Ok, Mars in the binoviewers just blew me away this morning – not to mention Saturn. I just could not get over the impression of Mars as the eyeball of a gargoyle – an impression that seemed enhanced by the binoviewer’s faux-3D effect.
But that all came after I did some serious experimenting to try to pin down this business of exit pupil and the apparent astimatism in my eyes. Essentially, exit pupil is the diameter of the cone of light that leaves the eyepiece and enters your eye. I have always been conscious of it in terms of low power in that if it is too large, you’re just wasting the light from your telescope or binoculars. Thus, for example, the typical 7X50 binoculars which are great for daytime use where gathering a lot of light doesn’t matter – are producing a 7mm exit pupil. For most of us, as we get older our eye just don’t open that wide – 5mm is more the norm. So that means you may be wasting half the light the binoculars gather – something that is very important for night-time astronomy viewing.
But now I discover from Ed Zarenski – and various astronomy texts I had ignored – that even a moderate exit pupil of 4mm will make it impossible for you to bring bright stars to a sharp focus IF you have astigmatism. And since I can’t get bright stars to come to a sharp focus in any binocular of any quality that I have ever used, I began to suspect I have astigmatism. And when I did a modest, self-administered test, it does indeed appear that I have it.
50mm Sparow Haek on LXD55 mount.
So I decided to attempt to confirm this from another direction – using a small scope at various powers to see how small the exit pupil has to be for my astigmatism not to matter. Exit pupil can be calculated by dividing the power of an eyepiece into the objective diameter of the telescope. In this case I choose a 50mm telescope that happened to be very “fast” – F4 – so it was very much like half a binocular.
I then systematically increased the power by changing eyepieces while the scope was pointed at Mizar, a bright star with a reasonably bright companion separated by 14 arc seconds – something even small binoculars should be able to handle. But I have never been able to split Mizar with binoculars in the past.
Bottom line – I’m loving using two eyes, but I’m also learning that the only way I’ll see sharp stars with binoculars is to lower the exit pupil, or use glasses that correct for my astigmatism.
I found that with steady skies and Mizar nearly over head I could get a wonderful split – nice and clean – with the 20X60 Pentax I had just bought at Ed’s suggestion. IF I was very careful about my head position. Not having my head correctly aligned and held steady, the image deteriorates. But this was very encouraging. The exit pupil on these was 3mm – quite small. The image wasn’t perfect, but darned good. Much better than anything I had seen before when using binoculars that gave as much magnification, but also a larger exit pupil – such as the popular 20X80 binoculars.
That’s when I brought out the 50mm F4 refractor, so very much akin to a binocular. I mounted it on the LXD55 (way overkill, but it was handy) and aimed it at Mizar.
I started with a 32mm Plossl. That creates a ridiculously large 7.8mm exit pupil – way wider than my pupil can open, so light is lost – but the fov is wide making it easy to find things. In any event, with a nearly full Moon in the west I could not even see the third star that forms a triangle with Mizar and Alcor. But I at least found the target. No prayer of splitting it, though.
Switching to a 20mm Plossl – 10X – and a 4mm exit pupil and I could not see a split. But from there it got better as I increased the power. A 17mm gave me 12X, and a 4.2mm exit pupil and I felt I could see a split, but it was very sloppy with lots of light spikes flaring off the primary and ghostly double images interfering with the view.
A 12mm Ortho gave me my first really good split. This was 16.6X and yielded a 3.1mm exit pupil – very similar to the 20X60 binoculars, so a 3mm exit pupil seems to be the starting point at where my astigmatism is not as much of a bother.
But the view continued to get better with each increase in power and subsequent diminishing of exit pupil – the 10 was real nice – 20X and a 2.5 exit pupil. With a 7.5mm Plossl I had “refractor like” images – that is the kind I would expect with a long focal ratio refractor – an F12 or F15. That yielded 26X and 1.8mm exit pupil – so down below 2mm is real good.
And the best images came with a 6mm ortho – 33x and a 1.5mm exit pupil. This gave the kind of performance I expect out of a real nice refractor when skies are steady, as these were.
So where does that leave me? Well, the binoviewers are no issue at the powers I’m using. For example, on the 8-inch SCT (200mm) I’m using a minimum of 100X and that’s a 2mm exit pupil – so that explains why the binoviewer images are sharp. Interestingly, though, if I could obtain lower powers I would increase the exit pupil and could run into problems.
Of course, I can get corrective glasses, but wearing glasses while observing is a pain, so if I can avoid that, I would prefer to – and by carefully choosing what I view with I may be able to avoid it. For example, the 18X50 IS Canons would give me an exit pupil of 2.7mm and probably result in a pretty satisfactory image – about like the 20X60 Pentax. But a better choice for me might be the 70mm right angle binoculars that Garrett Optical offers. I could put 13mm eyepieces in those and get a 2mm exit pupil at about 34X. If those are Naglers, then my field of view would be about 2.4 degrees – very respectable. What this boils down to is the 10X30IS are real nice for wide fields. The 20X60 ae great for a lot of binocular double star work. I could get nice, wide field, low power views with the Garrett 70mm and Nagler eyepieces. And from there the binoviewers would take over. In other words, i can see a way to always use two eyes.
20X60 Pentax binoculars mounted on a Parallelogram mount are a good fit for me now.
But, I want to move into this slowly. So I’ll first continue to experiment and use the Orion binoviewers with the eyepieces I have. But I am finding it is simply hard to tear myself away from the binocular view and when I returned to the 20X60 binoculars I discovered how critical head position was, for I could get near perfect images with those as well – if I held my head just right.
This was easiest to do if I actually was a little farther away from the eyepieces than the extended eyecups called for. Going down to the observatory, I put the scope on Mars – keeping in mind that I was in twilight by now, as well as fighting a nearly full Moon. And Mars – small as it is – was spectacular in the binoviewers – I went to two hundred power and feasted on a cosmic eyeball – a ghoulish cosmic eyeball with orange and greens in it, but the startling white polar cap appearing to distort it and give it the eyeball feeling Saturn was high in the trees to the south, pretending to be one of the Heavenly Twins , paired up with Spica. I had to do a double take when I first saw them, for I knew Gemini was not in that section of sky. The two-eyed view was simply mesmerizing. For the first time in along time I really wanted to just sit there – not change anything.
bottom line – this was a very enjoyable and productive morning. And yes, I need to explore this business of head position more, and I need to get tot he eye doctor .
A good pair of binoculars is a great investment and they become friends over a lifetime. Cheap ones just don’t really do it, though, go for midrange at the very least or even better – you’ll be happier with the results.