Sunday, May 03, 2015


Shootout at the 4-inch Corral

There is no denying that in this day of monster telescopes, when almost every amateur at least aspires to owning a 25-inch Dobsonian or a C14 Schmidt Cassegrain, a 4-inch telescope is a small telescope.  A 4-inch telescope can still be a useful telescope, however. It's not that, as my Jedi Master asserts, “Size matters not!” but there are things that matter more than size sometimes.

In truth, choosing a telescope for use in the backyard is a battle. A battle between what you are willing to set up and what you’d like to look through. It is the telescopic equivalent of the life of a giant star, which pits radiation pressure against gravity. When the star gets old, gravity wins out. As you get older, you may find a smaller scope wins out because of gravity.

Take me for example. I am probably luckier than some of you—maybe many of you—in that I have a backyard with a zenith limiting magnitude of about 5.0 on a good night. That is better than what a lot of amateurs put up with at home, yes, but it’s usually not good enough to inspire me to drag out the 12-inch truss-tube Dobbie or my C11. If I had to depend only on those scopes, I’d probably never observe from home.

Luckily I have several four inch telescopes. Actually, I have four of them. They seem to be multiplying like rabbits. There’s the Celestron C102 f/10 achromatic refractor, the Explore Scientific f/6.5 4-inch achromat, my old f/10 4.25-inch Edmund Scientific GEM Newtonian, and the StarBlast 4-inch f/4 mini-Dobsonian reflector. It’s an embarrassment of riches, with my only problem being deciding which is most effective in my backyard. That is what we are here to determine this Sunday morning.

I eliminated two from the running almost immediately. I love my Palomar Junior. It wasn't my first telescope, but it was my first good telescope, and was the telescope that kept me in astronomy. Unfortunately, while its optics (an f/10 spherical primary) are surprisingly good, the scope is a pain to lug around. Couple its long tube with a surprisingly heavy and shaky 1960s style German equatorial mount, and the scope is most assuredly not my choice for spur of the moment looks at the Moon during commercials in Arrow.

Then there’s (Dorothy's) Explore Scientific refractor. It could easily be a contender. At f/6.5, it is kinder to any mount than the C102 is. Yes, at that focal ratio there’s going to be plenty of color, but at a recent public outreach session I was surprised by how little purple I saw on the Moon. Still, the scope isn't much more portable than the C102, and I believed the C102's higher focal ratio might be best for the general purpose observing I do out back.

Finally, there’s the StarBlast, Orion’s (Synta’s, actually) powerhouse of a tiny Dobsonian. It is as portable as portable can be, even given that unless you are the size of GI Joe or Barbie you are going to need a table or something else to place the little thing on. It’s given me superb views of large deep sky objects. Particularly noteworthy were the looks I had of the North America Nebula and the California Nebula with it at the Deep South Regional Star Gaze one year. However, there was no way I’d be seeing those big, dim clouds from my back 40. Might the scope's fast focal ratio actually be a handicap there? Maybe, but its uber-portability won it a slot in what was developing into a shootout.

So, the contenders would be Amelia, the C102 refractor, and Yoda, the StarBlast (named that for obvious reasons). Yes, the scopes are as different as apples and oranges. A long focal length achromat and a fast neo-Dobsonian. But I didn't care. I was simply interested in which of these two telescopes would be best for the most casual of my backyard observing.

Hard as it is for me to believe, the StarBlast first came to me nearly eleven years ago. I wasn't out to buy one of the micro-dobs that summer of 2004, but I had heard quite a bit about the StarBlast at the Internet hangouts where amateur astronomers talked telescopes then—and now—the Yahoogroups and Cloudy Nights. It seemed the fast little Newt was making plentiful converts and garnering numerous thumbs-up reviews from respected observers.

Still, I wasn't in the market for one. Until the kindness of Herb York (Anacortes Telescope and Wild Bird) caused me to find myself in possession of a gift certificate from AT&WB. What to get? One thing I needed back then, desperately, since I was a resident of our city’s downtown Garden District, was a portable grab and go scope. One light enough that I could easily move it around to dodge streetlights and trees, but with more aperture than the Short Tube 80 refractor I had been using in that role. Anacortes was an Orion dealer and stocked the StarBlast. What to do with that gift certificate seemed obvious.

What I pulled out of the box when the Yoda arrived, and what you will find when you order a StarBlast today, are the same. The only difference I know of is that the price of the scope, just over 200 bucks, is actually slightly lower than it was when I bought Yoda. Otherwise, you get a cute little f/4 reflector with a (very) green metal-flake tube on a small particle board Dobsonian mount. The mount is a single arm affair, but the altitude axis has adjustable tension and ball a bearing race and the azimuth axis has Teflon pads ensuring the thing works pretty well.

The scope’s fittings are OK and about what you’d expect at this level. The 1.25-inch rack and pinion focuser has a metal drawtube and mechanism, but a plastic base. It works smoothly, however, and has never been a problem in a decade of use. The secondary is collimated with allen screws, which is a pain, but holds collimation pretty well. The center-dotted primary is adjusted by means of bolts and is secured with locks. Unfortunately, locking the mirror down will change collimation slightly, so you sorta have to collimate with the locking bolts as well. Not a big deal.

Accessories? Two Kellners, which are just about useless for such a fast scope. One of those little collimation caps that ship with many Chinese Newtonians, and which is also pretty useless for a low focal ratio Newt. One of Orion’s humble plastic “BB gun” red dot finders that nevertheless works. A so-so manual. That is it.

The Celestron C102 is officially only available at present as the Omni XLT 102 with a CG4 mount for 419.00, about twice the price of the StarBlast. You do get a lot for your money, however, including a mount that is useful for a variety of medium-small OTAs.  That’s not necessarily the best way to get a C102, though.

Celestron has been selling these 4-inch achromats since forever. Since the 1990s in their current Synta-made form, anyhow. They remain unchanged today save for cosmetic “improvements” like snazzier paint-jobs and more modern looking fittings (on some OTAs). Thank goodness the optics have remained outstanding. But the point is they have made so many of them that they are cheap and they are plentiful used. AND…that’s not all. At least one major dealer has C102 sales, seemingly yearly, which allow you to glom onto one of the OTAs (no mount) for less than 100 dollars (!).

Lucky me didn't even have to pay that. My good friend and fellow amateur, Pat Rochford, had a C102 he wasn't using. He thought the scope, which he'd repainted a cool brass color and outfitted with brass focus knobs, would look good in the living room of our new home, and gave it to me as a house-warming gift last spring. It did look good in the living room, but I soon found it was even better in a dark backyard.

A C102 of any vintage is a pretty plain Jane proposition. Long steel tube. Non-adjustable objective cell. Decent rack and pinion focuser (a 2-incher, yay!). Synta’s standard finder mount. My 102 came with the once-standard 30mm finder. I immediately replaced it with a red dot job and a 50mm RACI finder I got from Orion, which I alternate depending on my mood and what I am using the scope for.

While the C102 is sweet on a CG5 class mount, I’ve got mine on my SkyWatcher AZ-4 alt-azimuth rig. I like “point and shoot” these days, and while the long and surprisingly heavy tube is a bit much for the mount, the portability makes everything OK. It’s all of 2-minutes work to get the scope, Amelia, out of the Sun Room and onto the deck or into the backyard.

Time to get set up. I could have placed Yoda on the table on the deck, which would have been workable if not overly comfortable—I’d have to rearrange when going from one side of the sky to the other—but I wanted him in the yard to get the best view to the east, which is somewhat blocked by trees. What I put him on was the wooden "StarBlast Stand" Pat and I built years ago. It’s not great in grass, but it is OK. I have used the ‘Blast on the AZ-4 mount via a set of (overpriced) tube rings I got from Orion, but Amelia would be using the AZ-4, so it was unavailable.

With the two four-inchers set up side by side I waited for darkness, fidgeting more than a little, since it was obvious haze was building in advance of clouds. In the end, we only had time for four objects, but I gave the scopes plenty of time on each of them, and the four I chose were rather telling.


I was antsy to get going, and Venus, AKA, “Refractor's Bane” was shining bravely in the gloaming, so that is where we began. I thought if anything would give Yoda a leg up over Amelia, the Love Goddess would be it…

Yoda was first. I inserted the 7mm Uwan 82-degree ocular, lined up Venus with the little scope’s red dot finder, and had a look. My initial impression was “DARN! THAT IS BRIGHT!” Venus wasn't just bright at 64x; what the planet looked like was a chromed BB sitting in the Sun. Of course there was no spurious color, but the image was too overwhelmed by glare for me to say I was getting a good view. I could have rounded up a colored filter, but the other problem was that the planet, in its gibbous phase, was just too small. I ducked inside and returned with 2x and 3x 1.25-inch Barlows.

The 2x, an Orion Shorty, made things somewhat better. The planet was bigger, and while still a mite bright, was bearable. I don't normally expect detail on Venus—once in all my years of observing it I've seen the ashen light, and only a few times have I spotted darkish markings through filters—but I want there to be the possibility of seeing detail if it is there to be seen. In my judgment, I still needed more power. In went the 3x Barlow an Owl Astronomy Products Barlow I've had for a while, which, coupled with the 7mm, yielded close to 200x.

That made Cynthia bigger, but not necessarily better, I thought. Two things were obvious:  the seeing, while OK, was not perfect, and 200x was pushing Yoda. With Venus now descending, the planet was shimmering like hell. But more than that, the disk was slightly soft at 200x. This is an inexpensive f/4 scope, and there is only so much you can expect. Don’t get me wrong, though, the StarBlast did a creditable job.

Now it was Amelia’s turn on what is the most daunting target for any refractor. No point in fooling around. I’d start out at 145x with the 7mm Uwan barefoot in the Intes 2-inch diagonal. “Hmm. Well, that’s a little surprising.” I had to admit that at medium power the planet looked surprisingly good in the C102. Dare I say it? Better than in the Newtonian.

There was purple, but it was a purple haze (sorry, Jimi) that surrounded the planet but did not much impinge on the disk. Venus herself was sharp and clear, and if there'd been detail to see, I believe she'd have given it up. Chromatic aberration tends to get worse with magnification, but I didn't stint there, going to the 2x Shorty. At almost 300x, the planet held up well.


Full dark was now upon us, and the King of the Planets was sailing high overhead. Back to Yoda with the 7mm and the 2x Shorty. I didn't bother with just the eyepiece, since I knew the 64x that would produce would be way too little power. For detail, you start with 200x on Jupe. To begin, we’d see what the ‘Blast would do at 128x.

What the little scope did was impressive. Plenty of banding, though not much detail in the bands. The Galileans were looking a little like amoebas chugging along as the seeing suddenly began to degrade, but, yes, Jupiter looked good, with its color a slightly creamy hue  that looked very good and appropriate to me.

However, 128x isn't enough to see Jupiter's features easily. The human eye has an easier time with “bigger,” and even an experienced Jupiter hand will detect more with higher powers. I swapped out the Orion Shorty for the 3x, and had a look. Again, I had to admit that for the planets, anyhow, 192x was pushing it with the little green guy. I could make out more detail, but not much more. The planet was not nearly as sharp as it had been at the lower power (I'd collimated the scope that very afternoon and gave her an hour to cool).

The C102 began at 145x as before. Everything that had been visible with the StarBlast was visible with the C102. And then some. After just a moment of looking, I picked up the Great Red Spot, which was near to transiting the Meridian. Even in seeing that was now decidedly less than good, I could make out its current pale orange color, too. Back on the Starblast, I found I could detect the spot now that I knew it was there, but it was not easy.

At 290x with Amelia? I saw more than I did at the lower magnification, which is the true test of whether your scope is “taking” more power or delivering Empty Magnification. Single down-check? In the achromat, particularly at higher powers, the image looked more yellowish than in the reflector at comparable magnifications.


I thought we'd essay a couple of DSOs before the sky went totally punk, which I could tell it was on its way to doing. I had a look at a chart generated by Cartes du Ciel on my netbook, which seemed to be an appropriately simple computer to use with these simple telescopes, and pinned down the exact position of Gemini’s best open cluster, M35.

What do amateurs, particularly refractor loving amateurs like to say? That stars always look better—tighter and tinier—in a refractor. So, they ought to look like fuzzy ping-pong balls in a small aperture fast reflector, then? Nope. At both 64x and 128x, the stars of this gorgeous cluster looked very good in Yoda. When I switched over to the refractor (I used a 16mm Uwan in the C102 to come close to the reflector’s lower power) the stars looked good, too.  They were certainly, nice, hard points, but better than in the StarBlast? Actually, no. What did I note? The contrast was markedly better in the refractor. It wasn't night and day, but there was clearly a difference. Anyway, not surprisingly, with M35 getting low neither scope showed a hint of its more distant companion cluster, NGC 2158.


Messier 3, Canes Venatici's great globular star cluster, the premier glob of springtime, has always been a finding demon for me. Do I approach from the Bootes side or the Coma side? Either way, I generally fumble around for a while before I get the big ball of stars in the field. Same old story on this night despite the 16mm Uwan's big field in Yoda. When I finally had the glob, I upped the magnification to 128x and took a look.

Not bad, not bad at all. The cluster was at least a little more than a round fuzzy spot (it was quite a ways down in the Airport Boulevard light dome, so I didn't expect too much). There was an obvious core, surrounded by a dimmer halo. However, that’s all it was:  fuzzy core surrounded by fuzzy halo. There was some graininess, but not a hint of resolution.

In Amelia at 145x, the view was admittedly more the same than different. However, there was more of that graininess, and after staring for quite some time, using averted vision, jiggling the scope, and using the other tricks I know, I convinced myself I occasionally picked up a star or two. The bottom line was that the image was better in the refractor than in the reflector. And it was considerably better than the slightly higher power of the refractor would account for. M3 was our last object, with the sky suddenly closing down with a resounding thud.

Inside, after taking maybe ten minutes to put both scopes put to bed, I pondered. The beauty of Yoda is that he is so portable. Even when I have to use his little stand. Heck, he’s more portable than the C102 even when he is on the AZ-4. He’s lighter, and his short little tube makes it easier to maneuver the rig through doorways.

That’s the extent of his advantage in the suburbs, unfortunately. Out in dark country, the StarBlast can provide striking wide-field views (though not as good as those of the 4-inch Explore Scientific in my opinion), but wide-field performance is not a factor in judging a scope for use in my backyard.

So why is this? Is it because of optical quality? No, not really. The C102's objective throws up a good star test, but the StarBlast’s parabolic primary isn't bad, either. Touch of turned-down edge, perhaps, but only a touch. The bottom line is that the f/10 focal ratio of the C102 is just more versatile. 

With a 4-inch objective at f/10, I can still get to powers as low as I need in my suburban sky. And it is easy to reach higher magnifications without resorting to high power Barlows and uncomfortably short focal length eyepiece. In truth, that's the main fault of the StarBlast: it’s just too difficult to get it above 150x. It was not just that is was easier to get higher magnifications with the refractor, though. Amelia took higher power more willingly. At 200x, the StarBlast was pushing hard; at the same magnification, the refractor was just getting started.

Despite these facts, you might enjoy a StarBlast. I have, for years. Especially if you can occasionally get the little guy to a dark site where the wide-field part of his personality can shine. For backyard use, however, the C102 or a similar refractor seems a better solution. These scopes are cheap and easy to get, and while the longish tube can sometimes make me say a few Bad Words, I can still have it set up in less than five minutes and be ready to roll without worrying about cool-down. It is the standard for my briefest and most informal backyard observing runs. For those not so informal backyard runs? That is a subject for next Sunday...

I share the same sentiments as you Uncle Rod. My NexStar 102 (long focal length) always wins out for backyard use. The contrast is just that much better for DSOs and like you said they take power in comparison to my Orion Skyscanner and Astroscan (both true 4 inchers lol). The Astroscan (I stuck a red dot finder on it) is the perfect knock around scope for my lil 5 year old pricess though....I have a 25mm Orion Explorer kellner permanently living in it. In fact she loves it so much, she says it is HER scope (well it is...saving it for her)
Excellent post, Rod, both informative and entertaining, as always. I am just beginning my third year of astronomy, hence a low level of expertise so I was happy to see that your take on the C102 parallels my own. When OPT had these babies up for $59 in (I think) 2013, it seemed that everyone on CN was buying one, me included. I mounted mine on a Porta II and it's a near perfect g&g, out the door of its sunroom base and onto the patio is 10 seconds. It can shudder for a few seconds on the P-II due, I think, more to its length than its weight, but that's the only negative, and a trivial one at that. The optics are just superb, it takes, as per your report, magnification well and with a 32mm WA ep, serves up solid wide field views. Every image is scalpel-sharp. Its versatility has made the C102 my default 'scope. I wss going to say that every observer should own one. But then I realized that by this time everyone probably does.
Anyhow, thanks for yet one more excellent post. Your blog is a terrific resource and motivator.

For me its a SLT (upgraded software) and C5. Setup and running one trip carrying all in less than 5 minutes and with/without FR gives me as wide as I need plus good up to 300X. Even my 8" is too much to set up now unless I want to image at home. My C5 is an Orange one from the early 70's and has incredible optics.
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