Basic Optics question

Before I damn my (cheap) star diagonal can anyone help me on basic
optics?

It seems to me that the closer the optical advice is to the eye the
less perfect it needs to be. The imperfections in the primary
objective lens or mirror affect the light over the full light path, so
quite small aberrations can have severe effects. But the same level of
imperfection in a diagonal or secondary mirror or eyepiece has a much
shorter light path and the effects will be much less severe.

Does this mean that money spent on fancy diagonals or secondaries my
be better spent than on getting a better primary?

Eyepieces may have different optical characteristics - FOV etc - which
may make some harder to make and therefore more expensive, but money
spent on extra quality in the same type of eyepiece would be better
spent on a better primary.

Or am I wrong? (it is possible...)

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

Martin Frey wrote in message . ..


Or am I wrong? (it is possible..?)

Yes & yes.

Chris.B

(Chris.B) wrote:

Martin Frey wrote in message . ..


Or am I wrong? (it is possible..?)

Yes & yes.

Chris.B

Helpful bugger, as always ...

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

Martin Frey wrote in message . ..

Helpful bugger, as always ...


I didn't want to be rude about your faulty optical knowledge old
chap. Having written a complete & thorough "destruction" of your
patently false theories it seeemed cruel to post it. Actually, I
thought you were just practicing your trolling.

If you think it through...bending the light at 90 degrees (45 + 45)
is going to demand higher standards of quality on a diagonal
regardless of how near it is to the eye. A minimum of 1/10 wave is
required (or so it used to be said in the last century). You can get
away with much less accuracy in an objective lens. Though you wouldn't
want to. A mirror needs twice the accuracy of a lens (roughly
speaking) to remain usable since errors are effectively doubled by
reflection. Hope this helps?

I remain your most humble & devoted Servant Sir etc.

Chris.B

(Chris.B) wrote:

What about eyepieces? I'm sure they're not apochramatic - but you
don't hear APO owners whining about the colour fringing from there.

Why not? - I presumed it was because the eyepiece is so close to the
end of the optical train that the colours would not be spread enough
between eyepiece and eye for it to be noticeable.

Is this wrong?

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

Martin Frey wrote in message . ..

Is this wrong?

Yes.

Chris.B

(Chris.B) wrote:

Martin Frey wrote in message . ..

Is this wrong?

Yes.

Chris.B

Why?

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

hi....i'm a newcomer to this group so i don't really want to interupt but in
my experience it only takes 3-4 elements in an eyepiece to correct for TCA
(transverse chromatic aberration) but i guess that would make it abit
pricey....i've designed one for a directors finder (film industry type
thing) and didn't notice much TCA....i could dig up the design and post it
if anyone is interested?

Richard



"Martin Frey" wrote in message
...
(Chris.B) wrote:

Martin Frey wrote in message
. ..

Is this wrong?

Yes.

Chris.B

Why?

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47