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Broadband Battle Redux

Started by Rod Kaufman, 12/27/2017 03:21PM
Posted 12/27/2017 03:21PM Opening Post
I've previously reported on broadband filter performance with an eye towards galaxies but I decided for another round of observations in front of my moderately light polluted home on 12-26-17. Adding to the high ambient light from the Christmas lighting from my neighbor's homes, the moon glared down at me at first quarter so I would consider this outing as a severe test of the filtration effects of the broad bands in question. As an aside, I read somewhere, and I think it was in Astronomy magazine, that the moon has to be at or near first quarter before it significantly degrades the image in the eyepiece but I suspect most amateurs would probably disagree with that assessment. Speaking of Astronomy magazine, Phil Harrington wrote an excellent article of his experiences with light pollution filters several years ago, and I 'd certainly recommend it over whatever you might read or hear in the field or on the CN forums(especially the latter).The pdf is here:
http://www.astronomy.com/~/media/import/files/pdf/8/c/7/0805_nebula_filters.pdf
I decided to view several of the same objects that Harrington observed in his tests of "nebula" filters and report my results. My personal interest is in observing galaxies with broadband filters and although some will appear to be enhanced by a particular filter while others will not and nothing beats the views of them under truly dark skies, I still enjoy seeing them in front of my own home in forty-degree plus temperatures versus 20 degree temperatures up at Mt Pinos or at Lockwood Valley, California.
My first observation was of a bright galaxy in Perseus, NGC 1023, which was near the zenith and therefore well-placed for observation. Harrington used M81 for his tests but that wasn't visible from my location at the time of my observations. I compared four broadband filters, a Levenhuk Ra CLS model along with three conventional nebula filters, a meade unit made in Japan with individual transmission data, another from Astrotech which probably represents most filters originating in China and an Orion skyglow. In stark contrast to Phil Harrington's findings, I felt the Orion skyglow darkened the field the most making it unsuitable for observations of this galaxy and in general observations compared to the other models. Harrington liked it; I didn't. The meade and astrotech filters did enhance the view and I'd rate them about the same while the CLS filter appeared to have the greatest effect by blocking out most of the unwanted spectrum of light. While the CLS filter reportedly has its greatest effects and usage for imaging purposes, it does exceptionally well, in my opinion, for visual observations. I have found the 16-18mm eyepiece range of focal lengths to be especially useful in observations of galaxies with these filters with perhaps the 17mm focal length being optimum. The exit pupil here with my 16" f4.5 scope affords the best contrast that I've observed with with my scope and filters for most DSOs.In my opinion, one of the biggest mistakes amateurs make is failing to take the time to try filters with different focal length eyepieces with varying powers and exit pupils to determine the optimum level of image brightness and contrast for their equipment.
My second observation aimed at another Harrington target, NGC 2371-2, a dual lobed planetary nebula located in Gemini. At magnitude 13, it was surprising to me that I could see it all, rising above the cul-de-sac but still relatively low in position compared to the other winter constellations at the time of observation. With my CLS filter in place, the dual peanut like shape was readily apparent while it tended to blend together as a single object with the other filters in place.
Speaking of peanuts, another Harington object of opportunity was M76, the little dumbell. Although all filters helped enhance the view, I'd again give the edge to the CLS filter for the brightest appearing view of the lot.
NGC1514 is a beautiful planetary nebula in Taurus and all filters worked well on this particular object.
Turning to the final Harrington object on my list, M42, the great nebula of Orion, majestic in just about any filter-eyepiece combination, I felt the edge went again to the CLS filter in discerning more of the glow of the adjacent M43 nebula although not by much.
My final object of observation for the evening was NGC 2022, a fine planetary also located in Orion. My impression was that more of the egg-like appearance of this object was apparent through the CLS filter than though the others during my observations.
My take away again is to vary the focal length of the eyepiece along with filter usage to get the optimum level of brightness and contrast and do not rely on a single observation to make a determination on the effectiveness of a filter.
Posted 12/27/2017 10:10PM | Edited 12/27/2017 10:35PM #1
Hi Rod & All,

First let me say thanks for the good reference article on filters. I agree that this article is a good one to sort out the sometimes confusing "Marketing Spin" that manufacturers barrage us with to try and sway us to their products instead of their competitors. All filters are a compromise because the can only "throw away light" to possibly improve the contrast of the image.

Second, let me thank Rod for a very nice and well balanced review of the Broadband filters he tried in his situation. Good job !!! I especially liked the comments about the "Moon glow" and agree with Rod that any moon in the sky is bad for DSOs when viewing unfiltered, not just first quarter or more gibous. I also appreciate Rod's comments about trying the interaction of eyepiece focal length, exit pupil, etc. with the different filters he tried. The interaction is important to explore.

Third, after trying a number of filters from my back yard (which is on the edge of the Portland, Oregon "sky glow") I have concluded the following: (1) Broadband (LPR type) Filters in a 10 inch SCT give very nice results that work almost as well as the 100 mile drive to really dark skies with my 8 inch SCT. I've been pleased in the contrast improvement of M1, M42, M43, M51, M57, M81, M101, etc. (2) Narrow band filters (for me) boil down to a UHC does everything an O-III AND a H-Beta does for me, so after careful comparison with all three (LUMICON Brand filters)I sold two of the filters here and kept the UHC for use in my 8 inch SCT. (3) If I had the 10 inch SCT available to me again (or larger reflector even more so), I'd spend more time viewing from my backyard with filters, but with 8 inch SCT AND smaller scopes, I'm better off driving to dark skies than trying to make the filters work at (or closer to) home. (5) Both Narrowband & Broadband filters are very helpful to me when doing outreach to people who are newer to astronomy (example is an O-III or UHC can really make M57 "pop").

Best Regards,

Ed Blankenship