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ASTRONOMIKFILTERSVISUAL ASTRONOMYUNDER THE (LIGHT) DOME

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By Matt Harmston

Visual AstronomyUnder the (Light) Dome

Over the years, I’ve shared the eye-piece with family, friends, and myriadothers. Telescopes so often capture theattention of onlookers, don’t they? Re-gardless of attention garnered, inter-ested onlookers often don’t considergetting a telescope because they findthemselves within the pale confines oflight-polluted skies.

While light pollution interfereswith visual astronomy, all is not lost.Many outdoor lights glow due to ex-citement of such elements as mercury,sodium, etc. Effects from such light-ing can be blocked in order to revealdeep sky wonders.

This article is going to describe ex-periences under light-polluted skiesusing Astronomik’s 1.25” UHC,UHC-E, and CLS filters along withthe absence of a filter. A follow-up ar-ticle is being penned that discusses useof these filters as observing tools underdark skies.

Filters and the Passage of Light

Deep sky objects (or, DSOs) emitand/or reflect light along the electro-magnetic spectrum. As a result, visibledetails will vary as a function of wave-lengths being allowed to pass to youreye. Filters permit users to emphasizeselect wavelengths over others, therebyrevealing specific details by limitingpassage of unwanted light.

To use an analogy, we might enjoyeating a bite of salad - a pleasing blendof tastes arising from a multitude ofvegetables and toppings. Deep sky ob-jects are like this salad in that unfil-tered views provide a pleasing blend ofdetail across all visible wavelengths.

Using a filter to isolate wave-lengths of light is like pulling an indi-vidual tomato or slice of cucumberfrom that salad: A specific flavor, se-lected for attention in relative absenceof competing options. You might say

that filters help us tailor the view toemphasize our desired “flavors”.

Technical Details:Astronomik Filters

For this article, I used AstronomikCLS, UHC-E, and UHC filters (seeImage 1).

The purpose of these filters is toenhance visibility of deep sky objectsby increasing their contrast with thebackground sky. Contrast is enhancedby limiting passage of unwanted wave-lengths of light while permitting de-sirable ones (i.e., your selected“flavor”) to pass through nearly unim-peded.

Though a given DSO might ap-pear brighter when using a filter, this isan illusion. Filters do not brighten ob-jects. Rather, improved visibility is dueto enhanced contrast between DSOsand the background sky.

Under the dark of night, a typical

human eye might detect light withwavelengths ranging from about 400to around 600 nanometers, a trun-cated upper range compared to day-light conditions. To illustrate lightpassage by filters along this spectrum,Astronomik publishes charts for eachfilter that juxtapose percent of lighttransmitted with wavelength (see Im-ages 2 - 4).

Each graph’s elevated regions rep-resent wavelengths where most of thelight is allowed to pass. In contrast,the low-lying areas represent wave-lengths being blocked by the filters.For instance, sodium vapor lightsshine at a wavelength of 589nm, apoint where virtually no light passesthrough any of the three filters.

The CLS filter is a budget-friendlyfilter intended to block out light frommercury and sodium-vapor lampswhile permitting relatively larger por-

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knig

Image 1 – From left to right, Astronomik CLS, UHC-E, and UHC 1.25” filters

tions of the visible spectrum to passthrough. The UHC-E filter is alsobudget-friendly, yet is designed to fur-ther enhance contrast of emission neb-ulae and comets by blocking airglowand common artificial lighting.

With the UHC, blockage of selectwavelengths is even more aggressive.Where desired light is passed, theUHC is also the most efficient of thethree filters. The UHC is particularlyadept at transmitting Hydrogen-betaand Oxygen-III lines while aggres-sively blocking background sky.

Because progression from theUHC to the CLS filter means passingadditional wavelengths of light, overallfields of view will appear brighter withthe CLS than the UHC-E and UHC.But, brighter isn’t always better withfilters: Contrast is key.

Technical Details:Observing Site and Gear

Light-pollution testing was con-ducted at a friend’s suburban homeunder smoke-filled skies, ample man-made light pollution, and on one ofthe nights, a 31% illuminated moon.

Judging by the faintest visiblenaked-eye stars (my night vision is ex-cellent, and corrected acuity is betterthan 20/20), lower altitudes were lim-ited to roughly magnitude 2.9 andbrighter stars. At zenith, I couldn’tmake out stars fainter than roughlymagnitude 3.9. The 3.9 estimate wasobtained after the moon had set on thefirst night, and replicated under nomoon the second night. Direct lightfrom street lamps was blocked byhomes, but neighboring lights castsome illumination across the yard.

I wanted to use a telescope andmount fairly typical of what might befound in the amateur community,thus my SkyWatcher Pro 100ED re-

fractor (100mm aperture) and Cele-stron AVX mount went out for a nighton the town. Though the AVX is aGoTo mount, I did not use that func-tionality so as to more readily use star-hopping and sky panning in mytesting of the filters.

In spite of keeping mechanics andaperture fairly basic, I wanted to avoideyepiece aberrations while testing.Thus, the f/9 telescope was pairedwith two quality eyepieces: An Orion22mm Lanthenum Superwide and a13mm Tele Vue DeLite. These eye-pieces were chosen because they aregood quality, have comparable appar-ent fields of view, long eye-relief, andtheir disparate focal lengths enabledbrightness-impacting differences inmagnification.

Targets for the testing includedMessiers 8, 13, 17, 20, and 57, alongwith NGC 869/884. These were cho-sen for several reasons. They could allbe detected with the small scope with-out a filter under my bright skies. And,they represented varied ratios of emis-

sion nebulosity- and stellar-sourcedlight output. Finally, their altitudes re-sulted in piercing varied levels of lightpollution.

The View: Light-Polluted,Smoky, (Sometimes)Moonlit Skies

To test each filter, I alternated be-tween the unfiltered and filtered con-ditions and then, in some cases, acrosseyepieces. For those seeking fine de-tails, I’ve included some (edited) ob-serving notes in an appendix at theend of this article. For high-level de-tails, a summary of observations fol-lows:

• All three filters enhanced con-trast to varying degrees, thus detectionof and detail within deep sky objectswas enhanced by using the filters inmost cases.

• The already sharp focus of theunfiltered ED refractor was main-tained or improved upon by use ofeach filter. The UHC rendered starssharper than any other condition.

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Image 2 - Light transmission by wavelength plot for the Astronomik UHC filter

• When viewing bright emissionregions of nebulae, increased filtercontrast progressively enhanced detec-tion and detail regardless of eyepiece

or object elevation. In order of mostemission detail to least: UHC, UHC-E, CLS, and finally, no filter.

• The UHC filter provided a par-

ticularly pleasing 3-d effect whenviewing emission portions of M8.And, it revealed far more nebular de-tail than any of the other filters, elicit-ing a muttered, “Wow!”. M17 wasquite remarkable due to dramatic im-provement in observable detail withthe UHC.

• If you are panning light-pollutedskies in search of targets, the use ofany of these filters would be beneficial.M17 was difficult to distinguish fromthe unfiltered background sky, andwas more obvious against the darkerbackground from the CLS. However,the UHC-E and UHC filters morereadily extracted emission nebulaefrom the sky’s soup when star-hoppingtoward it.

• If you want to star-hop to emis-sion nebulae with a small telescope,the UHC-E gets highest kudos be-cause it markedly increases contrast

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Image 3 - Light transmission by wavelength plot for the Astronomik UHC-E filter

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yet cuts less starlight than the UHC. • A general point to mention: All

other things equal, higher magnifica-tion can help with detecting fine de-tails under filtered and unfilteredconditions - to a point. Too muchmagnification relative to aperture canbe counterproductive due to dimmingthe field of view, among other factors.

• Heavily populated star fieldssuch as the Perseus Double Cluster(i.e., NGC 869/NGC 884) most ben-efited from the CLS filter. It’s moder-ate background darkening, combinedwith greater maintenance of star illu-mination, made for an eye-catchingview.

When considering which filter

would get the gold star as a general-purpose accessory, I found myself in aquandary. I kept coming back to thephrase, “different filters for differentflavors”.

As a general-purpose filter, I mostoften found myself preferring theUHC-E - plenty of starlight for thesmall aperture, darkened background,and enhanced views of more targets ofmost interest to me. However, if myinterest had been specifically in em-phasizing views of brighter emissionnebulae with this small ‘scope, theUHC would have been my preferredaccessory.

Note, “brighter emission nebulae”is a relative label, as larger apertureswill reveal increasingly fainter objectsand detail than my little refractor. Hadmy interest been specifically in starfields/clusters, the CLS would havebeen my choice. In short, each filterhad its own application where it stoodabove the rest, making selection of thebest filter a personal choice based onconditions, equipment, and interests.

There are two important limita-tions to this article. First, the combi-

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Image 4 -Light transmission by wavelength plot for the Astronomik CLS filter

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nation of aperture, sky, and locationpushed accessible galactic targetsbelow my visibility threshold and/orbehind the neighbor’s trees. The sec-ond limitation was that there were fewexamples of accessible reflection neb-ulae, and even the most obvious (aportion of M20) was barely visible indirect vision. Thus, evaluation of usewith galaxies and more thoroughstudy of reflection nebulae will have totake place under darker, more openskies.

Other Filter Characteristics Beyond the Eyepiece

Though the emphasis in this arti-cle is on the eyepiece view, there aresome important observations to con-sider beyond contrasty views:

• The filters are accurately adver-

tised as being parfocal. • All three filters threaded into the

Orion and Tele Vue 1.25” eyepieces,permitting full, secure seating in allcases.

• The filters had secure lenseswithin their respective housings. Notightening of set rings was needed be-fore, during, nor after use in tempera-tures ranging from 60F to 83F.

• Astronomik website referencescoating and construction durability. Ican embarrassingly attest to this, as onedark sky session saw sweaty fingers dropthe UHC filter lens-down on a coarsegravel driveway. Short of my bruisedego and utterance of something best leftunprinted, no apparent harm camefrom the drop. Suffice to say, the filtersmust be cared for just as any other fineoptics, but this (un)happy accident un-derscored the durability claim.

In the EndViewing under light-polluted

skies may not be ideal, but formany, doing so cannot be easilyavoided. Testing these filters rein-forced my opinion that light pollu-tion need not preclude exploringthe heavens. Instead, we can suc-cessfully seek out a wide range ofcelestial targets.

So, are filters for you? If so, whichone(s)? Arriving at this decision in-volves personal tastes, interests, gear,and settings. But, like myriad eye-pieces in their cases, there are filters tosuit most needs and interests.

I encourage you to take the plungeand counter light pollution with fil-tered viewing. These filters from As-tronomik put a smile on my face, eachin their own way. You just might havethe same experience.

Target/Eyepiece No Filter CL

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M8/22mm Lanthenum Superwide

M8/13mm DeLite

M13/13mm DeLite

M17/22mm Lanthenum Superwide

M17/13mm DeLite

M20/13mm DeLite

M57/13mm DeLite

Perseus Double Cluster/22mm Lanthenum Superwide

The nebula appears as two separate star clustersunder direct vision, with averted vision revealing afaint fog in the core. This nebulosity would be veryeasy to overlook.

It is a beautiful star field against a bright sky, thoughthe background is darker due to increased magnifi-cation. Core nebulosity is obvious, nebulosity onthe other side of the lagoon is a suggestion…if youknow to look for it and use averted vision, you mightsee something.

Easily visible, with very fine stars flickering in/out ofvisibility. The best view came from using no filter.

Barely visible, a faint patch of fog. Oval in shape, nooff-axis extensions. Easy to miss if you didn’t knowwhere to look.

More visible than in the 22mm, but still just an illu-minated oval.

The star field is plainly visible, though reflectionnebulosity is not apparent with direct nor averted vi-sion.

Visible, but beyond circular shape, this was nothingremarkable.

Varied intensity pinpricks of light emerge from adusky, bright background. Magnitude 8 HD 14330has slightly different color than nearby stars, sug-gestive of ruddy color.

Improvement upon no filter though slight. The core is a groups of stars remain the are sharpened.

Surprisingly, views doesn’t background hasn’t changestars dimmed slightly and sstill visible, but not across tvisible only with averted vis

The cluster is more obvioubackground, but it looks likual stars have dimmed too with averted vision.

More obvious elliptical objeagain no structure, no protrlooked, but no hint as to its shape.

The view didn’t markedly imter. The nebula appears to background.

The background sky is darthree filters, and detection with averted vision. Neithe portions are markedly more

Not evaluated due to time ings in other sessions.

The most appealing view dcounts, star brightness droter, but star counts remain slightly dimmer than with nHD 14330 color difference not as ruddy as with no filte

Appendix A: Semi-Rough Observing Notes

LS UHC-E UHC

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r in terms of nebulosity, a distinct fog, but the two primary details. Stars

improve much. Thed appreciably, but the

sharpened. The core is the lagoon. The lagoon ission.

s against a darkened e a nebula. Most individ-

much for detection even

ect than no filter, but rusions. Not easily over-

s namesake omega/swan

mprove relative to no fil- dim in concert with its

kened the least of the requires lengthy studyer emission nor reflectione distinct than the other.

running short and find-

due to highest star ps a little relative to no fil-

high. Background sky iso filter. The Magnitude 8is still as obvious, though

er.

Improve upon the CLS, background markedlydarker but the core did not diminish. Not a "WOW!"factor, but still more obvious. With a little imagina-tion, there might be nebulosity near both stargroups. Stars are further sharpened.

A distinct improvement upon the CLS, nebulosity isvisible on both sides of a murky lagoon. The core isquite obvious, the other side is more subtle…yetimagination is not required. The nebula looks like acolorless, dense cloud of fine, spark-filled smoke.

The cluster is even more obvious than with theCLS, but here, too, it looks like a nebula due to ab-sence of fine stardust.

Darker background than CLS, body more promi-nent. Still a rough oval absent other details. Ifstarhopping, navigation to the nebula would be eas-ier with the UHC-E than the other three conditions.

The nebula is more obvious than with no filter andCLS. Still, it is just a visible patch of oval-shapedfog. No details emerged. This is the best option forstar-hoppers.

The background sky is darkened slightly comparedto CLS, and emission nebulosity is still a faint fogwith direct vision, less obvious than UHC. Reflec-tion nebulosity is a suggestion with averted vision.Aside from telltale star patterns, it would otherwiseeasy to overlook the nebula if panning the area.

The nebula is more obvious than with no filter, butstill not a ring…just a circular patch of fog.

A beautiful view in terms of contrasting backgroundsky and star field, yet noticeably fewer faint starsvisible in the core of NGC 869 (my point of refer-ence for star population) compared to the CLS.

The background is really dark and the lagoon be-comes quite obvious between two sharp, star-stud-ded regions of haze/nebulosity. Wow. This filterwas the best view in the 100mm scope/22mm eye-piece combination.

Forget any imagination…wow. The backgrounddarkened quite a bit, and the stars sharpened infocus just like the other filters. In addition, the in-creased contrast has brought on a 3-d impressionof stars being superimposed upon a dynamic neb-ula. This view has the most pleasing detail.

And again…an obvious object against the darkenedbackground, but it takes on a nebular appearance.

The head and neck of the swan take shape. Faintnebulosity off the tail begins to emerge. Back-ground is very dark. This view has the most pleas-ing detail.

The swan's neck and top of the head becomesclearly visible along with more obvious detail be-yond the tail. The nebula appears like a fat, funny L,and stands out starkly against a dark background.Faint surrounding stars nearly disappear. This is thebest nebular view.

The background sky is darkened slightly comparedto UHC-E, emission nebulosity most obvious. But, itis little more than a featureless, faint fog with directvision. The Reflection nebula is extremely faint withaverted vision.

The ring structure is obvious. Pronounced contrastbetween the dark background, illuminated ring, anddarkened center of the nebula. This filter providesthe best view.

Sharp pinprick stars in the field, but even moreslipped below the visible threshold than with theUHC-E. It is still a pleasing view, but the sparserstarfield makes this view the least interesting.