Sunday, June 03, 2012

 

Filter Frenzy


You may find it surprising, muchachos, but deep sky hound Unk went many a year before he turned on to Light Pollution Reduction (LPR) filters. They’d been coming on strong in amateur astronomy since the 1980s, but they were expensive and Unk is a suspicious sort when it comes to expensive gear, especially little pieces of glass that fetch as much as 200 fracking bucks.

I’d had a look or three through a filter or three at star parties, sure, but I wasn’t overly impressed. They either didn’t seem to help much, the target looking much the same or worse as it did without the filter, or I couldn’t see nuttin’, honey. The problem, it turned out, was not the filters, but Unk. I didn’t know about the different types of LPR filters, didn’t know how they worked, and didn’t know how to use them. In other words, I didn’t know pea turkey.

By the mid-90s I’d begun to feel left out—many of my buddies were using filters—did some research, and decided I’d get me one. Despite my constant preaching against it, I have sometimes fallen prey to the dreaded The Only Enemy of Good Enough is More Better syndrome. And that almost happened this time. I read that the OIII filter was the “strongest” LPR filter, and figgered that was therefore the one to get. Starting with an OIII worked out for me, more or less, but might not for you.

So, let’s talk the hows and whys of light pollution reduction filters this morning. Which does what, what they are good for, how you use ‘em, and which ones are for you in which order. Y’all know I like telling stories, so before we get to that, why doncha come on along with not-yet-so-old Unk when he bought his first “light pollution” filter?

As I said some time back, Miss Dorothy and I were pleased enough with our trip to the 1995 Mid South Star Gaze to want to come back the next year. Bad idea. Not because star party organizer Jim Hill did a poor job or because we were unhappy with our accommodations or didn’t like being around our fellow star partiers. It was a bad idea because of a four letter word: R-A-I-N.

MSSG ‘96 ran from April 17 – 21, Wednesday - Sunday, but, as was usually the case back then, workaholics Unk and Miss Dorothy couldn’t get away till Thursday, and, as in 1995, not till late Thursday afternoon, which would have put us in French Camp, Mississippi well after dark. We stayed in Meridian that night, just as we had in ‘95, and didn’t feel a bit deprived. The Weather Channel on the motel cable made obvious there would be no observing at MSSG Thursday evening. Apparently there hadn’t been any Wednesday night, either, and it was not looking so good for Friday and Saturday.

When we pulled onto the observing field around noon Friday, it was sadly obvious attendance was down compared to the previous year. There were some scopes set up, but nowhere near as many as in ‘95. For a couple of reasons. The person tasked with sending out mailings (snail mail, that is; in 1996 we were just on the cusp of  the Internet astronomy explosion) had forgot to do so till a few weeks before the star party, and quite a few people were unable to fit the event into their schedules at the last minute.

The main reason was the consarned weatherman. Many folks will not pull the trigger on a star party unless the long range forecast looks OK, and it sure didn’t look OK ten days before the event. Hell, some folks will not show, even if they’ve already paid, if it looks like there will be more clouds than clear. Not moi. I don’t care what the forecast is, I am going and I am going to have a great time, guar-ron-teed.

I said doing MSSG 1996 was a bad idea, but it really wasn’t. I wouldn’t count it among the My Favorite Star Parties ranks, but Miss D. and I had fun anyway. If nothing else, the low attendance, about 60 folks, tops, gave the star party a friendly, intimate character.

It even looked like we might get in a little observing Thursday night. The wind had been blowing strongly all day with gusts above twenty knots. That prevented us from pitching our tent-canopy, but it also seemed to be driving those consarned clouds away. The sky looked more than good enough to impel us to set up our Ultima C8, Celeste.

That done, we wandered the field a bit, since it was still too early to check into our accommodations at The French Camp Bed and Breakfast—no drafty chickie for us this year. In our progress around the field, who should we come across but our old DSRSG buddy, dealer Rex Allen of Rex’s Astrostuff.

He had some cool things on display, including a revolving rack of earrings and other jewelry that attracted Miss Dorothy’s interest till a huge gust of wind came along, knocking the rack off the table and scattering earrings everywhere. After we helped Rex gather up his non-astrostuff, I turned my attention to his astronomy-goodies.

I had sworn, SWORN, that if any of the star party vendors had an OIII filter for even ten dollars less than list price, I was a-gonna get it. Guess what? Rex had a Lumicon (the day’s big LPR filter maker) on sale for twenty dollars off. I still thought $79.00 was a lot of money to pay for a wee circle of glass, but at least that was better than the normal $99.00.

Field business done, it was off to the French Camp Bed and Breakfast, a restored 100-year-old log cabin home that had all the amenities, and which was just ten minutes from the observing field. In them days, it seemed odd to be staying off-site, but it was worth it for comfy beds and excellent food. By the time we were unpacked and settled, Sol was sinking and it was time to head back to the field. There was no question that we needed to abandon our comfortable room—the clear sky was holding.

As the Sun went down, a great curtain seem to rise in the west, where members of the Solar System put on a wonderful show just for us. Our binoculars and our old friend George Byron’s Astroscan brought us a very young Moon, a shy Mercury, and the still blazing Comet Hyakutake, all grouped within just a few degrees of each other. This was to be my last good look at the comet, who still sported a tail nearly ten degrees long. I’d often joked about George’s puny Astroscan, but it sure delivered the goods this time.

My new toy, the OIII? As we shall discuss shortly, “nebula filters” (another of LPR filters’ monikers) are really only good on, well, nebulae. Spring is not exactly a target rich environment for them, in other words. I did turn Celeste to the sword of rapidly sinking Orion, but the filter didn’t seem to do much for M42. I hoped it was just too low in the sky.

What do you get a lot of in spring? Galaxies, of course. I set out to navigate the Virgo cluster sans go-to and DSCs and had a lot of success hopping from one nightbird to the next with my “manual” SCT and 12mm Nagler Type II. What was really good? I liked Coma Berenice’s M100. A lot. Not only was the galaxy “bright,” other little sprites were scattered around its field. The vision of big M100 and a tiny edge-on in the big porthole of the Nagler was a memorable one.

That wasn’t the prize for the night, though. That came a wee bit later, after I’d run through every stinkin’ Messier in Coma – Virgo. Almost unbelievably, the sky was still clear, so it was on to the galaxy fields of that grumpy old bear, Ursa Major.

I’d obsessed over M101 for a long time. I’d had semi-good looks at The Catherine Wheel Galaxy over the years, but it never seemed as good as I thought it should, even in fairly large apertures. M101 was different tonight, way different. Something about the combination of steady seeing, transparency that was getting better and better, and inky black skies let it shine, even in my modest 8-inch.

The distant giant was startlingly detailed; the more I looked, the more I saw. The spiral pattern, the beautiful spiral pattern, was for once not something I had to guess at. There it was, set right before my wonder-struck eye. The nucleus was a tiny thing just blazing away, and the arms looked clumpy, festooned with HII regions. It was then that I had an idea. What if I tried the OIII on M101?

Even back then I knew OIII filters are not normally good on galaxies (!), but wouldn’t one make M101’s HII patches, its nebulae, stand out? I gave it a try. Bingo! Yes, the galaxy was dimmed to extinction, but I kept looking and eventually began to be able to trace its arms by the softly glowing nebulae along them. Now that was cool, muchachos, and is still one of the best visual looks I’ve had of any galaxy, ever.

I was pumped and ready to chase down every island universe in Ursa Major shown on Sky Atlas 2000. Till good old Jim H. walked onto the field and announced that we were under a severe weather warning.

Frankly, I wasn’t that surprised. Yeah, I’d just been gazing stupefied at a giant and distant galaxy, but I’d noticed clouds building along the horizon for the last half-hour. Conditions did not appear immediately threatening, but the air had the feel that suggests bad stuff is on its way. D. and I did not want to take chances with our new C8, and didn’t just cover Celeste up; we packed her in her case and loaded the case into the Camry’s trunk.

There was plenty of rain that night, but no severe weather, and at first we thought we’d dodged the bullet. Might the front pass through and bring clear skies for Saturday night? Not. It was still cloudy at breakfast (a really great breakfast; the bacon was just CRAZY). Lunch at French Camp Academy—the school that sponsored the star party—came and went and still the clouds hung on. I tried not to worry, spending the afternoon working on the eyepiece drawings of Virgo galaxies I’d done Friday night.  At supper the sky was grayer than ever. Maybe we’d get a repeat, with the skies clearing just at sunset? Nope. At dusk the weather was getting worse instead of better.

We didn’t even get a glimpse of the comet with binocs, and there was obviously no point in setting up the C8. Ah, well. Mr. Jim had scheduled an early evening talk for just such an eventuality. It was during this presentation, whose subject was “Vulcanism in the Solar System,” that the bad stuff arrived in the form of torrential rain, high winds, and ceaseless thunder and lightning. There would be no observing Saturday night.
Miss Dorothy and I and our Possum Swamp Astronomical Society buddies who’d make the trip to MSSG retired to the B&B for a little whiskey and a lot of tall tales. We spent the next several hours recalling old times as blinding lightning flashed and deafening thunder boomed long into the night.

That’s just the way the cookie crumbles in the astronomy game. We left Sunday morning feeling frustrated and disappointed that we hadn’t got more than a few hours of observing. But we’d enjoyed the bed and breakfast and had fun hanging out with our friends. And I’d got the cotton picking filter and even got the chance to use it for a few minutes. There just ain’t no downside to a star party, rain or shine.

Which brings us back to the subject of LPR filters and how they work. I’ve gone into fairly minute technical detail on ‘em before, in my books anyhow. But I ain’t gonna start spouting-off about nanometers and angstroms this morning. That would just make your eyes glaze over—mine too. If you want the nitty-gritty, you can find it on the web. Google “light pollution filter” and you will be rewarded. But you don’t need the nitty gritty to get the straight poop.

How they work is simple. An LPR filter is a piece of optically flat (hopefully) glass that has been coated with layers of various materials. Coatings are chosen that reflect certain wavelengths of light and allow others to pass. The “passband” of a filter, the range of wavelengths it will allow through to your eyepiece, is tailored to the purpose of the filter. That’s all there is to it.

One thing that should be obvious is that, contrary to what some novices think, a filter does not make M8 (or whatever) brighter. What it does it increase contrast by blocking the “bad” wavelengths of manmade light, making the sky background darker without dimming the nebula. More contrast makes the target it look better—if not as good, usually, as it would look under a dark sky.

Now that we know how they work, what are they good for? They are good for nebulae, emission nebulae and planetary nebulae. Yeah, I wish I had a galaxy filter, but there ain’t no such animal. Sadly, the light from human streetlights and other light pollution sources is in the same range of wavelengths as that from the stars, which means any light pollution reduction filter cannot help but dim the light of stars. Since galaxies and star clusters (open and globular) are made of stars, an LPR filter will not enhance them. Period.

“But Uncle Rod, but Uncle Rod, some folks are selling galaxy filters, and my good buddy Skeezix down to the club says his Orion Skyglow helps with galaxies.” Yes, people are selling what they claim to be galaxy filters, but they just don’t work. Neither do broadband nebula filters, not for galaxies. Over the years, I’ve often heard broadband filters like the Skyglow and Lumicon’s Deep Sky can darken the field just enough without compromising a galaxy or cluster too much to bring a little improvement—but I have never seen that.

Look over vendors’ websites and you will find a confusing array of LPR filters from numerous makers. No need for confusion. Filters fall into four general classes, and the filters in those classes are more alike than different (usually), no matter who makes them. What there are are broadband filters, narrow filters, line filters, and ultra-narrow filters (all are my simple-minded designations).

Broadband filters, represented by the Skyglow and the Deep Sky, are often called “mild filters.” Their passbands are wide, and they let in more light wavelengths than they reject. As above, they are not very useful for clusters and galaxies, and I question their value on nebulae, too. If you live where the sky is badly compromised, they don’t darken the field enough to help much.

Next up are the narrow filters like the Lumicon UHC. These are the bread and butter LPR filters for working amateur astronomers. They can spectacularly improve the appearance of emission nebulae, and can also help some planetary nebulae. They darken the sky background substantially, but are not so strong as to wipe out the stars in the field, and many amateurs, including Unk, think they provide the most attractive filtered images for that reason.

The OIII filter is referred to as a “line filter” because it is designed to suppress everything but the spectral lines of Oxygen III emission, the “forbidden lines.” The whatsit of the whosit? All you need to know is that these filters are designed to pass the light of planetary nebulae. Or anything else that radiates in OIII. Caveats? They will work on most but not all planetaries, and they will enhance some but not most emission nebulae. M42, for example, looks worse to me with than without an OIII.

Finally, there are the ultra narrow filters. The most popular of these is the h-beta, which is engineered to focus on the dim red light of the hydrogen-beta lines of an object’s spectrum. If the OIII does not work on all objects, the h-beta does not work on most. It is often referred to as the “Horsehead filter,” since it’s usually bought by observers pursuing that dim devil. It will help on a few other similarly dim objects like the California Nebula, but it is most assuredly not a general use filter. There are a few other types of ultra-narrows, like the “Swan Band,” which is designed to improve comets, but these are even less useful and popular than the h-beta.

Yeah, there are only four basic filter types, but there are umpteen makers. Who should you buy from? How much should you pay? Orion and Lumicon are probably the leaders. Lumicon was in the forefront of the LPR revolution, and much of the credit for the introduction of LPR filters goes to Dr. Jack Marling, Lumicon’s founder. He’s long since retired and sold his company, but the Lumicon filters are still good. Orion’s are very good, too, and my usual advice is “Get whichever is cheapest at the moment.”
Orion and Lumicon are not the only players. I’ve been very happy with my Thousand Oaks and Baader (whose filters are often sold branded “Celestron”) LPRs. I hoped I’d like the OIII I got from TeleVue. Their eyepieces are the best, so I figured their filters should be too. I found the TV OIII not to my taste, alas. It seemed too mild; more like a UHC than an OIII.

How much? Expect to pay up to 100 bucks for 1.25-inchers and up to 200 for 2-inchers. One of the things that makes Orion’s filters attractive is that they can often be found on sale. Can you live with 1.25-inchers or must you buy 2-inchers? Depends on whether you mostly use 2-inch eyepieces or not. A 2-inch filter can be used for 1.25-inch eyepieces via a filter-threaded 1.25 – 2-inch eyepiece adapter, but vice-versa will not work.

You are ready to bite the bullet and buy a filter, and you know who you want to buy from; which type should you buy? I started with an OIII, and that was OK. Enough nebulae are improved by one that I was purty happy. In retrospect, though, I’d probably have been happier with a UHC. One works on almost all diffuse nebulae and many planetaries, too. Get the UHC first. Get the OIII after that.

Save the hydrogen-beta filter for last—if you ever do buy one. Keep in mind that even with a good h-beta and large aperture you will probably not see even a trace of B33/IC434 you’re your suburban backyard. It is a filter designed to help with a few hard ones from dark sites.

From dark sites? Filters can be used under dark skies? Many novices think LPR filters are just that, filters to help our in heavily light polluted areas. They are, but can be just as efficacious out in the dark. Even the best skies are rarely perfect, and a filter will almost always kick the old contrast up a notch or three.

What did Unk get after the OIII? My next purchase was a 1.25-inch Lumicon UHC. Sweet. Used it a lot. I bypassed the h-beta for a while, instead concentrating on 2-inch filters. When I heard Lumicon was being sold, I became worried I wouldn’t be able to get a 2-inch UHC, and paid way too much for one from a vendor up in Tennessee at the 2002 Peach State Star Gaze. Good filter, but I’d have been just as happy with Orion’s narrow filter, the Ultrablock, for considerably fewer $$$, I reckon.

I wanted a 2-inch OIII, too, but decided I didn’t want another Lumicon. If I were going to spend all that money, I’d spend for TELEVUE. That didn’t work out for me. I put the thing on the dadgum Astromart and picked up an excellently priced 2-inch Thousand Oaks LP-3 Oxygen III from old buddy Gary Hand at the Almost Heaven Star Party one year. It works impressively well.

Next it was (finally) h-beta time. I’d decided that after all the years of kinda seeing B33 in my scopes I was going after the Nasty Nag in serious fashion. Being aware that this would not be a filter that would often be out of its box, I wanted to lowball it. I wouldn’t need a 2-incher, I didn’t think, and the Orion Hydrogen-Beta, while not cheap, was on sale at the time and saved me at least a couple of dollars. The results? It worked, it brought me the Horse. On the other hand, it has not been on my eyepieces more than a couple of times since then. And “then” is “2006.”

My final filter purchase? I had got tired of my original Lumicon OIII. The old filter with its pinkish tint just didn’t seem to work as well as the new 2-inch Thousand Oaks with its bluish cast. I needed a 1.25-inch OIII, since I like to use my 8mm and 13mm Ethos eyepieces with 1.25-inch filters (these eyepieces have 1.25-inch barrels and 2-inch “skirts”). My wanting one happened to conincide with the 2009 Deep South Regional Star Gaze, which allowed me to come full circle with Rex’s Astrostuff for my last (for now) LPR filter. Rex did not have a 1.25-inch Thousand Oaks, but he did have a Celestron (Baader) that seemed similar, and which I liked and still like a whole lot.

We now come to the subject of how to use LPR filters, how to use any filter type on anything. It would seem simple and obvious: screw onto eyepiece and look. As the Great and Wonderful Wizard of Oz liked to say, howsomeever, “Not so fast, not so fast!” There is a secret to using the things successfully, and if you don’t know it you’ll as little with LPR filters as Unk did at those long ago star parties.

If a filter is to do its best, there is one thing you must not allow to happen; you must not allow ambient light to enter from the eye lens end of the eyepiece. If you are observing in even minimally light polluted surroundings, that is likely to happen and you won’t like the results. When light enters the eyepiece from the eye lens side—which it will unless your eye is tightly jammed up against it—the ocular will be flooded with bad light and you will see less with the filter than without. Ambient light will be bounced off the eye lens facing side of the filter and right back into your eye.

The cure? Use a rubber eyecup on the eyepiece. If your ocular doesn’t have one, you can buy cups that fit over almost any eyepiece. Or cup your hands around the eye lens when you are looking. Best of all, use a dark hood to cover your head and the eyepiece preventing any stray light from getting in. Orion sells a nice piece of light-blocking cloth for that purpose, or you can get a square of black Nylon or muslin down to the fabric store for just a buck or three. Use it and your filter will be able to perform as well as it is capable of performing.

So there you have it, the light pollution filter story sans gobbledygook and fiddle-dee-dee. The bottom-line-a-roony-o for you greenhorns? Yes, you want an LPR filter or two. Don’t wait as long as I did to get one, either. They have improved my deep sky experience immeasurably, not just enhancing nebulae, but making the difference between seeing and not seeing some of ‘em, both from light and dark sites. That is most assuredly worth a few C-notes, muchachos.

Next Time: Me and Mr. Sun… 

Comments:
I posted here about a week ago, but it never came through. Is the comment system losing submissions?

It was a post pointing out that if you want to stick a toe in the world and filters and try several different ones without spending too much money, give the Zhumell filter set at telescopes.com a look. For $90, you get an OIII filter, a UHC filter, a variable polarizing filter that works great on Luna, and a worthless "SkyGlow" filter.

This may not be top-quality filters, but I am getting a lot of benefit from them. Without these affordable guys, I'd probably just have a single UHC filter.
 
I posted here about a week ago, but it never came through. Is the comment system losing submissions?

It was a post pointing out that if you want to stick a toe in the world and filters and try several different ones without spending too much money, give the Zhumell filter set at telescopes.com a look. For $90, you get an OIII filter, a UHC filter, a variable polarizing filter that works great on Luna, and a worthless "SkyGlow" filter.

This may not be top-quality filters, but I am getting a lot of benefit from them. Without these affordable guys, I'd probably just have a single UHC filter.
 
Though I am thankful for the accessibility of good cheap astro-gear, I have had horrendous customer relations with them. I would gladly eBay or Orion than choose zhumell just to save a few bucks.
 
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