Sunday, December 04, 2016
Issue #520: After the SCT Redux
Four years
ago, an article appeared here with the title “After the SCT.” In truth, it really didn’t have a darned thing to do with
Schmidt Cassegrain telescopes. What it was was a report on the 2012 Deep South
Regional Star Gaze. That year, the telescope I took to the event was my
hallowed 12-inch truss tube Dobsonian, Old Betsy. The fact that I had her on
the field instead of a Schmidt Cassegrain impelled one of my (shocked) fellow
star partiers to ask me worriedly whether my next book would be called After the SCT. I replied, “There ain’t no ‘after’ for [me]. I am still
Mr. SCT and I still use Schmidt Cassegrains more than any other design of
telescope.”
Yes, despite
leaving the CATs at home with the cats in 2012, it was true I still loved SCTs
best. Well, maybe that was true, or
maybe I was whistling past the graveyard. Those words were written just after I
completed the observing project of a lifetime, The Herschel Project, my quest to view all 2500 Herschel deep sky objects.
Chasing down more than two thousand galaxies, nebulae, and star clusters over
the course of a mere three years, especially given our weather and my work
schedule at the time, ensured I was sorta wrung out and ready for a change.
Oh, I
continued on for the next two years much as I had, using my SCTs to slowly,
ever so slowly, tie the ribbon on the H-project. Using my C11, Big Bertha, and my C8,
Celeste, to clean up a few objects here and there. Some that I wasn’t 100% sure
I’d identified correctly, and some that I wanted to re-image with my new color
Mallincam. That’s what I told myself I was doing anyway, but the truth, I
think, was really that I had no idea what—if anything—came next for me in (amateur)
astronomy.
I did try
to get various new observing projects off the ground. I started in on the ARPs, for example, observing them in much the same
way I had the Herschels. Some visual observing with the C11 and the 12-inch Dob
and lots of video observing with the C8 and C11. It, like every other project I
tried, however, fell flat. With a resounding thud. I was worried. Had finishing
the Herschel Project also finished amateur astronomy for me?
There it
remained until early 2015. That’s when things began to change. No, I didn’t
suddenly think up a new approach to amateur astronomy. That happened, but it
was the result of other changes. Mostly
me entering in upon a time in life when I began considering larger questions
than just “Hmm, I wonder how a C14 would do on a CGEM DX?” The big questions, questions like, “How did
I get here?” and “What is it all about?” and “What (if anything) comes after this
story is done?”
“Began to change”? Not hardly. The
astounding thing was that I went to bed one night in early 2015 (after imaging
that winter’s cool little comet) feeling much the same as always, and awakened
feeling entirely different. What concerned me now was no longer observing projects, but the
questions above and many more like them, which suddenly seemed more significant
than Arp galaxies. I also began to look with dismay at all the astro-junk I’d
accumulated over the last 25 years of my near 50 years as an amateur astronomer and began to thin the herd.
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| Big Eye of my 6-inch... |
I won’t bore
you with the answers to some (not all) of those questions I’ve come
up with so far. Every person has to wrestle with these things for themselves and
the answers you arrive at may be starkly different from those I’ve arrived at.
We’re talking astronomy here, anyway, and the point is that being preoccupied
with deeper thoughts meant I didn’t want to worry about, struggle with, or
obsess over telescopes anymore. I did still want to observe, though.
I just didn’t
want to be bothered by my telescope when
it was time to watch the skies. The C11 was out. So was the C8. I didn’t want
to spend 30-minutes setting up a scope. Heck, I didn’t want to spend 2-minutes.
In the months immediately following that 2015 winter’s night with the comet, the
yen to look at the stars often came on the spur of the moment, and our AR102 4-inch achromat or my C102 4-inch achromat on
a light SkyWatcher AZ-4 alt-azimuth mount didn’t bother me. I’d put one of the refractors on the mount and carry it
into the backyard in a single piece. No drives, no batteries, no computers, no aggravation, just me contemplating the cosmos.
Yes, I will
admit I missed the light gathering power of the SCTs at first. My solution was
to buy an inexpensive and ultra-simple 10-inch solid tube Newtonian, a GSO Dobsonian,
Zelda. She is almost as quick and painless
to set up as the refractors. Do I use her a lot? I did when I first got her,
but of late not quite so often
despite her simplicity. This past year, my refractors have been in the
backyard more often than the big 10-inch.
What
changed? I began concentrating on what I could
see with my "small" refractors rather than what I couldn't. When I learned to do that, it
was as if the deep sky became new to me again after all these years. I won’t
tell you I was seeing more details in my favorite objects, but I was seeing a different, a new—or at
least forgotten—aspect of them and I welcomed that.
It wasn’t just
my new mindset that impelled me begin a more relaxed sort of visual observing.
It was my body. No, age didn’t suddenly
catch up with me, or at least that was only part of it. I was washing down the
front porch of good, old Chaos Manor South just after we moved out. Stupidly, I
was wearing flip-flops, slipped on the soapy front steps, and crunched my back—but
good.
I was in
pain for a while, but it seemed as if I’d dodged a bullet and just gotten
bruised up and that would be the end of it. Guess again, Skeezix. Last summer,
it became obvious that was not the
case. I did something that aggravated that back injury. I don’t know what it
was, but I was in real pain for weeks. Carrying a C8 into the yard would have
been laughable. 3-inch and 4-inch refractors became my lifelines to astronomy.
And I had so much fun with them that when my back pain suddenly subsided—almost
overnight—my inclination was to stick with them.
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| The C102... |
What is that
special something refractors are supposedly imbued with? There is one
overriding advantage to them: they are
unobstructed systems. There is no secondary mirror getting in the way of the of
the objective lens. That has several benefits.
First, the
light gathering power of the scope is not compromised by the blockage of the
secondary. You get the full effect of the aperture you’re paying for; it’s not
“4-inches minus the secondary mirror.” That helps some, but not really a lot,
since the light gathering power of the telescope is dependent on the area, not
the diameter of the mirror or objective. Still, more light is always better
than less.
A refractor will deliver more light than a reflector of the same aperture not
just because of the absence of the light blocking central obstruction, but
because a higher percentage of light is transmitted through a lens than can be
reflected by a mirror. As the coating of a mirror ages, its reflectivity
decreases and a refractor continues to pull ahead, often to the tune of 1 or
2-percent a year.
Somewhat
better light gathering power aperture for aperture is not the largest advantage
of a lens-scope, though. What is more important is the better contrast offered
by them. The lack of that central obstruction results in sharper, higher
contrast images. Not only does that help with details on Solar System objects,
stars often look better in lens-scopes.
You often
hear refractor-philes say stars are “smaller” in a refractor than in a
reflector. That is due in part to refractors often being smaller aperture,
shorter focal length instruments than the average reflector. But that is not
the whole story.
As you
probably know, stars are not perfect pinpoints in any telescope. The merciless
laws of physics make that impossible. What we see is an “Airy disk” surrounded
by diffraction rings. Here’s the thing:
thanks to the missing secondary mirror, refractors distribute less
energy into the diffraction rings and more into the Airy disk than obstructed
telescopes. Stars tend to look “smaller.” If you’re trying to split double
stars at the limit of the telescope’s resolution, the less prominent rings in a
refractor may make the difference between success and failure.
Finally, one
of the most important advantages of a refractor for me these days is one’s
thermal stability. Despite their closed tubes, refractors require considerably
less time to cool down than reflecting telescopes. The lens is less affected by
changes in/changing temperature than a mirror and is ready to deliver good
images more quickly when moved from a warm house to a cold yard. Despite our
relatively mild climate, I find my 6-inch refractor is ready to go much sooner
than my 5-inch Maksutov Cassegrain.
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| Zelda... |
None of the
above is to say reflectors don’t have advantages; they do, of course. They are
cheaper per inch of aperture, available in (far) larger apertures, and are more
portable in larger sizes. These things are true, all things being equal, but all things are rarely equal.
A 6-inch achromatic refractor is still considerably more expensive than a 6-inch (or 8-inch) reflector, but 6-inch achromats are much more affordable than they have ever been. If you just have to have 12—or 20—inches, you don’t want a refractor, true, but many of us, especially those of us focused on imaging, find we don’t need more than 6-inches of aperture to be happy. Portability? A 6-inch Newtonian is nothing; a 6-inch refractor is something of a beast. Nevertheless, I find setting up my 6-inch on a GEM to be easier than setting up my fork mounted Ultima 8 was in the old days.
A 6-inch achromatic refractor is still considerably more expensive than a 6-inch (or 8-inch) reflector, but 6-inch achromats are much more affordable than they have ever been. If you just have to have 12—or 20—inches, you don’t want a refractor, true, but many of us, especially those of us focused on imaging, find we don’t need more than 6-inches of aperture to be happy. Portability? A 6-inch Newtonian is nothing; a 6-inch refractor is something of a beast. Nevertheless, I find setting up my 6-inch on a GEM to be easier than setting up my fork mounted Ultima 8 was in the old days.
There’s also
the color question. “False” color, the purple fringing around bright objects in
some refractors. This chromatic aberration is not necessarily a deal breaker.
APO/ED refractors that show virtually no false color are, like achromats,
cheaper than ever. Also, some people are more disturbed by color than others
(as we baby-boomers age and our corneas yellow a little, we become ever more
happy with “just” achromats). Finally, most of us don’t spend much time
observing that bane of achromats, Venus; usually we are viewing deep sky
objects that show little or no false color.
The ground
truth for me is that the refractor advantages, sharp images, the portability
of smaller aperture refractors, and their relative immunity to cool-down issues,
have made these telescopes more practical for me for visual use now. Most of the time. There are still times when
I just have to drag out my trusty
10-inch Dobsonian. BUT—and I hope I am not turning into a refractor snob—when I
look through that Newtonian, something, that ineffable refractor something, is missing.
While I have
not done a lot of video imaging since the end of the Herschel Project, that
doesn’t mean I haven’t done any astrophotography. Admittedly, I laid off for a while following
that night of the comet, but when I eventually picked it up again and began doing some simple, informal deep sky imaging with my DSLRs, I actually found it relaxing. Since I was using refractors
for visual work, it seemed a natural to continue with them for photography. And was I ever glad I did.
The best
thing about refractors, for me anyway, is their imaging capability. Affordable
apochromats may be the best thing to hit amateur astrophotography since the
death of film. Let me say here that I have recently been experimenting with
imaging with inexpensive achromats, and I’ve been amazed at what they can do,
but if you want to do astrophotography, you want an ED scope.
One of the
things I'd known for a while, but had filed away in a little drawer in the
back of my mind, and only recently taken out (along with some
non-astronomy related things) and examined in detail is that deep sky imaging
is just easier with a refractor. As you know, I've taken many a shot with SCTs
over the years, but my results were never, frankly, quite as good or easy to
get as what I can achieve with my 5-inch APO refractor given my modest skills.
Sure, if I could afford an AP or Bisque or 10Micron mount I might still be slapping a big CAT on the
GEM, but I can't afford a ten-thousand dollar mount these days. Refractors are more
usable for imaging on my Ford and Chevy mounts.
OK, so what makes
a refractor better for imaging? Three things, I’ve found, speed, focusing, and weight. By “speed,” I am referring to
the telescope’s focal ratio. As you may be aware, what matters when determining
how bright an extended object (a galaxy or nebula) will be to your camera and
how long your exposure will need to be to adequately record it is the telescope’s
f-ratio. F-ratio for f-ratio, the only thing more aperture gets you on extended
objects is larger image scale. The typical SCT is at f/10, and that requires
punishingly long exposures to properly expose dimmer objects. The average
apochromatic refractor, in contrast, comes in at f/5 – f/7. Getting these smaller
focal ratios with an SCT will require a focal reducer, which can cause
illumination problems, vignetting, among other things.
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| AR102 + AVX... |
Focusing as
done in SCTs, with the usual moving mirror arrangement, is also a problem for
imagers. I don’t just mean focus shift, the subject moving in the field as you
focus, but, worse, mirror flop. As the scope tracks, changes in attitude can
cause an SCT’s mirror to move slightly, producing trailed stars if you are
using a guide scope for photographic guiding as most of us do. Some modern SCTs
have mirror locks, and some of Meade’s newer scopes have improved
focusing systems, but almost all SCTs are still saddled with the old mechanics
and resulting flop.
Finally,
mounts always track better with lighter payloads. The GEMs most of us turn to,
which tend to be in the Atlas/CGEM class and below, may track very well during
imaging with an 80 or 100mm refractor onboard, but can have real problems with 8-inch and larger long focal length SCTs.
Certainly
you can take good pictures with SCTs; I’ve taken my share over
the years. It’s just easier to do, much easier, with a short, fast refractor.
If nothing else, astrophotography is less stressful—it can even be fun—with a
refractor. I don’t ever remember a time when I was doing long exposure imaging
with an SCT that I wasn’t at least slightly stressed out by something.
And there
you have it. More than a few of you have stared in open-mouthed amazement at the news
your old Uncle has become a refractor man. Believe you me, nobody is more
surprised than moi. That’s just
another example—in a rather long series—of how wrong I was a few years ago when
I just assumed this stage of life would be without surprises.
Don’t be too
sad about this if you’re an SCT enthusiast. I haven’t completely deserted
your camp. I still have my Edge 800 and my C11 (though I keep telling myself I
will sell Bertha, the 11). None of this says all that much about the basic
worth of SCTs. They are still the most versatile scopes, period, and I will
always love them (I even have plans for a Solar System imaging project for my Edge C8). No, what my current transition to refractors speaks volumes
about is me.
Comments:
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And your last point is where your SCT will still outperform most refractors and that is solar system imaging and to some extent, observing, where their long focal length and aperture comes into play..... Dwight
Great issue, Unk! I've followed your journey the last couple years closely, because it parallels my own. I''m a bit older (70) and priorities, needs, outlook and my aching bones have evolved in ways I never anticipated. So now me and my AR 102 are happily having fun in a more relaxed, easier style. You've taught me a lot about astronomy and observing, for which I'm grateful. The trick I guess, in all of this, is separating the wheat from the chaff. I think you've got a handle on it.
v/r
Pete
v/r
Pete
Even I am "idly thinking" (see http://www.cloudynights.com/topic/556452-idly-thinking-about-downsizing-a-bit/?hl=%2Bidly+%2Bthinking#entry7535837) about replacing my TeleKit with something logistically and physically (if hopefully not too much functionally) smaller. I've been having fun with my SW 100ED and I'm not done yet, but just the same I haven't reached the point of chunking my 14.5 incher either. However a 12" or so light(er) weight Dob? Hmmm... now there's something to consider. Less than a magnitude loss on the faint end, lighter weight, smaller to transport. Only thing is dew. But I'm still idly thinking about it. Selling my TK without shipping it would be probably the major challenge.
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