First impressions - Orion Short Tube 120 refractor

April 2, 2008 - 4:20 am - 6:35 am - 43 degrees, very windy, transparency excellent, seeing average

Good.

I don't dare say a lot more, except in terms of specific observations and general principles, simply because I've come not to trust my first impressions. But let's look first at the goal.

The question I'm trying to answer for myself - and, perhaps, others - is which would be a better choice - inexpensive, huge binoculars? Or a short-focus refractor? For example, Garrett Optical sells a 28X110 - their largest offering at this time - for $650. If it's anything like other binoculars in this price range it does a credible job at the low power it is using - but, of course, it would be far from being an apochromatic.

I suspect it's performance is rougly comparable to that of a short focus achromatic refractor. To that end, I purchased an Orion 120mm ST F5 refractor. So what am I giving up by going this route?

1. Binocular vision.

* more light grasp with two eyes - I woulod say that 110mm is at least the equal of the 120mm in light grasp and probably a bit better.

* The relaxation and just plain naturalness that comes from using two eyes instead of one. We were designed this way.

It's this last that is the key. For some people there is no subsitute for bninocular vision. I thought that for amany months - maybe more than a year. But I slowly slipped away from it and I don't miss it. There are some substantive arguments for binocular vision and I appreciate them, but there is also a huge subjective one that comes down to personal choice.

So monocular vs binocular vision aside, what advantage do you get by going the refractor route?

1. Less expensive. A 120ST with good quality diagonal, and a single, quality eyepiece costs less than $500. A good mount and tripod costs about $300. With the binocular the cost is $650 plus, conservatively, $500 for mount and tripod.

2. Magnification. Low power views are great. The 28X110 gets an advertised field of 2.3 degrees. Using a 24mm Hyperion with a 68 AFOV, the 120 ST should give 25X and a 2.7 degree field of view. (I need to check the fov - my memory fromt his morning says it is not that large, but I didn't evaluate this aspect carefully.) Given the lower power of the 120ST, I'd say that's roughly even. BUT. . . with the 120ST I can crank up the power. This morning I used 46x, 75x, 120x, and 171X.

So essentially you're trading off the advantage of binocular vision against the advantage of using any eyepiece and a wide range of powers. I think the 171X is probably the top end. seeing was not great this morning in high wind, but I still was able to split the Double Double at 120X and it still looked good at 171X. I didn't try the low end. I have a 30mm ClearVue with an 80 degree AFOV. That should yield 20X and a four degree field of view - huge. But it also means an exit pupil of 6mm and while not unreasonable, that's a bit much for my old eyes - I suspect when dark adapted I have a 5mm exit pupil and so this would effectively become an 100mm refractor - or less - at that power.

Of course, I could also use a binoviewer with it - but that too cuts the light grasp. Trade-offs, trade-offs, trade-offs!

What it all boils down to is the advantage of using two eyes rather than one, versus, the advantage of using multiple eyespieces and powers. (Yes, you can now purchase large binoculars with interchangeable eyepieces, but then you're spending money on two eyepieces for each power instead of one - the bottom financial line gets way out of balance with this approach. But if money is no object, it's certainly worth considering.

Having used several excellent binocular mounts with large - but not huge - binoculars, I have to say the refractor approach is simpler, lighter, and more likely to get used by me for those reasons. Part of me loves leaning back and looking up through binoculars - but another part of me doesn't like the lugging around of the binocular mount and tripod, plus the adjustments that have to be made when you look elsehwhere in the sky. Even with the excellent 15X70 Astro Physics binoculars I'm now using, I would grab the 120 ST for a quick look long before I would grab the binocular set up. Again - some objective factors involved, plus a healthy dose of personal choice.
subjective

OK - was the 120ST performance up to that of a decent, high-powered binoculars? Yes. But I need to do a lot more observing and comparing before I say it can, for all practical purposes, replace my APO refractors of 80mm and 100mm - though it might. See - that's also part of my agenda. I could sell my APO refractors, plus binoculars and mount, for around $1400-1500. That would finance this the 120ST, plus a 12-inch Orion DOB. Now that's an interesting trade-off ;-)

Anyway, here's what I saw in a quick hour-plus of observing this morning.

1. Albireo. Nice clean split. Charming at 28X. (Hmmm.. forgot. I started with the 21mm Hyperion rather than the 24mm. The 21mm is a good match for the binoculars mentioned. It would be interesting to do a head-to-head comparison because that would be 28X against 28X and almost the same field of view.

2. M27. As with Albireo I was getting a strange, 3D feeling here I can't explain. Maybe it was all the pinpoint Milky Way stars in the background. At 120X I could see one of the stars near the edge of M27, but nothing embedded in the nebula.

3. M5. Easily found. Took magnification nicely. At 120X the outer areas were granulated.

4. M51. This I really liked. Even at 28X the spiral nature and interaction of the two galaxies was apparent. I didn't have time to study it at multiple powers, though, because I wanted to see how the scope would perform on M81-82, targets for the Prime Time group.

5. M81/82 - These were low in a light-polluted north-west quadrant, but still very easily picked up at 25X and took magnification well, at least up to 75X.

6. M13 - a backbreaker - nearly at the zenith. Showed nicely at 25X and I didn't try anything else. Problem here is the Voyager mount simply isn't tall enough. Need to check to see if they're selling the extension yet. It's needed.

7. M57 - Unimpressive - but by then I noticed twilight was quite bright. Need to wait for dark skies to check it out.

8.Double Double - I was surprised this split at all, but I did get a clean split at 120X. The wind must be gusting around 25 mph - maybe higher.

I tried Vega and saw an expected purple fringe around it. I had left the fringe-killing filter in the house. Maybe I'll give that a try tonight.

Jupiter was lost in the trees, as was the crescent moon, so with the increasing twilight I put the scope away.

One last note - I could carry the scope - mounted - and my eyepiece box with 8 eyepieces, note pad, red light, etc. all in a single trip to and from the house. That's nice.