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Urban Variable Star Observing

OK, so your interested in observing Variable Stars but you live in a town or city and think your skies are too bright for visual observing.  Well you'd be wrong!  Unlike diffuse comets - where you really do need a dark sky to see them - or the faint fuzzy patches of nebulae and galaxies, observing point sources of light is pretty straightforward in even the worst light polluted skies this country can offer.  


The naked eye and binoculars reveal hundreds of stars to be observed, whilst a telescope increases this to thousands.  Your limiting magnitude will of course be affected, but don't let this put you off. Watching a stars variations over the course of a night, week, month or year is one of the great attractions of observing Variables. Does it really matter if it's a magnitude 8 or 16 star?  No it doesn't, so why let light pollution stop you having a great deal of fun and contributing to science at the same time!  I've done all of my observing from the bright skies of Birmingham - over 40 years and in excess of a quarter of a million visual observations.  All you need is a little patience and dedication.  Be warned though, once you get started you won't want to stop!

Limiting Magnitude

Let me say something about limiting magnitude as seen through optical aid - binoculars or telescope.


You may have seen tables published in various magazines and books, informing you of the faintest star you can see in instruments of various sizes.  My advice is to totally ignore this information and determine the limits yourself! There are just too many factors to take into account to say how faint you can see with an instrument of a certain aperture - light pollution, quality of eyepieces, quality of optics, whether you need spectacles to observe, your age and the most important factor of all, the experience of the observer.  


An experienced observer who uses a 20cm telescope in a location with 'average' light pollution, may break magnitude 14.  However a newcomer to astronomy who has no 'telescope time' behind them may well struggle to see magnitude 12 with the same instrument.  A few months later, this observer will push this to 13 and so on.  In a dark sky these values might be increased by a further magnitude.  There is no substitute for experience when it comes to visual observing.  So if you find that your new binoculars or telescope isn't showing you the faint objects you thought it might, stick with it and be patient!


Once you know how faint your instrument will take you, you can then plan your observing programme.  Choose stars whose range fall well within the limits of your telescope.  Pick stars whose minimum range is about one magnitude brighter than your limiting magnitude, and as you gain more experience you will be able to judge how faint you can confidently make an estimate and add new stars accordingly.  


Stray Light

It's not possible to do anything about the brightness of the background sky, as this is governed by street lights, security lights, illuminated buildings etc.  What we can do something about is the amount of light shining on us, the observer, as we make our observations.


If your neighbours are, like mine, care free with leaving bedroom lights on for long periods without curtains, or have a badly directed security light, think about constructing a light baffle on your fence, or a portable screen if you haven't got a fence or hedge. Anything which prevents stray light from shining in your eyes or on your telescope.  To the right are two  examples I have in my own garden - and they work fine! You might be surprised the difference it makes to you too!



One of the biggest problems you will find if using a Newtonian reflecting telescope is stray light illuminating the field of view as seen through your eyepiece.  This is caused by any light source shining directly into the light path from the secondary to the eyepiece.  An easy and cheap way to counteract this is to fit a 'light shade' to the top of the tube opposite the focusser which protrudes above the rim of the telescope to a distance which suits you best. The photograph to the lower right shows the one I use on my 51cm. It's simple plain black card which is attached to the tube by velcro, so I can easily remove it when not needed or when the wind is particularly strong.  It works perfectly, and enables me to observe variables much closer to the Moon than I could without it fitted!  It's also very light, so it won't affect the balance of your telescope!

So, how about a light shroud?  These are material covers which fit over the eyepiece and observers head to completely cut out stray light - left.  Do they work? Yes they do, but there is one problem which arises from these devices that retailers fail to pass on.  The climate in the UK can be described as 'damp' - an understatement to be sure.  If your observing on a cold or damp (or both) night and employ a shroud, your eyepiece will fog up in minutes unless you can hold your breath for world record times.  That's a simple statement of fact. There just isn't anywhere for your warm breath to escape too, and you find that you are constantly removing the shroud or having to use a hairdryer on the eyepiece to bring it back to working order.   They aren't cheap either.  If you fancy having a go, try a large towel or bin-liner before you buy one.  Better still, do without one!

So as you can see, there are things we can do to help reduce the problems of stray light.  These won't cure our urban light pollution of course - only a power cut can do that - but they do make things a little easier for us long suffering visual observers.  There is no reason why we can't make full use of the conditions we have, and have a great deal of enjoyment observing Variable Stars from heavily light polluted skies.  Just give it a go!

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