Deep Sky Filters

by Andy Ferguson

Gleaned from a variety of sources, this table shows the comparative performances of the four filters most commonly used by amateurs to observe deep sky objects.

The most basic filter is the Light Pollution, Sky Glow, Deep Sky filter, or Nebula Filter, depending on what a particular manufacturer calls it. It is fairly broadband and acts to suppress the effects of the sodium street lights and sky-glow we are all unfortunately familiar with. It increases contrast between sky and object and brings many objects into view normally rendered invisible by sky-glow.

The Ultra High Contrast filter (UHC) does what it says on the tin. It is a narrowband filter which darkens the view through the eyepiece quite noticeably and favours emissions from both O111 bands at 496nm and 501nm, and H-beta wavelengths at 486nm.

The O111 filter is narrower still and allows only the 496nm and 501nm wavelengths through, excluding all other emissions. As a consequence, the view through this filter is very dim but will show detail invisible with other filters, especially from a dark site.

The H-beta filter only passes light at 486nm and could be regarded as a luxury as it only works on a very few objects, notably the Horsehead and California Nebulae. Both the O111 and H-beta filters are also used to great effect by CCD imagers, along with the H-alpha filter (NOT to be confused with the Solar imaging type!! Deep sky imaging only!)

Descriptions of the visual effects of these filters can vary quite a bit, depending on the observer and their individual preferences. The best thing to do is to try to attend an observing session where fellow amateurs can show you the effects at the eyepiece, or arrange a 'special' session purely for that particular purpose.

A quick and easy way is to 'blink' the filter over the eyepiece - just hold the filter between your fingers (keeping them off the glass!) and pass it back and forward between your eye and the eyepiece. It saves all the time and palaver of screwing/unscrewing the filter onto the bottom of the eyepiece. It can get a bit fiddly with gloves on in sub-zero temperatures.

Narrowband filters (1.25") can range from £80-£100, depending on manufacturer, and it is advisable to satisfy yourself of the performance of a particular filter before parting with the cash.

The scores range from 1 to 5, with 5 being the highest. Where two filters have the same score, one has a '+', indicating a slight preference for its performance over the other. Scores should be regarded as a guide and not as hard and fast recommendations.
Scoring:

1. Forget it. Waste of time.

2. Slight improvement

3. Noticeable improvement

4. Large improvement, more detail seen

5. Very large improvement, maximum detail seen

Result:

From the list, the UHC filter comes out as the first filter purchase for the amateur, followed by the O111. If your observing is limited to a light polluted site, the UHC/O111 might be too dim - check them out first at your own site, if you can. The H-beta is limited in its application but, when it works, can show things invisible in the other filters.

OBJECT

UHC

O111

LP

H-beta

M1 - Crab Nebula, Taurus

4

3+

3

1

M8 - Lagoon Nebula, Sagittarius

5

5+

3

2

M16 - Eagle Nebula, Serpens

4

4+

2

2

M17 - Swan (Omega)Nebula, Sagittarius

4

5

3

1

M20 - Trifid Nebula, Sagittarius

4

3

2

4

M27 - Dumbbell Nebula, Vulpecula

5

4

3

1

M42 - Orion Nebula

5

4

3

3

M43 - North part of Orion Nebula

3+

2

3

4

M57 - Ring Nebula, Lyra

4+

4

2

1

M76 - Butterfly Nebula, Perseus

4

4+

2

1

M97 - Owl Nebula, Ursa Major

4

5

2

1

NGC40 - Planetary Nebula, Cepheus

3

2

3+

2+

NGC246 - Planetary Nebula, Cetus

3

5

2

1

NGC281 - Emission Nebula, Cassiopeia

4

4+

3

2

NGC804 - H11 Region in M33, Triangulum

3

4

2

2

NGC896/IC1795 - Diffuse Nebula, Cassiopeia

4

4+

3

1

NGC1499 - California Nebula, Perseus

2

1

2

4

NGC1514 - Planetary Nebula, Taurus

4

4+

2

1

NGC1999 - Diffuse Nebula, Orion

1

1

2

1

NGC2022 - Planetary Nebula, Orion

4

5

3

1

NGC2024 - Flame Nebula, Orion

3

2

3+

1

NGC2174 - Diffuse Nebula, Orion

4

4+

2

1

NGC2327 - Diffuse Nebula, Monoceros

3

2

2

4

NGC2237-9 - Rosette Nebula, Monoceros

5

5+

2

1

NGC2264 - Cone Nebula, Monoceros

4

3

2

1

NGC2346 - Planetary Nebula, Monoceros

3+

3

2

1

NGC2359 - Diffuse Nebula, Canis Major

4

5

2

1

NGC2371-2 - Planetary Nebula, Gemini

4

4+

2

1

NGC2392 - Eskimo Nebula, Gemini

4

4+

2

1

NGC3242 - 'Ghost of Jupiter' Planetary, Hydra

4+

4

2

1

NGC6210 - Planetary Nebula, Hercules

4

4+

2

1

NGC6543 - Cat's Eye Planetary, Draco

4

4+

2

1

NGC6781 - Planetary Nebula, Aquila

4

4+

3

1

NGC6804 - Planetary Nebula, Aquila

3

4

2

1

NGC6888 - Crescent Nebula, Cygnus

4

5

2

1

NGC6960-95 - Veil Nebula, Cygnus

4

5

3

1

NGC7000 - North American Nebula, Cygnus

5

4

2

3

NGC7009 - Saturn Nebula, Aquarius

4

4

2

1

NGC7023 Emission/Reflection Nebula, Cepheus

2

2

3

1

NGC7027 - Planetary Nebula, Cygnus

4

4+

2

1

NGC7129-33 - Diffuse Nebula, Cepheus

3

3+

2

1

NGC7293 - Giant Helical Nebula, Aquarius

4

5

2

1

NGC7538 - Diffuse Nebula, Cepheus

4+

4

3

1

NGC7635 - Bubble Nebula, Cassiopeia

3

4

2

1

NGC7662 - Blue Snowball Nebula, Andromeda

3+

3

2

1

NGC7822 - Diffuse Nebula, Cepheus

3

2

2

2

IC405 - Flaming Star Nebula, Auriga

2

1

3

2

IC410/NGC1893 - Open Cluster, Auriga

4

4+

2

1

IC434 - Horsehead Nebula, Orion

3

1

2

4+

IC1318 - Gamma Cygni Nebula, Cygnus

3

1

2

3+

IC1848 - Diffuse Nebula, Cassiopeia

4+

4

2

1

IC2177 - Diffuse Nebula, Monoceros

3

2

2

3+

IC5067-70 Pelican Nebula, Cygnus

4

4+

2

2

IC5146 - Cocoon Nebula, Cygnus

3

1

2

3+

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