NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/astro/13468

Text File | 1992-12-28 | 9.2 KB | 186 lines

Newsgroups: sci.astro
Path: sparky!uunet!spool.mu.edu!agate!linus!linus.mitre.org!mandolin!chaloux
From: [e-mail] (Dave Chaloux)
Subject: Astro Physics Refractors
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Sender: [e-mail] (News Service)
Nntp-Posting-Host: mandolin.mitre.org
Reply-To: [e-mail] (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Mon, 28 Dec 1992 17:55:07 GMT
Lines: 173

A friend of mine recently obtained a 6" F8 Astrophysics Refractor and he
brought it over to my Observatory for "First Light". He has a system 800
mount. We observed using Televue Eyepieces, specifically, a 35mm Panoptic,
a 24mm Widefield, a 9mm Nagler, a 7mm Nagler, and a 4.8 mm Nagler. We also
used an Astrophysics 2" Barlow and 2" Mirror Diagonal.

This gave me a chance to compare side by side a supposedly Top Quality
Apochromatic Refractor with my Obsession 20" F5 which is also a supposedly
premium quality telescope. I thought I would share my impressions.

I am well aware of the debate about refractors vs. reflectors, etc. and I have
previously given my view here. Basically I like both classes of telescopes.
I think they both have their place and if I have a bias it is toward top
quality instruments of any kind. Of course I also acknowledge the fact that
you could buy a fleet of around 5 Coulter 17.5" scopes for the price of the
6" refractor outfitted the way my friend had it. As some have pointed out,
fuzzy is not necessarily all bad if you are using the telescope for variable
star estimates where you often use out of focus images anyway! (Still, fuzzy
isn't for me.)

Portability:

The 6" F8 optical tube assembly was contained in a very nice padded case. The
case is not particularly compact but fit in the trunk of my friends Honda
with no problem. The mount disassembles into various pieces as well which all
fit in his car quite comfortably. There is no way on earth the 20" would have
fit despite the fact that Obsessions are very portable for big scopes. That
is the good news.

The bad news is the number of pieces involved with the mount. To maximize
portability, the mount breaks into a number of pieces. As I recall there are two
hex rings to support the Optical Tube assembly, a polar head, a separate shaft
with two counterweights attached, a pier, an assembly at the bottom of the pier
the three legs attach too, the three legs, and three rods with turnbuckles to
give the pier rigidity. I think it took about 4 trips between the car and my
observatory to move all of the pieces. There were also some cords and the
drive corrector involved as well.

Setup and takedown was about 1/2 hour. We did not bother to precisely Polar
align for a number of reasons.

1) My friend has no finder scope yet (not even a Telrad)!
2) I feel confortable starhopping even w/o a finderscope.
3) Since we weren't going to be using the setting circles, even a very
rough alignment was sufficient for the clock drive.

Would I class this as a portable instrument? Yes. However the number of
pieces involved to make it as car friendly as possible definitely increases
the setup and takedown time.

In comparison, my Obsession can be setup in slightly over 5 minutes.

Engineering:

The engineering of the refractor is simply 1st rate. The mount is extremely
solid and is a joy to use. No shakes! Everything fits together very smoothly
and precisely. Everything is quite logically laid out. Things like the set
screw in the diagonal have a wonderful feel to them. The focuser is very smooth
and has a tension adjustment. As much as I like the engineering on my
Obsession, the Astrophysics is even better. In the entire night of observing
I had only one real complaint. The velcro patch that is used to hang the
drive corrector on the pier is too small. I had to hunt for it a little bit in
the dark.

I feel compelled to point out that for the money charged and the amount of time
it takes to get the instrument (about 1 year) the engineering had better be
this good.

Optics:

Quite frankly, I have never seen better. I could find nothing to complain about
with the star test. Images inside and outside of focus were simply identical.
Not only that but the optics were obviously very smooth without the speckling
associated with most optics. Diffraction patterns in focus were also perfect. We
had the good fortune that despite the night being fairly transparent (about
6 Mag) Seeing was also good (at least at 6"). No color errors were noticed even
at 254x.

Observing:

We tried the scope out on all kinds of different targets from Mars to Double
stars to clusters and Nebula. Bottom line was images were very impressive.

Take for example the Trapezium in M42. You may recall how someone on the
net recently tested a 5" Astrophysics Starfire against a 17 1/2" Coulter.
The 5" revealed 5 stars in the trapezium while the 17.5" scope revealed only
4. I am happy to say the 6" Starfire brought out six of the stars in the
trapezium. Pretty good. It did help that I routinely see six in my 20"
Obsession so I know where to look.

Mars showed very nice detail, especially with the 4.8mm Nagler which gives
254x. I see more detail in my 20" during moments of good seeing but moments
of good seeing are harder to find with the bigger scope. Also having a clock
drive is certainly convenient. I did find that images were not bright enough
to use my 25 Red filter effectively. The 80A filter gave a better view.

Views of the Double Cluster and Pleiades were simply ideal. A 6" F8 using the
35mm Panoptic gives a magnification of about 35x and a field of view of almost
two degrees. The big field of view together with enough aperture to bring in
fainter stars was very nice. The Merope Nebula was obvious. The smoothness of
the optics, proper baffling, etc. really paid off here.

I would be remiss if I didn't mention that M43 also showed its comma shape very
clearly. It was much more obvious in this scope than in 8" SCTs I have looked
through. Direct vision was more than enough.

M35 and 2158 were a magnificent sight together in the same field. I am used to
not being able to see all of both at the same time in the 20". 2158 was not
resolved in the smaller scope.

Double stars such as Delta Gem, Castor, Zeta Ori, etc. were all very pretty,
clearly separated with nice diffraction rings.

I tried looking at Ngc 404 which is only 6' away from Mirach in Andromeda. This
small round galaxy showed with no problem.

We looked at numerous other objects as well such as M31, M32, M110, M78,
the Eskimo Nebula, and M79. Images were always very nice.

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Big versus small:

I thought I would address this question directly. In my view, every object has
an ideal aperture for it. This aperture depends on both the size of the
object and its brightness. Bigger is NOT always better. The Plieades and the
Double Cluster are good examples of this. Trust me, these objects are better
seen with 6" F8 than with 20" F5. You see more stars with the 20" but
aesthetically, the view is simply not as nice.

On the otherhand, assuming good optics, bigger gets you much fainter objects AND
better resolution. The 20" definitely does show better detail on Mars than the
6" refractor despite the fact I know the 6" Refractor had the better optics.

Bigger gives much better views of both Galaxies and Globular clusters in general.
This is because light grasp is so important for both of them. Resolving a
globular requires enough aperture to see the faint stars involved. However, you
need good enough optics to keep the stars separate instead of a glob of smeared
out fuzz.

Smaller has other advantages. 20" scopes simply do not fit in Hondas. 20"
scopes simply do not come on conventional equatorial mounts unless you have a
Universities budget. (You can put them on a Polar platform). Unless you have
an Observatory the difference in portability can mean the difference between
using a scope two or three times a year and several times a month.

Large scopes require step ladders that you can and DO fall off. Smaller scopes
do not.

Large scopes with their shorter F ratios have to be collimated very precisely
to perform well.

So if I had 5000$ to burn would I spend it on a premium large scope like an
Obsession or a premium apochromatic refractor like a 6" Starfire on a systems
800 mount? Well personally, I would get the large reflector and test the optics
to make sure they are up to snuff. I have the equipment to keep it correctly
collimated and I am fortunate enough to have an Observatory. I made my
decision that way and would not change it.

On the otherhand, I am convinced that my friend Ted who owns the Astrophysics
refractor also made the right choice for him. In fact he owned a large
reflector and found he was never using it. He ended up selling it.

I like the refractors enough that as I have said before, one of these days,
I fully intend to get one of them as well.

I have said before and I will say again that although quality costs money,
there are very real returns on that investment. I would not recommend that
a neophyte go out and spend 5000$ on any telescope. Many neophytes burn out
and find they do not enjoy the hobby like they thought they would. However,
once you know that you are hooked and can appreciate the difference that
quality offers, it is worth it.