Been done to death but...

Hi. Sorry to even bring this up but...

Is there a way to know how the f ratio will impact the view? I understand a
4" refractor with an f/5 fl will be "rich field" though probably bad with
false color. An f/4 reflector will show coma. An f/9 reflector will be
flatter though the fov will be smaller. OK.

Assuming a 14" Dob. F/4 is going to be a coma problem...f/5 is better but
still.... so at what point is the view pretty darn good. f/6? f/8?
Forget the ladder thing. How can one anticipate a view from a given
aperture at various focal lengths? Is f/5.5 much better or slightly better
than f/4.9?

Can someone give a little hard guidance on this?

Doink

On Tue, 28 Dec 2004 21:41:33 -0800, Doink wrote:

Hi. Sorry to even bring this up but...

Is there a way to know how the f ratio will impact the view? I
understand a 4" refractor with an f/5 fl will be "rich field" though
probably bad with false color. An f/4 reflector will show coma. An f/9
reflector will be flatter though the fov will be smaller. OK.

Assuming a 14" Dob. F/4 is going to be a coma problem...f/5 is better
but still.... so at what point is the view pretty darn good. f/6? f/8?
Forget the ladder thing. How can one anticipate a view from a given
aperture at various focal lengths? Is f/5.5 much better or slightly
better than f/4.9?

Can someone give a little hard guidance on this?

Doink


IIRC, the same angular field of view will suffer the same degree
of coma in a classical Newtonian (hence, the desire for coma-
correctors in fast dobs.)

Achromatic Refractors (without additional correctors) probably offer
superior performance over a wide field compared to newtonians.
However, color fringing becomes an issue on brighter objects at higher
powers.

But to answer your question, there is no sharp transition from bad
view to good, based solely on f/ratio. The trick is finding out what
you can detect, and how much coma, color, or other aberration you are
willing to live with.

Cheers,
Larry G.

I'm considering buying a 14" Dob and having it made to my desired FL. I
just don't know if f/6 or f/7 is worth the hassle of longer total scope. I
don't know how to evaluate this. I've had an 8" f/4 and it was very tough
to focus and keep collimated. I have an 8" f/4.9 and it's a lot
better---much sharper. A friend just got a 12" f/5 but my ED80 always blows
away his views---seems mushy. I need to collimate better for the guy but
would it be a lot better if it was f/6????

Doink

"Larry G" wrote in message
news
On Tue, 28 Dec 2004 21:41:33 -0800, Doink wrote:

Hi. Sorry to even bring this up but...

Is there a way to know how the f ratio will impact the view? I
understand a 4" refractor with an f/5 fl will be "rich field" though
probably bad with false color. An f/4 reflector will show coma. An f/9
reflector will be flatter though the fov will be smaller. OK.

Assuming a 14" Dob. F/4 is going to be a coma problem...f/5 is better
but still.... so at what point is the view pretty darn good. f/6? f/8?
Forget the ladder thing. How can one anticipate a view from a given
aperture at various focal lengths? Is f/5.5 much better or slightly
better than f/4.9?

Can someone give a little hard guidance on this?

Doink


IIRC, the same angular field of view will suffer the same degree
of coma in a classical Newtonian (hence, the desire for coma-
correctors in fast dobs.)

Achromatic Refractors (without additional correctors) probably offer
superior performance over a wide field compared to newtonians.
However, color fringing becomes an issue on brighter objects at higher
powers.

But to answer your question, there is no sharp transition from bad
view to good, based solely on f/ratio. The trick is finding out what
you can detect, and how much coma, color, or other aberration you are
willing to live with.

Cheers,
Larry G.

"Doink" wrote in message ...
I'm considering buying a 14" Dob and having it made to my desired FL. I
just don't know if f/6 or f/7 is worth the hassle of longer total scope. I
don't know how to evaluate this. I've had an 8" f/4 and it was very tough
to focus and keep collimated. I have an 8" f/4.9 and it's a lot
better---much sharper. A friend just got a 12" f/5 but my ED80 always blows
away his views---seems mushy. I need to collimate better for the guy but
would it be a lot better if it was f/6????

the 80ED blows away a 12" ? that's hard to believe.
--
md
10" LX200GPS-SMT
ETX105
www.xs4all.nl/~martlian

"Doink" writes:

Assuming a 14" Dob. F/4 is going to be a coma problem...f/5 is better but
still.... so at what point is the view pretty darn good. f/6?
f/8?

If I recall correctly, Tom Clark wrote in Amateur Astronomy that "F/6
is just a wonderful way to bend light" or something to that
effect. The paper was titled "Newtonian 101" IIRC.

pej
--
Per Erik Jorde

I'm considering buying a 14" Dob and having it made to my desired FL. I
just don't know if f/6 or f/7 is worth the hassle of longer total scope.

I don't know how to evaluate this. I've had an 8" f/4 and it was very tough
to focus and keep collimated. I have an 8" f/4.9 and it's a lot better---much
sharper.

I think Larry is right on the money, it's a personal choice/decision.

If you are building a dream scope, your dream needs to come from experience,
you may not have that experience to know what it is you really want.

If I were in your shoes, I would be planning on a Paracorr if I didn't already
have one. A Paracorr really makes an F4 scope quite nice in the full field of
view and with an F5 scope, things are truly wonderful... In my book, no need
to go over F5 if you have a Paracorr.

A used Paracorr is about $200 or so...

So what happened to that Coulter DOB you just bought. Its 13.1 inches so a 14
incher is not going to have much on that scope when it comes to DSOs and if you
got a good mirror like you said you did then why not use it for a while or
rebuild that 85 lb OTA and 40lb base into something useable...

Get yourself a Paracorr after you add a 2 inch focuser and see how you like
F4.5..

In the end it comes down to compromises, one normally wants the biggest scope
that will fit some need. Some requirements:

1. Fit in the car without much trouble. I think my F4.06 12.5 incher is at the
limit. Here, 50lbs OTA thats 52 inches long, sort of a slightly overgrown
XT-10....

2. Can be used without a ladder....

3. If I fall off the ladder I will probably not get hurt.,,

So, the decision comes down to something like this: Do I want an 6 inch F8
scope, an 8 inch F6 scope, a 10 inch F5 scope or a 12 inch F4 scope???

A few months ago I had all 4 of those and while there is no way to make that
call but there is no doubt in my mind that one sees more with an 12 inch F4
scope than with a 10 inch F5 scope and so on down the line...

So my dream scope would probably be something around 18 inches and F4.... It
could be built so it only requires a short step at most and it would have about
the same eyepiece height as a 14 inch F5 scope but gather about 65% more
light...



A friend just got a 12" f/5 but my ED80 always blows
away his views---seems mushy. I need to collimate better for the guy but
would it be a lot better if it was f/6????

If an ED-80 is "blowing away" a 12 inch F5 then either the scope has problems
(collimation/cooldown/optics) or the seeing is horrible.

A longer focal length is not going to take care of those issues... One thing
to be careful about when dealing with larger scopes in poor seeing is that they
are much brighter so the defects are more apparent. The view in a small scope
may be more "pleasing" than in a large scope but a careful observation will
show that one actually sees more detail in the larger scope.

Jon

HAVRILIAK wrote:
Newtonian 101" IIRC.

What is IIRC, is it International ____ Red Cross
If I Recall (or Remember) Correctly :O)

Havriliak wrote:
What is IIRC

IIRC, it means "if I remember correctly"

Doink wrote:
I'm considering buying a 14" Dob and having it made to my desired FL. I
just don't know if f/6 or f/7 is worth the hassle of longer total scope. I
don't know how to evaluate this. I've had an 8" f/4 and it was very tough
to focus and keep collimated. I have an 8" f/4.9 and it's a lot
better---much sharper. A friend just got a 12" f/5 but my ED80 always blows
away his views---seems mushy. I need to collimate better for the guy but
would it be a lot better if it was f/6????

When it comes to a Dobs and EQ Newts, choose your focal length solely on
your ability and willingness to manage and use the scope.

Collimation, focal ratio, central obstruction and the need of a coma
corrector for off axis performance are all secondary to the realities of
having a scope that's the right "size" for you to use comfortably, and
often.

I'm 5' 7", and my limit is right around 1600mm focal length. As long as
I hold that figure constant, the aperture that I am looking for will
dictate the focal ratio, and I will just have to live with it.

There is a point at which "best" performance for a given aperture is a
physical burden that you are unwilling to bear.

Stephen

Thanks for all the input. Yes, the ED80 to give much more pleasing views
than the 12" and as I said, I need to work with my friend on collimation.
He isn't good at it yet. Saturn was considerably sharper though smaller in
the ED80 than his 12" Orion Dob. That too was instructional. It took him
nearly 1/2 hour to unpack and set up. Cool down was probably an issue. I
have the ED80 on a light Alt/Az and just carried it out.

About the Coulter. Yes, I do enjoy it but it's a back breaker! The tube
and base each weigh more than I want to lug and storage is a pain too. I
has given me good Dob experience and will remain in the collection. I find
that I do enjoy evaluating instruments and tuning them up after I get them.
That's part of the fun for me and I've actually managed to net about 10%
return by tinkering and selling my surplus when I can part with them! But
I like to have a goal too and my goal is a really good quality deep space
instrument. Have been reading Suiter's book on optical testing and have
leaned a lot...this bit about the actual impact of focal ratio is a little
veiled in mystery. I would think there would be some mathematical
relationship to fov/coma. Something like (again, assuming a fixed aperture)
increase of fl by 1 yields 12% flatter field or something like that...I know
brightness diminishes with increased fl and on deep space objects,
brightness is a big issue. So, f/5 is probably the right spot for a bigger
Dob and Parrcorr is probably the solution if coma is an issue---which it
shouldn't be on galaxies anyway.

So, the discussion has been of great help.

Doink


"Doink" wrote in message
...
I'm considering buying a 14" Dob and having it made to my desired FL. I
just don't know if f/6 or f/7 is worth the hassle of longer total scope.
I don't know how to evaluate this. I've had an 8" f/4 and it was very
tough to focus and keep collimated. I have an 8" f/4.9 and it's a lot
better---much sharper. A friend just got a 12" f/5 but my ED80 always
blows away his views---seems mushy. I need to collimate better for the
guy but would it be a lot better if it was f/6????

Doink

"Larry G" wrote in message
news
On Tue, 28 Dec 2004 21:41:33 -0800, Doink wrote:

Hi. Sorry to even bring this up but...

Is there a way to know how the f ratio will impact the view? I
understand a 4" refractor with an f/5 fl will be "rich field" though
probably bad with false color. An f/4 reflector will show coma. An f/9
reflector will be flatter though the fov will be smaller. OK.

Assuming a 14" Dob. F/4 is going to be a coma problem...f/5 is better
but still.... so at what point is the view pretty darn good. f/6?
f/8? Forget the ladder thing. How can one anticipate a view from a
given aperture at various focal lengths? Is f/5.5 much better or
slightly better than f/4.9?

Can someone give a little hard guidance on this?

Doink


IIRC, the same angular field of view will suffer the same degree
of coma in a classical Newtonian (hence, the desire for coma-
correctors in fast dobs.)

Achromatic Refractors (without additional correctors) probably offer
superior performance over a wide field compared to newtonians.
However, color fringing becomes an issue on brighter objects at higher
powers.

But to answer your question, there is no sharp transition from bad
view to good, based solely on f/ratio. The trick is finding out what
you can detect, and how much coma, color, or other aberration you are
willing to live with.

Cheers,
Larry G.