Equipment: filters

Telescope and Eye-pieces SkyWindow and Binoculars filters accessories


Lumicon LPR filters: Light pollution reduction filters or LPR filters
(for visual deepsky observing)

The theory

To fight the severe light pollution in the area where I live I bought a set of light pollution filters from Lumicon:

  • Broadband filter (Lumicon Deepsky Filter)
  • Narrowband filter (Lumicon UHC Filter)
  • Line filter (Lumicon OIII filter)

As Lumicon states in the manual that goes with their LPR filters, the Lumicon filters have an optimum exit pupil (magnification) range in which they can be used best. The exit pupil can be calculated by dividing the eyepiece focal length by the telescope f-ratio, for example if you have a f10 (focal ratio of 10) telescope and an eyepiece with a focal length of 40mm, the exit pupil is 4mm.
Lumicon light pollution reduction filters


These are the optimum exit pupil ranges for the Lumicon filters (data provided by Lumicon in 2001) :
Filtertype
Deepsky
UHC
(O-III)
H-Beta
Bandpass
90 nm
22-26 nm
10-12 nm
8-10 nm
Opt. Exit pupil near cities
0,5-2 mm
1-4 mm
2-5 mm
3-7 mm
Optimum exit pupil dark skies
1-4 mm
2-6 mm
3-7 mm
4-7 mm
Exit pupil range (by Lumicon)

Quote from the manual provided by Lumicon:
“If you use an exit pupil too small, the background sky is too dark. If you use an exit pupil too large, the background sky is too bright and there is insufficient contrast”.


I own a telescope with a focal ratio of F10, and a focal length of 2000mm. I got a range of eyepieces from 32mm down to 7mm. Here is an overview of the different exit pupils and magnifications which I get using my own eyepieces and telescope, and a separate column for every Lumicon LPR filter that tells for every eyepiece separately whether or not the calculated exit pupil falls in the optimum exit pupil range of the Lumicon filters
(based on the data from the table above, provided by Lumicon (in 2001):

Eyepiece
Magnification
Exit pupil
Deepsky
UHC
OIII
H-Beta







32 mm Plossl
62.5
3,2
3
1
1
2
25 mm Lanthanum
80
2,5
3
1
2
4
20 mm Lanthanum
100
2
1
1
2
4
15 mm Lanthanum
133
1,5
1
2
4
4
12 mm Lanthanum
166
1,2
1
2
4
4
10 mm Lanthanum
200
1
1
2
4
4
9 mm Lanthanum
222
0,9
2
4
4
4
7 mm Lanthanum
286
0,7
2
4
4
4

Legend:

Always optimum exit pupil, under dark skies as well as under city skies

Only optimum exit pupil under city skies

Only optimum exit pupil under dark skies

No optimum exit pupil


Observing out in the field

So far for the theory…... ; what are my experiences in the field, using the Lumicon filters for deepsky observing.


The Broadband Lumicon Deepsky filter

I use the Lumicon Deepsky Filter when I observe from my light-polluted backyard. This filter, sometimes means the difference between seeing or not seeing a galaxy or nebula. It sometimes brings out a reflection nebula a little bit better. However when using this filter for visual use only, the differences are marginal most of the time, whatever magnification used.


The Narrowband Lumicon UHC filter

The UHC filter is the one I use most, not only in my backyard, but also on dark-sky sites. This filter brings out emission nebulae and planetary nebulae, without blocking out too much starlight. It can be used at all magnifications that I have available. However, the filter gives everything a greenish color. If I had to buy the filters again, this would definitely be the first I would aquire.

The OIII Line filter from Lumicon

The OIII filter works great on the small, planetary nebulae. I see the planetary nebulae much better. They sometimes jump out at you. The problem however is, that much of the visual light is blocked out. A lot of fainter stars become invisible. On a few bright objects like M42 I use it even with magnifications of 200 and more. But on the average planetary, at magnifications of 200 and more, there is not much left to see, using my telescope.


The H-Beta Line filter from Lumicon

I do not own the H-Beta filter. As you can see in the table above there is only one eyepiece where I could use it with. In the local astronomy group, I only know of one person who ones one, but even with his 20 inch Newtonian under the light polluted skies where we live, he told me the filter is of little or no use for visual deep sky observing. It works only on a few objects. In time I will give it a try, and I will update these pages with my findings.
OIII Line filter from Lumicon

Conclusion

After using three of the four Lumicon light pollution reduction (LPR) filters for some time, I can say that they definitely do work. When using the right eyepieces (exit-pupil) they can provide better views (more contrast) of deep sky objects, not only under light polluted skies but also under dark skies.

However, as with the color filters for planetary observing, you have to decide for yourself if you find them useful or not. They show more detail and contrast, but they also block out a lot of visible light and produce false colors. And then there is the cost. Do the differences you see in observing the deep sky-object with and without the filters justify the cost? Before buying them go to a star party or visit a friend who already owns them. It would be good to look at a few nebulae, and see the effect of the different filters, before buying on or more. It’s up to you to decide.


If you want to know more about these filters and about the use, please visit the following links:



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