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Observing tip: make a comparison chart for your eyepieces

October 16, 2013

MJW eyepiece comparison chartSherlock Holmes once exclaimed, “Data, data, data! I cannot make bricks without clay!”

I often feel the same way at the eyepiece. The more I know about what I’m looking through, and what I’m looking at, the more I understand what’s going on and can make meaningful evaluations and comparisons. At a minimum, I like to know what magnification I’m working at, and it is often helpful to know the true field of view (TFOV) and exit pupil (the width of the beam of light entering my eye). So I made a table with all of that information, for every combination of eyepiece and telescope in my inventory, and I keep it on a clipboard with a few other odds and ends.* So if I’m using a 21mm Stratus in my Apex 127, I don’t have to stop observing and do long division to work out the magnification.

* Single-sheet all-sky maps of the Messier and Caldwell objects (from here and here), often a tear-out map of the moon or the Milky Way from S&T, and the logbook for whatever project I’m working on.

As you can see, my table is a pretty bare-bones affair. I didn’t even give it a title,  just left it as “Sheet 1”. And when I got the C102 last week, I just wrote in the additional column by hand. But it’s a crazy useful thing to have along, and if you haven’t made one for yourself, I recommend it.

How do you calculate all this stuff?

Magnification is telescope focal length divided by eyepiece focal length. So in the XT10 (f/l = 1200mm), the 14mm ES82 gives a magnification of 1200/14 = 86x.

True field of view is apparent field of view divided by magnification. It’s important for star-hopping and celestial navigation; if you know that a certain object is two degrees west of a given star, that’s two eyepiece-fields if the EP gives a one-degree field. The ES82s have an 82-degree apparent field, the ES68 and Stratuses have 68-degree fields, the Expanse has a 66-degree field, and the Plossls all have 52-degree fields. In the same example listed above, the 14mm ES82 in the XT10 gives a TFOV of 82/86 = about 1 degree (0.95 to be exact).

Exit pupil equals the diameter of the scope’s objective lens or primary mirror divided by magnification. That’s pretty much what magnification is: taking a wide beam of light with a small image scale and turning it into a narrow beam of light with large image scale. To keep using the same example, in the XT10 (250mm diameter) the 14mm ES82 give an exit pupil of 250/86 = 2.9mm.

A lot of people, myself included, find that eyepieces become a lot less comfortable to use when the exit pupil gets under 1mm. In contrast, large exit pupils are very comfortable because you can move your eye around a bit without losing the light beam. Most veteran deep-sky observers recommend an exit pupil of about 2mm as the optimum for picking out faint details. This is explicitly a trade-off between brightness and image scale: lower magnifications offer a brighter image but smaller image scale; higher magnifications give a larger image scale but spread out the light so the image is dimmer. The only way to beat that trade-off is to move up to a bigger scope, which will let you have a brighter image at a larger image scale. That’s why aperture matters.


Hey, nice sketch!

October 16, 2012

Just a quick hit: Justin Balderrama, a fellow PVAA member who blogs at The Young Astronomer, had one of his sketches chosen for the Astronomy Sketch of the Day this past Sunday. Congratulations, Justin!


Back to Barsoom

February 6, 2012

I haven’t had a look at Mars through my telescope yet this year, but I have seen it with the naked eye a few times, when I’ve been out late at night. Mars has been much on my mind lately, because I’ve been rereading the Mars novels of Edgar Rice Burroughs. The first book, A Princess of Mars, follows the adventures of John Carter, an ex-Confederate officer who is mysteriously transported from the desert southwest to the desert planet. He is captured by warlike Martians, falls in love with a human princess, and goes through a series of chases, escapes, imprisonments, arena battles, and deadly duels. The tale was first published in serial form in 1912, when the “canal” theory of Mars was at its most popular. The Mars of Burroughs’ novels, known as Barsoom by its inhabitants, is only sustained in a habitable state by the high technology of the dwindling races of Martians, in particular the canal system and the “atmosphere plants” that produce and distribute breathable air. The canal theory is a historical curiosity now; when modern astronomers get excited about Martian water, it’s over braided fluvial systems that seem to change from year to year, based on high-resolution photos from the Mars Reconnaissance Orbiter.

Burroughs’ Mars books are all ripping adventure yarns and they inspired much of the pulp science fiction of the early 20th century–and many of the science fiction films of more recent years, from Star Wars to Avatar. That circle is about to be completed: in this 100th anniversary of the first publication of A Princess of Mars, the story is finally coming to the big screen, in Disney’s John Carter, set to be released on March 9.

Needless to say, I’m looking forward to the movie. But I’m also looking forward to hauling out a telescope and having a good look at the red planet. The thing that always gets me about seeing planets through a telescope is that I am forcefully confronted with how real they are. Of course, nebulae and galaxies and everything else “up there” is equally real, but as much as I love those things they don’t have the same mythic hold on me as the planets. Even when I look up with my naked eyes and see Mars, I experience a curious sense of dislocation, knowing that Mars is really there. The canals may be (human) history, but the ice caps and canyons and volcanoes and dust storms are all just as real as you or me. And at least a handful of Earthlings really have been transported to Mars and have left their tracks on its dry, dusty plains. The fact that these have all been robots so far should not discourage us. To paraphrase Carl Sagan, Mars calls to us, possibly in a more profound and mysterious way than any other heavenly body. I don’t know exactly when we’ll get there, but I think we will actually get there, and have adventures no less exciting than those of John Carter.

I’m going a lot sooner. I have this weird device in my garage. It looks like a small water heater, but it’s really a transporter. Very soon, I’m going to Mars. I’ll let you know if I ever come back.

If you’ve never read A Princess of Mars or the rest of Burroughs’ Barsoom novels, you can start right now, for free. Most are in the public domain, and you can find them at Project Gutenberg, and on Amazon in free Kindle versions, and probably elsewhere on the web as well. For more of my thoughts on the upcoming movie, go here, and for my previous posts on the real Mars, go here.


Observing report: Binoculars on Mount Baldy

August 14, 2010

Thursday night my buddy Brian and I drove up Mount Baldy to do some casual observing. Brian probably wouldn’t describe himself as an amateur astronomer (yet), but I’m working on him. We’ve been talking for months about going out with binoculars and a planisphere and just spending some time learning the sky. When I got back from Uruguay I realized that Brian had been in town for a year and we hadn’t been out observing yet, so I started bugging him regularly. Thursday night, we went.

It was just by chance that Thursday night was the peak of the Perseid meteor shower; we were going out anyway and the meteor shower didn’t affect our decision one way or another. But it was a nice perk, and we both saw some excellent meteors up on the mountain. Not as many as we might have if we had gone for that purpose, because the best meteor watching is done with both eyes wide open, laying on the ground or on the hood or trunk of the car. Even binoculars cut out so much sky that you’re more likely to miss meteors than to see them if you’re scanning the sky with binos. That said, Brian did catch at least one through binoculars. Brian had along his 10x50s and I had my 10x50s, 15x70s, and SV50. We looked at just about every good target with all three instruments. Usually we’d find things with the 10x50s, kick things up a notch with the 15x70s, and go to the SV50 for a steady fixed view and sometimes for more power. It was a useful, easy-to-use set of instruments that I thought complemented one another well; my only regret was not bringing the eyepiece rack for the telescope mount, because I spent more time than I wanted fiddling with end caps when I was switching eyepieces on the telescope.

We started out facing south, down the mountain, toward Scorpio and Sagittarius. Those are two of the most recognizable constellations, Scorpio because it actually looks like a scorpion and Sagittarius because of the striking ‘teapot’ asterism. They’re also prime territory for deep-sky observing, with binoculars or telescopes of any size. Our first target was M7, just above the “stinger” of Scorpio. M7 is a BIG, bright cluster, and it looked pretty darned good even though Scorpio was down in the light dome over LA. M6 is right next to M7 and looks like its smaller sibling. From there we went up into Sagittarius, to M8, M22, and M24. M8 is the Lagoon Nebula, and M22 is the brightest globular cluster in Sagittarius. M24 is “not a ‘true’ deep sky object, but a huge star cloud in the Milky Way, a pseudo-cluster of stars spread thousands of light years along the line of sight, perceived through a chance tunnel in the interstellar dust”, according to its SEDS page.

At that point I was doing something else–switching eyepieces on the telescope, as likely as not–and Brian was just cruising with the 10x50s when he ran across another bright cluster. We identified it, and several other “discoveries” of the evening, by the following process: one person would find something in binoculars, and then hold the binos with one hand while getting a green laser pointer on target with the other hand. Then the other person would follow the line of the green laser to the target using his binoculars. That first time, the target was M11, the Wild Duck Cluster, one of the true gems of the summer sky. Other “discoveries” sent me scrambling for the star atlas.

By that point we had been facing south for more than half an hour and we needed a stretch and a change of pace. We hit M13, Epsilon Lyrae (the Double Double star), and M15 in the mid sky before settling down to face north. Our  first northern target was M31, the Andromeda galaxy. It was grand. We also spotted its two satellite galaxies, M32 and M110, without much trouble. By that time the Double Cluster had cleared the treeline to the north so we spent a few pleasant minutes contemplating that celestial showpiece. Then we just panned around Cassiopeia taking in all the good stuff. Even with binoculars, you can spot clusters in Cassiopeia faster than you can identify them, unless you already have them committed to memory, and we saw a lot more than we logged. Specific objects that we noted or looked up included the open clusters Stock 2, M34, and NGC 457. Our last two objects were M33, the Triangulum galaxy, and the Engagement Ring of stars around Polaris.

We wrapped up about 12:30 AM after a solid hour and a half of unhurried observing, during which time we had seen several asterisms, one nebula (M8), one identified double star (Epsilon Lyrae) and at least one unidentified by us, seven identified open clusters (M7, M6, M11, the Double Cluster, Stock 2, M34, and NGC 457) plus several more unidentified, three globular clusters (M22, M13, and M15), five galaxies (M31, M32, M110, M33, and our own Milky Way arcing high overhead), and a galactic star cloud (M24). So we had seen at least one of just about every class of deep sky object except for planetary nebulae and dark nebulae. If I’d been more target-oriented I would have remember M27, the Dumbbell Nebula, and then we’d have gotten a planetary as well.

But I wasn’t target-oriented. I was just there to have fun surfing the sky with a friend. I had a heck of a good time, and I think Brian did too. I’m already looking forward to the next time out.


I’m back

August 6, 2010

Back from where?

Back from a 10-day trip to Uruguay for a conference.

Back from 10 days of very intense summer teaching before that.

Most of all, back from the summer doldrums. Like all of my interests, astronomy is not something that I pursue 365 days a year. From the very beginning I have kept an observing log, and looking back over it there are some pretty big holes. These usually fall at the ‘poles’ of the year–December is usually a fallow period, and I often find myself occupied with other things during the summer months. But I always come back, too, when the phases of the moon or the progress of the Earth around the sun bring something particularly lovely into view, or when I am overtaken by a random wave of enthusiasm (hat tip to Mike for that useful phrase), or when I get the chance to go someplace where the observing is particularly good.

The last case is the most relevant here. I went to Uruguay to present a paper on dinosaurs and whales at the 9th International Congress on Vertebrate Morphology. A strong secondary draw was the opportunity to see the southern skies. This was my first trip to the southern hemisphere and I don’t know when or if I’ll ever go back, so I was determined to make the most of it. That’s why, months ago, I put together the Concordiem Australis to give me a big observing list when I got down there.

It worked. But that’s a story for another post. For now…I’m back.


Temporary hiatus

October 1, 2009

Hey, this isn’t to announce that I’m going to stop blogging, it’s to acknowledge that I have effectively stopped already. But it’s just temporary; last week I was at a conference in England and this week is my big lecturing push in Gross Anatomy. With any luck I’ll be back to normal–and back to blogging–next week.

In the meantime, Jupiter is high and pretty every night, and the moon is moving toward full, so you can get some lunar exploring in now and look forward to chasing faint fuzzies in a week or so. I’ve got some more missions in mind so do stay tuned. Also, check out the excellent observing reports that David Delano has been leaving as comments, and feel free to post your own.

Clear skies!


Traveling in space and time

September 18, 2009

I’m going out of the country for a week, and I doubt if I’ll get anything posted while I’m gone. In the meantime, the Small Telescopes blog has some great targets for binoculars and, uh, small telescopes, that you might want to check out. Cloudy Nights also has a monthly highlights list and a constellation profile.

And speaking of Cloudy Nights, my first article was just published there today. I wrote that little piece ages ago and most of it will be familiar to readers of my blogs; this blog is sort of an extension of that into an ongoing project.

Have fun while I’m gone!