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Sunday, September 15 2019 @ 08:42 MDT

City Deep Sky Observing

City Deep Sky ProjectThe purpose of this blog topic is to provide a list of deep sky objects that are observable from within the light polluted confines of a city. This will allow people who are new to astronomy with a list of objects to hunt down and observe visually from their backyards. It will also be useful to more experienced observers who, like me, are sometimes just to lazy to drive out to a darker site.

Now there are obvious limitations to what can be observed from within a city. Light pollution tends to wash out most objects, and it would seem counter-intuitive that you can observe any deep sky objects at all. However there are many deep sky objects that can be observed from the comfort of one's own back yard. A couple of them are even visible to the unaided eye. That being said, some form of optical aid will likely be necessary to view most of the objects in the list. Note that this is a list of objects that can be seen by looking through the telescope and not by using CCD or film astrophotography.

Limiting Magnitudes:

Of course the problem with observing from within a city is that the sky itself has a visual magnitude. This limits what can be observed, though with larger instruments, you can see fainter objects. For example, from my light polluted back yard within the city of Calgary (or as we like to call it, light pollution central), my unaided eye, on a good night can see down to 4th magnitude which is much brighter than the 6th to 7th magnitude that you could see at a good dark site. At this magnitude, most constellations are missing stars and other than the moon and planets, there's not a heck of a lot to look at. Even so the Pleiades and the Hyades, for example, are visible in the winter time, albeit missing stars.

If I move up to my 10x50 binoculars, my limiting magnitude moves to around 7. This of course brings many more items into visibility, at least in theory. There are at least 50 deep sky objects that would fit into this category, that are visible from Calgary's, and hence Canada's, latitude. Now this is a theoretical 50, as for many objects, the actual surface brightness will be less than what the magnitude suggests. This is because for extended objects, i.e. non-stars, the magnitude is the aggregate of the total brightness of the object. The more spread out an object is, the dimmer it actually appears for a given magnitude. So two objects can have the same magnitude, but if one is spread out more, it would appear dimmer. This is one reason why this list won't be simply generated from a planetarium program as the magnitudes listed won't necessarily translate into observability at a given limiting magnitude.

I will also include objects visible with a small telescope. My 127mm (5 inch) Maksutov has a theoretical limiting magnitude of about 13. However due to light pollution within the city this becomes something around 9 to 10. Again with surface brightness differing from magnitude, it's unlikely there will be many 9th mangitude galaxies on the list, but what I can see through my scope will be.

The List:

Since there's a difference between what can be seen through binoculars and a much larger telescope, each entry in the list will specify for each of unaided, binocular or telescope if the object is visible under a light polluted sky. Of course as they say, your mileage may vary depending on how much light pollution you have in your night sky. You can determine what your limiting magnitude is by reading this.

The only order to the list is simply the order in which I make confirmation observations of the objects from my back yard. So objects are likely to be roughly in order by RA, but don't count on it as it depends on my schedule and weather. The list should be more or less complete in a year or so from the date at the top of this article, at which point I'll publish here a table of contents for the list.

So enjoy and get out there and observe! Feel free to comment on this project and the each object.

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