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If you own a scope 8 inches or larger then filters can make a big difference to the fine details you see in a range of non-stellar objects. They work by blocking out some wavelengths of light, while allowing full transmission of others. Depending on their type and composition, different nebulae emit light at different wavelengths and by using a filter matched to these the overall background and star brightness is reduced significantly more than the nebulae itself. While all filters cause some overall reduction in light transmission, the differential reduction of the background (especially under light polluted skys) and the transmission of the nebulae’s specific emission lines increases the relative contrast allowing the eye to see more detail than is possible without the filter.
Filters are not magic; their effect is minimal or negative for many objects because of the reduction in total light transmission. However, for some objects – especially marginal ones – the contrast increase can make it significantly easier to make out the details of an object. The Tarantula Nebula in the LMC is one of my favourites to view through a UHC or OIII filter: it reveals different and fascinating details through both. The OIII filter is also ideal for making planetary nebulae stand out from the background stars. NGC2438 in M46 is a excellent example for this.
David Knisely has done an extensive and detailed comparison of the UHC, OIII and Hb filters on almost 100 objects using a 10 inch scope. I’m not going to try and replicate this excellent analysis; instead I recommend you read the descriptions for objects you’re familiar with to get an idea of the improvement you can expect with the different filters.
Many companies make filters. Personally I have the UHC, OIII and Hb 2″ Astronomik filters mounted on a moonlight filter slide with a heater strip and temperature sensor set up to keep them dew free. The filters don’t appear to introduce a colour cast, or any other undesirable artefacts when viewing. They do however require a slight refocusing when changing between a filtered and non-filtered view.
Like many astronomy accessories filters are not cheap, so here’s my buying advice. If you’re only going to buy one filter, get a UHC. Next buy an OIII, especially if you enjoy planetary nebulae. An Hb filter is honestly an excessive luxury, but if you have a big Dob (15+ inches) and you want to be able to enjoy some very faint nebulae objects like the Horsehead it makes a noticeable improvement.
Last year we became members of the Astronomical Society of Victoria and one of the perks is access to the club’s dark site. Each year they run a few public access events to promote the club and astronomy, so we took it as a good chance to head up – check out the dark site and share our scopes and love of the night sky with some new folks.
We headed up under perfect clear blue skies and set up on the main field – I picked a spot between three big Dob brothers, an 18″, 20″ and 25″. Secretly I hoped they might act as a bit of a decoy for my baby 16″, to keep away most of the crowds, but once again the magic of observing with both feet on the ground seemed to draw people in. Alex set up our 5″ APO next door; she drew a bigger crowd than me!
It was fabulous to see so many people having their first experience of astronomy under a proper dark sky (SQM 21.5 last night). The local Lions Club put on food, the club put on a range of short and interesting talks as well as tours of the site, and after dark one of the night sky.
The evening started off pretty quietly with most of the visitors engaged with the night sky tour but that soon ended and for the next few hours the observing field was awash with the curious, interested and far too many far too-bright red lights. But hey, we’ve all gotta start somewhere.
In keeping with the Charles Messier theme of the evening I made a point to try and show people a few interesting Messier catalog objects as well as a number of the usual showpiece objects. I also always like to show people one or two of the more impressive galaxies that are up too; regularly seeing these objects which put everything else in the sky into context is one of the privileges of owning a big scope.
Over the evening I observed and shared these and more:
M46 OC + NGC2438 PN
NGC 3115 Gal – Spindle Galaxy
NGC 5128 Gal – Centaurus A (saw the bright strip through the dust lane)
M104 Gal – Sombrero Galaxy
NGC 2392 PN – Eskimo Nebula
NGC 3242 PN – Ghost of Jupiter
M42 and M43 Neb – Great Nebula in Orion
NGC 1365 Gal – Zorro is sadly chasing the sunset at this time of year.
NGC 2024 Neb – Flame Nebulae.
B33 Dark Neb – Horsehead (Hb Filter) – With Orion high in the sky it was the easiest view I’ve had of the normally elusive Horsehead, I managed to show Alex and 2 or 3 others who all saw it. Normally I struggle to see it myself, let alone help anyone else see it.
NGC 2070 Neb – Tarantula and region in the LMC
NGC 3372 Neb – Eta Carinae
47 Tuc GC
NGC 5139 GC – Omega Centarus
NGC 4755 OC – Jewel Box
NGC 3293 OC – Gem Cluster
While I wouldn’t want all my observing sessions to be like this it feels good to go and be part of an event like this a few times a year.
Posted in observing on January 18, 2011
It’s fair to say I have a bit of a reputation among friends for being a bit of a luddite. I collect vinyl records, only use cameras which take celluloid film and I think humanity’s greatest technical achievements may be the Apollo program in the 1960s. Given this it seems quite logical to me that I would find sketching more appealing than astronomical imaging, especially with one of those new fangled CCD cameras.
So why sketch at all? Well, a few reasons, the first is it’s a very simple and cheap way to capture and share what you see through a telescope. I think this is particularly valuable as it gives quite a good impression of what an observer sees visually through a telescope rather than the colourful but misleading candy we see from amateur images right through to professional images from the Hubble. I think the second reason is it makes you a better visual observer; the very act of building up a sketch forces you to take a second, third, fourth and many more looks at the object of interest. There is something too about the way the brain works which means once you have seen a detail once and noted it down, somehow it’s easier to see it again, even if just using averted imagination.
I started out sketching images of the moon. Partly this is due to our inner city home and partly because I think it’s probably the easiest target to start working on. The moon is so bright it’s quite easy to have an outdoor light on while you draw, in fact having a light on can help balance out the blinding bright appearance of the moon through all but the smallest of telescopes. Once I’d made a few drawings of the moon I was happy with I started working on other bodies in the solar system, mostly Jupiter. Lately I’ve begun sketching some of the deep sky objects, but this is still very much something I’m learning to do well.
Tempted ? Good. Here is how I suggest you start: get a copy of this book.
Buy some pencils – I have a box of 12 Derwent Drawing pencils but when you are just starting limiting yourself to a few pencils makes life easier. 4B, B and 2H is about right. It doesn’t matter though if it’s 3B or 2B and 4H etc. A B pencil is my standard go-to pencil. I never grab more than 4 pencils for any one sketch in the field. You also need to get a blending stump or three (you can get away without cleaning them all the time if you use the same one for really soft black pencils, and keep one pretty clean for light smudging). Also buy some different erasers, including a soft ball-style one and an eraser shield – the eraser shield might be the most important thing you buy. Finally you need a decent pad of A5 drawing paper. If you go to an art store all of this should cost you less than $40.
Beyond the tools the most important thing to practice is figuring out your reference points, and drawing those first. For the moon, big craters and the key shadows. For star clusters, focus on the brightest stars first. For nebula, the bright stars and basic shape. Once you get those down, you just keep refining, and refining sections of the sketch. Breaking it down makes it much less overwhelming.
Finally, for a bit of inspiration check out Astronomy Sketch of the Day.