So, I was thinking about:
- What filters I have
- What filters I use
- What filters I deem essential
- And what filters I might want to buy next
The short version: The only essential filter in my humble opinion is an neutral-density filter (maybe an ND09, sometimes called a ND13), and only if you want to view the Moon. Everything else*: you can get by without, somehow, and at least initially. (Though undeniably some filters will help depending on what you want to do, but you don't need them to get started.)
* 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
I have in my possession two UHC type filters: a "
Meade Broadband Nebula Filter" (#908B) and a no-name UHC filter. I have spectrally measured both. The Meade has a filter curve which matches the information on Meade's website, with a transmission of about 90% for O-III and H-B lines (not excellent, but good enough for starters). But the no-name UHC filter is a piece of junk: its passband lies completely below 480nm! This no-name piece of junk would completely cut off all O-III and H-B light from nebula!
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!
