DISCLAIMER: All this information is accurate to the best of my knowledge; if there are any ommissions or errors, please let me know. This document is intended to be an overview, not the end-all-and-be-all of a given topic. If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section.
(1) Do LPRF's work?
Light-pollution reduction filters (henceforth, LPRFs) *do* work -- on some
things. They are not a substitute for a clear sky, but they can and do provide
improved views of many objects. Furthermore, filters can be of use even in
very dark skies, not just ones that are significantly affected by city lights.
(2) How do they work?
Filters work by rejecting light in certain frequency ranges. As you may already
know, many artificial light sources emit over fairly narrow wavelength ranges.
Chief among these are the wavelegnths associated with LPS (low-pressure sodium),
HPS (high-pressure sodium), and mercury vapor lamps. Sodium lamps produce
light centered on the sodium D line at 589 nm (yellow), with HPS covering
a much wider range than LPS; mercury lamps produce a number of lines throughout
the spectrum, particularly below 450 nm and above 550 nm. All LPRFs are designed
to block the wavelength regions associated with these sources.
As you might have guessed, there's a catch. The LPR filter has to not only reject the undesired light, but also admit the desired light from your nice, friendly Messier objects (or whatever).
(3) What do LPRF's work best on?
The most favorable circumstance occurs with emission nebulas; these are things
like planetaries (e.g., the Ring and Helix Nebulas), supernova remnants (M1,
the Veil), and other large diffuse nebulas that emit, rather than merely reflect,
starlight (other good examples: M42, the Eta Carinae nebula, M17). These nebulas
emit light in only a few wavelengths, predominantly those of oxygen III and
hydrogen alpha and beta.
By a fortunate coincidence none of these wavelengths is near significant(*) light pollution lines. OIII and H beta are located near 500 nm well away from most interfering lines; H alpha, at 656 nm (red) is less important visually but important in photography and sometimes in very bright nebulas like M42. Consequently, LPR filters are also designed, in addition to having very low transmittances for lines associated with light pollution sources, to have very *high* transmittances for these nebula lines. Thus LPR filters tend to work well on emission nebulas.
(*) For the technically...shall we say, "thorough," I should point out that HPS lights do put out a line very close to the OIII lines. This line is comparatively minor and does not normally affect the performance of a LPRF designed to admit the OIII region.
(4) What don't they work on, and why?
As already mentioned above, filters work by rejecting the light from various
undesirable sources. In general, filters are designed to admit a fairly narrow
region of light, called the passband (or bandwidth), around the desired lines.
The rest of the visible spectrum, in general, is heavily blocked. As a result,
if an object produces much of its light outside the passband, it will tend
to be attenuated strongly, much as a light pollution source would be.
Sources of this sort include stars, and anything that is either composed of stars (galaxies, clusters, multiple stars) or reflects starlight (reflection nebulas, e.g. M78 and the blue portion of the Trifid Nebula).
The degree of attenuation, of course, depends on the object and how much of its light is being rejected. In particular, it is physically possible for an object like a galaxy to be attenuated less than a light pollution source, which should lead to its being enhanced. In practice, however, this does not occur often. Much of the time, in fact, the filter actually makes things worse. This is not a hard and fast rule, but it does hold enough of the time to merit strong mention.
In any event, enhancement of these objects is not as great as it is for nebulas. This last statement holds for all currently available LPRF's.
(5) Who makes them?
Many places. Most large telescope/accessory manufacturers or resellers, such
as Celestron, Meade, and Orion, offer some. The dominant company in the filter
market, however, is Lumicon, which has (at last count) five distinct LPRF's,
all with different technical specifications and performance on various types
of objects.
(6) What does all that technical stuff mean?
The most important technical specifications for a filter are:
a) the width of the passband(s),
b) the transmittances in and outside the passband(s).
For determining general usage, (a) is the most important, as this detail tends to dominate the filter's effectiveness (or lack thereof) on various objects.
LPRF filters are characterized as being either broadband or narrowband, depending on how wide the bandwidth around the desired lines is. These descriptions are a bit vague; here are some figures taken for some popular filters.
Name Type Bandwidth
(at 50% transmission)
Celestron Type A Br 47nm
Meade #908 Br 35nm
Lumicon Deep Sky Br 68nm
Lumicon UHC (Ultra High Contrast) Na 27nm
Lumicon OIII (Oxygen III) Na 11nm
Orion Sky Glow Br 85nm
Orion UltraBlock Na 24nm
(figures taken from _Astronomy_, Feb. 1991, p. 77)
Extremely narrowband filters, such as the Lumicon OIII and H Beta, are sometimes called "line" filters, since they only admit one or two emission lines and reject nearly everything else. These are the ultimate for enhancing dim objects -- provided said objects happen to emit an appreciable amount of that particular line...
In general, the following holds true for narrowband filters when compared to broadband ones:
1) Superior enhancement of emission nebulas.
2) Darker sky background and greater blockage of light pollution generally.
3) Worse performance on non-nebulas (galaxies, stars, clusters, comets, etc)
You would thus want to use something like the UHC, OIII, or UltraBlock for viewing dim planetaries (e.g., the Helix) under difficult conditions, but using one on a galaxy or globular cluster is not a swift move!
(7) Which one should I get?
If you have only enough money for one filter, I suggest getting a good narrowband
version, such as the Lumicon UHC or OIII. One of these will optimize viewing
of nebulas under all sky conditions. If you have enough to buy two, you might
consider a complementary pair -- one broad, one narrow -- but in this case
I'd recommend trying some filters first (e.g., at an astronomy club meeting)
before committing to a purchase. This is particularly true for broadband models,
since the benefits are more subtle.
Some filters are "specialty" filters, which work well on a small (but possibly important) class of objects. The most notable example of this is the Lumicon H Beta filter, which strongly enhances a few emission nebulas (notably IC 434, the emission nebula surrounding the Horsehead in Orion), but does not work as well as other narrowband filters on most others. These are not recommended for a first purchase.
(8) Other possibly useful information
Light pollution isn't limited to cities. There are naturally occurring sources
of light (no, I mean *besides* the Sun and Moon, etc. :-), most notably something
called "airglow" or "auroral" glow. This is fluorescence produced by the air
itself (more precisely, by molecules in the upper atmosphere). As with many
light pollution sources, airglow predominatly occurs at a small number of
frequencies, particularly at 465, 558, 630 and 636 nm. Since many filters
block these regions, or at least reduce it significantly, they can be helpful
even in very dark skies with little artificial light. The line at 558 gets
munched by all filters; most narrowband filters will also eliminate the one
at 465 nm as well. Judging from transmission tests, filters that attenuate
the red (630 and 636 nm) airglow lines include the Orion Ultrablock and (to
a noticeably lesser extent) the Lumicon UHC and DeepSky filters.
Dim nebulas seen from dark skies frequently benefit from filtration because of the removal of this natural airglow. This is most noticeable on large, low surface brightness nebulas such as the Veil Nebula and many planetaries, e.g., Jones 1 or any of those little #$%@#$&*!! things labeled with a P-K number in Uranometria.
Another tip: Unlike ordinary color filters, which work by absorption, LPRF's are highly reflective. As a result, observing with a LPRF requires some extra care in screening out stray light. Light that enters the eyepiece from the outside, e.g. around your eyes, will be reflected back by the filter and interfere with viewing.
Since LPRFs work on the same principles as interference filters, the exact position of the bandpass depends on the angle the filter's surface makes with the incoming light. (You can demonstrate this for yourself by taking a LPRF, looking at a light through it, then watching its color change as you tilt the filter.) This is not a problem for normal use, with the filter inside the eyepiece, but if you're using it to "blink" for nebulas (i.e., rapidly moving the filter in front of and away from the eye end of the eyepiece), you'll want to make sure the filter is parallel to the eye lens. Furthermore, if you're "blinking" with a wide-angle eyepiece (a Nagler, say), light emerging from the edges of the field of view might experience enough of a path length difference at the filter than light coming from the center. This applies only when the filter is used *after* the eyepiece, as in blinking; there is no problem when the filter is mounted normally. In theory this could lead to a change in the position of the bandpass, making nebulas harder to see at the edge of the field of view. I haven't observed this effect myself; owners of wide-field eyepieces and filters are encouraged to try this and comment.
Finally, remember that filters never make things brighter; they make *everything* dimmer, to varying degrees. What happens, of course, is that things like nebulas are scarcely dimmed at all, whereas the sky background (containing light from artificial sources, airglow, etc.) is greatly darkened, thus leading to improved contrast. However, the resulting image may still be fairly dim, especially in a smaller telescope. Patience is advised. Before hunting dim, challenging objects with a filter, practice on easier targets first. Try a big bright nebula like the Lagoon or M42 to get a feel for how much the filter improves things.
(9) Where can I find more information?
Several places.
(1) Telrad vs. Conventional Scope
(2) How big should I go?
(3) What other features are relevant?
(4) Who makes them?
(5) Where can I find out more?
(1) Telrad vs. Conventional Scope
Most people have seen conventional finders before -- they're just little telescopes
that have cousins in areas well distant from amateur astronomy (monoculars,
rifle scopes, etc.). Like all telescopes their main purpose is to gather light
and magnify the image -- but not too much, since the main goal is to have
a wide field for finding.
Newcomers often hear about this thing called a "Telrad" and wonder what it is. Telrads are well known to experienced amateurs, but are often less familiar to newcomers to amateur stargazing. In a nutshell: A Telrad isn't a telescopic finder. It's a heads-up display. You look through it and what you see is a bullseye pattern about 4 degrees* across, which appears to be focused at infinity. By centering the bullseye on a star that's also in the field of view of the telescope, you align the Telrad and are ready to go. Using one on naked eye objects is easy -- you simply push your scope until the bullseye is directly centered over the object, and presto, there it sits in the scope (at least with a low power eyepiece).
*The diameters of the circles in the bullseye are 4 degrees, 2 degrees, and 0.5 degree -- the last is small enough to ensure that centering the object will put it in the field of view of a low power eyepiece (assuming the Telrad is aligned properly).
TeleVue, which makes a variety of eyepieces and other astronomical gear, offers a similar finder, except that it projects a single spot onto the sky rather than a bullseye.
Which is better? This is one of the perennial religious wars on the sci.astro groups. I personally like Telrads on Dobsonians and conventional finders on equatorially mounted scopes. If you have enough naked eye stars around to make "star hopping" feasible, and a good set of charts, using a Dobsonian or other alt-az scope with a Telrad becomes very fast and natural with only a little practice. On the other hand, on equatorial scopes, a conventional finder of known field of view simplifies finding, since you can "step off" in known increments of right ascension and declination towards your target -- handy if you know the coordinates.
Try both and decide for yourself.
(2) How big should I go?
Depends on your scope. For medium size scopes, say about 4" - 8", a 50mm finder
is a fairly common choice. Owners of large aperture scopes often put a large
finder on, such as an 80mm or even a small conventional refractor, to make
finding dim targets easier. IF you have a small telescope such as a 60mm refractor,
you probably don't need more than a 30mm finder or so.
If you have a finder, smaller than 30mm, e.g., 5x24, odds are it's poorly made. Save up the bucks and buy a good 30mm or similar finder from one of the major outfits, e.g. a Celestron or Meade dealer.
(3) What other features are relevant?
Commercially made finders often have cross hairs, for easier centering of
objects. Many models are available in either a straight through version or
a right angle version (accomplished with a diagonal inside the finder itself).
The right angle versions have the advantage of being more comfortable from
more positions, but have the serious disadvantage of introducing an extra
reflection in the image, so that the sky appears mirror-reversed when compared
to charts.
Sometimes, you can get finders with Amici (erecting) prisms, which don't suffer from this reflection.
(4) Who makes 'em?
Conventional finders: Same places that make or sell telescopes. Orion, Lumicon,
Astronomics, 'most any Celestron or Meade dealer...
Telrads: Steve Kufeld, 7092 Betty Dr., Huntignton Beach, CA 92647. Normally, though, they're available from major retailers, so you shouldn't have to bug Steve.
(1) Huh?
"Collimation" refers to the alignment of optical elements in a telescope.
In portable telescopes, particularly reflectors, these elements can be moved
enough out of position (through minor bumps and jolts when moving or transporting
the telescope) to seriously affect the quality of the images they produce.
(2) The procedure
This is a very brief overview and is not a substitute for a more detailed
account (see the References below for several very good ones). In brief, for
a Newtonian reflector, you need to do the following:
There are two basic tools: the sight tube and the Cheshire eyepiece. The sight tube is designed to make the preliminary stages of collimation easier. It's nothing more than a long tube with a pinhole at one end. It fits in the focuser, like an eyepiece. What it does, in effect, is projects the focuser downward, so you can see more easily if the secondary mirror is concentric with the focuser. Many such tubes also have cross hairs at the far end, which makes it easier to align the tilt of the secondary and get the reflection of the primary properly centered.
The Cheshire eyepiece is used to adjust the primary mirror -- the last major step. It's an eyepiece-like plug with a washer-like insert tilted at 45 degrees to the axis. When you illuminate the washer (there's a hole in the side of the eyepiece for this purpose) and look through the top, you see a donut shaped reflection in addition to the usual reflection of the primary. You then turn screws on the primary cell until the center of the primary lines up with the center of the washer. Many amateur astronomers (carefully!) mark the centers of their mirrors to make this adjustment easier. In fact, it's very difficult to use a Cheshire eyepiece without some sort of reference marker.
There are a number of other tools out there too. There is a device called an autocollimator which can be used for very fine adjustments (no, it does *not* automate the collimation process, I am sorry to say) -- it's much less important than the others I mentioned. There is also a new breed of laser based collimation tools, about which I know relatively little.
(4) Where to get them
You can make a sight tube yourself out of a plastic 35mm film canister, by
poking a pinhole in the lid and putting it in the focuser. This *does* demand
some respect for mechanical tolerances (that pinhole should be accurately
centered, and the canister shouldn't have any slop in the focuser). Other
tools have to be made or purchased elsewhere. The principal vendor for collimation
tools in the US is Tectron Telescopes, which makes a line of large Dobsonian
reflectors. The address is
Tectron Telescopes 2111 Whitfield Park Avenue Sarasota, FL 34243
They also sell a booklet which goes into gross, disgusting, and useful detail on the procedure for collimation.
(5) Other resources
(1) What solar filters are safe?
The simplest rule to use is the following: If you don't know its transmission
characteristics in the ultraviolet and infrared spectrum (as well as the visible!),
don't use it.
A solar filter is safe if it blocks out a reasonable (see section 2) fraction of all solar radiation wavelengths that can damage the eye. An important thing to remember -- IR and UV can be just as dangerous as visible light. IR, in particular, poses a severe hazard since too much IR, focused on the retina, will cause a thermal burn consisting of permanent damage to the cells therein. UV is generally less immediately dangerous to the retina -- largely because it tends to get absorbed in places like the lens, where it too can cause damage.
(2) How can I tell if a filter is safe?
Easy -- find out if it violates the rule above. The simplest way to do this
-- if you have the requisite equipment -- is to measure its transmittance
over an appropriate wavelength range. In general, the fraction of light transmitted
should be well under 1%, and preferably under 0.1%, over a wide range including
the visible and both the near UV and near IR.
Most people can't do that, so the next best thing is to stick to filters which have either been deliberately designed to pass such tests (i.e., filters specifically designed for solar work), or which have not been so designed but nonetheless give suitable protection.
Safe filters include the following:
Unsafe filters include anything with high transmittance in UV or IR over a range including at least the near portions of each region (to ~250 nm in the UV to ~2 microns in the IR). "Filters" of this sort include things like crossed polarizers, color film, floppy disk media, many homemade substitutes like smoked glass, etc.
(3) What should I use to view a total/partial/annular/etc. solar eclipse?
You need to use a safe filter whenever a portion of the *photosphere*, the
brightest part of the Sun, is visible. The photosphere is the part of the
Sun that is visible through a good filter as a disk about half a degree across;
it's also the part that you see during sunsets through haze, or when projecting
the solar image using a telescope onto a screen. As far as eclipses are concerned,
cases in which some photosphere is visible are the following:
Contrary to widespread belief, viewing the total phase of solar eclipses does not pose a threat to vision -- the intensity of the corona and chromosphere is much too low to cause damage. The total light output of these outer regions of the sun is comparable to that of the full Moon.
(4) What do the numbers in welder's glass specifications mean (e.g.,
#14)?
The shade number (SN for short) is based on the optical density of the glass
in the visible region. If the fractional transmission of light through the
glass is T, then
OD = -log T SN = 1 + (7/3) OD
Consider, for example, a filter that blocks 99.9% of the visible. T = 0.001 and hence OD = 3, and SN = 1 + (7/3)*3 = 8. This would thus be typical for a #8 welder's glass.
If you plug 14 into the formula above, you find that the OD is 39/7, or 5.57... A #14 welder's glass transmits about 3 millionths of the light hitting it.
Note that this formula only applies to transmission in the visible; for IR and UV the specifications are a little more complicated, and not given by a nice neat formula like this. In the US the requirements for welders' glasses are given in the ANSI publication listed in the references. In general the tolerances for UV and IR are not quite as strict as those in the visible, which is part of the reason astronomers recommend the darkest shades of welders' glass for solar viewing.
(5) Given that I have a safe filter, is it suitable for naked eye/binocular/
telescopic use?
All safe filters can be used as naked eye filters.
Welder's glass is not optically flat. It is good for naked eye viewing, and possibly low-power binoculars. Used at high powers, it tends to produce undesirable distortions. The highest power I've seen anyone use a welder's glass on is 16x, using a small Newtonian RFT. In addition, the small size of welder's glasses (the largest sizes are about 6 inches square) makes them awkward on large telescopes anyway.
Both the mylar and the metallized glass telescope filters work well for telescopic use at a wide range of magnifications. There are two main difference, for practical purposes. The Mylar filters tend to be cheaper, and there is a difference in the color of the image -- aluminized Mylar renders the Sun blue, whereas metal-coated glass gives it a dull orange-red color. The difference is mainly cosmetic; some observers use normal orange colored filters in conjunction with aluminized mylar to give the Sun a more natural color.
(5) I have one of those in-the-eyepiece solar filters. What should I
do with it?
Take a large sledgehammer and make filter confetti out of it. Since these
filters absorb nearly all of the energy passing through your scope, they can
get dangerously hot and (if you're not fortunate) crack while in use. The
aforementioned over-the-aperture filters go on the sky end of the telescope,
thus rejecting most of the solar energy before it even gets into the scope.
Most scopes that come with such filters are small (60mm) refractors. This is a common size, and replacement filters that go over the aperture of such a scope should be available from major filter manufacturers. In particular Celestron offers a line of over-the-aperture filters; since this company also makes a number of 60mm telescopes, one of their filters ought to fit. See also under 5) below.
(6) Who makes 'em?
Lots of places. For welders' glasses, visit a local welder's supply store.
Note that #14, being an extremely dark shade, is rarely used by welders and
so you will probably have to order it specially. Cost is about $2 for a rectangular
filter about 2" by 4".
For telescopic filters there are a number of major manufacturers and resellers; your best bet is to get an issue of Sky and Telescope or Astronomy and check the ads. Some common names include Solar-Skreen, a Mylar based filter produced and sold by Roger Tuthill, Inc., and Thousand Oaks, a company that makes metal-coated glass filters. Celestron also makes a number of solar filters. Also, Questar ships its telescopes with a metal-coated glass solar filter, designed specifically for use with the Questar.
(7) References
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