Is 1/4 wave good enough for a rich field?

I once owned a 6" f/4 that had a terrible mirror made by a former famous
maker whose name (starts with a C, ends with an E) shall remain nameless. I
couldn't get a "round" star with it to save my life.

I'd say if the one your interested in is a decent, smooth 1/4 wave mirror,
you'll have no problem getting decent images.

Howard Lester

"Howard Lester" wrote in message
...
I once owned a 6" f/4 that had a terrible mirror made by a former famous
maker whose name (starts with a C, ends with an E) shall remain nameless.
I
couldn't get a "round" star with it to save my life.

Could that have been some kind of weird collimation problem, such as
non-concentric primary and secondary, so that when it looked collimated, it
was actually crooked?

I'd say if the one you're interested in is a decent, smooth 1/4 wave
mirror,
you'll have no problem getting decent images.

Agreed!

"Michael A. Covington"
wrote in :

I'd say if the one you're interested in is a decent, smooth 1/4 wave
mirror,
you'll have no problem getting decent images.

Agreed!

Thanks for all the replies. I looked at the ad again, and it's an f/5
mirror, not f/4. The correction is listed as "1/4 wave lambda" which means
???.

The mirror is less than $25, and I have decided to go ahead. The ocular
will be a Meade 25mm Plossl. The only expected usage will be low power
views of the Milky Way and bright deep sky objects.

I agree with the "less than 7 mm exit pupil" argument. I have astigmatism
anyway, and have never been able to get much satisfaction from a 7 mm exit
pupil. 6 or maybe even 5 seems to work much better for me.

Hey, even if the mirror stinks, at least I will have built a telescope that
needs only a decent mirror to be successful. Having a mirror gets one
moving on the mount, tube and peripherals, right? Since winter observing
opportunities are very rare here, it will be a pleasant diversion during
the winter, and a real reason to get outside once spring and clearer skies
arrive.

donutbandit wrote:


The mirror is less than $25, and I have decided to go ahead. The ocular
will be a Meade 25mm Plossl. The only expected usage will be low power
views of the Milky Way and bright deep sky objects.


Hey, even if the mirror stinks, at least I will have built a telescope that
needs only a decent mirror to be successful. Having a mirror gets one
moving on the mount, tube and peripherals, right? Since winter observing
opportunities are very rare here, it will be a pleasant diversion during
the winter, and a real reason to get outside once spring and clearer skies
arrive.


If the mirror stinks, you wash it in Drano, make a pitch lap and fix
it for next to nothing. And then you can have a 1/10th wave mirror.
The beauty fixing the mirror is that 99% of the work has already been
done for you (usually).

Good luck.
Etok


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Etok wrote in -
news.com:

If the mirror stinks, you wash it in Drano, make a pitch lap and fix
it for next to nothing. And then you can have a 1/10th wave mirror.
The beauty fixing the mirror is that 99% of the work has already been
done for you (usually).

I thought about that, but then it would have to be realuminized.

I thought about that, but then it would have to be realuminized.


This need not be too expensive.

http://home.covad.net/~alcoat/al_1.htm

jon

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.
----------------
Yes--you can use a very poor mirror acceptably at only 40x. However, you
will spend as much time getting it built and improved over time as you will
with a much better mirror that will allow 200-300x on planets, thus be
useful for more types of observing. The larger problem with an f/4 is that
no matter what kind of eyepiece you buy you will be observing with
horrendous (70%) coma, which is made even worse with the typical, cheap,
wide angle, low power, astigmatic eyepiece. I've been there and done that,
so I am not just putting the project down. You can always start simple and
improve as you go. But if all possible try to start with a better mirror
that will allow for later improvements in the optical/mechanical chain.
Alternatively, you might consider the 6" f/5 ratio is much easier to deal
with straight out. I like and use mine a lot and I have several larger
scopes. With a 35mm Pan I still get a 2.75 degree wide field at like 24X,
and an old favorite Nagler 20mm provides an absolutely fantastic tack sharp
42x. But that Spooner mirror is likely at least 1/8 wave. The upside is
that little scope will do nice planets at 300X.
Best wishes,
Bob Cuberly

If the mirror has a smooth surface, it will perform well even with a 1/4
wave error on the mirror. In other words if the mirror departs from the
ideal parabolic shape by 1/4 wave but is free of serious zones , turned
edge, etc., it will probably perform quite well, although not to the
theoretical diffraction limit. A case in point are virtually all
imported 4.5 inch F/8 newtonians. At f/8 their spherical mirrors depart
from the ideal parabolic shape by over 1/3 wave at the surface!! Yet I
can testify personally that many of them perform surprisingly well, with
resolutions of about 1.5 arc sec with very good planetary contrast.
Years ago Edmund Scientific advertised all of their mirrors as
accurate to 1/4 wave or better. Apparently they believed that 1/4 wave
was good enough for most amatuers at that time. However remember that
claiming that a mirror is 1/8 ,1/10 or 1/20 wave or "diffraction
limited". does not mean that it necessarily is. In many cases surface
smoothness and freedom from zones and turned edge is more important than
the departure from the ideal parabola. The vast majority of mirrors out
there probably fall in this catagory. Mirrors that do not depart from
the ideal parabola by 1/8 wave are probably rare.
-Karl-

Bill Becker wrote:

"donutbandit" wrote in message
...
I ran into a great deal on a 6" f/4 parabolic mirror. All the stats on it
look good, except that it is a 1/4 wave mirror.

I was raised believing that no less than 1/8 wave will suffice, but how
about a low power rich field scope? The magnification would never be
greater than 40.

I don't know but....if the total correction for the "system" was 1/4 wave,
that wouldn't be too bad at all but...
for the primary itself to be 1/4 wave, it doesn't sound very promising.

Best regards,
Bill

Why use the Drano? It can etch the glass and is not needed. The Al coating
is so thin it will polish off with your first bit of work.

Clear Skies

Chuck Taylor
Do you observe the moon?
Try the Lunar Observing Group
http://groups.yahoo.com/group/lunar-observing/

"donutbandit" wrote in message
...
Etok wrote in -
news.com:

If the mirror stinks, you wash it in Drano, make a pitch lap and fix
it for next to nothing. And then you can have a 1/10th wave mirror.
The beauty fixing the mirror is that 99% of the work has already been
done for you (usually).

I thought about that, but then it would have to be realuminized.