This little dude should not perform that well! I mean, it’s not just that it’s a 50mm. I have a 50mm Tasco I love – but that’s an F12, a much different beastie! This is an F4.1 achromat so the focal length is just 205mm. It is nothing but half a binocular – a glorified finder – glorified because it has a helical focuser and will take a variety of 1.25-inch eyepieces – and it’s a Stellarvue. But it’s even using a diagonal prism so you get a RACI and prisms aren’t known for handling power well. Oh – and it’s a terrific finder and Stellarvue sells a slightly different version of this as a finder. They’re great. But asking a scope to perform well at 9X is one thing. Asking this kind of fast scope to perform well enough to split Castor, which requires relatively high power, that’s quite another.
Yet I won’t keep you in suspense – it passed the test. Using a 6-3 Televue zoom which at 3mm was yielding 68X it delivered a lovely image of the the Aa and Ba components of the super double, Castor. (For more on Castor see this post.) But I didn’t start with Castor and I knew this could do fairly well in any event. I have owned three of these, but in previous tests some time ago I had decided that 40X was its practical upper limit. I was wrong.
What inspired me to grab this for some early morning star splitting was a debate I have been having with myself – and boring John with – regarding binocular telescopes. (I tend to get super-focused on a subject like this and drive everyone around me crazy, starting with myself.) In this case I was looking hard at the Garrett Optical 70mm binocular telescope that takes 1.25 inch eyepieces and includes this curious note in it’s web site description – “you always have the ability to expand your magnification range by inserting virtually any 1.25” telescope eyepiece from 40mm down to about 9mm.” Hmmm. .. since this is a 70mm F6.2 that would put a limit of about 48X. How much enjoyable star splitting could I do if I were limited to 48X?
To answer that question my first thought was to use an 18mm eyepiece in my Unitron 60mm – that would give me 50X. But I would be comparing an F15 achromat with an F6.2 and that didn’t seem fair. That’s when I thought about the Little Rascal. It had all the potential fault points of the binocular telescope and then some – 90-degree prisms rather than mirror diagonal – very fast achromat – and, of course, apparent difficulty in using really high power. I don’t know, but I suspect the limit Garrett seems to be assigning here might reflect the difficulty in aligning two telescopes at high power. That would not be a problem with the Little Rascal, of course. So I will assume that at 9mm – 48X – the Garret can still deliver a sharp star image. My quest then was to develop a feel for what I could see at these low powers using a similar instrument. (I have this thing about using two eyes – really want to try it – but the price is steep. I’ve already explored binoviewing and for various reasons that doesn’t work well in my situation. )
So – it’s 19 degrees out. That’s cold. But my enthusiasm for the test is running hot. I decided to start easy. Mizar was well placed in the East as the Big Bear clawed his way up the morning sky. I put in a 24mm Takahashi (8.5X). Mizar and Alcor split, of course. Heck, people with better eyes than mine do this without a scope. I could also see the third star that’s not part of this famous pair, but turns them into a memorable triangle. But I couldn’t split Mizar itself. Went to a 12.5mm Tak – 16X – and got a solid hint of a split of Mizar. These two are 14.3 seconds apart and magnitude 2.2 and 3.9. OK – 10mm and it splits! That’s 20X and fits what I read in one of Ed Zarenski’s wonderfully detailed and scientific binocular reports on Cloudy Nights. Essentially Ed found that most binoculars are limited in their ability to split stars by their power, not lens diameter. And at 20X the best you could hope for was a split of stars separated by 7-10 seconds of arc and, of course, even that depends on the difference in magnitude between the two stars. So it’s know real surprise that with a 10mm (20x) it splits, with a 7.5 mm (27X) the split was very nice and with a 5mm (40X) it was super! Great start.
Orion was over in the west, sohow about the Trapezium? I grabbed the scope and walked to the front yard where no house or tree blocked the view. Uh oh – lower than I thought. A bit less than 15-degrees above the horizon. This would not be easy. Even when clear that’s a lot of atmosphere to look through. But I discovered something here. Having gone portable – the first observation was from my deck where I have observing chair, table and plenty of eyepieces – I found the Little Rascal would work with an 24-8 zoom and a 6-3 Nagler zoom – so I didn’t need to carry much with me to pretty much cover the reasonable power range. Boy – talk about grab and go! I wss using this on a Bogen with a Universal Astronomics MicroStar mount. That was overkill and a tad heavy. I could have been using a light weight tripod with pan head. Or maybe not at these high powers? Hmmm – just as an aside, I did test a lighter weight rig the next night. I just put it on an old photo tripod.Here is is trying to look big and confident next to the 60mm Unitron.
But the Trapezium low in the sky, was difficult. I couldn’t see anything worthwhile with the 24-8 zoom. With the 6-3 I got a split at 3mm – 68X – but for this test I really wanted a split at 4mm (51X) or a bit less. BUT – the issue here wasn’t so much the separation between the stars – the closest pair int he Trapezium are 7.5 seconds. The problem was not enough light grasp when looking through that much atmosphere. I could get a hint of more than one star at 5mm (40X) even. But this was not a satisfying view. Have to try it another night when Orion is higher. but Castor is still high right now – about 60°, so no problem there.
Again, lack of light grasp kept Castor Ca out of the picture. And remember, the brighter pair is magnitude 1.9 and 3 and split by just 4.2 seconds. That’s a serious challenge for this little scope, but with the 3mm (68X) it was absolutely charming. It barely split at 4mm (50X). Perhaps the 70mm binocular telescope could do better. Then again, they make a 100mm that’s similar and could take higher power and …. oh my. This could end up costing significant bucks. I need to remember how much fun I’m having just testing this idea without spending any money 😉
OK – if Castor splits, why not Algieba? Back to the deck – and in the house to warm up and write a few notes, then back out to try Regulus, Algieba, and Leo’s double double.
First up, Regulus. And it’s reasonably easy. No trouble with the separation, of course – that’s 176″ – but we’re dealing with a magnitude 8.2 star that even at that distance is going to suffer some from the glare of the 1.4 mag primary. At least I assume that was the cause. In any event, using 8mm (26X) I had a split, but the secondary was so faint I needed to quickly sketch it, then check it against the PA which is 308° to make sure I wasn’t fooling myself. I wasn’t. It split, but not in a way to excite me.
Now for Algieba. I love this star as much as Castor, though it doesn’t have such a fabulous back story, though the colors are wonderful. In terms of difficulty it should be just about the same as Castor – the separation is 4.6″ and the magnitude difference is 2.4-to-3.6. But it will not yield. It’s actually a bit higher int he sky than Castor was, but I think the seeing – which was above average when I started – is deteriorating. Otherwise I have no explanation. I was using the same scope and same 6-3 zoom and I just could not get the clean split I had seen with Castor. Ah well!
Now for Leo’s double double – charming! Using 20X-40X I got an easy split. Of course, these stars are all orettyc lose to one another in brightness and the closest of the two piar is separated by a comfortable 29 seconds of arc. So i didn’t view this as a challenge. I really wanted to evaluate the aethetic of seeing it with this power and withr elatively little light grasp – and I liked what i saw. I can’t explain why I liked it – it’s just that the gestalt was appealing to me.
Last on my list was Cor Caroli, a long-time favorite that I suspected would be easy and it was. The separation here is a comofrtable 19″ – the only challenge is the difference in magnitude – 2.9-to-5.5. But it wasn’t a problem, It split at 20X and was real nice at 27X.
So the bottom line is this. The Little Rascal is a fun telescope. With the right conditions and eyepieces you can push it’s fast lens to higher magnifications than I thought, though we’re still only talking 34X per inch – a good long focal length refractor will do twice that. It also has some issues with not all eyepieces coming to focus in it, though I had no problem witht he Taks and the two zooms. But I would not recommend getting this for double star work. It’s just fun to know you can press it into duty in that capacity and it’s light weight and small size can’t be beat when you want something, quick, easy, and incredibly transportable – plus it has an incurable case of the cutes.
What I learned
And what did this teach me about binoculars as double star tools? Well, I suspect you just cna’ttake themr eal seriously – though there is a deifnite advantage to using two eyes and that has not been tested here. However, I did try some of these same stars the next morning using 20X80 and 15X70 binocular mounted on a Parallelogram mount. First, a mount is absolutely essential. Second, I had real difficulty bringing the 20X80 (they’re i an inexpensive pair) to sharp focus. The grease in them didn’t like the 21-degree temperature any more than I did. My test stars were the Leo double double. I didn’t get a good split until I switched to the 15X70s – also an inexpensive pair, but these had been in th ehouse and the grease was warm 😉
I got a nice view using them mounted and both pair split at 15X. The 70mm objectives showed me all the stars, but not to great advantage. I liked the higher power view in the 50mm better.
Bottom line? Something like those Garrett binoculars mentioned near the beginning of this post would be real nice and with 90-degree prisms they could go on any sturdy tripod making them simpler than using a parallelogram mount. But with my interest focused on double stars i don’t think I could justify the cost. It’s the implied power limitation that stops me – and I think in practical terms that’s a real limitation you can’t get around because the focal length is just too short. with doubles we generally want long focal length because we are pushing our scopes to the highest power that the atmosphere will allow us. So I had fun with the Little Rascal. And I’ll use it again. but mainly what it did for me was scratch this binocular itch enough so I can put that idea aside and cous on some other needs that are high priority to me, such as simple clock drives for EQ mounts that hold small – but long -scopes.
Filed under: 1. Star-splitting Scopes |