepiece focal length, you get the magnification. The maximum useful magnification of a given telescope is either 50x the diameter of the telescope in inches, or 20x the diameter in millimeters. Anything above this will give you a bigger image, but it will be fainter and show no smaller details. Of course it is up to you whether you go for a small bright target or a faint but easier to see one. Did you use a diagonal (something that pops into the eyepiece holder so the eyepiece points upwards, making it easier to use at some telescope positions)? Alt-az or equatorial mount? Motor(s) on (tracking the motion of the heavens) or undriven? Maybe the observation was made using binoculars or the naked eye. In which case the magnification and diameter plus focal length of the binoculars is still required, though the eyes can just be written down as being used alone…
Also include all you can on any imaging devices you might use. Did you do a quick sketch of what you can see? Maybe you held a camera, or mounted a camera on the telescope – say which you did and any information on the camera, such as focal length etc that you can get off the web or just the make (if you have a few cameras, why not write this information down on an early page or a back page and give it a name, so you don’t have to copy everything out twice). Did the camera look through an eyepiece (afocal viewing) or was it acting as the eyepiece (prime focus viewing). Maybe the camera was out on its own without a telescope, doing star trails, getting the Moon with a planet or meteors etc. Write down what lens or lenses were used.
Other things to include, if you are using a telescope eyepiece combination. The field of view is the angular size of what you can see through the eyepiece. If there are several objects in the area you are looking at, it is sometimes useful to know how many should be in range of a given eyepiece, or in the case of large objects (like comet Lulin) whether or not the whole thing will fit in the fov of a given eyepiece. This can be calculated through something like an online calculator, or checked at the eyepiece. To check at the eyepiece, remember that the sky rotates (roughly) 360 degrees in one day. That’s 15 arc-minutes every minute and 15 arc-seconds every second (there are 60 arc-seconds in one arc-minute and 60 arc-minutes in one degree of arc). So if you measure the time taken for an object to pass from one side of the field of view to another (with no motors running), then multiply the times in seconds by 15, that gives you the f.o.v in arc-seconds, which can then be converted to degrees, minutes and seconds. Knowing how big your f.o.v. is will then tell you how big it is compared to the objects you might be looking at.
Another reason you might want to switch off the motors is to check the orientation of the things you are looking at. This is always important to put on a sketch or image to tell others which way up it is meant to be. If you, for example, want to know what satellite was on the left hand side of Jupiter and which was on the right and the next day you compare your image to where Stellarium says the moons were, it helps to know if your telescope has flipped the image left to right, or top to bottom or both or neither. To do this, firstly turn off your motors if you have any. Then watch the direction the stars drift in. This is the direction of the rotation of the Earth, they are headed towards the west and an arrow can be drawn to say whether this is left or right. If you’re using an equatorial mount, point the telescope down in declination. The direction the stars move is then the up direction (positive declination), which can be denoted with another arrow. If you’re using alt-az, do the same in both alt and az directions (up-down, left-right, respectively) to get your two arrows.
For objects in the sky such as planets, comets, stars, deep sky objects and the like (things that aren’t really moving that much). The name, if known, and basic co-ordinates (again if known) of the target you are looking at is always helpful. Some telescopes with control pads or setting circles will give you the RA and Dec co-ordinates, in other cases a compass and estimate of the angular altitude will be helpful. Mention what type of thing you are looking at (maybe even why if you have a reason). Is there an even like a occultation going on? Did someone say it was a pretty thing to look at? Are you ticking off everything on Messier’s list one by one?
Of course not all objects require this sort of an observation. Writing down the co-ordinates of the aurora might not be so helpful as writing down the forms of the aurora (corona, sharp curtain, sheet, flickering, green, red etc). Meteors you might not want to sketch every single one, but get an idea of where in the sky they are coming from (their radiant) and an idea of how many per minute are likely to fly overhead (Zenithal rate). Think about what you are seeing/going to see before making notes. If it’s a sporadic meteor happening during observations of other things, then get its direction, where it went to and from and maybe an idea of how bright it is.
The magnitude of an object is how astronomers denote how bright an object is. Originally, the 4000 or so visible stars were divided into six magnitudes of brightness. A sixth magnitude star was the faintest visible to the less polluted skies of the ancients, now you’re lucky to get a fifth magnitude sky. The magnitude scale has since been revised a little and now given a proper footing. Each magnitude represents a drop or increase in brightness of two and a half times. Magnitude one is two and a half times fainter than magnitude zero and so on – the bigger and more positive a number, the fainter the thing is. Some stars in the sky vary in their brightness, whilst others shine fairly constantly. I mentioned in yesterday’s skies over Kendal post that there are some good variables up this month. Why not pick a few stars in the area of known constant magnitude, compare Algol’s brightness to each of them and determine its change in brightness (its ‘light-curve’) during one of the dimming episodes?
Post observation notes
Of course observations don’t end just because you’ve packed away the telescope for the next cloud filled month. Further analysis can be done on what you’ve seen. Compare your notes with Stellarium’s predictions. Do some data reduction on your images to correct for light pollution (sky removal), thermal interference in CCDs, flattening out warping of the image by fast lenses (flat fielding) and so on. Note down what you’ve done and what the result is. Get names for what’s in your sketch or your image to see what you might not have noticed you had seen. Remember all the prediscovery recordings of Uranus and Neptune as background stars. Although you’re unlikely to have seen a new planet out there, there’s many an asteroid, satellite or comet waiting to pop up unexpectedly on a set of images. This may even provide you with a new target to observe the following night.