Bye bye BV – it’s been good to know you!

 

OK – I’ve decided this before, but I have to learn a lesson at least three times before it takes hold: Binoviewing is not for me. Oh – and the Denkmeir “Big Easy” is big, but it’s not easy.

Alright, in fairness to Denk, when they said “Big Easy” I think what they meant is that we’ll sell a beginner package that will come to focus in all the major types of scopes – no need to break out the hack saw and shorten the tube 0f that 100mm APO.  And they did – and it does. But it’s not easy.  As you use this package and attempt to change powers using the various attachments you may find, as I did, that fumbling with these various screw-in thingees in the dark and the cold isn’t that much fun. What’s more, achieving focus isn’t all that straight forward and may call for really major adjustments.

For example, in switching to high power mode in a refractor – 2.5X normal power –  I had to back the focus way out and when I did that – given the weight of the binoviewer – the scope went out of balance so I had to change its position in the clamshell. So changing power in this case meant:

• removing two eyepieces and replacing them with two others
• removing the binoviewer from the diagonal and replacing the “nosepiece” with a different, screw-in “nosepiece”
• loosening the clamshell, sliding the scope forward, tightening the clamshell (repeat until it balances)

Right – and in mono mode you would simply swap one eyepiece for another.

Just call me Cyclopian!

That’s a big price in what Pooh would call “bother” to pay for being able to use two eyes instead of one.  Makes me feel a lot more comfortable with the habitual, one-eye approach.

But what if I limit the use? What if I say I’ll just use this one with the TV85 at  relatively low power and treat the combination like a super sharp binocular? In this mode I simply go with the low power view using the 25mm Plossls.

Well, the field’s pretty small for a binocular, but I can live with that and if I really like this idea, I could put a couple 24mm Pans in there for another $200 or so. That would give me a wider field – but . . .

What about the old business reported by many that the binoviewer cuts the light in half? You know what? It does!  OK – I know this is controversial, so breathe through your nose all you binoviewer fans. I can’t speak for anyone else, but I spent a whole lot of time this evening with the Pleiades – particularly a little segment between Alcyone and the center of the cluster – well – that little arrow asterism near the center.  Right here:

Click image for larger version - prepare from SkySafari screen grab.

There’s a triangle of  twelth magnitude stars near there and guess what – I could see them while in binoviewer mode with the TV85.  (M45 was near the meridian and transparency was average – maybe a bit better.) That’s good – except, I could see them just as easily with the 60mm Unitron set up next to it!  (I could not see the 13.57 star noted in either scope.) Bottom line – as near as I could tell there wasn’t any difference in the light reach of the 60mm  Unitron and the 85mm Televue.

This, by the way, is exactly what the math shows. Figure the area of the 85mm lens. (Just square the radius – you don’t need to throw pi in there.)  Now divide by two and take the square root of your answer. That give you the radius of an objective that would produce half the light gathering area of the 85. Or in math-speak: 42.5 squared is 1,806 – divided by two is 903 and the square root of that is 30.05 – times two and you have a 60mm objective. Don’t you love it when the geeky math actually shows you the same thing as your eyes and common sense!

Downsizing the TV 85

So by putting binoviewers on my TV85 I was turning it into a TV60 – as far as light grasp goes. I should hasten to add – and this was obvious on Jupiter – that the added resolution of the 85mm is still there despite the binoviewer, so Jupiter certainly looks better in binoviewer mode in the TV85 than it does in the Unitron at a similar power. Which is why the common wisdom is that binoviewers are really great on the Moon and planets, giving you a sense of 3D even though at astronomical distances that really isn’t the case.

So – why not just limit binoviewing to the Moon and bright planets – and bright doubles?

Maybe. It’s tempting. The binoviewers do a good job on bright objects – but I did run into some unexpected CA problems which I can only attribute to the binoviewer – or maybe my difficulty in reaching precise focus with them. I must admit, at high power I actually had to ease the binoviewer out of the  diagonal a bit, then clamp it tight, in order to reach focus  in the TV85. Different eyepieces might have solved this problem. But that’s really no way to hold the system together and I had no remaining outfocus, so let’s ignore that CA business.  It wasn’t there, for example, with the 25mm Plossls – just with the 17mm Plossls.  But whatever the cause it was irritating.  And by that time my patience was wearing thin. See, when it’s clear I like to observe – not screw with equipment.

And really – I am neither a planetary or lunar observer. I look from time to time and enjoy both activities, but I don’t think that is reason enough to have $400-plus  invested in binoviewer and eyepieces.

Oh – a few other notes from this observing session – which was really three sessions starting at  about 9 and ending after 2 am with significant breaks between as clouds came through.

• I could not split  Polaris with the 60mm Unitron or the TV85 in high-powered binoviewer mode. I could split it with the TV85 in cyclops mode.
• I could not split Mintaka – frustrating – with the 10X30IS Canons, or the 15X70 Celestrons handheld. (The mount for the Celestrons was in a shed 100 feet away and I was too tired at that point to go get it.)
• I could split Mintaka with a 50mm F4 and a 50mm F12 on a double mount – but what I was really trying to do was to get a handle on the linear size of the star image   – something that the optical experts says is controlled strictly by the focal ratio of the telescope.  But I had a brain freeze about this time – it really w as time to go to bed – and I just couldn’t make good sense of what I was seeing or even remember what I thought I saw when I went to making notes.
• Still, the last thing I did was a quick tour of familiar sights with the 10X30IS binos – I do like them – and I meandered long enough to get some idea of their lingering magnitude -in the neighborhood of 9. I was checking what I think of as the “house” in the Hyades – what Daphene calls the “bell.”  Here are the test stars.

Screenshot from SkySafari on my Ipad showing the 8th and 9th magnitude stars I was able to note with the 10X30IS binoculars. Less tired and more patient I'm sure I could go deeper with these. Click image for larger view.

Actually the number I’m getting are pretty fantastic – enought o make me wonder about whether SkySfari is giving me the right magnitudes. Of course,t here are various opeinions on limiting magnitudes for different size instruments. Here’s what respected observer Clay serrod has to say about it on his web site:

 

There are formulae available in all the books that I will not bore you with; from that formulae, I have prepared a MEAN value, an average of sorts, of all of them and offer the list below. My 32 years in astronomy has shown me that this list is, indeed, VERY close to actual performance.

Under the darkest conditions (see below)

            HUMAN EYE             - 6.5            5.0"         - 12.8
            2.5"                 - 10.5            6.0"         - 13.2
            3.5"                 - 11.4            7.0"         - 13.6
            4.0"                 - 11.7            8.0"         - 13.9

Hmmm . . . 30mm is about 1.2 inches and if 10.5 is the limiting magnitude for 2.5-inches  I have to think 8.7 is a respectable figure for 1.2 inch binoculars. Of course there are two of them, so let’s multiply 30mm by 1.3 – that gives me 39mm – more like 1.5 inches. well, 8.7 is good there too.  (That 1.3 multiplier is another compromise – binoculars increase the light grasp over a single scope by a factor of 1.2-1.4 accoridng to conventional wisdom.)

What I’m more surprised at is my 60mm (and the 85 inmbinoviewer mode) delivering a magnitude 12 star when Dr. Sherrod’s compromise chart above would set the limit at more like 10.5 – that’s a huge difference.  And Starry Nights Pro has the same, or slightly lower magnitudes for those three stars.

So what’s going on? i think there’s just too many variables and so this limiting magnitude business is all over the charts. For example, according to another web site with a very sophisticated looking formula I should see significantly deeper – and yet another one has me seeing deeper than I  should.  None of which matters for what I was trying to learn with the binoviewers. The simple truth is they made the 85 perform like a 60 – no matter what the absolute values involved, this was a side-by-side test.

 

 

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