Pros: Easy to use, defeats night vision and achromatic telescope color problems
Cons: Photographic use only, somewhat expensive
The Orion Hydrogen Alpha Filter is a powerful tool for photographing nebulas in the night sky. What it does is block out all light in the visible spectrum except for extreme red, which has the effect of blocking out all light pollution from the insane number of lights used today in favor of the deep red (656.3 nanometer wavelength) color of the hydrogen clouds stars form in when they are lit by the intense ultraviolet light from new stars. This filter works well and is fairly economical, and will effectely defeat light pollution, including the bulk of the sky glow caused by having the moon in the sky.
This filter WILL NOT work for safely viewing the sun. There is no configuration where this filter will allow isolating solar hydrogen alpha in a telescope. Attempting to do any solar observations without a specially made solar blocking filter, which securely covers the front of the telescope and blocks over 99.99% of sunlight from ever entering the optical tube will result in permanent blindness to observers and/or damage to equipment. This is the most dangerous thing you can do with a telescope, and must be taken seriously. If you do a search on the web, you will see hundreds of posts to user groups suggesting doing this on the net: THEY ARE ALL WRONG. DO NOT TRY IT.
-End of Safety Note-
OK, you've got me- it's a photographic filter. Your camera gets to see red; you get to polar align the scope. The way this filter is used is to install it between a camera and a telescope on an equatorial mount tracking a spot in the sky. I have been doing my own work with this filter using the excellent Minolta Maxxum 7D. Since this camera is a digital SLR, there is a major difference in the results I am seeing from what a monochromatic astronomical camera would see; the image in the Maxxum is the color of the image from the telescope, where the image would be black and white in a monochromatic camera. This has the effect of pre-registering the results to what color they should be, so my review does not include techniques on how to color-correct your results.
Hydrogen alpha is a color of red caused by only one thing in nature: hydrogen gas getting hit with lots of energy. Depending on whether it is out in space, or in a star, it can show up in two ways: (1) thin clouds of hydrogen gas between the stars glow as their electron gains and loses energy when hit with ultraviolet light. (2) Hydrogen gas heated in stars to temperatures over 6000 degrees C absorbs this color in the reverse process. This is sort of like the differnece between the cup of coffee heating your hands, or the cup of coffee being heated by a hot plate.
As similar as these sound, there are a few differences in the processes which make seeing hydrogen alpha in deep space, which lets us see star-forming nebulas, a very different project from seeing hydrogen alpha light in the sun, which makes solar prominences and other dramatic features visible. In the case of deep space, nebulas are the only thing emitting light anywhere near the color of ionizing hydrogen, so blocking off light from about 650nm (red, but a little more towards orange than hydrogen alpha red) to 350nm (violet) is all you have to do to make this color visible. The sun, on the other hand, is a fiercely heated globe of gas emitting light continuously from infrared through ultraviolet and into gamma rays, and in this environment, the fact hydrogen is sensitive to this wavelength actually makes it absorb rather than emit this color, so it is mostly visible in hydrogen gas as it reaches out past the edge of the sun in solar flares and prominences. In this case, the fact the sun is emitting all other colors of red means a filter has to look at exactly hydrogen alpha, so manufacturers such as Coronado make telescope filters which only do this, and will call out light pass amounts of 7 nm (.7 angstrom) or less to make this visible.
For nebulas, depending on their composition, they may have characteristic colors from Hydrogen Alpha (red), Hydrogen beta (a different electron energy state which causes a blue-green photon to be emitted), Oxygen III (a green color from oxygen atoms), and sometimes a few other elements such as Nitrogen (green) and Sulphur (deep red). What these colors mean is the cloud of gas has different ingredients in it. So, for example, the Orion Nebula is largely composed of hydrogen gas, and some amazing hydrogen alpha images show this, such as in this famous image by master photographer Robert Gendler: http://www.robgendlerastropics.com/OriondeepfieldS.html.
Hydrogen is what the stars are forming from, and if it is lit from behind, it will glow. Other elements mean the gas cloud includes material from stars which have formed, fused hydrogen into heavier elements, then either exploded or blew theis material out as they aged and died. So, these colors can indicate what a nebula can form- hydrogen makes stars, other elements are the material needed to make planets.
Not all nebulas are visible this way; if the gas and dust lit by nearby stars is on the other side from them, the nebula will appear blue in color and is a reflection. The Pleiades star cluster actually has one of these near it, again easily seen in a photo from Robert Gendler: http://www.robgendlerastropics.com/M45STLmosaicS.html. This sort of nebula is not helped by using a hydrogen alpha filter.
Isolating hydrogen alpha from deep space can be done with the Orion hydrogen alpha filter for about $80. Isolating hydrogen alpha in the sun can be done for $600 and up with a specally designed solar hydrogen alpha filter set, or a telescope specially made only for this purpose with an integral hydrogen alpha filter. This is actually a very special set of matched filters designed to prevent most sunlight from even entering the telescope, and then tuning the small sliver of the spectrum they allow in to see only hydrogen alpha light from prominences and flares. So, it is sort of the difference between finding a needle on a glass table and finding it in a haystack.
The Orion Hydrogen Alpha filter arrived in a small box like a photographic filter. Inside was the filter in a padded storage case and a set of instructions. This filter looks like a semi-opaque piece of glass, and looks like a mirror in some orientations. If held up to a household light, it simply looks like an all-red filter. The housing is a well made machined ring with threads to directly attach to the barrel of an eyepiece or 1 1/4" T-adapter.
In practice, the Hydrogen alpha filter is a little difficult to use at first. The first telescope I attached it to was the Astronomy Technologies AT-66ED, a small 66mm diameter f/6 telescope, which I have been using this winter to produce my own images of the Orion Nebula. This is one made without the filter, where I have had a consistent problem with Hydrogen Alpha fighting to be seen in the presence of Sodium street and building lights: http://www.buytelescopes.com/gallery/view_photo.asp?pid=10915&c=35913
The AT-66ED is an apochromatic refractor (note, I have asked Epinions to list this product, so when they have a chance to do so, I will review it). This scope has a focal length of 400mm, so in photographic terms, it acts like a 400mm telephoto lens at f/6. This brightness is similar to Schmidt Cassegrain (SCT) telescopes such as the NexStar 5 or NexStar 8 when they are used with the f/6.3 Focal Reducer, though with their longer focal lengths, they show a smaller field of view, but the little AT-66ED gives a sampler of what sort of sensititivity the larger scopes will have.
The problem with light pollution is the blue and green bands are far less affected by light pollution than the red channel, which can see sodium lights, as the more blue Mercury vapor lights have been replaced over time. The Orion Hydrogen Alpha filter offered a solution to this since I was losing about 25% of my data depth to light pollution when the filter only takes an 8% toll. More importantly, the single color offers the possibility of getting high resolution images by using telescopes faster than f/6 in the city, which normally pay such a large price in light pollution as to be nearly unusable.
So, for my first experimental series I set up the Maxxum 7D with the Hydrogen Alpha filter on the T adapter and installed it in the telescope. Immediately, I could see just focusing would be difficult as I could not see stars in the eyepiece. Even Sirius, the brightest star in the night sky, did not come through. I was trying this with a half-illuminated moon high in the sky, so I used its light to get the rough focus done. I put the little scope back on Sirius and found it as a little red dot. I fine tuned the focus and marked where it was on the focuser (if you have the ability to do this, it will help). I moved to the Sword in Orion, where the Orion Nebula is, and could see absolutely NOTHING. So, I shot a 30 second frame at highest sensitivity and out came a red image of Orion of to one side of the frame. Using this, I adjusted the mount in the direction to center my field. Note, a GOTO computerized mount will work well for this part since they usually are close to being centered on the first approach, though it is probable you will want to rotate the camera and center the scene manually. I took a few shots with exposures up to 2 minutes to get a feel for what happens.
With this done, I programmed the camera to take a series of 30 second frames in interval mode, and since it was already 35F outside, I went in to warm up. After shooting the series, I went out and retrieved the telescope and camera.
The images of Orion taken with the filter are brighter and more detailed in the red band than previous sets. Since even the frames taken outside the city have some light pollution, it does seem the filter is improving absolute sensitivity, even with the light loss. The camera does have its own sensor artifacts, especially in long exposures, where the top left of the frame is lit as if by white light. One very interesting thing was the stars of the trapezium at the center of the Orion Nebula, which usually are washed out by the glare of the bright gas cloud around them, were sharp and distinct in the green and blue channels on the camera. Since the camera can see infrared, and the filter passes all light below Hydrogen Alpha, the green channel contains an IR view of the scene. This is a nice plus since an IR scene can be used for emphesizing stars if you are using a digital SLR, but if you are using a black and white astronomy camera, it means you should use an IR blocking filter in series with this one.
The next experiment was to see if a refracting telescope with a the same focal length but poor color correction could be made to work as a single color scope. The idea here is to overcome the main problem of refractors, where all colors in the spectrum try to come to focus at a different point due to the same refraction effect used to show the spectrum with a prism.
In practice, the red color does all come to one focus in a telescope such as a Short-Tube 80mm refractor, an inexpensive f/5 telescope which produces an image about 47% brighter than the f/6 66mm scope I was using, but which would produce disasterous photographic results if I just tried to shoot an image with no filter since the colors for green and blue come to such different focal points in this telescope, the image would be very indistinct with a lot of false color haloes around stars. Since the two telescopes have the same focal length, the idea is to have a high quality all-spectrum image from the 66mm enhanced with hydrogen alpha red.
The results here were interesting, since the color of red you are focusing is relatively close to infrared, the focus ends up being sharp for the red and still fairly sharp for the infrared as well. I found it is very tempting to leave the IR channels in an image, but strangely enough, the IR light is poorly focused enough in the portion the blue channel ends up recording to actively degrade the full-light image if combined with it at full brightness. On the other hand, it did separate out some features, such as the trapezium, which are fully washed out in the full spectrum image. More importantly, since red is all the way at one end of the focus region, the image ends up being taken at a slightly different focal length, so the whole field is enlarged by about 1.5% Image programs such as Gimpshop are full able to correct this, but it means you will not be combining frames taken this way with the other data set in an image stacker; they will have to be handled separately. The end result is the data is subtle, but must be combined with care.
One of the strange things is how the green channel only shows part of the IR image. Taking the frame to extreme brightness showed dark holes in this band corresponding to where much of the nebula and stars were. Apparently there is an IR compensation algorithm used by the camera to control the signature in the green channel. More interesting than this, the results directly contradict an assumption made by people who have started modifying cameras to make them more sensitive to red by replacing their IR filters on the front of the camera sensor. They are assuming the IR blocking is a function of this filter, when simple experiments such as taking a remote control and taking a picture of it while holding down a button show the cameras can see IR. The truth is the cameras have no trouble seeing IR OR hydrogen alpha; the photographer needs to set a white balance where the camera will write the data to the file. So, for example, a fluorescent light white balance results in a much brighter Hydrogen Alpha image than the tungsten setting, which assumes the light on the scene has a strong IR and red component out of balance with the true color of the scene.
The most impressive result with this filter is how it makes urban light pollution vanish from the scene. Full color photos are still difficult in the city since light pollution interferes with them. But for Hydrogen Alpha red, a color the signature of sodium lights usually interferes with, the filter is a new lease on life for urban imaging for me. At the same time, its properties allow comparitivly inexpensive telescopes to be used, rather than attempting imaging with extremely expensive large aperture apochromatic refractors.
An unexpected benefit of this filter is the ability to take IR images with a Digital SLR. While this feature means someone using a monochromatic camera will have to put in an IR blocking filter, the Orion Hydrogen Alpha filter lets the observer get a separate IR image which can be used for spot enhancement of features lost in other wavelengths. For someone ready to get into advanced imaging, the Orion Hydrogen Alpha filter is a key to the door.