h1

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.

astigmatism-of-the-eye

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.

bresser-ar102s-with-2-inch-ep

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.

Advertisements

5 comments

  1. Thanks for the further review, Matt. Good to know about the 102S being able to take 2″ EPs very easily like that. I like that the 102S is brighter than the 80ED, which means that it’s brighter than my Orion ST-80 as well. Of course, this is to be expected with the increase in aperture, but given the reflactor design of the scope, it might not have actually turned out that way. I’ll have to think about adding this scope to my herd.

    A couple/few of things, either for you (although I’m pretty sure you already know all this) or for your readers (who may not).

    First, have you ever seen people in old movies reading something and squinting? That’s because they have astigmatism. The squinting has the same effect as “stopping down the field” so that the astigmatism goes away.

    Second, and I’m not trying to be a jerk, but it appears from the number of elements (5) and price ($90) that the Astro-Tech Titan is a relatively simple Erfle design. The problem with an Erfle is that – for reasons above my optical paygrade to understand, much less explain – it only works well in higher focal ratio scopes – f/8 and higher. In short scopes, like the 102S, it introduces visual aberrations; especially edge-of-field blurriness. (As Uncle Rod mentions in the article you referenced.) This is one of the reasons why expensive eyepieces are expensive – they are better-corrected than cheaper ones, so that the stars stay in focus, either from edge to edge, or at least over more of the field of view than cheaper, oops, less expensive ones.

    FInally, yeah, you gotta watch out for EP exit pupil so that the light cone coming out of the EP isn’t larger than the ability of your eye’s pupil to let that light in. Kids and teenagers start out with entrance pupils in their eyes of about 7mm, but as we age, that width decreases as the muscles in the eye get less and less flexible. In your 20s and 30s, the eye’s pupil opens to only about 6mm, and in your 40s and 50s, it’s only 5mm. (These are averages across the population – your pupils may vary.) So you always have to carefully match the exit pupil of your EP/scope combo with your own eyeball so that the light cone coming out of the EP doesn’t just hit your iris and bounce off, wasting light and dimming the image.

    By the way, what is up with the spate of blog posts recently? I’m not complaining, but you’ve become a regular publishing fool recently! And I mean that firmly in the Frank Zappa sense. Good stuff. Keep it up!


  2. Good to hear that the Bresser AR102S can take 2″ eyepieces. Thought about getting one but decided not to, given my abundance of F/4s and F/5s already in my stable of scopes.

    Sorry to hear about your astigmatism – thought you were just nearsighted (like me).


  3. Thanks for the further review, Matt. Good to know about the 102S being able to take 2″ EPs very easily like that.

    It’s possible, but it’s not perfect. With the 1.25″ adapter off the top, it’s just a bare drawtube, with no set-screw to hold the eyepieces in position. I’m not overly worried about that – since this scope is always side-mounted, I’d have to rotate it backwards until it was pointing past the zenith before an eyepiece could slide out (if it will even go that far – the tube might collide with the central hub of the mount first), and the drawtube is sufficiently solid and well-machined to hold the eyepieces straight and centered.

    More worrying is that 2″ eyepieces won’t just slip down into the drawtube until their own wider barrels collide with the top of the focuser drawtube. That I would be content with. Instead, there’s a little screw for the focuser rack whose head protrudes a fraction of a millimeter into the focuser drawtube, and if you drop in a 2″ eyepiece, it slides down until it hits that screw. I doubt that will be a good thing for either the eyepiece or the screw over the long run.

    So I have thought of a couple of options. One is to find a 2″ focuser top with a set-screw and ideally a compression ring, to screw on top of the focuser tube. I doubt it’s easy to get one of those without a whole 2″ focuser attached, which is more money than I’d like to spend. Maybe someone will turn up online with a wrecked 2″ focuser for parts.

    Another possibility would be to make a sort of zero-length extension tube out of two nested cylinders. The smaller cylinder would fit down inside the drawtube and have a cutout to avoid that rack screw. The larger, outer cylinder would stick up above the drawtube and hold the eyepiece. It would mean the 2″ eyepieces would sit a cm or two higher, but the focuser has plenty of travel so that’s not a problem. I *think* this is something I could build. If I do, you’ll read about it here.

    I’ll have to think about adding this scope to my herd.

    I’ll have a full review up soon – hopefully in no more than week or so, if we can get some good weather and I can get out someplace dark for a decent test under optimum conditions. The short, short version is that this scope is rather strongly optimized for two things: wide fields and portability. With only one or two relatively minor exceptions, design and execution are excellent within those constraints.

    First, have you ever seen people in old movies reading something and squinting? That’s because they have astigmatism. The squinting has the same effect as “stopping down the field” so that the astigmatism goes away.

    Ha! I did not know that, but it makes perfect sense. Thanks for the info!

    Second, and I’m not trying to be a jerk, but it appears from the number of elements (5) and price ($90) that the Astro-Tech Titan is a relatively simple Erfle design.

    Sure. I picked it up on a lark when it was on sale. It doesn’t have clean edges, but it’s still a fun eyepiece, especially in the XT10. I’m not much of a purist when it comes to edge-of-field aberrations. Maybe someday I’ll get enough expensive glass that I won’t be able to tolerate such things, but neither of those things has happened yet.

    By the way, what is up with the spate of blog posts recently? I’m not complaining, but you’ve become a regular publishing fool recently! And I mean that firmly in the Frank Zappa sense. Good stuff. Keep it up!

    Thanks. It’s a complicated series of things coming into alignment. It won’t last, so I’m striking while the iron is hot. I’m working on a post explaining all of that, which will hopefully be up soon.


  4. […] I could see the entire field of the 8-24mm zoom at all magnifications while wearing glasses. Which I have to do now. In fact, the other night at the Salton Sea I made almost all of my observations with glasses […]


  5. […] the best of my non-zoom eyepieces, particularly the Explore Scientifics. However, my eyes are now the weakest link in the optical chain, even with glasses. So although I don’t get super-sharp pinpointy star […]



Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: