Astronomy – Page 187 – A Darker View

Public Astronomy Laser Power and Safety

How bright a laser beam is needed to allow good public presentations and a good astronomy education experience? In the United States legal green lasers are limited to 5 milliwatts (mW) by the Food and Drug Administration. Many of the lasers sold as 5mW are actually 3-4mW as it is necessary to stay below that limit if you wish to import the laser into the United States or to sell the laser across state lines. 5mW units are fine if you are under fully dark skies, but often you are not, there are city lights, or moonlight and the 5mW beam ceases to be usably visible. This is worse if you are working with a larger group and the distance from the presenter is larger, also making the beam less visible. In practice I have found a beam in the 20-30mW range is about ideal. These lasers are available from a number of sources for around $100. Good visibility in moonlight, good visibility in light polluted surroundings and good large group utility. But importantly, not powerful enough to be truly dangerous.

Laser and Stars — Deb pointing out the star βPhoenicis to VIS volunteer Joe McDonough

Where does the 5mW limit come from? The FDA, being a very conservative organization, set the safety threshold at 5mW based on animal and human studies and medical injury data. The 5mW limit is a national standard and applies to interstate or customs transactions, not all states have adopted it. Hawai’i does not appear to have strict laws regarding lasers, licensing or use, so my 30mW unit is legal. This contrasts with my previous home in Arizona, which is more typical of many states. The use of a class IIIb device was subject to licensing, training requirements, and yearly fees. The agency responsible being the Arizona nuclear regulatory office. Sheer bureaucratic overkill that made it practically impossible to legally use a device in the power range I needed, just a few milliwatts over the limit. I am always surprised there is no intermediate class, that a 20-30mW device is classed with devices of up to half a watt, devices that are quite dangerous.

How do I come up with the 20-30mW number as being relatively safe? Being an engineer I understand safety margins and over specification and figured there must be a compromise. The trick was to find undistorted data, without the bias of overcautious bureaucrats. The data is out there on the web, but took some digging to find, many medical journal articles are hidden behind subscription services. Fortunately there are sources like PubMed that are freely and publicly accessible. Eventually I found and read several very informative papers on lasers and eye damage, looked at pictures of laser damage on monkey retinas and looked a damage done by visible lasers in the same sort of power class as those available for public work. I was surprised by actual human damage studies, done on patients who were having eyes removed due to conditions like cancer, where the researcher could do damage without harming the patient. What I was looking for was just what level of laser radiation is dangerous in practical use.

Lasers at the VIS — Green lasers in use at the Mauna Kea VIS nightly observing with the Milky Way high overhead

The conclusion I came to was that a 20-30mW unit could cause damage, but was not excessively risky. The data in the papers showed you could generate damage to a retina with even a 5mW device given a long (60s) exposure. But there were a lot of caveats, the beam had to be well focused, the eye could not be moving and it had to be focused for a substantial length of time. In reality a number of things protect the human eye, the first is the fact that the eye is constantly moving, this spreads the power over more than one spot, not allowing cumulative damage at one site. The second is that a bright visible laser will initiate a blink reflex, keeping any exposure in the tens of milliseconds time scale. To prevent these protective effects the tests in the references were sometimes done with anesthetized subjects. At the 100mW power level damage occurred with very short exposures and was quite dramatic.

A 20-30mW laser needs to be treated with respect as injury is possible, but no more than any other dangerous tool we use every day. In practice damage with a laser at a sensible power level (20-30mW) would require prolonged exposure (>0.5sec) on a single site on the retina, requiring staring into the beam. The bright green light would elicit both a blink response and an aversion of the head and eye, particularly in a visually dark adapted environment. I would strongly discourage use of any laser 50mW or greater for public astronomy work. Some of the references showed significant damage inflicted with 50mW lasers and sub second exposures. Lasers at all power levels should be kept out of the hands of anyone too young to understand the implications of the danger posed by a laser.

Retinopathy From a Green Laser Pointer, Dennis M. Robertson, MD; Jay W. McLaren, PhD; Diva R. Salomao, MD; Thomas P. Link, CRA, Arch Ophthalmology 2005;123:629-633 [Note: direct testing of a 5mW green laser pointer on a human subject]

A comparative histopathological study of argon and krypton laser irradiations of the human retina, J Marshall and A C Bird, Br J Ophthalmology 1979 October; 63(10): 657–668

Histopathology of ruby and argon laser lesions in monkey and human retina. A comparative study, J. Marshall, A. M. Hamilton, and A. C. Bird, Br J Ophthalmol. 1975 November; 59(11): 610–630

Maculopathy From Handheld Diode Laser Pointer, Clive H. Sell; J. Shepard Bryan, Arch Ophthalmology, Nov 1999; 117: 1557 – 1558.

Assessment of Alleged Retinal Laser Injuries, Martin A. Mainster, PhD, MD; Bruce E. Stuck, MS; Jeremiah Brown, Jr, MD, MS, Arch Ophthalmol. 2004;122:1210-1217. [Note: good general discussion on the issues of laser injury and great reference list]

Of Lava Flows and Lunar Eclipses

A good lunar eclipse high in the sky. This was something I have not seen for a while. I have seen several lunar eclipses over the last few years. But they always seemed to be low in the sky from Tucson, rising with the eclipse already in progress. Thus as the date of this eclipse approached I was planning to view it properly. The eclipse would be high in Hawaiian sky, transiting with the Moon in totality. Perfectly placed to view the entire eclipse under the most ideal conditions.

Eclipse Revelry — Everyone at the VIS was enjoying the night!

The timing was highly convenient as I was scheduled to be on the mountain in any case that evening to help prepare for an interferometer run that night. Simply pack the telescope in my vehicle and drive to Hale Pohaku at the beginning of the day so it is waiting with what I need that evening. The scope I chose was my 90mm APO, the focal length was appropriate to frame the Moon well on the Canon 20Da camera. This I mounted on the Losmandy G-11 mount quickly polar aligned with the polar scope and set to track at lunar rate. This particular setup had not been tested together and I thought it would work. I was pleasantly surprised when everything not only worked, but worked quite well.

There was quite the crowd gathered at the Mauna Kea VIS. No surprise, the VIS is the best place on the island to view anything astronomical. Above the clouds and tropical haze the side of Mauna Kea offers a clear view of pristine skies and several telescopes available for anyone to look through. What was surprising was the weather, while the skies are usually clear the VIS can be very cold, and if you add wind the conditions can be miserable. This was not the case! It was cool but not cold and there was no wind beyond a slight breeze.

Eclipse Observing — How to properly observe an eclipse, set the camera on automatic and sit back and enjoy!

We had a couple busloads of high school students, quite a few local folks who knew where to go for an eclipse and the usual group of tourists. The atmosphere was rather festive, a couple musical instruments had appeared and everyone enjoyed the night as the moon slid out of the light.

The proper way to watch the several hours of a lunar eclipse is in comfort. Thinking ahead I brought a lounge chair and setup properly where I could monitor the camera and see the frames on the LCD screen as they came in. This worked perfectly. I could view each frame as it was taken without getting up and adjust settings on the camera simply by reaching over. Between each frame just lay back and enjoy the view.

Well, maybe I am understating the amount of work, things were not that relaxed and the camera took a fair amount of tending. I could not get the camera to autoexpose well with a single bright object in an otherwise black frame. The camera insisted on overexposing the Moon badly, even with automatic bracketing the situation did not work until I just put the camera in manual and adjusted the exposure regularly. The scope did not track perfectly and I did need to adjust the position a few times each hour. But the setup and tending were worth it as the resulting pictures are quite satisfying.

Lava flow from the MKVIS — Lava flowing from the July 21 vent of Kilauea as seen from the Mauna Kea VIS about 30 miles away, taken with the TV-76

This eclipse was darker than any I have seen lately, the Moon dimly seen high in the sky, my camera exposures running to 30 seconds to get a decent image. Once the view of totality had worn off it’s novelty the other telescopes began taking advantage of the truly dark conditions to show deep sky wonders to the crowd. The scopes jumped from Galaxy to globular clusters with the view being fully as good as a moonless night. It seemed of to be looking at M31 and other deep sky objects with the unaided eye on a night of a full moon.

As the eclipse wound down word of another spectacle made its way to me, the glow of something odd seen to the south. This got me out of my comfortable chair to go and see. Surprisingly the usual solid clouds on the east side of the island had parted and the brilliant golden glow of fluid lava was to be seen! There was some confusion as to what it was, some thought it was the lights of Hilo, but there was no confusion to those of us familiar with the view from Hale Pohaku.

The latest lava flow from a rift on the eastern flank of Kilauea was a river 100m wide and several miles long, even from our vantage point thirty miles away we could clearly see the stream. We could see the lava falls near the head of this river with binoculars as well as most of the course. The view of an eruption and an eclipse was and extraordinary reminder of the dynamism of the earth and universe around us.

Above is one of my photos from mid-totality taken with the 90mm APO and a Canon 20Da DSLR, note that several stars frame the moon. Check out a few more pictures of the eclipse from the visitor center by Simona Vaduvescu

Lunar Eclipse 28Aug2007 — Total lunar eclipse, photo is a 8sec exposure with a Canon 20Da on a 90mm f/12 APO

Of Pu’u and Perseids

Pu’u dot the landscape of the Island of Hawai’i. Pu’u, (pronounced poo-oo) is an interesting Hawaiian word that can mean hill, bump, pimple, wart, or any similar concept, but in this case generally means cinder cone. These reminders of the volcanic origins dot the sides of the big volcanoes like pimples on the face of the island. They are everywhere and each has a traditional name. Locals have used them since the dawn of civilization on the islands to give directions, describe legal land boundaries and name roads throughout the island.

Learning your way around the island is often learning the names of the Pu’u. I use them to mark my progress along my morning commute to work and note the weather above Wiamea. A large pu’u stands above the end of Saddle Road where it turns into the center of the island and another pu’u marks the intersection of Saddle Road and the Mauna Kea access road that climbs to the summit winding through the pu’u that cover the flanks of the mountian. Keck Observatory itself sits on the rim of Pu’u Hou ‘Oki.

The West Hawaii Astronomy Club’s dark sky observing site sits directly beside a small pu’u that goes by the name of Pu’u Kuainiho in a unit of State DLNR land named for Pu’u Anahulu. Nothing fancy, a large gravel lot just off a major state highway, but far anough off to avoid the headlights. Easy to find along the road from Waimea to Kona. It sits at about 2,000ft elevation, high enough to be above most of the low altitude tropical haze. Being in the rain shadow of a 14,000ft peak the site offers surprisingly reliable weather, it is often cloudy in the evening but almost always clears after dark. The only real issue is the often heavy formation of dew and some ground mists that will plague observers. The site is a comprimise between fairly decent skies offered by lower elevations here in Hawaii and the truly spectacular observing that can be had from sites high on the side of Mauna Kea at 9,000ft. The only issue is that those perfect Mauna Kea skies are accessed by a rough hour long drive up Saddle Road and are often windy and quite cold. Sometimes a warm site 15min from home on a good road wins the toss.

This time the target was meteors, a shower I had often not observed because it generally occured in the middle of Tucson’s rainy monsoon season. But a dark sky with no Moon and access to a decent dark site a few minutes away was simply too attractive. Except, of course, for the 2am setting in the alarm clock. I had expected a few other observers to be out for the peak, but when I got to the site I was alone. No matter, nothing new for me. I had brought along the Losmandy mount and the DSLR for a little attempted meteor photography and a lounge chair for relaxed meteor observing.

Set the mount up, a quick polar alignment with a polar scope good enough for wide field photography, bolt the camera on and let it go. Just lie back in the chair with a couple blankets and I am set!

OK, start the show now…

…I must have waited five minutes for the first meteor.

But meteors did appear. A few dim ones widely scattered and a few bright ones from time to time to annouce that something out of the ordinary was going on. I kept a few rough counts to do a quick estimate of rates using the camera’s shutter interval as a timer. I would get three to five every five minute interval giving rates of around 30-60ZHR. Nothing spectacular, just a decent show as the Perseids are so well known for. Though after the true Leonid meteor storm I witnessed in Tucson a few years back anything else does seem a little tame.

As for the photographs? A careful examination of every photo shows a staggering number of meteors were captured by my extensive photographic effort. I had to painstakingly go though each photo to come up with a grand total of…

zero.

I did get a nice photo of the Perseus-Cassiopea region of the Milky Way however…

Cassiopeia and Perseus region of the outer Milky Way, Canon 20Da, 14 5min exposures stacked, M31 and the Double Cluster are easily visible

Unaided Vesta and a Moonbow

See a naked eye asteroid? Why not? I had never seen an asteroid with the unaided eye before and here was a good chance. Was it worth setting the alarm clock for 2am? Sure, you cannot answer that with a no if you are truly an amateur astronomer.

Off goes the alarm… wife starts grumbling… throw a few things in the vehicle… a few more complaints from my wife now up and awake… feed the cats… A kiss… and off into the dark. I setup at the end of the development where the streetlights have yet to be turned on, at the end of a dead end road surrounded by lots that have been graded but not yet built on. While setting up the gear I discover I am not quite alone, listening to hooves clattering up the road towards me. Someone riding by moonlight? No, just three feral donkeys that wander past. Later, the calm was broken by a male donkey being, ah, quite energetic, just up the hill, and willing to tell the world about it.

Conditions could have been better, Jupiter was a swimming ball, so much for a little planetary viewing to pass the time awaiting moonset. Transparency was only so-so with the usual low altitude tropical haze and the occasional cloud scudding through. So just sit back and enjoy the night for a while, and try to ignore the loud braying that occasionally disturbed the otherwise peaceful night.

As I waited for the moon to set I noticed a bank of clouds approaching from the northeast. The usual cloud bank over the Kohala volcano was reaching out a little further than it usually does. This was a bit of a concern, when the trade winds are blowing this cloud bank usually forms over the Kohala and with it a heavy misting rain that drizzles constantly in Waimea. I looked about at my gear and decided to cover some of it up in case this cloud bank got closer.

And it did, and as usual the mist was falling ahead of it driven by the steady breeze. Combine a wall of mist and a setting bright moon and the unexpected can occur. I looked out to check the cloud’s position and quickly did a double take. Hanging there in the wall of mist was a beautiful, ghostly moonbow. Grab the camera time!

Camera, tripod, remote shutter release… dash 50yds to where the big power lines would not dominate the picture… hope not to find the donkeys that are around here somewhere… find a spot on the rocks of the ancient lava flow where I could setup the tripod without endangering the camera or my ankles… in the dark… focus the camera… in the dark… frame… program the shutter release for a one minute shot… fire! Time for two frames before the moonbow faded from sight. A quick check of the frames and victory! Got it!

Waimea Moonbow — Moonbow! 1min, Canon 20Da, 17mm@f/4 with the lights of Waimea at center

With the fading moonbow and the setting Moon I was able to return to my mission objective, Vesta without optical aid. Finding the asteroid was trivial, just starhop up from Jupiter or down from ζOph and there is was. Swung my little TV-76 to the correct location and there it was, right on the location plotted. Easy to see at about 5.5 mag. a little brighter than HIP80793 at 5.6 mag. that was located about 1°sp. Naked eye was tougher, there was a line of 4-4.2 mag. stars coming up from Sco that was relatively easy, but it took averted vision to find Vesta and then only occasionally as it would appear and dissapear before my eye. I was clearly being hampered by the lousy seeing and poor transparency down lower in the sky. I am sure from a better site, possibly up on the side of Mauna Kea without the moonlight it would be quite simple to spot. I will need to try again before this opposition is over. That will have to wait a bit until after full Moon.

Vesta 28May2007 — Vesta (marked), Jupiter and Summer Milky Way, 40s, 17mm@f/4, fixed tripod

A Daytime Comet!

C/2006 P1 McNaught is currently passing close to the Sun. At a mere 18 million miles (0.197 AU) from the Sun the intense solar radiation is causing the comet to boil and vent violently. The resulting cloud of gas and dust is reflecting enough sunlight to brighten the comet dramatically. Estimates are placing the comet at mag. -5 or even mag. -6. This is much brighter than even Venus! Some observers are reporting easy unaided eye visibility, but in the sky over Tucson a lot of dust and aerosols in the air are causing a great deal of glare around the Sun. Thus binoculars or a telescope is required to see the comet.

The challenge is that Comet McNaught is just five degrees east of the Sun. Observing so close to the Sun presents distinct dangers, I do not want to fry my retina! So I carefully positioned my TV-76 on the alt-az mount so that the Sun was just behind the edge of my carport. Thus I could pan around and locate the comet with little fear of direct sunlight down the tube. A few frames with the Canon 20Da turned out OK but not great, those dust and aerosols in the sky and the resulting glare is reducing the signal to noise in the images. The photo to the right is a stack of three images in an attempt to improve the signal to noise, then greatly cropped in and levels adjusted in Photoshop.

The view was better in the eyepiece. A sharp point of the coma with a fan shaped tail. This really looks like a classic comet, I had not expected such a good tail in the bright sunlit sky. This is my first daytime comet, and since the opportunity may not come again anytime soon, this may be my only chance to see one, truly a once in a lifetime event!

Mercury Transit

About a dozen times a century Mercury passes in front of the Sun as seen from Earth. The event is observable with a modest telescope and a solar filter, Mercury can be seen as a small black dot crossing the surface of the Sun. If half of those happen when the sun is below your horizon the average person will have the chance to observe five or six in a lifetime. Since the next opportunity will not occur until May 9th, 2016 I didn’t want to miss this one!

I took the day off.

Considering that Mercury never gets very far from the Sun means that most of the time you can observe Mercury it is low on the horizon and is typically seen through a great deal of atmospheric distortion. A transit is one exception to this, during a transit mercury is a sharp disk, very different from the multicolor jello ball that is usually seen.

The 2006 Transit was well timed for observation across western North America, starting just after noontime and ending at 5:09pm MST. This put the Sun high in the sky for all but the last part of the event. Our weather cooperated as well, delivering a cloudless blue sky the entire day in place of the clouds that had been forecast. The air was reasonably steady as well allowing good photographic and observing conditions.

I took advantage of the weather and photographed almost the entire transit, all but the very end when the sun sank below the trees in my neighborhood. I used the Canon 20Da and setup a timer to shoot every 5min. The only issue was the inability to do a polar alignment on the mount when setting up in the middle of the day. The result was I had to manually guide the scope every 10-15 min to keep the sun centered.

I got plenty of good photographic material, enough for a few single photos as well as an animation of the transit. A transit is an impressive demonstration of the scale and arrangement of our solar system. Not hard to visualize the reality of those textbook drawings of planetary orbits after you have had such an opportunity to see the real thing.

No complaints on my second Mercury transit.

Mercury Transit 8Nov2006 — The Mercury transit of 8 Nov 2006 in progress. Mercury is about halfway between the center to the bottom, a large sunspot complex is visible on the left edge. Photo with a 90mm APO refractor, a Thousand Oaks full aperture filter and a Canon 20Da camera.

DSLR Astrophotography

Digital SLR cameras make surprisingly good astrophotography cameras on bright objects. Good sensitivity, low noise and a convenient form factor make these camera a good choice for shooting the night sky.

One useful modification to the camera is removing the standard IR cutoff filter present in cameras and replacing it with a filter that is tuned to let in more of the red. The new filter should allow light at 656nm, what astronomers call hydrogen alpha, or Hα, the light emitted by neutral hydrogen atoms, the most common element in our universe. This is the red glow that makes the emission nebulae so colorful. An astrophotographer can do the filter change themselves, send it to a specialist who can do the work, or buy an already “modded” camera. Canon has produced two special models specifically for the astrophotography market, the EOS 20Da and 60Da, with this special filter.

Below, one can see the results of using modified, and unmodified cameras and relatively small telescopes…

The Eagle and the Swan — M16 and M17, the Eagle Nebula and Swan Nebula, single 8 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

Total Lunar Eclipse 28Aug2007 — A total lunar eclipse on the night of August 27-28, 2007 with the Moon deep in the Earth’s inner shadow. A dark eclipse, this exposure shows a number of stars surrounding the Moon

The Snake Nebula — B72, the Snake Nebula, single 8 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

Lagoon Neighborhood — The Lagoon Nebula and The Trifid Nebula, single 8 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

Baade's Window — Baade’s Window, single 8 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

The Andromeda Galaxy, M31, single 8 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

Omega Centauri, NGC5139, single 2 minute exposure at ISO 800, Canon 20Da DSLR camera mounted on a Televue-76 APO telescope with a 0.8x focal reducer/field flattener

2005 Sentinel-Schwaar Star Gaze

With the first cold weather of the year I planned a trip to Sentinel, one of our favorite winter observing sites. The site features dark skies and good company. The forecast was for clear skies, so Saturday afternoon I packed up my big dob and headed down the freeway.

Dob Silhouette — Steve Dillinger’s 20″ Dob awaiting full dark at Sentinel, AZ with Venus and the Moon shining behind

The sky started a little rough, with high cirrus and contrails scarring the blue lit by the the glow of sunset. The thin crescent Moon and Venus were gorgeous among the bright gold wisps. Shortly after dark these annoyances quickly cleared leaving a clean sky. Seeing was soft all night, transparency was decent, but poor at low altitude as views into Fornax or lower demonstrated.

Violet Awaiting — The author’s setup awaiting a dark December sky with Deep Violet

I counted over 20 vehicles a little before sunset, but a least 10 more rolled in at sunset or just after. After a few hours there was a steady rate of departure as the cold night took its toll on Arizona observers.

I spent the first half of the night touring old friends and a couple new objects, but was basically killing time waiting for H400 objects to rise. After midnight the objects I had been waiting for had risen high enough to appreciate properly and I started to work, cleaning out Pyx, Lyn and Pup of a few remaining H400 objects as well as chewing on western UMa a little. It looks like I have 28 objects left, almost all in UMa, the end is in sight!

Around 0300 the breeze had become a steady cold biting wind and I finally packed it up and pulled out. I had already had around nine hours of good observing so I can call it a success. I wasn’t the last, there were at least two observers still going when I pulled out.

Telescope Line — The telescope line at Sentinel for the 2005 Sentinel-Schwaar Star Gaze

2005 All Arizona Messier Marathon

A Messier Marathon, one of the crazier ways we celebrate our love of the night sky. Try to find all 110 objects on Charles Messier’s catalog in a single night. It is possible, barely, to do this. It is a challenge of skill and perseverance, and a lot of fun. In 2005 I joined a group of friends at Arizona City at the 2005 All Arizona Messier Marathon. This site has hosted the most successful marathons anywhere, a great site and a large group of very skilled observers makes the competition tough.

Ready to Marathon — The author waiting for dark

My immediate competition was my young friend Carter Smith. Carter and I marathoned together, often racing on objects and comparing scores through the night. I had meant to quietly work on H400 galaxies in Virgo, but Carter sucked me into marathoning with him. Great night with good transparency and for once this winter and spring, NO clouds. Seeing was great at sunset with wonderful views of Saturn while we waited for full dark. The seeing deteriorated badly after dark with cool and warm breezes warring over the site. But transparency is what you want for a perfect marathon which is what we got.

Marathon Briefing — Briefing the attendees on the rules of our messier marathon

I managed to stay ahead of Carter most of the night using my 18″ while he was using a 10″. Actually parts of the marathon are much harder with a big scope, particularly the Virgo cluster where a big scope sees all of the dimmer galaxies and not just the M’s you want. I used the setting circles only to confirm objects a few times, so both of us ran this one manually. We ran on purist rules, Telrad and charts the only tools allowed to find the object! The All Arizona Messier Marathon allows setting circles and GOTO scopes, but we are free to make our own rules a little tougher.

Late in the night Carter found my weakness, the 18″ will not depress below 5 degrees elevation, so he could get objects coming over the horizon before I did, after that he was ahead most of the time. Except a few occasions when guile won out against youthful enthusiasm and young, sharp eyesight. In the end we both scored a 109, the best that was possible with M30 being impossible. We had to work on M74, M73 and M72 together to be sure, but the spottings were confirmed by both of us. All in all a great marathon!!!

Messier Marathon Mosiac — The observing field at the Farnsworth Ranch, with the Silverbell Mountains in the background and Kitt Peak just visible at far right. The field is unusually green after heavy spring rains. The clouds are rapidly departing to the east.