OR: Visiting some old friends, but now armed with more aperture (umm, long)
From: Axel (ritesh_laud_at_hotmail.com)
Date: 07/15/04
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Date: 14 Jul 2004 21:36:56 -0700
Date/Time: 10 July 2004, 9:30 PM to about 4:00 AM CST
Location: near Eagle Lake, TX
Telescopes: 14.5" f/5.4 Newtonian reflector, 90 mm apochromatic
refractor
Seeing: 7/10 to 8/10
Transparency: Mag 6, barely, as confirmed at zenith
Having been to my usual dark sky site near Eagle Lake many times with
my 8" Dob, I was greatly anticipating the new 14.5" Dob's performance
on deep sky objects. The few showpiece objects I'd attempted from
central Houston over the last two weeks, frankly, didn't impress.
Randy and I arrived at the site soon after sunset and began setting up
for the session, taking a break in between to watch the International
Space Station cruise across the sky, brighter than Jupiter, at a speed
suitable only for naked eye or binocular observing. Through my
handheld 10x42 bino, it was merely a bright non-circular blob, shaking
far too much to have a hope of resolving anything. It took only a few
minutes to cross from the southwest to the northeast, seeming to start
off very quickly and slowing down as it faded into the distance. This
was my first sighting of the ISS; it was much brighter than I'd
expected. It would pay a visit again during the session.
Randy had his apo set up in virtually seconds and took a look at
Jupiter, which was perhaps at 25° elevation. He said the view wasn't
too hot. It took me a while to get ready, mostly because I'm still
getting the routine down for putting this tall truss scope together.
Once I get the hang of it, I'll probably get it down to less than 15
minutes.
------------------------------------------------
Scope Setup Notes, DSCs
The most awkward part of the setup is the light shroud, which is
pulled on like a tight sock over the secondary cage and poles: slowly,
one side at a time. It tends to snag on the focuser, finder base(s),
and cage bolts, so it's pretty much impossible to pull it on quickly
without the risk of tearing it. The shroud isn't strictly necessary
when there aren't local light sources, but it serves the secondary
purpose of shielding the optics from anything blowing sideways into
the scope. An important accessory, but it takes a good few minutes to
get it on right.
The other kind of scary bit is attaching the secondary cage. It's not
a big deal when someone's there to help, but if observing solo one
must rest the cage on the tops of the truss poles before being able to
align and tighten the bolts. While it's resting there, it doesn't
take much imagination to see it falling off to one side, with a truss
pole going up the middle and banging right smack into the face of the
secondary. It's essential to ensure that the bottoms of the trusses
are firmly bolted onto the mirror box, so that the frame forms a
square structure that the cage will rest on without slop. Still, it
makes me nervous.
I was impressed that the collimation was nearly dead-on after the
hour-long trip, the last bit over a rough dirt road. It was much
better in this respect than my 8", whose Protostar spider did not hold
the secondary in place nearly as well between sessions, though far
better than other scopes I've used.
The next step was to align the Argo Navis DSCs, which I'd used for the
first time during an urban session last week and was highly impressed
with. For a Dob sans EQ platform, this is a three-step process.
First, the scope must be positioned in altitude to some mechanically
reproducible angle. For most Dobs, this is straight up or nearly so,
because the base or rocker box includes a board that acts as a stop to
prevent the scope from going through vertical and flopping over
backwards. This angle, 90° or thereabouts, is entered into the
controller through a menu option. My scope's altitude travel stops at
about 92° I figure; next time I'll actually try entering that value.
Tonight we pulled the scope back from vertical just a tad and told the
controller it was at 90°; it seemed to work pretty darn well, getting
well within 0.5° of objects for most of the session.
Then it's two alignments on bright stars out of a moderate database of
35 or so, and it's done, the DSCs are aligned. The process takes
perhaps two or three minutes. I went through this a couple times and
wondered why the readout for the altitude was almost completely
unresponsive to movement. Oops, I hadn't tightened the nylon screw
for the altitude encoder.
The DSCs are an extremely handy option to have. When I used the scope
in the city last week, with only a measly few bright stars and a
Telrad for getting around with, the DSCs were invaluable for finding
deep sky objects, and in fact, Uranus and Neptune as well. Tonight,
the DSCs saved the session because the Telrad was dewed up and useless
by about 11:30 PM. I'd forgotten its dew shield and have not yet
purchased any heating system except for the secondary, so at that
point we were completely dependent on the DSCs to get around with.
Even before the Telrad dewed up, the DSCs were very handy. There's
such a gross difference between the Telrad's limiting magnitude and
that of the scope, that the stars visible in the two are utterly
unrelatable, with the exception of perhaps the brightest stars. This
combined with the scope's maximum field of view of about 1.3° made it
painful to star hop the way I'm used to doing with the 8": Get to
somewhere within five or ten degrees of the target, then meander my
way there through the eyepiece using the MSA as a guide, stopping at
many sights along the way (mostly doubles), with frequent glances up
into the finderscope to make sure that I'm following those critical
Mag 8-9 guide stars.
This style of navigation is all but impossible with the Telrad, since
those guide stars are invisible. Through the Telrad, there will be
perhaps a handful of stars visible to navigate by. Through the 14.5"
Dob, any star brighter than about Mag 8 looked bright enough to make
me wonder if that was the star I saw through the Telrad or not. Or
was that the one a half degree over? Or how about this one over here?
And then only when I happen to stumble upon a blinding first to third
magnitude beacon, does my position in the sky instantly become clear.
I don't wish to review the Argo Navis until I've explored its
capabilities more thoroughly. Suffice it to say that it (or any good
DSC package, for that matter) greatly expands a Dob's utility for
various situations. In effect, it has the intelligence and
capabilities of a GOTO controller with several notable advantages: no
noise, less power consumption, no dependence on motors, ability to
slew at whatever speed one wishes to. To me, the Argo Navis is easily
worth the $750 or thereabouts that it goes for. One feature of the
product that came in handy a couple times during the session was the
ability to identify an object that the user stumbles on in the
eyepiece.
---------------------------------------------
Observing Notes
Jupiter
With Jupiter at about 15° elevation, I wasn't expecting much. Indeed,
the planet was a shimmering mess, very bright but showing next to
nothing. The two equatorial belts and the prominent hollow in the SEB
that cradles the GRS were the only features I noticed. I didn't spot
the GRS itself, despite it being close to transiting.
M51, Whirlpool Galaxy
This was a surprising one. Frankly, I was expecting to be blown away,
easily identifying spiral structure and spotting the apparent bridge
to the companion galaxy NGC 5195. At 90x, all I saw at first was the
two bright cores of the galaxies, each surrounded by patchy
nebulosity. With averted vision, I was able to tease out a couple
spiral arms and dark lanes. I did not see the bridge, though Randy
caught a hint of it. Higher power (166x) didn't seem to help; in
fact, the view seemed a bit better at 64x.
My notes for a session last summer at Eagle Lake with the 8" describe
essentially the same features. The only explanation I have for this
is that the sky at Eagle Lake must have been significantly better that
night. Randy and I did remark that the skyglow seemed unusually
strong this night; the sky just didn't seem "dark", whether due to
humidity, encroaching light pollution from Houston, or what have you.
However, I did confirm that I could barely see a Mag 6 star directly
overhead. M51 was at perhaps 45° or 50° elevation, too high to make
atmospheric extinction a big factor. Perhaps we were looking through
a patch of haze when looking at M51. Whatever the explanation, I'm
sure that it's a knockout in truly dark skies.
M13, Great Hercules Cluster
M13 was nearly directly overhead. Holy smokes! A huge difference
over the 8", not even comparable. Best view at 284x, where it was
still amazingly bright and seemingly fully resolved to the core. I
won't bother describing it because I couldn't concentrate on the
structure, the view was so overwhelming.
M92
Another superb GC. My, I've never seen M92 like this. It's *much*
better than M13 through the 8". Outstanding view at 284x. More
condensed than M13.
Struve 2947 (number from RA-ordered version)
This is a close double and rather faint. I think the separation must
be closer than the 1.8" given in the Struve Catalog; I estimate it at
a little over 1". Using 166x, it was split without much fuss.
M11, Wild Duck Cluster
We stumbled on this one after deciding to point the Telrad at the
"fuzzy star". There were so many stars in the field that I didn't
recognize the famous Wild Duck Cluster, even though it only occupied
the central half of the field and was framed fairly well. We used the
Argo Navis's Identify feature to clue us in.
A stunning sight all the way from 64x out to 284x. We compared views
between the 90 mm APO and the Dob. The APO required averted vision to
glimpse the many faint members; this open cluster requires aperture to
really shine. The view through the 14.5" was far superior to that
through the 8". In the 8", most of the members were a bit fuzzy due
to their faintness and the central red star was the only hard point.
In the 14.5", nearly *all* the members are hard bright points and the
central star is brilliant. What a difference.
M22
Huge nearby globular cluster. At 284x, it was so well resolved that
it looked more like an open cluster than a globular!
M7
Even at 64x, this open cluster is too large and coarse for the 14.5"
Dob. It's much more suited to smaller scopes.
NGC 6453
This is a very dim and small globular cluster on the western edge of
M7. It was unresolved at all powers, but grainy with averted vision.
It must be at least 50,000 light years away, I reckon.
NGC 6444
A faint open cluster west of M7. The members are about Mag 12.
Bolide
At this time, just past midnight, Randy happened to be looking west
and spotted a bright bolide breaking up into several pieces, glowing
green. The breakup happened just off the Big Dipper's handle.
M57, Ring Nebula
Very nice! This planetary appeared as a smoke ring filled with fairly
bright nebulosity. At 398x, the central star popped in and out of
view. We simply couldn't see it at lower powers; the central
nebulosity was too overpowering.
M56
A small fairly dim globular cluster in Lyra. Well resolved at 284x
but not very spectacular after the showpiece GCs we'd already
observed. We noted that the brightest star in the field has a
companion.
M8, Lagoon Nebula
This large object was quite a sight, but with the OIII filter it
really blossomed. Lots of structure was visible at 64x. Without the
filter, the "arm" of the nebula extending through the open cluster and
down around to the north isn't really evident. The dark lagoon itself
is sculpted and looks much like a channel between two land masses.
The nebula ends abruptly towards the east, even though long exposure
images show it extending for quite a ways beyond.
M20, Trifid Nebula
Not as large or as detailed as M8, but the three major dark lanes were
apparent with averted vision and with the OIII in place.
Alpha Herculis
A stunning double at 90x, due to a nice color contrast. The companion
appears bluish green against the golden primary.
M24, Great Sagittarius Star Cloud
This object was overwhelming in the Dob. *Way* too large to fit in
the field, it was instead fun just to pan around amid the bewildering
mass of suns at various distances.
NGC 6603
This was a great find, an attractive but very distant open cluster
situated on one end of M24. The member stars are mostly in the Mag
12-13 range. Great contrast of distances with the foreground stars.
M18
This is a small and unspectacular open cluster.
M17, Swan Nebula
A bright emission nebula, spectacular with the OIII! It looks very
much like a swan: body, neck, and head. With the filter, the
nebulosity extends way out past the swan shape.
M28
A good "classic" globular cluster. Fairly condensed core, resolved
well at 398x.
M13, again
Enamoured with the far greater magnifications that the 14.5" could
handle than the 8" I was used to, we returned to M13 and put the
Nagler Zoom eyepiece in. At the 3mm setting, the power was 663x, and
amazingly, M13 was still well resolved, reasonably bright, and
completely filled the field.
NGC 7009, Saturn Nebula
This nebula did look a bit like a silhouette of Saturn. The
extensions on either side were visible with averted vision, but the
central star was not seen. To Randy, the nebula was distinctly
bluish, but to me it was colorless. Then again, even the Blue
Snowball nebula looks colorless to me.
Uranus, Neptune
We ended the session with these two distant gas giants. Uranus was a
nice crisp sizeable disk at ~200x using an 8-24 mm zoom eyepiece, pale
green with no detail visible. Neptune was smaller and bluer. Even at
64x, we both remarked that Neptune was obviously not stellar compared
to the field stars.
Epilogue
The only main thought I was left with after the session as we drove
home and discussed the experience, was that the 14.5" really is a
giant leap over the 8". On the Messier objects, particularly
globulars, it seems to have a nearly bottomless reserve of
magnification compared with the 8". It seems like the Nagler 7mm
(284x) is going to one of my most-used eyepieces with this scope.
Whereas with the 8", on DSOs my favorite power was generally 101x.
That's a huge difference.
In addition, the new scope provides a great boost in resolution.
Stars are much brighter, yet resolve to smaller pinpoints. This was
most evident in M11, which I was initially expecting to not be much
improved from the 8". But also in the globulars, where the larger
Airy discs in the 8" would cause the stars to get diffuse and faint
with high powers. With the 14.5", even at 663x, the member stars of
M13 were pretty much hard points.
In summary, I really don't see much of a point in taking the 8" out to
observe with anymore! For my situation, it's only barely more
portable, but vastly inferior in nearly all aspects of visual
performance that are important to me.
Cheers,
Ritesh
- Next message: Chris L Peterson: "Re: Cassini: where are colors and detail?"
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