Leica 8x20; Nikon Action 7x35; Vixen Apex Pro 8x42; Orion 15x63; Docter Nobilem 15x60
WO Megrez II 80 FD / APM 107mm f/6.5 / Mewlon 210 on DM-6 + Berlebach Planet
Quote:Kasia Narrow Band Super Nebula Filters, made to fit under the removeable eyecap of the Fujinon binocular.
Ray Teeter 18" f/3.5 Dob D&G 5.375" f/20(?) refractor 天体観測にも威力を発揮する、本格仕様の大口径双眼望遠鏡です。 興和ハイランダー
So this cabbie says: "Know who I had in this cab last week? Bertrand Russell! Wisest man in the Western World. And I says to him 'Bertie, what's it all about?' And do you know, he couldn't tell me..." Kenneth Williams
Filter performance does not depend on aperture, but rather it's the exit pupil diameter which is the first concern. And if course the nature of the target. All aperture does by itself is impose a limit to the smallest objects observable. Why, even the unaided eye looking through such a filter as an H-beta can bring out something like the lambda Orionis cloud.
Testing filters on such bright targets as M42 is not really useful. It's the dimmer stuff that many filters were made to better bring out of the the sky glow. In the case of emission nebulae, in order to work most efficiently filters *must* of necessity highly attenuate any continuum sources like stars, and of course the offending sky glow. A dark, seemingly near starless view is just what the doctor ordered.
I've attached 2" UHC filters to the front of commercial 10X50s and enjoyed awesome views of many nebulae, such as the California, the Heart and Soul, North America and Pelican, to name just a few. For my current right-angle bino (50mm and 60mm interchangeable objectives) I use even H-beta filters.
I stress again, aperture is no impediment to the use of narrow-band filters as long as you observe suitable targets, size wise.
Quote:The whole effect of contrast and enhancement with nebula/LPR filters is pure trickery to the eye. Even a strong emission object is actually slightly dimmer, as are all objects viewed with filters. The more narrow the filter bandwidth, the lower the optical throughput, and the dimmer the object, and background stars.
First and foremost observing love: naked eye.
Last but not least, telescopes.
And I sometimes dabble with cameras.
Quote:Quote:The whole effect of contrast and enhancement with nebula/LPR filters is pure trickery to the eye. Even a strong emission object is actually slightly dimmer, as are all objects viewed with filters. The more narrow the filter bandwidth, the lower the optical throughput, and the dimmer the object, and background stars.
There are three statements here. The first is false, the second is true, and the third is false.
The increase in contrast is not trickery; it's real. Contrast is the ratio between the object's brightness and the background brightness. A narrowband filter decreases the background brightness much more than it decreases the brightness of an emission nebula; therefore, it increases the nebula's contrast.
That's true both in theory and in practice. I doubt that anybody could see the California Nebula naked-eye or through small binoculars without using a filter. Holding an H-Beta filter in front of each eye, it's immediately obvious.
Filters do indeed decrease everything's brightness, because no nebula shines 100% in emission lines -- nor do all the lines make it through any given filter. However, the decrease is quite small in many cases.
It is not true that the narrower the bandwidth, the dimmer the object. For a strong O III emitter, any filter that encompasses the O III line will make the object similarly bright. Cut the bandwidth in half and the O III line still comes through just as strongly, though the extraneous light is halved.
The background stars are indeed dimmed, and that's often the whole point of the exercise. For instance, the main obstacle to seeing the North America Nebula clearly is the fact that it merges with some very rich star fields. Get rid of the stars, and the nebula stands out.
This is also often the case for clusters surrounded by nebulosity, like M16. Or for planetary nebulas where the nebula is significantly fainter than the central star.
Quote: It's good fun touring the sky and seeing how different objects respond to different filters. The very aggressive OIII filter is a nice one to experiment with, certain nebulae - particularly planetaries and SNR - just pop in a good OIII filter. M97 and the veil being memorable ones.
Quote:Dan, i'm talking telescopes mainly. I use a UHC and OIII with 5" and a 12" as I find the best effect with filters for me occurs with at least 100mm of aperture or more. I don't have very dark skies here so a pair of nebula filters on my 16x70s or 10x50 don't work too well (I tried briefly once).
In the case of emission nebulae, for all intents and purposes the *only* light we see is that emitted at the specific wavelengths passed by the filters we use. From a visual perspective (especially when dark adapted) we can eliminate the deep red H-alpha; our eye is so insensitive to it that it can never contribute more than a couple of percent to the visible light (except for a couple of very unique objects.) To our eyes, the vast bulk of nebular light is O-III and H-beta emission. Depending on the relative intensity of each emission component, we either employ an O-III, H-beta or 'UHC'-like filter, depending on whether we wish to pass one, the other or both.
In any case, because the emission lines are effectively of sub-nanometer width, the narrowest we can humanly make the passband the better, for then we increase contrast the most. And no desired emission light is lost.
Quote:Quite correct, Dan. Light entering an interference filter too far from perpendicular becomes 'de-tuned.' However, for even a fast scope of f/4, marginal rays enter a little over 7 degrees from perpendicular. A visual nebula filter can well handle this.