ETX70 vs. 900mm refractor?

Being a complete amateur, I dont like spending much on,what for me is only
a minor hobby, anyway, a local shop has the Meade ETX70 at a good price
at the moment.
I currently have a Skywatcher 900mm/70mm/f12 refractor,
which TBH, I'm happy with, apart from the sheer size of it, and the
'shaky' tripod.It takes up too much room when stored is the main problem.

The ETX is much more portable, and shouldnt suffer from the wobble, so
long as I site it somewhere stable,and, yes, I know the computer control
is just a gimmick! On the downside, is that it is a 350mm refractor, but
with an f5 aperture.

So, does the smaller focal length just mean that objects will appear
further away, as with camera lenses? Or am I missing something here?
What is the advantage of a shorter focal length of 350mm over a 900mm?

And would it be an upgrade to get the shorter ETX?
Thanks
Alan.
--
To reply by e-mail, change the ' + ' to 'plus'.
http://www.dvatc.co.uk - Off-road cycling in the North Midlands.

On Sat, 05 Mar 2005 10:18:36 +0000, "A.Lee"
wrote:

So, does the smaller focal length just mean that objects will appear
further away, as with camera lenses? Or am I missing something here?
What is the advantage of a shorter focal length of 350mm over a 900mm?

Telescopes used visually are afocal systems, so focal length of a
telescope doesn't matter the same way that the focal length of a
camera lens matters. The magnification depends on the ratio of the
focal lengths of the objective and the eyepiece. You can get roughly
the same power on a 900 mm objective with a 22 mm eyepiece as on a 350
mm objective with a 9 mm eyepiece. There's not any advantage with the
shorter focal length of the ETX70 except for the fact that a smaller
telescope is handier on a table top. With refractors, one with a
longer focal length has better color correction than one with a
shorter focal length, assuming the same kind of glass in both
objectives; therefore, a shorter refractor can be at a disadvantage.

I think the 90 mm Maksutov-Cassegrainian ETX90 would be better than
the 70 mm refractor ETX70. There's no point in getting stuck on 70
mm.

A 6" newtonian would be better still, but only if you are not going
for the ultra-small telescope.

Magnification is determined by dividing the focal length of the objective by
the focal length of the eyepiece, so the same set of eyepieces will yield
smaller images in the ETX. (For example, a 10mm eyepiece will give you 90x
with the Skywatcher but only 35x with the ETX.) You could, of course,
offset this with different eyepiece selections, but you might not be as
happy with the results. Generally, smaller f.l. scopes are intended more
for wide field viewing, not critical planetary observing where higher
magnifications are necessary.




"A.Lee" wrote in message
...
Being a complete amateur, I dont like spending much on,what for me is only
a minor hobby, anyway, a local shop has the Meade ETX70 at a good price
at the moment.
I currently have a Skywatcher 900mm/70mm/f12 refractor,
which TBH, I'm happy with, apart from the sheer size of it, and the
'shaky' tripod.It takes up too much room when stored is the main problem.

The ETX is much more portable, and shouldnt suffer from the wobble, so
long as I site it somewhere stable,and, yes, I know the computer control
is just a gimmick! On the downside, is that it is a 350mm refractor, but
with an f5 aperture.

So, does the smaller focal length just mean that objects will appear
further away, as with camera lenses? Or am I missing something here?
What is the advantage of a shorter focal length of 350mm over a 900mm?

And would it be an upgrade to get the shorter ETX?
Thanks
Alan.
--
To reply by e-mail, change the ' + ' to 'plus'.
http://www.dvatc.co.uk - Off-road cycling in the North Midlands.

A.Lee wrote:

So, does the smaller focal length just mean that objects will appear
further away, as with camera lenses?

For a given eyepiece, yes. (And that also means a wider field of view.)

Like the camera lens, the "native" magnification of a telescope is
dependent on the focal length of the telescope.

But, as has been pointed out, you can simply insert a shorter focal
length eyepiece, to make objects appear closer again.

The eyepiece is an additional magnifier lens, which you bring to focus
"on" the focal plane of the telescope.

The disadvantage to the ETX70, is that it will have more false color
(violet, blue, and/or yellow halos) on bright objects, due to the
additional demands of refraction when bringing a lens to focus in a
shorter distance. The outer edge halos are not only distracting, but
those halos exist around each "point" on the surface of bright extended
(large, resolved) objects like Saturn and Jupiter. This color
"overlapping" reduces contrast. On Saturn, it won't seem quite as bad as
it will on Jupiter. Saturn is a high contrast object for the most part,
with its finer details more easily detected. Jupiter on the other hand
has a lot of fine, low contrast details, which will likely not appear
with this color smearing going on.

That said, if you aren't spending a lot of time observing planets, the
shorter focal length is less of a disadvantage, provided you compensate
for the difference, in your eyepiece collection.

/smp

William Hamblen:


William Hamblen wrote:

[clip]

Telescopes used visually are afocal systems, so focal length of a
telescope doesn't matter the same way that the focal length of a
camera lens matters.

[clip]



My understanding is that the problem of the relationship of the low focal
ratio, f/1:5.6 vs. f/1:16, or low f/x number, f/4.5 vs. f/5.6, to precision
displays of images requires more explanation.

Telescopes are cameras, and all the same principles apply to their optical
systems that apply to photographic cameras.

1.) Mirror and especially lens systems that have low f/1:x ratios and
higher optical surface curvatures, are much more difficult to make to the
high accuracy standards required for the purposes of astronomy.

2.) Higher angles of incidence at mirror and lens surfaces produce more
color abberations, and those lens systems are most difficult to design. I
suspect that the more sophisticated lens systems have multiple lenses
simply to lower the angle of refraction and each lens surface. That is an
oversimplification, probably, because there are other reasons for multiple
lens systems, e.g., lens materials and light frequencies.

The general conclusion that telescope makers have drawn since not long
after the discovery of the telescope is that long focal lengths for a given
aperture have lens systems that are easier to make to accurate
specifications, and that also result in less color abberation. All other
factors being equal, e.g., aperture, a longer focal length will produce
better quality images.

Compactness of design is a primary value for product marketing purposes.
For a given focal length, that may be folded, a larger aperture is possible
in a compact design. Larger apertures for a given F.L. yield higher f/x
numbers. The consequence, however, is that greater refractive abberations
are the result. Some scope makers offer some of their units with the same
aperture, e.g., 10 in. diam., in two models each of which has a different
f/number, e.g., f/5 or f/6. All other things being equal, and if
compactness of overall size isn't an issue, the scope with the longer F.L.
will generally produce the better images.

With two scopes of the same F.L. the scope that has the greater aperture,
or a higher f/x number, will gather more light and produce brighter images.
A brighter blurry image is next to useless when star fields are not visible
at all. Better to have the smaller aperture and get equivalent optics to
preform within a higher performance range and produce clearer images.

Accurately made quality optics make a huge difference in creating better
images. Your $800 Nikon binoculars produce better images than your $45
(Ebay) to $13 (S&H Photo) Bushnell binoculars. [Purpose is important. The
Nikon is better for seeing ship masts beyond the horizon and the Bushnell
at the construction site.]

Therefore, your statement is true:

"With refractors, one with a longer focal length has better color
correction than one with a shorter focal length, assuming the same kind of
glass in both objectives; therefore, a shorter refractor can be at a
disadvantage."

Ralph Hertle

A.Lee wrote:
Being a complete amateur, I dont like spending much on,what for me is
only
a minor hobby, anyway, a local shop has the Meade ETX70 at a good
price
at the moment.
I currently have a Skywatcher 900mm/70mm/f12 refractor,
which TBH, I'm happy with, apart from the sheer size of it, and the
'shaky' tripod.It takes up too much room when stored is the main
problem.

The ETX is much more portable, and shouldnt suffer from the wobble,
so
long as I site it somewhere stable,and, yes, I know the computer
control
is just a gimmick! On the downside, is that it is a 350mm refractor,
but
with an f5 aperture.

I would not dismiss the Autostar feature as just a gimmick. When I
"eyeball" level and north with my ETX-70AT, it's often a close enough
approximation that, after doing the two-star alignment, objects will
actually appear in the field of view when I "goto." [I would presume
that a more careful initial set up would produce a better final
alignment].

The one good thing about the ETX-70 is that it's got a wide field of
view, so that even a "messy" alignment is often good enough. It
actually was kind of fun to punch the "goto" and watch M-13 slide into
view, for example.

One thing I don't like about the ETX-70 is the focusing mechanism.
It's a tiny little knob that can be difficult to reach if you're
looking up near the zenith and if your fingers are a little pudgy.
Switching from one eyepiece to another can put the image so far out of
focus that it the stars are not even visible as fuzzy blobs. Only
after much twisting of the little knob do the stars turn into those
fuzzy blobs that you can then try to focus down to a point of light
[which, with the ETX-70, you can never quite achieve, at least not
across the entire field of view at once--that's another thing I'm not
too thrilled about].

So, does the smaller focal length just mean that objects will appear
further away, as with camera lenses? Or am I missing something here?
What is the advantage of a shorter focal length of 350mm over a
900mm?

The main advantage will be field of view. With the standard 25mm MA
lens, your field of view is supposed to be something like 1.5 or 2
degrees. The same lens in your longer refractor would probably be only
1/2 or 1/3 that size.

And would it be an upgrade to get the shorter ETX?

Depends on what you're going to do with the telescope and what
accessories you have with your Skywatcher. ETX has the Autostar, first
of all. That's a plus. The ETX-70 has motors on both axes, so it will
track objects good enough for visual purposes [at least at the
relatively low magnifications you're likely to use on the telescope].
It's also more portable. However, I would suspect that you can get
sharper views [better resolution] with your Skywatcher than you can
with the ETX-70. With the ETX-70, it seems like even when you're in
focus, things just aren't that sharp. Using a plosal instead of the MA
eyepiece makes a noticeable difference in sharpness, but even with a
plosal [an admittedly cheap plosal], the image is still not as sharp
[and has more false color] than I recall getting with my long-departed
60mm f11 refractor. In fact, in playing around in the backyard, I have
discovered that a 50mm 12-36x zoom Tasco spotting scope I own produces
sharper images than the ETX-70.

Since I don't own your Skywatcher and don't know exactly how wobbly
your current tripod is, I can't say as to if the loss in image
sharpness you'll experience on planets with the ETX-70 is offset by the
ability to get wide-field view of assorted [bright] deep sky objects
that would be much harder for you to locate using your Skywatcher.

TK