NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/sci/astro/9488

Text File | 1992-09-07 | 5.8 KB | 125 lines

Newsgroups: sci.astro
Path: sparky!uunet!gatech!destroyer!caen!spool.mu.edu!agate!linus!linus.mitre.org!mandolin!chaloux
From: [e-mail] (Dave Chaloux)
Subject: Telescope wars
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Reply-To: [e-mail] (Dave Chaloux)
Organization: The MITRE Corporation, McLean, Va
Date: Fri, 4 Sep 1992 21:35:51 GMT
Lines: 112

Ah, yes: the semi-regular telescope war has broken out again. We have almost as
many broken ceasefires as a certain European country(s).

We have the usual participants with SCTs, Refractors, Newtonians, and Maks all
represented.

Being unable to resist taking up the challenge, I thought I would add my two bits.

Apochromat Refractors:

These are absolutely wonderful instruments. Supposed advantages are
A) Lack of central obstruction. This leads to more of the light occurring in
the Airy Disk. This advantage is really not disputeable. It has been put forth
that a perfect reflector of size Objective - Secondary/2 is as good as the
equivalent refractor. I have no reason to doubt this. However, it should be
pointed out that the refractor achieves this with smaller aperture which is a
guaranteed advantage when it comes to seeing. Also, high quality refractors
seem to be built with more regularity than high quality reflectors. Also,
the reflector has to be perfectly collimated to achieve what is supposed to be
capable of. Every article I have ever read on collimation indicates the average
shmoes telescope is far from perfectly collimated. Some of them (SCTs come to
mind) cannot even be completely adjusted. You only have access to moving the
aim of the secondary. On the flip side, if you have a big light bucket, you can
always use an off axis mask and avoid the central obstruction.

B) Great contrast. Dark background. Frankly, my ivestigation of this
indicates this one is overblown. I will grant that images thru a 6" Refractor
are dark at 300x and that images thru a lightbucket at 300x are not. Stop
the lightbucket to 6" and guess what. Images are dark.

C) Pinpoint images. I was looking thru a Televue Genesis just yesterday with a
9mm Naglar. Result, images were pinpoint. However magnification is only about
56x or 14x/inch. To achieve 50x/inch you would have to go to a 2.54 mm eyepiece.
When you get these scopes up at high magnification the images nice (stars have
there nice little Airy disks with diffraction rings.) So what. So does a good
lightbucket with a 6" off axis mask.

D) Large flat fields for photography. I can't really argue with this one. They
are great for Medium format. On the other hand, do you have the scratch to
do Medium format. (Quite possibly if you can afford the scope.)

Downside: Cost per inch of aperture is a killer.


Conclusion, refractors are great scopes for double stars, planets, moon, not
so deep, deep sky. It is true you can do planets with a big Dob stopped down
and get just as good a results. On the otherhand, you don't need a step ladder
for the refractor, it probably has a clock drive, and you can do photography.

SCTs:

Supposed advantages:

A) Good photography. Good yes. Great no. To much field curvature. To get
fully illuminated large fields takes big central obstructions that diminish the
usefulness as a visual instrument. Still, most of the pictures in the Mags by
amateurs are taken with SCTs. But if you look at the really exceptional photos,
they were normally taken with something else.

B) Good portability. True. Downside is the shakes. Compared to a well mounted
dobsonian these telescopes are on the move. I mean, I sometimes wonder how I
manage to get mine to focus. The fact I can proves just how wonderful the
human body, mind, etc. is made.

C) Good visually. Lets face it. A lot can be seen with a 8" SCT. A lot more
since it can be picked up and moved to a dark sight with relatively little pain.
1000+ lists over 1000 objects that can be seen in one. This isn't so shabby.

Downside: A good newtonian is much cheaper per aperture, can be made with a
smaller diagonal relative to the aperture, and can be made as portable if it
is not equatorially mounted. The dobsonian wont do photography however.

Newtonians:

Supposed Advantages:

A) Cheap per unit aperture. TRUE. This makes them great deepsky scopes.
B) Can be great planetary scopes in longer focal lengths or with off axis
masks. True, but then you need to make it yourself. You can purchase the big
light bucket and use an off axis but then you have a very unwieldy instrument
for the affective aperture. Furthermore, without an equatorial platform you
don't have a clock drive either. Yeah, I know you can track with some of these
beasts at 600x but then try showing your 9 year old what you just looked at.
Try drawing at the same time.
C) Can be very steadily mounted if a dobsonian. Mine is like the rock of
gibralter.
D) Relatively easy to modify and customize.

Disadvantages:

Large ones on equatorial mounts are huge and unwieldy, especially if the
mount is trully steady. Dobsonians suffer less from this but then you do
without the clock drive.

Fast ones suffer from coma. At F5 this is only noticable with really high
quality eyepieces or else astigmatism from the eyepiece dominates. On the
other hand astimagtism in the eyepiece is made worse by the sharply converging
light cone. Nothing a Nagler wont solve (and with the money you save buying the
Newtonian, you can get yourself a decent eyepiece or two.)

Personally, I own a Dobsonian and a SCT. I would love to get a 6" or 7"
apochromat Refractor some day as well. Decide what kind of observing you want
to do and then buy or make the telescope to suit.

By the way, I know that a number of the combatants in this thread are active
observers or photographers. That is what counts. They obviously have found
telescopes suited to what they want to do.

Clear skies.