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Go Back   IceInSpace > Beginners Start Here > Beginners Talk

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  #21  
Old 28-09-2018, 11:54 AM
75BC (Brendon)
Always in the dark.

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Enjoy reading these observing lessons Alex.

I have a question or 2 about filters for nebulae. I have a cheap Kson 2” UHC filter that serves my purpose just fine. Only used it a couple of times so far but do they all give a colour tinge to what you’re observing as this one does?

A new shopping centre has been built about 1km from my house – getting bigger by the week – and they have installed those nice bright LED lights in the carpark that has noticeably brightened the sky from my backyard. Do I understand correctly that my filter will not be able to block this light as it is a full spectrum type of light being emitted?

Thanks in advance and looking forward to the upcoming planetary lesson as they are a favorite target for me.
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  #22  
Old 28-09-2018, 12:25 PM
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mental4astro (Alexander)
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Hi Brendon,

Colour that you are seeing is no mistake. Two factors at play here, one is your eyes and their capacity to make out colours at low levels of light, and two is the actual colour/colours that these filters transmit. The light they transmit is not white. It is in the red and greenish range.

If you see an intense red red around individual stars this is an artifact of the way filters are produced transmitting internal reflections. Most obvious with brighter stars. A clue to their filter origin is this colour is too deep and intense, where actual red stars are more orange in appearance.

With respect to LED lights, what "light pollution filters" do is center their wavelength absorption around those of mercury vapour and sodium lights - older technology street lights. These broadband light pollution filters will still work, however their effectiveness is reduced because these LED lights transmit across the whole spectrum, as you said. Narrow band filters, OIII & UHC are less affected as they are more aggressive, but their effectiveness is still reduced in the Big Smoke.

It then becomes a balancing act between the light pollution vs aperture and filter you are using. While aperture is King, in the Big Smoke smaller apertures are the Knight in Shining Armour! Telescopes collect light, and hence light pollution too. Me, from my home in Sydney I now never set up my 17.5", instead limit my aperture to no more than 8".

There's also another issue with Bigger Apertures - they are also more sensitive to the prevailing seeing conditions of the night. Particularly if you are maxing out the magnification. Dropping the aperture sometimes means the difference between a productive session or a night of frustration...

My last dark sky session, seeing was abysmal. Two of my mates had their 18" dobs, but I only took my 8" f/4 dobbie. They had a crap of a time as even their lowest magnification was too much for the conditions. Yet I was only doing low power wide field observing and the image I had was sensational!

For those of you not aware of this, ripping the maximum amount of magnification out of your scope is not the best use of any scope. There's a lot of factors at play, seeing (atmospheric stability), transparency (clarity of the sky, humidity, fog, smoke, etc), aperture size, object size andx brilliance. And it takes a bit of time to understand ho w they all come together to play nice or give you grief!

Alex.

Last edited by mental4astro; 28-09-2018 at 03:26 PM. Reason: Typo
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  #23  
Old 28-09-2018, 02:39 PM
75BC (Brendon)
Always in the dark.

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Thanks Alex. Learning so much here and realizing I still have a lot to learn.
Even things that can seem obvious once you know such as why everything looked green through the filter I use.

Must be a common newbie mistake of cranking up the magnification but it took me a long time to learn, use less – see more. On planets this (eventually) became obvious. A bigger image does not equal more detail.
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  #24  
Old 18-10-2018, 03:09 PM
stevous67 (Steve M)
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Visual astronomy can really bore me, but this was great to read. Thanks for the huge effort!

Regards

Steve
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  #25  
Old 19-10-2018, 10:48 AM
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mental4astro (Alexander)
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Thanks Brendon and Steve

I'm glad to read that you have found my work useful.

It occurred to me a few years back that there must be hundreds, if not thousands, of scopes sitting in boxes that haven't been used in years, and only pulled out of their boxes just two or three times because of the unrealistic expectations that people had, in not knowing how to handle their scope, not knowing about how seeing affects things, and impatience.

I've also rescued a couple of scopes from the Big Rubbish Council Cleanups, fixed them up, made a new dobbie mount for them and found new homes for these scopes that were on death row. I just couldn't bear the thought of just letting perfectly good scopes, however modest, be trashed. These little scopes have seen more use now than they ever did

Alex.
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  #26  
Old 06-11-2018, 09:03 AM
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mental4astro (Alexander)
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In my Observing the Moon & planets thread, I've provided a series of Challenges. I thought I'd do something similar here with this thread, but more along the lines of a theme than a Challenge. This is mainly because of the wide variety of apertures that scopes come in so I don't want to create a set of challenges that immediately excludes small scopes. Instead, it will be a series of themes with (I'm hoping) two or three different objects that will be as both examples, but also offer a range of difficulty so to test your visual acuity and yes your scope/aperture - some level of challenge is a good thing!

Theme No. 1

Dark pillars

1, The Fish Mouth in M42: With Orion making its way back into our evening sky, the easiest Dark Pillar to see is an excellent way to start out!

The "Fish Mouth" is the name of this dark pillar. It is seen as a dark "finger" that points towards the Trapezium. The Fish Mouth is held together by a bunch of protostars that are hidden deep withing this column of gas and dust, and the collective gravitational pull of these protostars is resisting the erosive power of the enormous energy blast that comes from the Trapezium. The Fish Mouth is not a dumb black lump either. It is very much a 3D cylindrical shape, and if your scope and eyes are up to the task it is possible to see the variation in shading that the Fish Mouth has.

Another great thing about the Fish Mouth is it can be easily seen under urban skies!

1A, The Trapezium. While we are looking at the Fish Mouth, lets look at the Trapezium - this pocket sized dynamo packs a real punch not just in terms of power output, but there are many more than just four stars that make up this small open cluster. Four stars are easily visible even in a 2" scope. 6 stars are visible in larger scopes from 4". Here's the real kicker - push your aperture to 14" and over, and you will be able pick out at least 10 component stars!

2, Eta Carina's skeletal fingers: In the very first post I made in starting this thread, both the NASA Tour and my sketches, show the skeletal-like fingers that are a couple of dark pillars in this huge nebula. These dark pillars are visible in scopes larger than 4", and if you are under urban skies they are JUST visible without any filters in an 8" scope. Under dark skies, these are easier to identify.

3, The Pillars of Creation of M16: Ok, this one is for BIG scopes! The dark pillars that form the Pillars of Creation, also known as the Eagle that gives the Eagle Nebula its name, is a real challenge to see. Aperture is one, but also good transparency is necessary. If transparency is not up to scratch, you will see the glowing mass that is this nebula, but the dark pillars will not be distinguishable. The night I first saw these I saw them through my 17.5" scope. Later on that same night I had a look at it through a 25" scope, and the pillars were totally invisible all because in the hour between my last view of them in my scope and looking through my friend's scope, the transparency of the sky went to pot!

I think this is a good start to this series of observation themes. If there is any theme or object you would like me to offer, please just ask!
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  #27  
Old 06-11-2018, 09:42 AM
gjr80 (Gary)
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Thanks Alex, looking forward to running through this theme, but you do realise this is the equivalent of buying new equipment, I am now doomed to days (nights) of cloud...


Gary
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