Sunday, April 24, 2016

 

Issue #490: The Refractor Way Part 5: Is One for You?


As you have probably noticed if you are a regular here, over the last year I’ve been revamping my telescope lineup (amongst other things). I don’t just mean I’ve been reducing scope head-count, though I have been doing that. I’ve also been developing a radically different take on telescope aperture and, especially, telescope design.

To recap the past year’s minus column, almost unbelievably my much-loved 1994 12.5-inch Dobsonian, Old Betsy, was sold. Also out the door went three freaking C8s including my 1995 Ultima 8, Celeste, who’d been to more star parties with me than even Betsy, I believe. My old-time classic Criterion RV-6 Newtonian also found a new home. Finally, my C11 may go as well.

The pluses, the newcomers? First there was Zelda, a 10-inch Zhumell Dobsonian. After her came Hermione Granger, a beautiful and bewitching SkyWatcher Pro 120ED APO refractor. Finally, there was Big Ethel, a 6-inch achromat of somewhat uncertain parentage (though there’s likely some JOC in her heritage; she looks an awful lot like the old Meade AR refractors).

What was the why and wherefore of all these changes? In Betsy’s case it was guilt. Guilt that she was sitting in my shop (a.k.a. “The Batcave”) unused week after week and month after month. The ground truth was that even though my friend Pat had done a lot to lighten up the old-style truss tube 12.5-inch telescope, she was still more of a handful than I wanted to handle, even for star parties and other special observing runs.

The C8s? I was holding onto three 8-inch Schmidt Cassegrain OTAs because…well…because I could. Honestly, while I’d had the Ultima 8 OTA, Celeste, out a time or two since I bought my Edge 800 C8 in 2013, it was only a time or two, and the other pair of 8s had not been used at all. Why would I? The Edge 800 (“Mrs. Emma Peel”) is the best C8 I have ever owned or used.

Now for the incoming telescopes. My acquisition of Zelda, a GSO 10-inch Dobbie, is easy enough to explain. I like to have a large—well large for moi—scope in the inventory. 95% of the things I want to see/like to look at are just fine in a 4 – 8-inch instrument. But there’s that remaining 5%, which is comprised of dimmer stuff. And sometimes I also want a little more horsepower on the bright objects, horsepower provided by 10 – 12-inches of aperture, which will make Messiers “spectacular.”

“Well, Rod. You’ve got a C11.” That I do, and I tried to make myself start using Big Bertha more than I have over the last four years. I removed her from her old GPS fork mount and put her on a Celestron CGEM. That did help encourage me to get her out a little more, since I no longer had to lift 66-pounds onto a tripod. But only a little more. The switch to the CGEM just didn’t help enough.

Setting up the 11 and the CGEM is still enough work that I rarely undertake it. Since the end of my observing program of a lifetime, The Herschel 2500 Project, which was mostly undertaken with Bertha, she has, like Betsy, sat unused. I haven’t yet worked up the nerve to sell the carbon fiber C11 OTA, but it’s become evident I am probably not going to use her much anymore.

Enter Zelda. As I mentioned last time, 10-inch Chinese Dobsonians are both affordable and manageable. For 500 bucks delivered, I got not just a scope with surprisingly—maybe even amazingly—good optics, I got a couple of usable eyepieces, a cooling fan, a laser collimator, and more. Best of all, I don’t mind setting Zelda up in the backyard on any but my laziest evenings.

Now the hard part. Has your old uncle gone from being Mr. SCT to Mr. Refractor? Let’s get one thing straight:  I’ve actually used refractors for a long time, including a pair of spectacular William Optics APOs, an 80mm fluorite job and a 66mm “SD” (ED) baby. There’s also been a 4-inch f/10 achromat, a 4-inch f/6.5 achromat, an 80mm f/11 achromat, and, the ancestor of all of them, the Short Tube 80 who came to live with me in 1999.

But why do I seem to be emphasizing refractors now? Simple, boys and girls: they are just so easy. My 11-pound 120mm APO, for example, is wonderful on the CGEM when I want to do serious imaging. She is fine on the Celestron VX when I am not quite so serious. And she is usable on my uber-portable SkyWatcher AZ-4 alt-azimuth mounting when I am not serious at all, just wanting a quick peek at Jupiter. The light weight of the scope is just the lagniappe on top of her other refractor advantages, like very little cool-down required and no need to even check collimation.

There is still more to it. Not only are the visual looks I get through the 120 very much on a par with what I see in the Edge 800, they have that almost indefinable refractor je ne sais quoi. Yes, some of that “refractor images are so sharp; their stars are so tiny” business is due to their (usually) shorter focal lengths when compared to SCTs.  But, in my opinion, refractor images really are sharper. There’s also more contrast compared to an obstructed scope, and refractors’ thermal characteristics really allow them to capitalize on their advantages.

Even more important to me than the visual advantages offered by a refractor, however, are the imaging characteristics of the lens scopes. That’s a big deal for me now, since I am on an astrophotography jag again—hell, I even have an AstroBin account now. There’s no focus shift or mirror flop to worry about and so no need for add-ons or work-arounds to exorcise those SCT gremlins. There’s also the usual short focal length and wide-field character of APOs to consider. In addition to allowing wider imaging vistas, if that is your bag, the smaller amount of millimeters of the average APO makes guiding mucho easier.

Are the images produced by a refractor better, though? Better than those I can get out of my Edge 800? Optically speaking, probably not that much. Deep sky imaging is pretty forgiving of optics anyway. However, the pictures I can turn out with the 120ED are nevertheless better than the ones I can do with the Edge. That’s because of the inherent ease of focusing and guiding the refractor. A refractor is just a dream for me to use for imaging after coming off 35 years of using SCTs for deep sky work.

So, here is the bottom line: I ain’t that old, but I am well into my spring semester, and I expect the ease of use and portability of the refractors (and the 10-inch Dobsonian) means they are likely the telescopes that will see me into Final Exam Week, if you know what I mean. In the amateur astronomy game never say never, but I simply cannot see myself acquiring larger/more difficult telescopes.

Does this mean I am against SCTs now? No, not at all, not hardly. For many, many of you, especially novices, an SCT is still the best telescope. Can’t help but be. While one is not the best scope at anything, one is good at almost anything. And if you don’t know which area of observing interests you most yet, a Schmidt CAT is definitely the telescope for you. Even if it is not always the best telescope for you, it may be the best telescope for you for a long time. I sure had a good long run with the CATs, 37 freaking years if I date the beginning of my transition to lens scopes to about 2013.

Just because a refractor is the telescope most useful for me doesn’t mean it is the most useful telescope for you, however. Let’s see if one is…

A Refractor May be for you If:

You are interested in wide field viewing and imaging. Sure, you can use an f/6.3 focal reducer on your f/10 SCT to open it up a bit, but you are never going to get the wide open spaces delivered by the average f/6 or f/7 APO. Use an eyepiece longer than about 25mm with your reduced SCT and you’ll get severe vignetting.

Certainly, if, like most of us, you live where light pollution is a factor, you won’t get to exploit this refractor strength often—the sky background will just be too bright at low power—but when you can get out to a dark site, you will be terribly impressed at what a four or five inch f/6 will show.

You are more interested in the aesthetic quality of images  than in seeing the dimmest, most difficult details. Only you can decide what is more important to you, the pinpoint stars and high contrast of a five inch refractor or the light gathering power of a 10-inch (or larger) SCT or other reflector. Or you can, like me, have the both of best worlds, and keep a low-cost Dob in reserve for those times when you want “deep” more than “pretty.”

Ease of setup is important. Yes, there comes an aperture point where refractors become difficult. That point doesn’t come until 6-inches, however. A 5-inch can provide most of the horsepower of a 6-inch, however, and can be remarkably easy to mount and awfully forgiving of the mount. And as hefty as she is, I’d still rather set up my 6-inch f/8 refractor than my C11.

"Low maintenance" is a draw for you. There might come a time when you might have to collimate some refractors. But that is certainly not a common thing in the lens scope game. And you will occasionally have to clean the outer surface of the objective lens, but only occasionally. Also, an an objective, like a camera lens, is a reasonably tough thing and easy and safe to clean compared to a first surface mirror.

You like pretty things. Yes, I think my Edge 800 is a very attractive scope, but, c’mon, there’s just something about a refractor out on an observing field pointed up at the sky in the gloaming that spells a-s-t-r-o-n-o-m-y.

You are into imaging the deep sky and are more focused on medium-size/larger objects than smaller galaxies and planetary nebulae.  This is where refractors really  pull ahead. As above, they are generally much easier to use for picture taking, particularly by beginners, than an SCT (or a Newtonian). And remember: on extended objects more aperture doesn’t get you “brighter,” it merely gets you “bigger.” The required length of an exposure depends only on the f/ratio of the telescope.

A Refractor May Not be for you If:

You are after the dimmest of the dim objects visually. There’s an old saw you will hear repeated frequently in places where hardcore visual deep sky observers gather: “aperture always wins.” There is no denying that is true. All things being equal (they seldom are), you will see more with a larger aperture scope. Chasing PGC galaxies? You want a 20-inch Dobsonian, not a 5-inch refractor.

You are a planetary imager after the highest resolution images you can get. How do you make high resolution planetary images today? You take thousands of short exposure frames in as short a period as possible. For them to be well exposed, you need plenty of light. The most efficacious way to do that is with a 10 – 14-inch SCT.

You are interested in a turn-key telescope. Something that appeals to beginners who are struggling to keep their heads above water in the murky sea of amateur astronomy gear is the modern SCT. You get a good scope on a fork mount with everything included. There are fewer ready-to-go refractor packages being offered. Usually it is a la carte.

You want to take detailed pictures of smaller objects and don’t mind suffering for your art. There comes a point where you need focal length if you are going to do high resolution shots of galaxies and planetaries, when you want a picture of M51 that fills the screen and is just popping with HII regions and curdled dust lanes. That point is where you want a C11 or M12 or C14 or M16. Which is not to say it will be easy to get good results with that much focal length, but that much focal length is definitely what you need.

Finally, you may be, as I was for many years, an astro-dilettante. If you want to take pictures of Saturn one night, spectrograms of Rigel the next, and chase the Hershel 2500 the following evening, an SCT could be your scope. In fact, I will say an SCT is your scope.

You know what? There’s actually only one way to decide if a refractor is for you: get out and use one. Join your local club if—horrors—you are not a member, and look through the refractors some of your fellow members are sure to have. Then do some long and hard thinking. If you decide on a lens scope after that, I salute you. Come on in; the water’s fine. 

Comments:
Great info!
I have thought of replacing my old Hardin optical 12 inch F5 with a C11.
It seems to be a mid point. I will miss getting the whole moon in one shot!
A big dob doesn't track as well as a C11.

Mike Boyle
 
Thanks, Rod for another good blog. I too have noted that my 12.5 Truss Dob has gained weight. I've very much enjoyed my new Sky-Watcher 100 Pro ED. Who would have predicted your could get such a fine 4" refractor for $650 (during the 2015 Christmas sale)?

John O'Hara
Oil City, PA
 
Hi Rod. I have learned a lot of SCTs (and many other astronomy topics) in your blog. And know I am very glad because it seems that I am going to learn a lot of refractors... Thanks for this interesting and delightful blog!

Saludos from Chile.




 
Even though this is about M objects, I do like NGC 1907 near M 38. It reminds me of NGC 2158 near M35. Both clusters (38 and 1907) fit in the same low power field. All these. clusters look great in big binos too and these are refractors too. My 25X100 binos blow my 6" Newt away, especially on open clusters.
 
Hi Rod, I've just picked myself off the floor and stopped laughing. I thought you were the one who coined the phrase "aperture always wins"? It's certainly in all your books I've bought and read over the years!

But, like you, I am starting to get a bit too old for lugging massive SCTs around. I will follow your further posts on this subject with much interest! Cheers, Phil.

 
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