- What filters I have
- What filters I use
- What filters I deem essential
- And what filters I might want to buy next
* What I won't write about is filters for solar observation, because this is an whole different beast.
What follows are a couple of groups of more common filters:
Neutral Density Filters (ND)
When you want to look at the Moon with a telescope, you need to reduce the amount of light, as the Moon is rather bright. One could "stop down" the scope, but the more aperture one has, the higher resolution one gets. So the image has to be attenuated by some other means.For this I have several "Moon" filter, colorful pieces of glass that came with different scopes, but frankly I do prefer a ND filter for viewing the Moon. The reason is that the "Neutral" Density Filters do not change the color – hence the "neutral" in the name.
I have what is basically an "ND 09" filter, which is OK for most cases, in most of my scopes from 2.something-inch to 6-inch: Seldom do I need "more light throughput", most of the time I'd wish for even less light!
BTW: My filter is a photographic filter, which follow a slightly different nomenclature. Astronomic filters should have better optical quality, but my photographic filter seems enough for me. So getting one or two cheap astronomical ND filters should be enough.
Here is a list of some ND filter values (but not all values are commonly available!), where the naming of the filter commonly is derived from the optical density:
| ND-Filter | Alternative names | Optical density | Trans-mission (%) | Trans- mission (fraction) | F-stops |
|---|---|---|---|---|---|
| No Filter | – | 0.0 | 100% | 1 | - |
| ND 03 | ND50, 2x, 0.3, #101 | 0.3 | 50% | 1/2 | 1 |
| ND 06 | ND25, 4x, 0.6, #102 | 0.6 | 25% | 1/4 | 2 |
| ND 09 | ND13, 8x, 0.9, #103 | 0.9 | 12.5% | 1/8 | 3 |
| ND 12 | – | 1.2 | 6.25% | 1/16 | 4 |
| ND 15 | – | 1.5 | 3.125% | 1/32 | 5 |
| ND 18 | 64x, 1.8, #106 | 1.8 | 1.563% | 1/64 | 6 |
| ND 21 | – | 2.1 | 0.781% | 1/128 | 7 |
| ND 24 | – | 2.4 | 0.391% | 1/256 | 8 |
| ND 27 | – | 2.7 | 0.195% | 1/512 | 9 |
| ND 30 | 3.0, 1000x, #110 | 3.0 | 0.098% | 1/1024 | 10 |
!!! WARNING !!!
Some companies use the transmission for their ND filter names. A ND50 is filter with 50% transmission (and equivalent to an ND 03), an ND25 has 25% transmission (ND 06), and an ND13 has 12.5% (ND 09).
A variation of "normal" ND filters are "variable polarizing filters", which offer the advantage of being adjustable – at a higher price though. Though 1 cheap polarizing filter costs new less than 3 cheap ND filters would cost new. Furthermore the single polarizing filter exits that offer to remove some glare (and might be beneficial on helping to remove blue sky).
My opinion is that everybody should at least once in her/his life have looked at the Moon with its varying phases, its craters, mountains and mares through a telescope – it IMHO a wonderful target for starting astronomy. And getting one ND09 filter is a good starting point for Moon observations. If you decide that looking at the Moon is not for you, not much is lost. And if you find interest, than you might upgrade you equipment with an polarizing filter.
Two tips:
Firstly don't combine two filters directly, as usually glare will be the result. What you might be able to do is add one filter to your Barlow and one to your eyepiece. Though having only one filter is preferable.
And secondly if you look at the Moon, even with a proper ND filter, you will loose your dark adaptation. It will be some time before you will be able to enjoy other celestial objects after a view of the Moon…
Color Filters
These are typically pieces of colored glass, that let through only certain colors (or rather only certain ranges of the light spectrum). There are "normal" versions with said colored glass, and there are more fanciful (and more expensive) versions that utilize dichroic filters.The utility of such color filters is usually reported as small.
What I found helpful is using a red filter to view the Moon during the early evening sky – there it helped cutting down on the blue sky and increased contrast quite a bit. My filter is a photographic "090 red" filter, which is equivalent to an "#25 red filter", and works quite well with the 6-inch Newton. The #23a filter might be an alternative for smaller scopes, letting in a bit more light, but probably having a bit less contrast. A "#29 deep red" might be overkill for small scopes, but is maybe another option.
BTW: These red filters (#25, #23a and #29) should come in handy when observing Venus or Mercury against the blue sky.
Other applications are using color filters to mask the chromatic aberration of refractors. And with planetary targets like Jupiter, Mars or Saturn one can increase the contrast a bit (maybe).
If you have some filters, go and try them, but I would advise not to build up a collection of color filters – but it depends on your taste, your targets and your scope.
Light Pollution Filters
These are dichroic filters which cut out light pollution, like from mercury and sodium lamps – these filters should therefore work (more or less) for all astronomical targets in the presence of light pollution.These filters go by many names like "Clear Sky (CLS)", "Galaxy Contrast Enhancement (GCE)" or "Light Pollution Reduction (LPR)", and there are differences in how much they cut out (and conversely how much they let through).
With regards to such light-pollution filters (and UHC and nebular filters) Steve Waldee has much information to offer, all of which I find highly recommendable.
I have not yet such a filter, it is on my list and I find it sensible to get one (not too expensive) such filter that deals with light pollution. If you become a more avid observer you might want to upgrade to two or three different filters and run your own tests on what suits you best for your needs.
A cheap filter to tackle light-pollution should be enough for starters, though a filter with better transmission curves might be worth its money.
Broadband Nebula Filters (UHC)
These are dichroic filters specialized on nebula. Unlike the narrowband filters below, these Ultra High Contrast (UHC) filters are designed to let through all three relevant emission lines from nebula, and reject the rest. They are broader than the OIII or H-B filters (hence the name), but are more narrow than the LPR-type filters above (which is why people like Steve Waldee call them "General Narrowband High Contrast Nebula Filters") – this confused me and I had to rework that part of this article…And then there are "intermediate" filters (such as Lumicon's "Deep Sky" filter) that are a bit more inclusive than UHC filters, but more exclusive than the light-pollution filters above – if I had to buy only one filter for deep-sky observation, the "Deep Sky" from Lumicon would probably be it. Though the Lumicon's "Deep Sky" filter is a bit too expensive for my taste (after all this is cheap astronomy here and my most expensive scope, with two Plössls and a mount did cost less than than that one filter…).
BTW: I think the spectrum that Steve Waldee shows for the UHC filters is wrong (it resembles more a "Deep Sky" filter from Lumicon), unfortunately I have no way of contacting him…
Narrowband Nebula Filters (O-III and H-B)
These dichroic filters are specialized on specific spectral lines of nebula.There are two types:
Once you start observing nebula (and start to get the hang of it), you should consider getting either one (ore more) broadband nebula filters, or even some narrowband nebula filters.
Warning on filters with narrow bands
If you decide to get filters with narrow bands (UHC, O-III or H-B): These nebula filter are one type of astronomical equipment where you should make sure you get quality. If you buy such filters, make sure you can evaluate them, either by- having a (trustworthy) filter curve of the filter you want to use (and you having the ability to evaluate said filter curve!)
- or by being able to test them, on the objects you want to observe
The problem with dichroic filters is that not only the design, but also the manufacturing process leads to differences in the position, width and height of the passbands (the "form" of the filter curve) – if the manufacturing processes are not controlled enough, these filter curves will vary from batch to batch. So you need to either have the filter curve of the filter in your hand, or you need to test the filter you got – as these dichroic filters tend to be more expensive, having a way to choose a filter that actually does its job (before you buy), or having a way to return a "faulty" filter, is good practice.
tl;dr
So to end this, I would suggest on not going crazy with filters, and taking it slowly.If you like lunar observation: Get one or two ND filters, or maybe a variable polarizing filter. Get maybe an red filter for daytime viewing of the Moon (and Venus and Mercury).
To tackle light-pollution consider getting a LPR-type filter. For nebula observation consider getting an UHC filter – but first get some experience on viewing those targets. Make sure you are able to evaluate the narrowband filters. And go find out yourself what works best for your needs!
Thanks for your very nice comments on my article about nebular filters. But I would like to reply to this statement:
ReplyDelete>>I think the spectrum that Steve Waldee shows for the UHC filters is wrong (it resembles more a "Deep Sky" filter from Lumicon), unfortunately I have no way of contacting him…<<
I use Windows; therefore opening myself up to the usual email is like ASKING for somebody to wreck my computer; so I no longer publish an email address; sorry.
Now: in my opinion the spectrum for the UHC type that I show is actually the right one. We disagree.
I have given a 'general' color bar spectrum for each of the filters discussed in the article, and try to show the stop-band in the specific broad filters, and the narrow passband of the narrow filters, with a black zone at the approximate location of the lowest response. I made every attempt to place that at the correct spot -- which does slightly vary if you examine a more critical plot with calibrated x and y axes. Some filters of each type have narrower or wider stopbands than others, and some have straighter curves, others more slanted ones. The charts are 'average' responses that are typical of the TYPE of filter, not any specific brand.
I've personally examined the performance of filters used in a four-party double blind test that I conducted back in the late 1980s, working on behalf of one company that marketed them. I used a visual optical band spectrum analyzer and a very wideband light source, and examined the spectra of both the Lumicon UHC and the Orion UltraBlock, as well as all the other filters used in the test (Deep-Sky, SkyGlow, and various others such as O-III and H-Beta.) I also did photographic tests back in the 90s; so my own perception and measurement of the spectra is what I relied on when creating the little general spectrum charts in my article.
The UltraBlock or UHC types, *generally*, have somewhat steeper stopband curves and a wider stopband than the Deep-Sky or SkyGlow. But they have to pass the green range to encompass O-III; so they cut off just above green. If you were to test various brands you might see that some have a bit of red light reponse while others also cut out the red; this is generally inconsequential to visual observers with either "normal" night vision, or without *very* large scopes--as we typically don't get much response to faint red with dark adapted vision. (It is somewhat inconsistent over a population of observers.)
The worst damaging light pollution, at any rate, isn't in the red range, as shown by the many astoundingly detailed h-alpha narrowband nebular photos that people have made, sometimes even from the heart of a city!
The UltraBlock (UHC type) filter therefore has a bit WIDER stopband than a SkyGlow (Deep-Sky) type. This is what I tried to show in my *general* little color charts.
If you can be more specific about what you believe IS wrong, then I'll think about this some more and try to figure out what might seem to be misleading about the chart from your perspective.
And--do you mean the one chart that is annotated, or the little "color bands" that show the stop-band region, or the narrow passband region, of specific filters? Believe me, I've been working on this subject since the mid-80s and once worked quite closely with Dr. Jack B. Marling of Lumicon, the "godfather" of amateur astronomers' nebular filters (as well as in the testing and marketing department of his closest competitor.) This is the first time I've seen any criticism of this information, which is based on an article I published back in 1997, derived from the earlier explanation about filters that I included in the (now discontinued) computer program "Lumiview" that I wrote, in *cooperation* with Marling, as a tool for Lumicon to use in assisting users in deriving correct exit pupils for using his filters.
Thanks for your remarks. Best wishes,
Steve Waldee
Dear Tony:
ReplyDeleteOn recalling my 1980s four-part double blind filter tests, done with controls; my visual spectroscopic analysis of competing filter brands; and my later work on many reflection nebulae -- some of which would receive enhanced contrast via the UltraBlock but NOT by the Lumicon UHC filter -- I have decided that the issue you bring up is PRIMARILY probably "how much light is transmitted in wavelengths shorter than green".
The UltraBlock had more sloping curves than the UHC type, which is readily demonstrated by Maurice Gavins' spectroscope photos, a link given in my article. Other filters that Maurice tested had even much more response down into the violet-to-blue region than the UltraBlock. Furthermore, this varies from unit to unit that one might buy from even ONE maker; in the filters made and sold over, say, 15 to 20 years' time; and in scopes with widely differing focal ratios--AND, the eye's response to faint out-of-band stimuli not right in the strong range of the passband will drop off as the exit pupil is diminished, and increase as the exit pupil size is increased--there are enormous changing variables to be considered here!
So, what I did was to show THE PASSBANDS AND THE STOPBANDS of 'generic' filters, as it was really impossible to show how they fall off at the high and low frequency visual spectrum.
I have decided that, indeed, my color band image is OK and not misleading; there is a definite discernible difference in the passbands of the LPR and UHC types, as I showed.
The "problem" (if there is really one) then, is entirely in the violet-to-blue, and the orange-to-red zones. This widely varies. I did not even see it very strongly by daytime color-sensitive vision, using the spectrograph: I could still discern deep blues and reds, with my eye to the optics that showed the diffraction grating. But, the eye isn't at all like a chart recorder registering flux values...
Ergo, it's really HARD and somewhat difficult to show the violet/blue end, and the red end, response -- generically -- for a very wide variety of commercial "UHC" type filters. I could have made a stab at it, and greatly dimmed those colors while not cutting them off sharply, as happens in the stopband. But, my experience in the instrumental and visual testing showed that it was hard -- perhaps actually not possible -- to give a readily discerned "believable, accurate" depiction of the high and low frequency (violet/blue, and red) ends of the spectrum in that very simplistic color band chart.
So I showed the UNAMBIGUOUS and GENERALLY EXPERIENCED stop band of typical UHC filters, pretty common to most makes.
This is not, in the original article, explained carefully. At the risk of introducing complicated things that will burden the text and confuse the beginner, I have, now, thanks to your objection, added much more on this to the text of the article. But, I'm not changing the color band charts. I have enough of an explanation of this phenomenon, and links to actual color photos through filters, in the text to overcome -- I hope! -- your objections!
Best,
Steve Waldee
Dear Tony:
ReplyDeleteTHIRD response! After cogitating further, and doing some tests on 1/25/14 with an 18" scope and a DGM Optics "NPB" filter -- similar in stopband performance to the UHC and Ultrablock, but having a very high transmission also in the red color range, causing stars to acquire bright, jaggedy reddish halos, I have decided to *very* slightly modify my color band pictures of the UHC type (full color range, and mesopic color detection), adding a question mark in the blue/violet and red extremes of the spectrum, while not changing the general stopband, correctly shown in my opinion.
This might, indeed, *confuse* some readers, while helping others; one can never tell.
The more we tweak the article to allegedly improve it, the more we risk obscuring the basic messages that are important to beginners!
Steve Waldee