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Deep sky filters, Ultrablock vs. UHC

Each day a greater number of observers are using interferential filters during their observation sessions. What is the advantage of using such filters ? Those filters are very useful to increase details in deep sky objects (DSO), isolating a spectral line where they shine of all their fires.

By nature DSOs are by far less bright than planets, they are often of very low density and many of them are of very small size. Worse, in our urban or suburban cities DSO appear in front of a bright background sky due to the light pollution. Sometimes this background is also affected by the presence of haze, moisture or dusts in the air. The DSO can also be dim because it resides in a bright area of the sky, in full center of the Milky Way for example. All these parameters reduce the contrast between the DSO and the sky and do no help to distinguish them.

To download : Spectrum of public lighting (.xls)

Emission spectrum of public lighting

Low pressure Natrium lamp

High pressure Natrium lamp

Mercury vapor lamp

Incandescent light bulb (IR not showed)

White light LED (high grade)

Fluocompact lamp (CFL)

Documents T.Lombry

The best think to do is thus to find a way to reduce all this "pollution" to improve the DSO contrast over the background. The best filters manufactured to reject the light pollution, also called LPR filters, are the narrow-band filters Lumicon UHC and Orion Ultrablock. Of course you can also use an H-beta for dim nebulae and the DeepSky to look at all reflexion nebulae. You will find an overview of all these filters in another page of this site.

Those filters must be made of high quality polished optical glass, but not lamined or plastic. They are constituted of over 40 layers of coating reject a serie of spectral lines and are protected with a broadband anti-reflection coating to prevent ghosting and to increase the contrast. They look like an aluminum or a mylar sheet.

Characteristics and performances

David W. Knisely from Prairie Astronomy Club, U.S.A, did some extensive comparisons between the Ultrablock narrowband filter and the Lumicon UHC end 1999 and thanks to his tests we can answer to many of the questions amateurs ask themselves about the usage of such filters.

For this comparison David used a Multi-filter adapter from Lumicon fixed on a 10" scope, as well as spectroscope to see the bandwidth and characteristics of both filters. The Multi-filter adapter allows instant comparison between sets of filters, so there is no ambiguity caused by the time it takes to take one filter out and put another in. Here are his comments.

Although both the UHC and the Ultrablock have very similar characteristics and performance, the UHC was the better of the two filters overall, as it was slightly higher in-band light transmission.

It consistenly showed a slightly larger area of nebulosity in most nebulae than the Ultrablock did, although the overall views looked fairly similar. In the spectroscope, the UHC shows an almost flat-topped passband extending from just a little on the blue-side of the H-beta line to just a little on the red side of the Oxygen III emission lines. The Ultrablock has a more gaussian (bell-curve) shaped passband and a slightly lower in-band transmission. This lower transmission is why the Ultrablock yields a darker sky; its just not letting as much light through. This tends to yield an "apparent" increase in contrast, when in fact there is little or none. In particular, the Ultrablock's light transmission at the H-beta line seems to be somewhat less than the transmission at the OIII lines, which may have an impact on objects with significant H-beta emissions.

Broad and narrow band

Deep Sky filters

Orion UltraBlock filter

in various sizes.

Both filters work quite well on most emission nebulae, so other than price, there isn't an enormous difference in performance, although again, if you are pushing things to the limit, the UHC is probably the filter of choice for an all-around nebulae enhancer.

As for the O-III and H-beta, the O-III tends to be the more useful of the two, as it tends to give a more significant improvement on more objects than the H-beta does. The H-beta is definitely *not* a good filter for the Veil nebula, as it dims it significantly over the view in the O-III or UHC. Similarly, the North America nebula is better in the UHC or O-III than it is in the H-beta, although the view is still improved by the H-beta to some degree over no filter at all.

The H-beta also tends to almost wipe out many planetary nebulae, so again, the O-III or UHC would be better here. The H-beta is best used on the Horsehead nebula, the California nebula, and the Cocoon nebula, but does not offer the kind of performances improvement on other objects as the UHC/O-III do. However, the H-beta is still somewhat useful for examining some of the structural details in some of the larger emission nebulae.

Larger planetary nebulae do benefit more from the O-III than the UHC, but for most smaller planetaries, filters are not really needed (except when trying to find the smaller ones in a crowded field).

In the field descriptions

M1: Difference between the UHC and Ultrablock is very slight. Ultrablock seems to make everything a bit darker, but doesn't show more nebulosity than the UHC. Ultrablock might be narrower than the UHC, but this would probably only be of benefit in town under light pollution, and even then, the difference is small.

M8 : Similar performance in both filters, but M8 shows a slightly larger area of nebulosity in the UHC compared to the Ultrablock (especially on the east and west ends), with the nebula appearing slightly brighter. Ultrablock gave a darker sky background, but a bit less of the outer nebulosity was noted.

M16 : A very slight difference, with the UHC showing a bit more nebulosity on the edges

M17 :Darker sky with the Ultrablock, but the UHC was again brighter with a little more outer nebulosity in the outer loop. Neither filter is quite as good as the OIII on this object.

M20 : Slightly brighter in the UHC than in the Ultrablock, but the difference is small. For faint nebulosity near the limit of vision, the UHC is probably the better filter.

M42 : shows a slightly larger area of nebulosity in the UHC compared to the Ultrablock, with the nebula appearing slightly brighter. Ultrablock gave a darker sky background, but a bit less of the outer nebulosity was seen. Otherwise, the views in the two filters are quite similar.

NGC 6888 : Seems to have very slightly more contrast than in UHC (darker sky background), but not nearly as much as in the O-III filter. This suggests to me that since NGC 6888 is mainly an "Oxygen III" emitter, the Ultrablock is probably narrower in passband than the UHC, which way be a factor in objects with significant H-beta emission.

NGC 7000 : UHC shows a bit brighter nebulosity in the North America nebula. Otherwise, the views are quite similar.

Lumicon filter series

For decades, Lumicon sell many different selective filters showing particular spectral response. For example, if aprochromat refractors are well color corrected they are not for the violet or UV light before 4400 for which it remains a residual aberration (mirror telescope are not subject to this chromatic aberration). To reduce this aberration mainly revealed on bright and young warm stars, Lumicon sell Minus Violet filters providing visually crisper stellar images and sharper pictures. 

These filters cut also mercury lines at 4050 and 4350 . The High Performance Blue filter provides similar results with a 90 % transmission in the blue. On the opposite side, the UV-370 filter only transmits ultraviolet light (U-spectral band) from 3200 to 4200 . At last with a passband a bit wider the Central Star Filter is transparent between 3400 and 4700 .

Those filters give the best results on chaotic or reflexion nebulae where young stars are evolving : M20, M43, M45, NGC 2024 that emit an intense UV stellar light. Without to forget their famous UHC, Solar prominence, polarizing and color filters.

Prices : in 2015 count about $160 (230 ) for a 1.25" filter, $240 (387 ) for the 2" model and 159 for a Astronomik Clip filter for Canon EOS. Prices increased of 60 % in a dozen years but some very appreciated models have seen their price doubled in 10 years.

This report has been published with the courtesy of  David W. Knisely from Prairie Astronomy Club and updated.

For more information

Useful filters for viewing deep-sky objects (on this site)

Bright-Sky Imaging (PDF), Ron Brechter, 2017

Manufacturers : Astronomik, Baader Planetarium, Lumicon, Thousand Oaks Optical

Dealers : Orion Telescopes & Binoculars (and french site), Optique Unterlinden, AstroShop, Teleskop Express, Telescopes.net

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