f/5 or f/8 newtonian?

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks.

In article ,
(Patrick) wrote:

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks

How was that 1/10 wave measured? Is it a smooth mirror? Remember, the
1/10 wave is a claim, it's highly unlikey that the mirror you will
receive was actually measured. If you want the details on testing, visit
http://www.rfroyce.com/

The f/8 will more likely be the better mirror. The amount of coma at f/8
will be below your ability to see it. Collimation will be much easier.
You will be giving up some true field, but still have a respectable
amount.

--
Jeff Morgan
email: eliminate the number and substitute mindspring for nospam

I don't know the quality of the Antares scope. I have used a Telescopics
6"f/5 Dob for many years and find it is a great scope, owing to its wide
maximum field of view, a big advantage, and its ability to take high
powers for the moon and planets. It is a very compact and easy to use
scope, another big advantage. Owing to the large (almost 3 degree)
maximum field with a low power eyepiece,, it is comparatively easy to
find things, ar huge advantage. I consider this focal ratio to be a lot
better than f/8 in the 6" size. I believe that the main reason the f/8
became a standard for 6" Newtonians is that it is a lot easier to for an
amateur making his first scope, to grind, figure and polish than a shorter
focal ratio would be.
I got a good mirror in the scope I got from Telescopics, now
unfortunately long defunct, and then I had it refigured to near perfection
by Richard Wessling, of the Pines Optical Shop in Milford Ohio, which has
proved to be a very good investment.. If you find you don't like the
Antares mirror quality, you can always have it refigured to superb quality
by someone very good, such as Wessling or R.F. Royce.
My view is a minority view, however. Most people accept the 6"f/8
standard as being preferable. I feel this is largely tradition but they
like the larger coma-free area in the (smaller maximum) field of view an
f/8 provides.
Hope this helps,
Bill Meyers

Patrick wrote:

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks.

You will be giving up some true field, but still have a respectable
amount.

Only if you use the same eyepieces. Use longer FL's and still get a good
field.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

In article ,
lonk (Bill Foley) wrote:

You will be giving up some true field, but still have a respectable
amount.

Only if you use the same eyepieces. Use longer FL's and still get a good
field.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)


My point was that for any given eyepiece, you will have somewhat less
true field in the instrument with longer focal length. This is a basic
fact of optics.

Yes, you can get more field in the 1200 mm scope with a longer fl
eyepiece, but that same eyepiece used in the 750 mm scope will have a
larger true field.

Since the original post was asking about low-cost Newts, I doubt they
are in the market for a highly corrected eyepiece like a 41 Panoptic. To
get that wide field, they more likely will use cheaper designs like
Erfles. These will give a much cleaner image at f/8 than f/5.

In any event, if max true field is what you're after, go with a TeleVue
Genesis.

--
Jeff Morgan
email: eliminate the number and substitute mindspring for nospam

(Patrick) wrote in message . com...

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks.

Can't guess from the specs. There is no reason a 6" F/5 scope shouldn't
be every bit as good as a 6" F/8, but the only way to tell is to try it.
I wouldn't trust that 1/10th wave spec farther than I can throw it,
although there's no particular reason it shouldn't be true.

The only thing you can say for sure, based purely on optical theory,
is that a 6" F/5 will exhibit more coma towards the edge of a wide
field than a 6" F/8. Also, it is very problematic to get a small
secondary into a 6" F/5, and a large secondary limits the contrast.
But all of that would pale beside the difference between a good mirror
and a bad mirror.

- Tony Flanders

On 19 Sep 2003 07:50:41 -0700, (Patrick) wrote:

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that.

My guess is that the Antares would be of Guan Sheng manufacture, and
known to produce good mirrors for a mass produced product.

If it were me, I would go for the f8 allowing quality widefield views
with much cheaper eyepieces, and a longer focal length to allow higher
magnifications for a given eyepiece.

Pat

Id go with the longer focal length scope. Its true that you can get a
very nice mirror at f5 but at f8 your chances of getting a good mirror
go way up. Plus if you do any planetary work the extra focal length is
nice for magnifications sake.

Rich


On 19 Sep 2003 07:50:41 -0700, (Patrick) wrote:

I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks.

(Patrick) wrote in message . com...
I'm looking at an Antares 6" f/5 newtonian and an Orion 6" f/8
newtonian. Both have parabolic main mirrors, but the Antares is said
to be 1/10 wave, while the Orion doesn't say anything like that. Can I
assume the Orion will give better on and off-centre sharpness due to
it's higher f/ratio? Or will the Antares, despite being much faster,
be just as sharp on and off-centre because it's 1/10 wave? Thanks.

A "1/10-wave" mirror can be anywhere from very bad to excellent. Even
though the vendor of the f/8 didn't state a "wave-rating," for a 150mm
aperture, it is much easier to end up with a good mirror at f/8,
either accidently or intentionally, than it is at f/5, so on that
basis, I would choose the f/8 if there was no more information
available.

Even if the f/5 were of higher quality than the f/8, its coma and
astigmatism would still be worse, although maybe not objectionable.

Optical quality aside, you also need to consider features such as
focuser,eyepieces, finder, stability of mount (very important,)
whether the manufacturer has made any major design flaws (i.e. stupid
mistakes,) and of course the return policy, when comparing 'scopes.

Thank you....that's pretty much what I thought. I like the idea of the
f/8 better too, but my problem is that my EQ3-2 mount isn't sturdy
enough to hold it, so I might have to get the f/5.

In the typical parabolic, 6", "1/10 wave" f/5, will coma and
astigmatism be unacceptable, or not too bad? If it's not too bad, I
won't mind.