Archive for the ‘Eyepieces’ Category


Get $5 off your first purchase from

February 21, 2017

Hey, I was temporarily without a 10mm eyepiece (long story) and I have been sufficiently happy with the Bresser 20mm 70-degree that came with my AR102S Comet Edition that I plunked down thirty bucks for the 10mm version (sale price, down from $50). It was only my second-ever purchase from (after the Bresser Spektar spotting scope a couple of years ago), but they rewarded my ‘ongoing support’ with this deal. You can use this link and unique code:

to get $5 off your first purchase, and if you do, I’ll get a $5 kickback. As far as I know, there is no limit to how often this can be used by people making their first purchase there. So if you’ve been tempted by something at that store, here’s your chance to save a little dough. Happy shopping!


The 28mm RKE in action

February 20, 2017

Still cloudy here, but we got a gap earlier this evening, a persistent sucker hole right over Orion, and I got a whole 10 minutes of observing in. I was using the Bresser AR102S Comet Edition and for eyepieces the 20mm 70-degree that came with it, and my new 28mm RKE from Edmund.

Both eyepieces will just fit in the belt of Orion, with Alnitak and Mintaka in the last 5% or so of the field on either side. So the belt turns out to be a good test of edge characteristics. The 28mm RKE is way sharper at the edges, by the way. You might think that its 45-degree apparent field of view would feel positively claustrophobic after the 70-degree field of the Bresser eyepiece.

But it doesn’t, because of the magical floating stars effect. It’s real! It’s one of the most arresting things I have experienced in almost a decade of observing. As your eye gets closer to the eyepiece, you begin to be able to see the image. As you move in until you can see the entire field, the point where the eyepiece barrel disappears from view coincides exactly with the point where you are far enough to see the field stop of the eyepiece. If you hold up right there, you see the image created by the eyepiece floating in space, with a thin ring of unresolved darkness around it, which if you back out a bit will be the eyepiece barrel, and if you move in a bit will be the eyepiece field stop. In either case, the eye relief is great enough that you can still see the rest of the scope in your peripheral vision, past the thin ring of darkness at the edge of the field.

I have never, ever seen anything like this. It is exactly as cool and immersive as the legends have it. I can imagine building a whole observing kit consisting of this one eyepiece and a series of Barlows of various magnifications.

Anyway, if you have been on the fence about this eyepiece like I was, just get it. It’s amazing.


Me and the ‘Stig

February 19, 2017

This story started a few nights ago. I had been monkeying around with the AR102S, both at its native aperture and stopped down, and I decided to see how it compared to the C80ED. In particular, I wanted to compare the rich-field views of both scopes (such as they are here – I was observing from the driveway after all), so I was looking at the belt and sword of Orion. The results of that comparo were not very surprising – with it’s wider aperture and shorter focal length, the AR102S goes significantly wider and brighter, but the longer focal ratio and low-dispersion glass of the C80ED produce a better-corrected image.

What was not only surprising, but actively alarming, was that at low power I was getting ugly star images in the C80ED. Even in the center of the field, stars were not focusing down to nice little round points, but to crosses and shapes like flying geese. I wondered if my diagonal might have gotten banged up, so I swapped diagonals. The problem persisted. The scope will not reach focus without a diagonal or extension tube, and I don’t have an extension tube, so I couldn’t try straight-through viewing. Still, it was exceptionally unlikely that both of my good diagonals got horked in the same way.

I didn’t know what to make of that. I figured maybe the scope had gotten out of collimation somehow, and I was pondering whether to mess with it. It’s always been optically excellent and mechanically solid (overbuilt, in fact), and I was loathe to take it apart (as opposed to the TravelScope 70 and SkyScanner 100, both of which were crying out for disassembly).

Then a few days later I ran across this thread on CN, in which a guy was having the same problem I had. It sounded like it was more likely astigmatism (aka the Stig) in the eyes than in the telescope. Apparently it’s worse at low powers where the exit pupil is large, which makes sense – astigmatism is caused by having corneas that are out of round (football-shaped rather than basket-ball shaped), but as the exit pupils get smaller, the less of the cornea is involved in vision, and the more likely it is that the ‘active’ portion will approximate a radially even curvature.


One commenter recommended making a little diaphragm between thumb and forefinger to stop down the exit pupil. I tried that, but it was awfully difficult to hold my finger and my eye all steady and in alignment. Then I had the idea of using a collimation cap from one of my reflectors. That stopped down the exit pupil to a 1mm circle, which made the image d-i-m, but the star images cleaned right up. Then I took away the collimation cap and tried the view with and without glasses, and the glasses also cleaned up the star images.

It wasn’t the scope, it was me. I have astigmatism, and it’s bad enough that stars look ugly at low power unless I wear glasses.

On one hand, that’s a big relief, because the C80ED scope has always been a rock-solid performer. Along with the Apex 127, it’s my reference standard for good optics. I was feeling a bit queasy at the thought that it might have gotten out of whack.

On the other hand, I now need to prioritize eye relief in my eyepiece collection. I have a bunch that are too tight to show the whole field when I’m wearing glasses. So I have some decisions to make.

That was the first major discovery of the night.

The second was that the AR102S can take 2″ eyepieces with the most minor tinkering. The 2″-to-1.25″ adapter at the top of the AR102S focuser drawtube screws right off. I had been worried that it might be permanently affixed, but when I tried turning it, it spun with remarkable ease. Once I had it off, I dropped in the 32mm Astro-Tech Titan, which is my only 2″ eyepiece, and the views were pretty darned good. Way wider than with any of my 1.25″ eyepieces, and pretty clean as well, although I need to a little more head-to-head testing on that score. Possibly the star images looked good because they were so small at only 14x.


In any case, the 32mm Titan gives a significant boost in true field, from 3.6 degrees in the 32mm Plossl and 24mm ES68, to a whopping 4.88 degrees.

I don’t think there would be any advantage in going wider, at least in the AR102S. Astronomics seems to be out of Titans, but the equivalent 70-degree EPs are available through Bresser and Agena. The next step up would be a 35mm or 38mm, giving 13x and 12x, but those would push the exit pupil to 7.7mm and 8.5mm, and that’s just wasted light. At least in the AR102S – in the C80ED, longer 70-degree eyepieces would yield the following:

Focal length / magnification / exit pupil / true field

  • 35mm / 17.1x / 4.7mm / 4.1 degrees
  • 38mm / 15.8x / 5.1mm / 4.4 degrees

Either of those would be a good step up from the 3.7-degree max field that the 32mm Titan gives in the C80ED, without pushing the exit pupil uselessly wide.

Anyway, I’m just noodling now. The big news is that the C80ED is fine, I need to prioritize long eye relief in future EP purchases (and maybe thin the herd a bit?) so I can observe with glasses on, and the AR102S can take 2″ EPs after all.


Unboxing the Edmund 28mm RKE

February 17, 2017


Look what came in the mail today.


Something small, in a gold box.


An eyepiece wrapped in paper, and a rubber eyeguard.


And here they are.


That is a big honkin’ eye lens. And that’s why I got this eyepiece. The 28mm RKE from Edmund is legendary for its “floating stars” effect where the big eye lens, the sharply raked barrel, and the long eye relief combine to create the impression that the eyepiece has disappeared and the image is simply floating in space. I’ve never experienced this, because I’ve never gotten to look through one of these before. But the reputation of this eyepiece, illustrated by several glowing threads on Cloudy Nights (like the ones that follow), was enough to convince me to take the plunge:


It didn’t come with a case, so I made my own out of an old prescription pill bottle. A little bubble wrap stuffed in the bottom and taped inside the lid, and I’ve got a nice padded case for free.


And I need that case, because the new gear curse is in full effect. How does this eyepiece work in practice? No idea yet – with any luck, I might find out next Wednesday, when the clouds are finally supposed to part. I’ll keep you posted.



What’s in my eyepiece case

January 9, 2017


In the 9.3 years I’ve been stargazing, I’ve had three eyepiece cases. The first was a Plano tackle organizer with a thin layer of bubble wrap taped into the lid, which held half a dozen 1.25″ eyepieces. After that I got one of the cool foam-lined purpose-built eyepiece cases that Orion and everyone else carry, but that one didn’t last long – probably less than a year. The problem was that although it did a fine job of holding the eyepieces, it didn’t have room for all the other stuff I wanted to cram inside.

Then in 2012 or so I got the eyepiece case that I’m currently using, and the one that I’ll probably be using for a long time to come. It’s not bespoke – it’s a $20 Craftsman toolbox I picked up at Orchard Supply and Hardware. I think this particular model has been discontinued, but there is something almost identical on the shelves today, and there probably will be from now until the end of time (or at least civilization). This one is probably the current incarnation, and hey, it’s only 10 bucks and has a better latch.

The exterior doesn’t deserve much comment. I put my name on it, and its contents, mostly to make it clear to anyone who might find it among my stuff if they’re going through the garage looking for tools of the terrestrial variety. I don’t fully trust the single latch so I keep a zip tie run through the hole where the lock would go. The zip tie goes in the top shelf when the case is open.


The top shelf, which is removable, holds my red flashlight, Astro-Tech dielectric diagonal (previously discussed in this post), eyepatch, Barlow, and quick-look and outreach eyepieces – various Plossls, the 6mm Expanse, and the dreadful 4mm VITE that I haven’t yet thrown away. Not shown in the photo are a spare pen and a little Sharpie, both buried under the bag containing the diagonal. You can see that all of the eyepieces are still living in the boxes or cases they came in, and they’re held in place against rocking or tipping by a thick layer of bubble wrap taped into the lid of the tool box.


Another sheet of bubble wrap sits below the top shelf and cushions the gear in the bottom of the toolbox.


The bottom of the toolbox holds my ‘top shelf’ eyepieces and a lot of spare gear besides. The three Explore Scientific eyepieces came clamshelled in foam, and each one rests in the bottom half of its original clamshell. One of the top halves forms a bed for the 5mm Meade MWA. The two slots in the middle used to hold my Stratus eyepieces before I let them go – the ES models are smaller, easier to handle, and do a significantly better job. Now those slots hold the 32mm Astro-Tech Titan, my only 2″ eyepiece, the GoSky iPhone adapter I blogged about here, and a cord to hang my eyeglasses when I’m observing.

Around the edges I have all kinds of stuff crammed into the spare spaces. Clockwise from the top:

  • Contact info, just in case the case ever gets lost and found by someone decent. Has my name, address, email, and cell number.
  • Lens cloth, just in case.
  • Spare AAA batteries for the green laser, the red flashlight, and the laser collimator.
  • A ziploc. Never know when you’ll want a small waterproof bag. Sometimes holds spent batteries if I have to do a field swap.
  • Laser collimator. Reminds me, I need to blog sometime about how to collimate a laser collimator.
  • A set of hex wrenches for collimation.
  • Small pliers for the same purpose – I’ve swapped the hex bolts on a lot of scopes for standard hex-head bolts that I can tweak with pliers. Much better than farting around with hex wrenches.
  • Green laser. Super-useful when stargazing with newbies and old hands alike.
  • Tiny atlas – so I’m never without one. This is the Collins Gem Guide to Stars, which has little charts of the constellations and a short list of the most impressive DSOs for each one. Unlike Sky & Tel’s Pocket Sky Atlas, this thing truly is pocket-sized, and small enough to take up essentially no space or weight in the case. It has saved my butt a couple of times when I forgot all other atlases.

There is one other thing. In the third photo you can see a light blue bag through the intermediate layer of bubble wrap. I think that’s the bag the eyeglasses cord came in. Now I use it to hold a set of iPhone earbuds, which serve as a remote trigger when I’m taking pictures with the iPhone adapter, as shown and explained here.

That’s it – an inexpensive, sturdy, and above all roomy case for my eyepieces, with nooks and crannies for a whole lot more.

What’s in your eyepiece case?


Observing Report: All-Arizona Star Party 2014

October 30, 2014


AASP 2014 - loaded for bear

If it’s late October or early November, it must be time for the All-Arizona Star Party. London and I headed out for it this past Saturday, Oct. 25. As in 2012, we were joined by the indefatigable Terry Nakazono. Here Terry and London pose for the obligatory “look how much crap we crammed into the car!” photo.

We arrived at the site about an hour before sunset, plenty of time to set up camp and chat with the neighbors. As usual, we set up not far from Darrell Spencer and AJ Crayon, but irritatingly I failed to get a picture with Darrell, my first such lapse.

AASP 2014 - setting up in the shade

When we arrived the sun was still well above the horizon and temperatures were in the mid-90s. London and I set up our scopes on the east side of the car so we could sit in the shade. Here London is tinkering with his AstroMedia 40mm “plumber’s telescope”, which we just built last week. More about that scope in a future post. The scope behind London was another AASP newcomer.

C80ED newly arrived 1600

This is my new Celestron C80ED. This scope originally retailed for about $500. Celestron donated all of the remaining stock of the spotting scope version to Astronomers Without Borders, and AWB sells it for $350 with free shipping. Vicki got me one for our anniversary last week (and I got her some leather boots–in both cases, the choice of gift was, ahem, heavily influenced by the recipient). The package arrived on Thursday about half an hour before the partial solar eclipse was to start, so I just had time to take this photo before I ran out the door to London’s school.

I got this scope because it filled a hole in my lineup. My Maks have sharp optics but can’t do wide fields. The TravelScope 70 can do wide fields but still has limitations, even after its tune-up. And the C102 is a wonderful scope but not exactly small, and although its chromatic aberration is minimal it is still there. I figured a small ED scope could be a grab-n-go that could deliver wide fields like the TS70, take magnification on planets and double stars like the Maks, in a more convenient and false-color-free package than the C102. Plus I’d just always wanted to try an ED scope. I was going to get an AstroTech AT72ED but they are out of stock and have been for ages. The C80ED offered a small but significant aperture boost for less dough, so I bit–or rather, encouraged Vicki to do so.

I was going to bring both the C102 and the C80ED, but as the date got closer I decided that what I really wanted to do was put the C80ED through its paces under those dark Arizona skies, and another scope would just be a distraction. I had briefly set up the C80ED on Friday night to make sure the scope didn’t have anything seriously wrong. It didn’t–in fact, it star-tests as well as any scope I’ve ever owned.

AASP 2014 - refractor city

Turns out we were all rolling with small refractors. From left to right they are the C80ED, London’s 60mm Meade refractor, Terry’s Orion Short-Tube 80, and London’s 20×50 Orion spotting scope (reviewed here). Terry had been going to bring a 4.5-inch reflector but the Clear Sky Chart said that conditions were iffy. Also, like me he had been interested to see how deep he could push a small refractor under dark skies.

Incidentally, after bringing my XT10 to the AASP in 2010 and 2012, I brought the Apex 127 last year and now an 80mm refractor this year. At this rate, in a couple more years I’ll be down to bringing just a finderscope. (I jest, but I have had a longstanding interest in going to a dark site with only the SV50 or GalileoScope to see how many things I could see with a small scope under dark skies–so far, greed for photons has always won out, so this project remains unattempted).

AASP 2014 - moon in C80ED

Our first target of the evening was the waxing crescent moon. I got a few shots with the iPhone shooting through the C80ED. Here’s the best one. All I did was crop it and flip it left to right–other than the orientation change, the actual pixels have not been tinkered with at all. Note the absence of false color. I also put the scope on Vega early in the evening and could not detect any false color–very impressive.

On the drive out, Terry asked me if I had any plans or goals for the evening. I did have a few:

  • above all, spend some time observing with London;
  • look at some familiar objects to get a feel for the scope;
  • track down some southern objects, since I’d be at a dark site with a clear and dark southern horizon;
  • to the extent that I could, test the scope on challenging targets like globular clusters and close double stars.

And that is more or less what I actually did.

A word about the sky conditions before I get into actual observations: they were not fantastic. Seeing was lousy the whole night, with the stars twinkling visibly all over the sky. Transparency was good in the early evening but around 9 or 10 a very light haze set in across the whole sky. It wasn’t ghastly, but it noticeably knocked down the contrast–where the Milky Way had blazed overhead at 8:00, by 10:00 it was just sort of there, visible but not nearly as prominent. In my notebook, I rated the seeing at 2 out of 5 and the transparency at 3 out of 5.

I only used four eyepieces for most of the night:

  • 24mm ES68, which in the C80ED gives a magnification of 25x and a true field of 2.7 degrees
  • 14mm ES82 (43x, 1.9*)
  • 8.8mm ES82 (68x, 1.2*)
  • 6mm Expanse (100x, 0.67*)

I did use a 32mm Plossl to drop the power down to 18.75x to see if Polaris could still be split (it couldn’t, but read on), and I used a Barlow once. Other than that, it was just these four, and out of these four, I used the 24mm and 8.8mm EPs significantly more than the other two. I had planned to use the 8-24mm Celestron zoom, but in testing the scope Friday night, I could tell that the Explore Scientific eyepieces were noticeably sharper. Good heavens, I think I’m turning into a refractor weenie and an eyepiece snob.

After the moon we visited Mars, but it was tiny and featureless and fairly burning in the bad seeing. Then I swung next door to Sagittarius and got my first surprise of the evening: the big glob, M22, was partially resolved even at 25x with the ES68! I love globs–they are one of my chief joys in observing with the XT10, and I expected them to be dim, featureless cottonballs in the C80ED. That I was getting partial resolution on one in a small scope at low power was pretty arresting. I had a quick look at M28, M8, and M24, and then helped London get his 60mm on target on M22, M28, and M8. London was interested in seeing a double star so we wheeled the scopes around and had a look at Mizar and Alcor. Then we looked at M13, M57, the Pleiades, the Double Cluster, and Stock 2 in his 60mm.

AASP 2014 - our camp

Highlights of the Evening: M13, M57, M27

After all that, London went to lie in the lounge chair and watch for shooting stars–he got 17 before he went to sleep around 10:30. I went on to M13, the Great Globular Cluster in Hercules, and had my socks knocked off. Like M22, it was partially resolved even at 25x, and much better at 68x and 100x. It wasn’t fully resolved, of course, and the XT10 will blow away the C80 on this or any other glob, but it was at least a ball of many, many stars and not just a fuzzy blob. Here’s one of the nice things about widefield eyepieces and short focal length scopes: you get huge fields even at reasonable magnifications. At 68x in the 8.8mm ES82, I could park M13 comfortably inside the field stop and watch it drift across the field of view for more than four minutes. Even at 100x in the 6mm Expanse, I could watch the cluster drift across the center of the field for a bit over two minutes. I commented to Terry that if I hadn’t had other things I wanted to see, I could have kept watching M13 all evening and been very happy.

Lyra was still pretty high overhead so I went there next. Epsilon Lyrae was shimmering in the bad seeing. It was elongated at 68x and almost split at 100x, but I had to Barlow it up to 200x to get a clean split. You may recall that under better conditions, the TravelScope 70 split the Double-Double at 133x, and I know that it is often split at well under 100x by high quality small refractors. So the high magnification required for the split here reflects more on the quality of the seeing than on the quality of the telescope. I’m looking forward to seeing how the C80ED performs on Epsilon Lyrae on a better night.

M13 was probably my favorite view of the night, but a close runner-up was M57, the Ring Nebula. It was clearly ring-like at 68x, but I liked it even better at 25x–the expansive 2.7-degree field of the ES68 showed the nebula nicely framed between Beta and Gamma Lyrae (the stars that mark the south end of the constellation stick-figure) and their attendant stars. It reminded me of the view of the Ring at 12.5x in the TravelScope 70 back in 2012, which is what got me into refractors in the first place.

After that I spent a few pleasant minutes rocking through the Lyra-Cygnus-Sagitta axis, observing M56, Albireo, Brocchi’s Coathanger (Cr 399), M71, and M27. Interestingly, the view of the M27 was very similar to the one I had through the C102 at the Salton Sea last year: I could not only see the “apple core” extensions, but also some of the “football” nebulosity between those extensions. That is a lot of nebulosity to pick up in an 80mm scope. I wonder what I could see on a night with better transparency.

By now it was about 8:50 and I knocked off the serious observing for a while. First I went to hang out with London, and while he watched for shooting stars, I used the 15×70 bins to sweep up many of the same summer showpieces I’d just seen in the telescope: M57, M56, Albireo, Cr 399, M71, M27, M13, the Double Cluster, some of the nice NGC open clusters in Cassiopeia. Then some folks from the other end of camp stopped by and we chatted for a while. Darrell came over and had a look at M13, and London and I went down to the center of camp to get some hot chocolate. When we got back, London sacked out. I had a quick look at M11 before it set, and tracked down the asterism DeLano 1 just to make sure it was still there. Then, at Terry’s suggestion, I tried M15, the big glob off the nose of Pegasus. Here are my unedited notes:

M15 – tough nut to crack. Starting to look grainy at 100x. Also pretty grainy at 68x in 8.8mm ES82. Even though it only gives about 2/3 the magnification of the 6mm Expanse, I think the 8.8mm ES82 shows almost as much. It’s just a superior piece of glass. Another ES82 or 68 in the 3-5mm range should be priority.

Now, this idea that the 6mm Expanse is maybe not 100% awesome–hold onto that thought, we’ll revisit it at the end of the evening.


Go South, Young Man

Ever since my incredible Salton Sea run with David DeLano last fall, I have been painfully aware of how much I’ve neglected the southern sky. So from 10:45 to 12:30, that’s where I went. My first southern target was NGC 7293, the Helix Nebula. It was dead easy to see once I got there, but it took me an unusual amount of faffing about to get on target. I was using the 6×30 straight-through correct-image finder that came with the C80ED. I’m normally a RACI man so using a straight-through finder took some getting used to. But I kinda like it, now that I have the hang of it.

After that it was onto some galaxies and planetaries: NGCs 55, 300, 288, 253, 247, 246, 720, and 779. NGC 288 and NGC 253 were nicely framed in the same field. NGC 288 is a globular cluster circling our own Milky Way galaxy, about 29,000 light years away, whereas NGC 253, the Silver Coin galaxy, is 11.4 million light years away, almost 400 times farther, and rivals our own Milky Way in size. So that pair has a bit of the M97/M108 ‘odd couple’ thing going on.

The not-quite-edge-on galaxies NGC 720 and NGC 779 were my only new objects for the evening. Both of them are on the Herschel 400 list, and bring my H400 tally to 175. I am starting to wonder if I will ever finish the Herschels–the only new ones I’ve notched in the past couple of years have been bagged at the All-Arizona Star Party. I gotta get out in the spring more. I’ve just about exhausted the fall Herschels, but there are hundreds of spring galaxies to observe in Ursa Major and the Virgo-Coma Cluster.

C80ED AASP 2014 2000

Orion and Points North

After almost two hours of faint fuzzies, I was ready for a change of pace. I turned east, toward Orion. The view was pretty great–the Trapezium was split into four components at only 25x, and the nebulosity seemed to go on forever. And yet, the subtle gradations in the nebulosity did not seem as pronounced as I had observed on other nights. Terry noticed the same thing observing Orion through his ST80. He thinks that the poor transparency was leaching some of the contrast out of the view, and I am inclined to agree.

Without a doubt, the strangest observation of the night was of NGC 1980, the field of nebulosity around Iota Orionis. When I looked right at the nebula, it was steady, but when I looked back at M42/M43, NGC 1980 would flicker in my averted vision like a bad fluorescent bulb. At first I thought maybe it was just my eyes, but I called Terry over and he reported seeing the same effect.

Now, I don’t think that the nebula was actually flickering. I suspect that through some quirk of eye/visual system physiology, it only seemed to flicker in averted vision.

Just to rule out the obvious distractors: we were parked on the very east end of the airstrip so there was probably no-one between us and Phoenix. Neither of us were using flashlights or any electrical gear at all while we were observing in Orion. Our nearest neighbors were about 50 yards to the NW and SW, and they’d all turned in for the night. So I’m about as certain as I can be that it wasn’t some terrestrial source that just happened to be shining into the eyepiece or objective lens. Also, we only noticed the flickering on NGC 1980, and not on the extended “wings” of nebulosity from M42, which were of similar brightness at their extremities.

Has anyone else seen anything like this, either for NGC 1980 or other DSOs? If so, I’d love to hear about it–the comment thread is open.

After Orion’s Sword I bounced around a few northern Messiers–M78, M1, M35 with NGC 2158 just starting to resolve behind it, M81 and M82 in the same field, and M97 and M108 in their own field. Midway through that tour I stopped to split Polaris. It was continuously split at 24x in the 24mm ES82, not split at 18.75x in the 32mm Plossl. This illustrates just how seeing-dependent double star splitting is–Friday night from my driveway, the seeing was even worse, and that evening Polaris was not continuously split at 25x, but it was a 43x in the 14mm ES82, and even at 28.5x in the 21mm Stratus. As indicated above, the seeing out in Arizona Saturday night was not awesome. One of my quests with the C80ED is to see how low I can go, magnification-wise, and still get clean splits on some of the classic double stars. Watch this space.

M97 and M108 were my last DSOs of the evening. After that I turned to Jupiter, and even at 68x I could see at least 4 belts. The Galilean moons were spaced about evenly, two on each side of the planet. Terry and I compared views of the planet through the C80ED and his ST80. We could get similar magnifications with our favorite short eyepieces: the 8.8mm ES82 gave 68x in the C80ED (FL = 600mm), and the 6mm Expanse gave 67x in the ST80 (FL = 400mm). So how did the scopes compare? Well, obviously the ST80 was throwing up a lot of false color, but I could detect the same four belts that I could in the C80ED, albeit not quite as crisply. More informative was the comparison of eyepieces. Terry had a 6mm Expanse clone from While were swapping all of these eyepieces between the two scopes–the 8.8 ES82, the 6mm Expanse, and the 6mm Expanse clone–I noticed something I had never spotted before: the 6mm Expanse threw up a huge circle of glare around Jupiter. Perfectly circular, like a lens flare, centered on Jupiter, and spanning out to the outermost moon on each side. The glare circle was there in the 6mm Expanse in both scopes. It was not there in either scope in the ES82, nor in the AgenaAstro Expanse clone. These are the Agena Enhanced Wide Angle (EWA) 6mm, which goes for $45 (you can find it here), and the 6mm Orion Expanse, list price $68, street about $59. So if you’re in the market for a 66-degree EP, you can save about 25% and get noticeably better performance from the Agena version. I’m tempted to get one myself, and hock the Orion EP. Until now, the 6mm Expanse has been one of my most-used EPs, but now that I can see its faults…like I said, eyepiece snobbery is taking hold.


Settling Up

After one last look at Jupiter in the ES82 at 3:00 AM, I shut down and went to bed. The next morning, London and I went on our customary “bone hike”, and we did find several bones, including a couple of cow limb bones, and the jackrabbit lower jaw shown in the photo. More exciting were the Western diamondback rattlesnake and the horned lizard that we found.


My final tally for the evening was 45 telescopic objects:

  • 2 planets (Mars and Jupiter)
  • 22 Messiers
  • 13 other NGCs
  • 2 asterisms (Brocchi’s Coathanger, DeLano 1)
  • 1 other catalogued DSO (Stock 2)
  • 5 double/multiple stars (Mizar/Alcor, Albireo, Epsilon Lyrae, Trapezium, Polaris)

…plus a couple of meteors.


Irritatingly, I realized later that I had completely missed out on some real gems. I never once pointed the scope at the Andromeda galaxy or its satellites–detail in M31 would have been a good test of the C80’s optics. And I skipped the nice open clusters in Auriga–M36, M37, and M38–which maybe more than any other set of clusters give that “diamonds on black velvet” feeling in a sharp telescope. We set up early enough that I could have rocked through all of the Sagittarius Messiers instead of the handful I actually saw, but I deliberately traded that time away to help London find some things, so I don’t feel bad about that particular omission. The others are a bit galling.

Even with those omissions, I still met all of the goals that I had set for myself: I got in some good observing time with London, I had fun touring the southern skies, even if most of the things I saw there were revisits, and I both got a feel for how the scope performed on average targets, and got to push it on some challenging ones. The biggest revelation to me was that an 80mm scope would start to crack open some of the bigger globs. M13 and M22 didn’t just look good, they looked stunning. I wish I was observing them right now.

In sum, a great night of stargazing, and a pretty thorough field test for the C80ED. I think I am going to have a LOT of fun with this scope.



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.