Oh boy! Been having a discussion on Cloudy Nights about astigmatism, etc. and the last post. What was bothering me was, among other things, how to separate the impact of power increase from the impact of exit pupil change.
See, in my last experiment, detailed here, what I did was take a 50mm objective and repeatedly change the power. This resulted in a smaller exit pupil – good for negating the impact of astigmatism in my eyes – but also a higher power which, of course, makes it easier to split doubles. So the experiment is interesting, but inconclusive. Someone on Cloudy Nights suggested that what I needed to change, of course, was the aperture – not the eyepiece. Keeping the eyepiece – and thus the power – the same, and cutting down the aperture would give me a shrinking exit pupil and a better guide – at least with bright stars – as to the impact of the smaller exit pupil.
So off I went immediately to make a 40mm mask for the 20X60 Pentax binoculars – which I did and the skies cooperated by giving me a cloudless early evening – something that wasn’t predicted, but I was delighted to take.
I used the masked 20X60 Pentax as is and stopped down to 40mm, first on Albireo, then the Pleiades, Jupiter, M34, and the Rams Eyes. Very interesting – and I’m a happy camper because at last I’m seeing pinpoint stars with binoculars! (You can’t imagine how many excellent pair of binoculars I’ve owned and sold because they did not deliver this – and they all had fairly large (4-5mm) exit pupils!) But there are still a combination of factors involved, the most important one being the relatively small exit pupil which seems to overcome my astigmatism and the second most important, finding the correct head position. I did play some with IPD, but couldn’t see that as an issue.
The most surprising situation came after spending about 20 minutes trying to get a really good view of Albireo. That wasn’t a matter of splitting it, but one of trying to come up with clean, refractor-like stars – bright, round little bullet holes in the night sky – and in this case, showing lots of color.
As I settled on Albireo I kept fooling with head position – the binos were on a p-mount, of course – and focus and just couldn’t get the primary to settle down completely. Then suddenly I accidentally moved the image off center – about half way to the top of the field, and bingo! There were my two perfect stars, bright orange (not gold, as I usually see it) and blue.
This blew my mind and I kept repeating it – bringing the image back to the middle of the field where there was still significant flare on the primary, then moving it up and when I did so, having it clear. I can’t explain this. I don’t think it had anything to do with the binocular. I think it had something to do with my head position. When I moved everything around and went after the stuff in the east later I could not repeat this in any form – in fact, as time went on I was getting good crisp stars throughout the field of view, so it just didn’t matter.
Could it be the binocular cooling down? I doubt that very much for I had left it out for at least an hour before going out to observe.
The stopping down produced obvious and predictable results in all the tests. I got a little less light, but I got sharper star images with it stopped down to 40mm and thus yielding a 2mm exit pupil. I did try 15X70 glasses as a test case and while I could split Albireo with them, the split was very, very sloppy and there was no way I could get it to even remotely look like what I was seeing in the 20X60/40.
Binocular doubles in the Pleiades - modified from SkySafri screen shot.
The Pleiades provided a terrific experience. I’m planning a post on binocular doubles in the Pleiades for the double star blog I share with John Nanson, so I was real pleased to go over several doubles, some of which I had split before – they’re ridiculously easy – and some of which I had never split with binoculars. One thing that revealed itself nicely is that triangle near Alcyone towards the center of the cluster. (See the inset in the above image.) It’s easy enough to get the 6th mag star there, but the other two are fainter and I hadn’t seen them before because of the glare from Alcyone. I could just detect them with the 60mm stopped to 40mm and could see them clearly when using it unmasked. This was the clearest indicator of how much light I was losing by going from 60 to 40mm objective.
But most satisfying was the whole cluster of sharp stars.
Jupiter was certainly better when I stopped down to 40mm. That way I could just pick out Europa which had recently transited and was still quite close to the planet. Ganymede and Callisto were obvious and Io was in eclipse.
Being in the neighborhood I decided to give the Ram’s Eyes a try. There the split is 7.5-seconds and I just couldn’t do it, though with the 40mm masks on I did have a distinctive figure eight that was oriented in the correct north/south direction. Maybe with more practice and a better night . . .
With M36 I could pick up two or three of the pairs that make the body of what I think of as a Klingon War Cruiser.
Bottom line – many thanks to Ed Zarenski for telling me about astigmatism and exit pupil and others here on CN who joined the discussion and have helped me work through this to the point where I can now enjoy binocular astronomy a lot more. It’s good to see sharp stars and it’s good to have some rationale for why low power views have never worked well for me with binoculars or telescopes.
