View Full Version here: : What are they all used for?
29-01-2011, 10:30 PM
G'day, I've been looking at Filters, and it is seriously blowing my mind what different filters are out there for Astronomy. I like photography a lot, and the norm, as most of you know is, UV (good lens protector), Polariser and ND at various ratings. Now I've bought the scope, and eyepieces, a Hydrogen Beta filter as well as a Circular Polariser. Now that I'm learning a bit about Astophotography, I'm finding out, that there are more filters out than I can poke a stick at.
1. Is there a listing available that explains what each filter does?
2. I understand that some bring out colour, that we would normally not see. So is there a "standard", for lack of a better word (like "the Horse head is actually that colour"), or is there scope for artistic license, because we wouldn't normally see them like that anyway?
3. If I could cruise around Orions belt, what would I see? When showing photographs of different Nebulas and the like, is there an "ethic" when saying "that this is what it is and looks like", but not realy, because we as humans wouldn't see them that way.
Sorry for the 2 heavy questions. I understand that Astronomy is a science, which I would know 5% of. It just seems like it is a type of Photoshop before we load it up to Photoshop or Stacking program and it led me to think about what you would say to people.
I love the Astrophotographs that I see people post here and elsewhere, and I hope to capture some great shots myself in the near future. I see now it doesn't stop with what I have spent already, it is only the beginning of a lower bank balance :screwy:. No wonder Ron from Sirius says to me "Hi, Dutch, am I eating steak tonight?" Cheeky Bugger :rofl:
30-01-2011, 01:49 AM
1: Have a look at this Lumicon filter site. It explains everything about what EACH filter does, and their best application:
2: In the visual world, we, as humans, are limited in how sensitive our eyes are to light. If the illumination, or brightness, falls below a certain level, we see things in "black and white", really in shades of grey. The colour of things is still there, just that our eyes cannot perceive them. Have you ever noticed this in a very dimmly lit room, like your bedroom with all the lights out? Another surprise our eyes have for us is that their most light sensitive region isn't our central vision, but it surrounds it. Remember hearing about "rods and cones" in our eyes? That is why "averted vision" is the trick to viewing deep sky objects - the central vision is packed solid with colour receptors, sacrificing light sensitivity, ;) . Light sensitivity is then a perifferal sense, which allows us to "see out of the corner of our eye", tiny variations in light is what this region is best adapted to.
Then there is the added variation in colour sensitivity, but this is of no consequence in astro-visual as DSO's are only seen in shades of grey.
3: Long exposure photography allows the colour of dim objects to be seen, again, not because they are not there, but we just can't see them.
However, if your scope is large enough, we can begin to see colour in only the brightest of DSO's, like M42, The Great Orion Nebula in Orion's belt. Through, say, a 17.5" scope, you can actually begin to see shades of pink tinge some of its filaments. It also helps to be in a dark site as light pollution also impeeds contrast. The Horsehead, in contrast, is a shadow imposed upon a very, very dim emission nebula. The nebula glows in red, but we can never see its colour as it is so dim, except in photos.
There is another thing about "colour" in objects, particluarly nebulae. There are two types of nebula: emission and reflection. Emission type glow because their gas is being 'excited' or ionised by the nebula's related star or cluster, and glow in the red end of the spectrum (the Horsehead nebula). Reflection, are just that, the gas is reflecting the associtated star light, this being blue in hue (the Pleiades). Then there are nebulae that have both emission and reflection components, so they glow both in the red and blue (like the Orion nebula). That is why there are various nebula filters listed in the Lumicon guide, different filters transmit different wavelengths according to the type of nebula you are trying to see or photograph.
30-01-2011, 02:06 AM
Wow, great link Alex!
Far better than the one i was going to give.
Boy, that is very comprehensive. I have Bookmarked it thanks :thumbsup:
30-01-2011, 08:22 AM
Yeah, excellent info! :2thumbs:
30-01-2011, 11:14 AM
Wow Mental, that is the most simplest of all explanations that I have heard to date :thumbsup:. Thank you so very much for that response, it has simplified what became a complex subject for me. As for the "ethics" debate, I suppose that if questions get asked about my images, your explanation covers the subject beautifully. I get it now 100%. I already knew about the rods and cones and also periferal vision, but it is still useful information to any Newbie who didn't know this.
That link you provided to the filters is absolutely brilliant, and has gone straight into my favourites. That site explains and covers everything I need to know about the filters, and it looks like others are going to benefit as well.
A thousand thank you's :bowdown: you have taken days of research away with that link and stopped the question regarding presenting images right in its tracks. I hope to catch up with you one day, Alex, because the first dozen rounds are on me. I deeply appreciate your comments and help.
To the moderators :- I think Mentals explanation regarding the filters is well worth being in the Beginners section as a "Sticky", with the link to the Lumicon site. I understand that it is brand orientated, however they explain it very well.
An example of performance between a UHC & OII filter, is probably well shown here in an observation report I did on the Orion Nebula, comparing magnification & filter usage. Different nebulas use different filters. Using the OIII oxygen filter on Orion certainly brought out bright Oxygen areas, however though, it took away much of the detail and contrast that the UHC provided. In general, as a whole, a UHC is the filter of choice for this neb.
I've found the following link very helpful to me, and certainly cuts out a lot of the time trying on filters to get the best performance. It gives a comprehensive list of nebulas and which filter is best suited to each one.
And here is another link that I find particularly useful. It goes into detail on how all the different filters perform. I've had this link for ages, but after just re-reading it, I see I had previously missed the bit about using the OIII filter to split double stars that are bright! :eyepop: Antares in particular (as it's difficult to split as it's so bright & large). It says the filter makes the two stars different colours making it easier to split the brighter star from it's counterpart. The brighter star turns red and the star turns blue. Interesting!
Have lots of fun with your filters, Dutch & enjoy! ;)
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