Lumicon Deep Sky Light Pollution Filter
Some Background Strictly Speculation First (Rejected) Light By The Light of the Silvery Moon More On The Ring By Moonlight Parting the Veil Welcome to North America! Taking the Pup Out for a Walk Taking Stock: Weather on the Way Probably an 8"SCT... Bode's Galaxies & the Pup Bode's Galaxies & Ms. Vicki Bode's Galaxies & Argo
Some Background
"The Lumicon Deep Sky Filter is THE foremost broadband filter efficiently blocking ALL mercury-vapor and high & low pressure sodium-vapor lamp light, neon lights and airglow, while allowing the rest of the visible spectrum through. The superior contrast gain between the object you're viewing and the black background of space allows you to view galaxies, nebula, star clusters, comets, etc. wonderfully -- when observing from light-polluted urban skies -- making it the best all-around visual light pollution filter." - http://www.lumicon.com/dsf.htm.
Christmas came early for me this year - Astro-bud El Marko was feeling particularly generous and graciously handed me a Lumicon Deep Sky filter at this last SCAC star party - well before the first Christmas Lights were turned on. Now the question is? How well will the Lumi block the assorted purples, reds, greens, blues, oranges, yellows, and whites cast against the vaporous expanse of the night sky above Backyard Boulder Creek before, during, and after, the Christmas season?
Well according to the distributor - you can forget the purples, oranges, and yellows - these have been axed. White however, has been denuded of these same colors and a sort of "aqua blue-green" is about all you will see of such a polychromate. How do I know? Well, just take that Deep-Sky filter, pinch it between thumb and index fingers (at the rim - of course) then hold it up to any white light source.
Of course any filter will block some frequencies of light and pass others. Meanwhile neutral density filters simply reduce the amount of light passed to the eye without changing a things apparent color - only its intensity. A red filter will cut off all light shorter than - say 650 nanometers in wavelength. A yellow filter will pass light in the 550-600nm range. A green filter passes light in the range 490-550nm. Blue presents the eye with light whose wavelengths lie in the 440-480nm region. But a truly deep-sky kind of filter must perform a special trick - it must reflect light emanating from common terrestrial sources while accepting light emanating from celestial ones. The ideal such filter is tuned to reject a very narrow range of wavelengths while passing all others - something impossible for commonly available materials and certainly not to be ordered off the web for $100 american.
So the Lumicon Deep Sky filter makes do by rejecting a broad range of wavelengths in two main regions - the yellow+orange 520 - 640nm and violet (400-440nms). This enables it to pass excited wavelengths associated with the sky (Hydrogen-alpha, Hydrogen-Beta, Doubly-ionized Oxygen (OIII), and Cyanogen - CN) while suppressing photons typically reflected back to earth from common sources of illumination such as mercury and sodium-vapor lamps.
In space of course, most illumination is provided by stars. Of the many types of stars, some are luminous in wavelengths shorter than the 400nm susceptible to ocular detection. Others irradiate in wavelengths too long to stimulate receptors in the retina (greater than 700nm). Neither the ultra-violet of post-noval stellar nuclei, nor the infra-red of low-mass brown dwarf stars pass through the earth's atmosphere of course, BUT many nebulae are rendered visible to the eye after being stimulated by ultra-violet light from sources invisible from the earth's surface. And it is those blue-green wavelengths - such as the Hydrogen Beta and OIII - that make it possible for the various emission nebulae - such as the Cygnus Loop - to be seen. Meanwhile red-wave Hydrogen Alpha is often associated with star-birth nebulosity - such as portions of the Great Nebula in Orion and the North American Nebula of Cygnus. But stars in themselves are in most cases polychromatic and some of their light is bound to fall into regions rejected by the Lumi. For this reason we can't help but lose telescopic limiting magnitude with such a filter in use.
NOTE: Most stars appear “white” but others actually emit quite a bit of their light in regions rejected by the Lumi. Thus stars with stellar spectra in the F/G, and K ranges will no doubt prove visibly dimmer through the Lumicon Deep-Sky.
Meanwhile "Cyanogen"- in its form as the diatomic radical CN - is commonly found both in interstellar space (the interstellar medium or ISM) and within comet tails. Like many other simple molecules, CN is easily torn apart or chemically dissociated by ultraviolet radiation. And because of this - again like other such molecules - CN tends to concentrate deep within the protective shells of dense nebular regions whose outer layers are comprised of simple gases (such as oxygen and hydrogen) - themselves ionized by nearby ultraviolet stellar sources. Thus we can think of CN as a secondary radiator - stimulated by light thrown off by other sources - and bound to be seen by the observer more toward the core regions of extended nebulosity than at its frontier...
With this background our stage is now set and we probably now know more about the theory of light-filters than is necessary. How will the Lumicon Deep Sky filter perform under the night sky? And under what circumstances might it profit the observer to thread the thing into diagonal or eyepiece?
Strictly Speculation
I must admit it: I like seeing celestial thing-a-mabobs shining against a pitchy-blacky sky. Now one way to do this is to use a small scope at low powers or a large scope at high powers. Sure! That old 43mm aperture stop on the 80mm Pup achro has its place! 1mm exit pupils generally throw up decently dark background skies from Backyard Boulder Creek. And in a 43mm scope that equates to 43x! But unfortunately it also equates to a telescopic limiting magnitude a bit shy of magnitude 10.5 - but at least you get 1 plus degree fields - large enough to take in the Eastern Veil Nebula - if you can see it!
So OK what about a large scope at high powers? So I go out and buy a 250mm newt and dial in the power. Holy Moly Starman - that's 250x! Now we're talking at best 15 arc-minute sized fields - M43 encompassing... But I get my dark background through 5.0 ULM skies I do! And I can hold stars down to magnitude 14. But now I've gone and spread all that faint nebular light all over the field of view. There's got to be a better way - short of giving up on deep sky from the 'ol backyard...
So here we go. If the Deep Sky filter really does the trick, I'm going to get one degree fields through 6 inch Argo at 50x - 3mm exit pupils - from 5.0 backyard Boulder Creek. Meanwhile I'm still going to just hold stars down to magnitude 12 - on the very finest nights of stability. And through 102mm Vicky I'm going to be able to drop the magnification down to 40x and traverse the North American Nebula sucking in all them 650-700nm photons well enough to trace the entire frontier - Yucatan to Hudson Bay...
Meanwhile at 120x or so Argo is going to resolve at least a few stars in 8th magnitude M56 in Lyra, or M72 in Sagitta while displaying the full glory of the rich Milky Way stars that grace their presence. This as views of the Dumbbell, Crab, Veil Complex, and Merope nebulosity are enhanced without the annoying occular peculiarities associated with the use of the Ultrablock filter. Then, of course, it sure would be nice to readily pick out NGC891 - or detect luminosity extending outside M31's first dark lane. Or what about a high contrast view of the frontiers of the Leo Trio or Bode's Galaxies? - especially the latter which hangs out in the San Jose light-dome north of Boulder Creek...
Yep - a lot to ask. Will the Lumi rise to the opportunity? To filter or not to filter - that is the question!
First (Rejected) Light
I did in fact use the Lumicon filter extensively on that first night out at Bonny Doon airport. As I recall, seeing conditions were in the 5.5 ULM range. Stability passable, but not expecially good – say 6/10. Strangely - other than an favorable overall impression – there’s not much to recall from the occasion. One thing – alluded to above – does have some importance. For you see, I already have an Orion Ultrablock filter in my possession. Although bandpass information on this filter is hard to come by, by my own estimate I would say that it can be compared to the Lumicon in the following way: The Ultrablock is formulated to pass a narrow region of wavelengths associated with Hydrogen Alpha and OIII - rather than broadly constructed to include all possible light from sources other than those common to light pollution. Thus the Lumicon Deep Sky is a general-purpose wide-bandpass filter while the Orion Ultrablock is a more specialized narrow bandpass unit. Because of this, one of my main reasons for avoiding the use of Ultrablock is obviated. For basically light of very narrow frequency ranges tend to cause a form of “eye-strain” I find unpleasant. But that first impression of the Lumicon was that this particular neuro-optical response was not present. And it is for this reason that I am willing to spend more time with the filter and discover where its “edges” are…
One other thing of note: I used a red flashlight to inspect the filter after unwrapping El Marko’s gift. As one might suspect – the filter was completely transparent. On the occasion El Marko also mentioned the fact that one surface of the filter – presumably the inward face – is very soft and susceptible to damage during cleaning. Heads up!
By The Light of the Silvery Moon
150mm MCT Argo is my instrument of choice for trips and it was through Argo that my first – favorable – impressions of the Deep Sky were gained. In the Backyard however I tend to leave 102mm Achromat “Vicki” assembled for “grab and go” viewing while Argo rests comfortably in his travel case. It was with Vicki that I set out around sunset on Wednesday, November 17, 2004 to begin characterizing the Lumicon.
Over the last few years I have learned something about myself observation-wise. For you see, I've discovered that if I do not begin observing early in the evening and follow the sky into darkness it is unlikely that I will either take up observing later in the evening or stay with it should I break for some reason. And such was the night of the 17th. With the setting of the Sun, a five-day Moon could be seen to the south. I had Vicki setup on the work-out deck and assessed seeing stability. The Lunar edge quivered only slightly with atmospheric distortion. Meanwhile the luminosity of the early evening sky washed away all contrast. I installed the 25mm eyepiece, and 2.6x configured Shorty Barlow along with the Deep Sky filter (giving some 110x – a little more than 1mm exit pupil). The sky remained bright beyond the lunar limb. It was also a distinctive aqua color. The deepening blue of the sunset sky became an ocean of light bathing Selene’s comely crescent. The terminator lay just east of craters Theophillus and Cyrillus. I pulled out my various colored filters to get a sense of which gave the best detail contrast and turned Vicki on Posidonius. 13Km wide Crater Posidonius A was obvious - along with Posidonius’ northwestern rima. The Rima itself was very luminous - even at this early observing hour - due to the solar illumination angle which just cleared the craters western rim and highlighted the rima’s western flank.
Cycling through all my filters (except the Ultrablock), it became clear that the medium-density green revealed the most detail within Posidonius – rivaling even that of the unfiltered eye. But it was also clear that contrast suffered visibly with the early hour and improved only as the sky darkened. At 3 km’s in diameter, Posidonius C – northwest of Posidonius A - could at times be resolved as a craterlet with and without the green filter - but only during moments of great stability when the atmosphere – some 40 degrees above the horizon - became particularly still.
NOTE: A 3km sized crater – such as Posidonius-C - displays roughly a 1.5 arc second angular-size along its major axis. Interestingly, a 4 inch scope is capable of resolving (as distinct airy disks) two sixth magnitude stars 1.2 arcsecs in separation. Seeing stability outside the skies middle third must have been exceptional indeed to make craterlet-C’s resolution as a crater possible.
In evaluating the other color filters I found that the medium blue and Deep Sky filters were not quite able to rival the green filter and unfiltered eye in resolution. Meanwhile the red filter – passing longer wavelengths almost exclusively – revealed the least detail of the four evaluated on this occasion…
About the time that 3rd magnitude Alberio could just be held overhead, I relocated Vicki to a spot to view the Double Double (Epsilon Lyrae). Using the finderscope I centered on the wide pairing at some 270x (10mm Ultrascopic plus 2,7x barlow configuration. All four stars showed distinct airy disks with three of them revealing a bright-ish first diffraction ring. Dropping in the 25mm eyepiece (110x), I found that I could resolve both pairs without difficulty - with and without the Deep Sky filter. However, with the Lumi installed, I could no longer see the diffraction rings of the bright 5th magnitude components. Meanwhile, I also noticed that – under 3.0 magnitude ULM conditions - neither the filtered or unfiltered 25mm eyepieces could hold the 10.5 magnitude come that triangulates with the wide bright pairings of the Double-Double. Later, as the sky darkened I was surprised to see that - at the point where I could just hold the come unfiltered - it could be held with the slightest aversion using the filter. Thus magnitudinal reach with the filter was no more than .5 magnitudes off from the unfiltered view. In the past I have done the same kind of experiments with the narrowband Ultrablock and found a 1.5 to 2 magnitude delta between the filtered and unfiltered view. Certainly the broadband unit is significantly advantaged when it comes to limiting telescopic magnitude…
NOTE: I also noticed something peculiar in this instance about telescope optics. Previous to observing Epsilon, I star tested the achromat on Vega. That test showed Vicki to be barely diffraction limited with pronounced under-correction. This is substantiated by the fact that the first diffraction rings could be easily held surrounding the three brightest stars of the Double-Double and yet the 10.5 magnitude come remained undetected. I imagine that a perfectly corrected unobstructed scope would easily show the 10.5 magnitude at about the same surface brightness as the first diffraction rings of the brighter components. So those observers who have a fine apochromat please take the opportunity to test this theory and let me know how it turns out...
About this time a considerable amount of moisture condensed on Vicki’s OTA. Knowing the evening was about to end, I turned the scope on M57 and compared filtered and unfiltered views at 110x. Due to the relative brightness of the sky, there really was no advantage to the filtered view. BUT since the bulk of the sky glow was polychromatic (solar-sourced) and not reflected from terrestrial sources I don't consider this to have been a valid test of Lumi’s capabilities.
More On The Ring By Moonlight
Seeing stability on this occasion (November 19, 2004) was a bit poorer than my last outing. Although the Double-Double could be easily resolved at 110x unfiltered – it's airy disks appeared to have a luminous haze about them. By my lights this means 7/10 stability overhead. Under these conditions filtered resolution was “challenged” at the same magnification – confirming my notion that the filter itself does not possess the degree of optical perfection that would make it useful as a lunar-planetary filter. But hey, ya never know…
Having evaluated the seeing, I switched over to M57. Stars to magnitude 4.0 were at the limit – given the early-hour and presence of an 8-day moon in the southern ecliptic. Since the moon – like the Sun - is polychromatic I doubted that Lumi would offer any advantage on this occasion – and I was right. The wash of colors across the night sky from that particular broadband celestial source negated the filters capacity to offer much of an improvement over the unaided view – at 1mm plus exit pupils. On a whim I tried the Ultrablock filter and the background sky darkened perceptibly. This filter really did perform under the conditions – due to its narrow bandpass. However many dimmer stellar sources dropped from view – even as the sky darkened significantly.
I was however able to confirm that the Lumi looses no more than .5 magnitudes on most stellar light sources. Referencing the 12.3 mag star in the “Chalice of the Ring” I found it could be held with moderate aversion using the filter and slight aversion unfiltered – again about a .5 delta. – Not bad at all…
Parting the Veil
December 1, 2004: The sky was just about perfect - no not perfect but perfect. Perfect for seeing how well Lumi does under run-of-the-mill 5.0 ULM transparency skies.
By 6:30 PST, things were about as dark as it was going to get. Christmas lights had sprung up all over the neighborhood. I was hard-pressed to find a spot in the backyard where they could be avoided. Meanwhile, what had risen earlier in the season as The Great Swan was now coming about to plant itself in the ground as the Northern Cross. Although still well within the skies middle-third, Cygnus offered a nice opportunity to turn up the Veil through Vicki at 40x.
But I get a head of myself...
My observing session actually started at about 5:00 PST. With the Lyre heading west - even at this early hour - I turned up the Double-Double at some 110x. Gorgeous! Four fine jewelike orbs (the faintest E1B at magnitude 6.3 visibly smaller than the others) displaying walking first diffraction rings - 8/10 seeing stability.
Of course such fine conditions also meant that Vicki would have an opportunity to resolve widely disparate Delta Cygni. Even at magnitude 2.8. Delta itself could only just be seen on eye movement at this early hour. Centering the finderscope, I popped in the 10mm Ultrascopic-barlow combination for some 270x. What'd I see? Nothing definitive - just a faint fixed widening on the first diffract northwest of the primary. Vicki's 1/4 wave optics just don't seem to be up to resolving the secondary as a star...
NOTE: Well er, hmmm... Northwest of the primary eh? In fact Delta-B lies at a PA=~230 degrees (southwest).
One thing you have to be careful about though when attempting to resolve wide disparates near the zenith - body heat alone degrades image quality. Add to this the challenge of setting precise focus hunched over an eyepiece suspended 18 inches of the ground. Yep, I know excuses, excuses...
Had to take a break from observing at 5:30 to help out an acquaintence. Back by 6:15, turned Vicki on 52 Cygni at 40x (25mm Ultrascopic) unfiltered. Through them 5.0 ULM skies, no sign of the Western Veil - no handle, certainly no broom. Dropped in the Deep Sky filter - same magnification. Yep, the brighter northern extension could be seen on moderate aversion. No sign of the fanlike southern region. With the Ultrablock in place, could hold the shall-like broomstick direct. No question here, the narrow bandpass filter really puts out the lights.
Shift 52 Cygni off to the south, then due east to the Eastern Veil (The "Batlief") no filter in place. Sure I could find it - but that be it. Deep Sky in Place - easily held but very little structure. Ultrablock - structure!
With Cygnus exiting the skies middle third made a quick run on the North American Nebula - based on Memory. Easily found the dark obscuration region comprising ther "gulf" also plenty of star-studded hazi luminosity but decided to save this one for another night.
By 7:00 the night air had become very cold and the sky above Boulder Creek went into "vapor phase". Turned Vicki on M31. Did a quick comparison. Globularesque M32 lost some punch but there was in fact a better sense of structure to The Great Andromeda Galaxy itself with the Deep Sky filter in place. Like the North American Nebula, The Great Galaxy calls for a longer visit in the nights to come...
Welcome to North America!
Locating North America is pretty easy. Just fly west from Xi Cygni. No turns required and there it is. Well flight conditions do matter. Too much of turbulence only shakes up the passengers. But should things get "cloudy" well you might miss Yucatan if your flying at 10,000 meters...
December 2, 2004: Last night - except for the 35 degree outdoor temperatures this passenger was pretty comfortable - 7/10 stability. As for clouds? Well transparency approached 5.5 ULM - and there's the rub. At some 40x even 4 inch Vicki had no trouble turning up Yucatan unfiltered. Adding the Lumi only made the "landmass" more obvious. No test here. Sure even under decent transparency the background sky darkened perceptibly. But the view was still good without the filter - even at 2.5mm exit pupils.
All this speaks volumes (decibels - not literary) to my thinking. What's the real value of such a filter when the sky is already accommodating? Well, if you have even lower power eyepieces available then the Lumi is useful. If not, well then perhaps not so. Another way of saying this is that if the sky is dark enough already then your options are limited. Either reduce the power, increase the aperture, or remove the filter.
Last night I dropped the 35mm Ultrascopic into Vicki and that did the trick. The North American Nebula (NGC7000) is huge and there is no way to encompass it without mounting the 80mm Pup - for some 12x/4 degree fields. But dropping the 35mm in the 4 incher was a good start - 29x/1.7 degree FOV. Voila - Lumi became relevant again.
But the real kicker was the eastern Veil revisited. At 29x the entire batlief could be encompassed. And more importantly with the Lumi installed there be structure. Very nice view.
However the prognosis on globular clusters (M56 in the Lyre) is not good. At 110x and through decent skies the filter is not the prefered choice. 1mm exit pupils (110x in this case) just don't need filtration and that .5 magnitude delta is no help.
Next stop The Great Galaxy in Andromeda - Last Call for flight 224...
Taking the Pup Out for a Walk
What these three babies needed was image scale. I mean lack of same... The Cygnis Loop, North American, and Great Galaxy are all extended studies. The Loop because its two main features (the Eastern and Western Veil Nebulae) are the now well-separated remnants of a supernoval display some 2K light years and 50K years distant. The North American, because its impossible to get a sense of its shape without some 3 degrees apparent fields of view. And the Andromeda Galaxy because this baby is perhaps 25% larger than our own and despite being some 2.5 MLY's distant sprawls across more than 3 degrees of the sky.
At best the F10 4 incher Vicki is able to achieve 1.7 degree fields. Meanwhile F12 150mm Argo doesn't quite squeeze a single degree of sky into the same 35mm eyepiece. What to do?
Take the Pup for a walk - sure, the Pup. At 80mm's in aperture the F5 achromat doesn't exactly capture a lot of photonage. But the Pup does low power very well. Using the 35mm Ultrascopic some 4 degrees of spherical sky can be pulled into the field stop at 11x. In fact early on I could almost encompass the entire parallelgram of the Lyre in a single view.
But what to do as the sky darkened? Why not lay two barlows and a 10mm Ultrascopic on the Pup's (visual) back and focuser for 210x. Do a star test on Vega - surprisingly slightly better than Vicki although color correction is significantly poorer. Resolve the Double Double (cleanly) and spend some quality time on Delta Cygni (same result a more or less fixed brightening on the first diffraction ring to the northwest). Drop down to 120x and check limiting telescopic magnitude (11.5 just before skydark). At best 4.5 ULM at the time and later after skydark closer to 5.0.
NOTE: This "northwest" idea just keeps popping up. What could be the matter here? Hmmm.
First stop the veil. At 11x just not enough image scale. 16x (25mm Ultrascopic) - no improvement. 26x (15mm Ultrascopic) - the magnification of choice. Through the Orion Ultrablock, structure - though less than through 4 inch Vicki at the same power. Through the Lumicon Deep Sky - a bit of a disappointment.
On to the North American! Due West of Xi - no problemo using either filter. Yet NGC7000 is large and needs lowest power possible. But the Lumi just didn't have the legs at 11 or 16x and required 27x for the right amount of contrast against the sky. Meanwhile the narrow bandpass and Hydrogen Beta transmissivity of the Ultrablock easily framed the entire continent at 11x. But if stars were wanted the Deep Sky could jump through the hoop successfully...
But the study of this evening (Friday, December 3, 2004) was The Great Galaxy. 11x was simply too low a power. Sky brightening made following the extensions difficult and the Ultrablock is unusable for stellar lightsources. Meanwhile there was no advantage to the Lumi either. Jumping up to 16x was a better choice. So swapping in and out the filter I noticed immediately that the little extra light seen unfiltered was better for tracing the extensions out toward the limit one and a half degrees from the core. That extra light was also useful in detecting that M32 was something other than a fuzzy star - while M110 was more easily held direct as a condensing patch of faint luminescence.
But the filter too had its advantages... It was much easier to detect the first lane north of the core. In fact there were suggestions of faint luminosity outside the lane as well. Meanwhile textural variations in luminosity were made plainer using the filter. This included hints of NGC206 triangulating with the core and M32 along the axis of the galaxy. And too there was a better notion of "spiralicity" to the whole view - something not really seen just sensed.
And finally the aesthetics of the view were significantly improved. Much of the gauziness of the sky was eliminated - and despite some loss of magnitudinal reach I found myself spending mote time viewing the Great Galaxy with the Lumi than without it...
Ain't that what its all about?
Taking Stock: Weather on the Way
My guess is probably an 8 inch SCT. Weather here in the SF Bay area favors overcast. Rain is also expected over the next four or five days. This reality leads me to conclude that some earnest amateur - probably a TACky - has taken receipt of a new scope this weekend. If the overcast lasts say 2 weeks - then I'd say more like a 20" Obcession. 16" SCT, 10" MCT or 7" APO. You can halve those numbers if it lasts a week. If the weather hangs out no more three days then we were simply due for a change...
Since I won't be getting out with the Lumi again soon now might be a good time to make an initial stab at evaluating the value of acquiring such a filter and its potential uses. Given the MSRP on the thing, I would like to point out that $100 will buy you a really good apochromatic barlow, a complete set of lunar-planetary filters, a narrow bandpass filter - such as my Orion Ultrablock, a minus-violet filter (for your achromat), one quality eyepiece, add a fine slow motion control / clock drive to your equatorial mount, an excellent 50mm finderscope for the main tube or pay for a round trip by POV to the deepsky site of your choice...
What it won't get you is a pair of 80mm binoculars, a fast achromat 80mm's or larger in aperture, a chromacorr for that achromat, a super wide field eyepiece, a stable equatorial mount for a four inch or larger scope, or round trip tickets to Mauna Kea...
Of course the real value of the Lumi can only be assessed based on the peculiarities of your own observing and observing site. My sense is that if you regularly observe through 5.5 plus ULM conditions the filter is innecessary. If you simply do lunar-planetary and double star observing - Lumi is equally unnescessary. If all your observing is done at 1.0 exit pupils or greater magnifications the filter is equally superfluous - this includes resolving globular clusters or chasing down 2 arc-minute apparent sized galaxies. Most planetary nebulae are out too and if a nebula is of the emissive type you are probably better off with a narrow bandpass filter - such as the Orion Ultrablock.
What's left?
5.0 ULM or lesser transparency skies or superlow magnification - wide field observing. Large scale open clusters, expansive reflection nebulae, or relatively bright and large scale galaxies remain in play.
One final caveat. I don't observe from an urban location so I leave it up to someone else to discuss the virtues of the Lumi through heavily light-polluted city skies. No doubt such environs make the Deep Sky if not an absolute necessity a necessity...
Will I continue to use Lumi. Yes. But only at super low magnifications where the Ultrablock's narrow bandpass rejects too much light. That basically means for me and my skies around 3mm exit pupils: 50x in Argo, 35x in Vicki and 25x in the Pup. Will I write up more experiences with Lumi?
Sure, if something noteworthy turns up!
Probably an 8"SCT...
It was in fact one week before skies cleared enough to again "Seize the Night". But there was a twist to the whole thing - for last Saturday (12/4/2004) and Sunday evening could have been observing occasions since there was unexpected clearing after sundown - strange weather for these parts during the rainy season...
My guess? The aforementioned SF Bay Area weather-bending scope was not actually delivered until Monday...
Friday day was beautiful - warm, sunny, inviting. I was up early driving Sharon to work so I could have the car for a job interview (anybody hiring?). By sunrise I had already consumed my first Double Mocha with Whip of the day. Cruising to Watsonville along Hwy 1, I saw the last of the clouds being chased away by the Sun - glorious golds and orange-bottomed cotton candy fair-weather clouds they were!
Since my interview wasn't scheduled till nine, I found a sunny spot and worked on a flamenco improvisation using the 3/4 scale traveller guitar. (What tiny fingers those 3/4 sized humans have!) After the interview (went fine by the way - but generally I am not cowed by such things...) I swung on by Orion Binocular and Telescope. Why?
Cause a couple things got flagged while using Ms Vicki and the Pup for Lumi tests. Now if you recall both Ms Vicki and the Pup are achromats. That means of course that these two refractors are only designed to bring the colors red and green to a common point of focus. Both scopes have trouble handling the color violet since refractors "bend" light and each refractive element bends light differently depending on the frequency (wavelength) of the light involved. Violet of course, is a short wavelength - which means the light cone is steeper than green and much steeper than red (the longest visible wavelength to human occular perception).
So where am I heading with this? Why during daylight testing of both achromats I noticed that the sharpness of both scopes improved with the Lumi in place. Get my stellar drift??? So I picked up a minus violet "V-Block" filter from Orion (along with a second low-profile 2 to 1 1/4" adapter for Ms Vicki who can't handle my 35mm Ultrascopic without borrowing Argo's).
Now as of yet I am not convinced the V-Block will do the job I want. After all, do you not, Dear Reader, recall that I did some moon testing early on with the Lumi and found that it did not resolve Posidonius-C as well as the medium green filter? Well then - the V-Block may not be "up-to-snuff" optically and I may find that it causes more problems than it solves. But of course, that is a different article than this one (look for one later on this month when I get some "Selene" for testing...)
But back to the Lumi... Does anyone get the impression that I am less than thoroughly pleased with it? So OK here's the reality, the thing is meant for skies flooded with sodium and mercury vapor lamplight after all. That is not so much an issue for Backyard BC (well not until Bode's galaxies are accessible anyway...)
Bottom line: Even here the Lumi does improve "blacksky aesthetics" at low powers. BUT, I have yet to find a single thing through the filter that could not as easily been detected without it. In fact there are studies that I could locate (galaxies in particular) that were undetectable with the Lumi in place.
Example: Last evening - using 150mm Argo through them 5.5+ ULM 7/10 stability skies hanging over El Marko's Private Night Sky Sanctuary (EMPNSS) - I could just turn up 12th magnitude galaxies NGC16 and NGC23 (near Alpheratz in the Great Square) at 50x without the filter. By adding the filter, Argo lost that .5 magnitude edge and both galaxies dissappeared into the night...
Once again - I weren't looking through no sodium/mercury vapor lamp haze neither...
Bode's Galaxies & the Pup
M81 & M82 were first noted by the German astronomer JE Bode in December, 1774. Like other such isolated discoveries (Praesepe, the Pleiades etc.) Charles Messier went on to add them to his list of "cometesque" objects later in the same century. The fact that Bode's Galaxies were ever discovered at all is a marvel. After all, even those of us who have viewed this wonderful galactic duo numerous times over the many seasons of the sky find it hard enough to turn them up without a bit of a struggle!
Despite difficulties navigating the northern sky - with its paucity of guide stars - I did manage to turn up the pair using the 30mm finderscope mounted on the Pup. Of course I pretty much inspected every faint smudge revealed by that same modest glass north of Upsilon Ursa Majoris through the main tube in order to do so. The time was about 5:30 am PST. Date: December 18, 2004. I had loaded a 25mm Orion Plossl in the Pup's diagonal. The diagonal itself was snugged nicely into the 3x Ultrascopic Barlow hanging from the Pup's focuser. The magnification? Some 50x.
Now 50x is 1.6mm's of exit pupil at 80mm's aperture. The sky was visibly brightened by the San Jose light dome north of Boulder Creek. I could make out a 5.0 ULM star near overhead trailing the Big Dip across the sky. Stars to 4.5 were possibly in the vacinity of the galaxies. Looks like the dome was covering up about .5 magnitudes of sky. Would this be a good test of Lumi?
OK, you gotta remember that 1.6mm's of exit pupil. The view of Bode's Pair was very nice. No filter, 1.6mm exit pupil - two cat's eyes glowing in the night sky. Sure, the background field was slightly greyish - but hola those gal's were a pretty sight. Structure in edge on irregular galaxy M82. Spiral extensions well spun on M31'ish M81. No sense of frontier though on the spiral. But I could hold the frontier of the spindle galaxy direct - though structure only jumped out at me during eye move,
As soon as I saw that view I knew Lumi was sunk. Basically if you're happy with the view you got why bother threading a filter into the barrel?
In this case I didn't have to thread anything - the filter was already loaded into the 25mm Ultrascopic. Drop in the Ultra and yep - the background sky darkened nicely. But nope - no real improve in the view. In fact M82's stilletto-like shape softened - and structure was hard to come by. Meanwhile I could follow M81 outbound a bit better using the filter - so yah - it had its moment...
Of course being up at that hour meant that Sat and Jupe were well-positioned. I tried the Lumi on both. At 150x, Saturn is just too faint with the filter in place. Jupiter however - wow! Lumi really brings out the contrast on the two equatorial belts. It also cuts down immensely on irradiation. Much easier - despite soso (6/10) seeing stability - to get a lock on focus. But, as you recall from my lunar explorations, the filter looses out a smidge on faint detail. So yah, wanna see them belts? Drop in a Lumicon Deep Sky!
But as for deep sky through the Deep Sky? Hmmm. I need more sodium and mercury vapor lights! Wait a minute - what am I saying?
NOTE: The next morning I repeated my observations of Bode's Galaxies using the Pup at 30x (2.7mm exit pupil). On this occasion with the sky perhaps a bit stabler - but no more transparent - I found the amount of detail indistinguishable between the filtered and unfiltered views. In fact, nearby irregular galaxy NGC3077 could be held with moderate aversion in both views - even displaying some slight central condensation. Meanwhile neither view could turn up larger/lower average surface brightness NGC2976 located a degree south of spiral M81. So in this instance the better aesthetics of a darker background sky won the night for Lumi. However image scale at 30x fell below the threshold of what I'd consider the "optimum" view of this pair - which really needs some 45x magnification for the definitive low power, all-encompassing view...
Bode's Galaxies & Ms. Vicki
We here at Astro.Geekjoy would like to think that the definitive write up on observing Bode's Galaxies is to be found at M81&82: The Great Galactic Duet!. So if you - as a result of reading this article - are hankering from some Bode - check out Tammy Plotner's superb discussion...
Meanwhile my own quest for observing this delightful pair of cosmic dance partners continued using 102mm F10 achromat "Ms Vicki" at some 5:00am PST on Monday, December 20, 2004. On this occasion and using a pair of 25mm eyepieces (one Orion PLossl without Lumicon Deep Sky filter, and one 25mm Ultrascopic with filter) I ventured to the northern hinterlands once again and - discovered a comet!
Well not really, I actually discovered a "cometary" object. Not quite that either, it was actually a galaxy - NGC 2985 to be exact and how I came across this 10.5 mag study fell out of the fact that Vicki's 30mm finderscope dewed out on me and I just started sweeping the sky looking for Bode's and ended up near 5.1 mag 27 UMA - which on this occasion I could just detect averted... Wow, sure packed a lot of info in that sentence!
So of course I compared the views of this barely holdable 13.1 magnitude average surface brightness face on spiral. And yep it was equally just direct with and without the filter. So Young Lumi - despite losing that half a magnitude - did pretty well...
Now the fact that I was picking out such faint stars in the light dome says something - BUT it was even darker overhead the difference being - say .5 mags.
After tracking down my elusive discovery in Tirion's Sky Atlas and accepting the ignonanimity of once again NOT discovering a comet it was an easy drop due south of 2985 to locate my intendeds.
At 40x the view was superb. M82 - a fine tipped dagger in the sky - M81 expansive and elegantly engaging. The main difference between the two views lay in the fact that Lumi showed the pair against a coal black sky while - not unexpectedly - its twin said "lighten up a little". There were some other distinctions - again the unfiltered view did a tad better on revealing structure in edgy M82 while Lumi brought out the evanescent expansiveness of stately M82. Do we have some kind of theme playing out her???
Of course, once again I turned the two views on neighboring peripheral galaxy NGC3077. Through 4 inch Vicki both held the galaxy direct and hinted at a central condensation. Meanwhile - as with the Pup the night before - lower surface brightenss 2976 remained - at best a wish and a dream...
Bode's Galaxies & Argo
December 21st is traditionally the date upon which ancient rites are solemnly performed to encourage the "Grand Man of the Heavens" to turn course and return once again upon His upward way to grace northern skies with His luminous and warming presence. This particular ritual - some call "sun-return" - has never failed to bring about its intended purpose for thousands of years. And it was on this day that I broke sleep at an early hour to once again continue my own personal quest of bathing my retina in the faint light of a distant pair of island universes where our own Sun's light is blended feebly with millions of others...
With Argo already assembled in the study, I grabbed the counterweight shaft and "slepped" OTA and mount out the door onto the cold wet ground facing Polaris and things north. Above my head the central meridian lay very close to second magnitude Dubhe - brightest star in Ursa Major and tip-star of the Drinking Gourd. Following the line due west I came upon 3.8 magnitude Upsilon. Sighting on Upsilon, I shifted the scope some seven degrees due north to 4.6 magnitude 24 UMA. Some two and half degrees west of 24 through Argo's 35mm finder, I could easily make out the faint patch of nebulosity which betrayed the location of Bode's Galaxies to my questing eye.
Perhaps in explaining this I have now just made out one of the best arguments for the traditional equatorial mount and finderscope (John Dobson and the makers of unit finders I bear you only goodwill on this holiday season...)
At F12 and 150mm in aperture, Argo is a scope unsuited to low power views. In fact my longest focal length eyepiece (a 35mm Ultrascopic) isn't quite able to achieve 50x - or even 1 degree fields. And unlike my 25mm Ultrascopic, it has no 35mm "peer" to compare views filtered and unfiltered without having to refit the same eyepiece with the Lumi filter. My first look then at the pair was unfiltered. And the first thing I noticed is that I preferred seeing the pair at an apparent separation of 40x (as seen through Ms Vicki on an earlier occasion.)
At some 3mm exit pupil and through 4.5+ ULM skies (5.0+ overhead) the background appeared a "graphite grey". To my eye however most of the detail I am pleased to decry in the pair were present. Edgewise M82 displayed a reasonably well delineated southern frontier - while that to the north bled a bit at the edges. The western "blade" of M82's stilleto-like form was quite bright while its thin eastern "tang" remained thin and inobstrusive. During aversion I could see the tang extending out considerably further to the east while at the same time a faint starlike nucleus could be held along the galaxies spine closer to the blade. There was a sense of the dark notch breaking into the luminosity very near the core along the southern frontier. However I found it ducedly difficult to fix its position - possibly because the seeing stability was so bad that I could not even bring neighboring 7th and 8th magnitude stars to focus!
Meanwhile M81 - some 40 arcminutes further south - gave a nice oblique dinner plate presentation. Luminosity almost seemed to swirl well away from the galaxies bright center, core, and paracore regions. Through Argo a counterclockwise sense of "swirling" proceded from the south, swung east, then north, finally terminating northwest of the seemingly "sunken core" of the galaxy.
Removing the 35mm, heads down, I spun the Lumi filter into place, dropped the ep back into the diagonal and yuch! All that faint detail was gone and even the star count fell off precipitously. Looking overhead I saw that a high thin cloud had dimmed the sky. After a few minutes it passed and I resumed my quest. Now that's more like it! The background sky was now coal black. Sure, all the field stars were visibly fainter and of course turning up M82's starlike core was more difficult. The notch was equally challenging to pin down. But M81 - with its broad swath of luminosity gracefully bleeding to the frontier remained unperturbed.
Again the aesthetics were better - but some of the nuances were lost...
I decided the ultimate test was to determine if elusive NGC2976 could be found through the filter. Now I had, in fact, already inspected NGC 3077 south and east of M81 without the filter. At 150mm's Argo held this face on irregular galaxy direct and even revealed condensation toward the core. This was true both with and without filtration. But as it had been unfiltered so it was filtered. The only way to call up the larger lower surface brightness 3077 was to use a faculty that can easily be abused in our High Art and Science - you know what I refer to of course...