Short Focal Ratio Dobs

I think it's clear that newtonians with their smaller central obstruction,
quicker cooling and larger aperture for the same dollar provide some
compelling advantages. I bought an SCT (N11, my second Celestron SCT) for
several reasons. I wear glasses and don't like to observe with them so I
wanted goto (I have a hard time star hopping taking my glasses off and on
all of the time). I like to observe sitting down (the Starmaster hybrid
14.5 is a very tempting option), I wanted tracking. I know I can get
tracking and goto in a Starmaster 14.5 but the cost has now increased
dramatically. I've looked through about half a dozen Starmasters and have
always been impressed with the construction and mirrors. All but one I've
looked through was miscollimated to some extent. I believe folks just want
to set up and observe quickly and maybe aren't as picky. My N11 holds
collimation unless I four wheel with it to get to a dark sky/camping site
(which I've done) or tried to tweak it a bit more sometimes not improving it
(which I've also done.). You pay your money make your choice, lots of
great options out there including refractors ....

"Jon Isaacs" wrote in message
...

Yes. Not as good as an f4 with a reducer, but a lot better than
an f10 there. In addition, and this was my point, albeit poorly
expressed, an SCT tends to give better views, at f10, than an
f4 newtonian barlowed up.

Not to start a war but....

Some things to consider:

1. A Newtonian is a simple device, on flat mirror and one parabolic
mirror.
Both can and are routinely made to close tolerances. Basic Asian DOBs are
spec'd as to having a minimum of an 1/8 mirror and premium mirrors are
much
better. Probably likely that commercial SCTs are not this good.

2. Collimation. A standard Newtonian can be optimally collimated because
there
are sufficent adjustments to both the primary and the secondary. The
collimation of a commercial SCT is a compromise because it only has one
adjustment, tilt of the secondary while it has three optical elements that
need
alignment.

3. A fast Newtonian can be used with a Paracorr to correct for coma. The
Televue unit has a 15% increase to the focal length. Now add a Barlow as
you
suggest and the effective focal ratio will be 9.2.

When it comes to dealing with the aberations in a less than perfect
eyepiece,
this will perform like an F9.2 scope which is to say that the long focal
length
of the SCT is not advantage in this regard. Also apparently a Newtonian
has a
flatter focal plane and there are no chromatic aberations caused by the
corrector plate.
---

So what this tells me is that certainly a fast Newtonian can provide views
that
are superior at high magnifications that an normal SCT.

As I understand it, Todd Gross's list of the best planetary scopes he has
tried
includes several fast Newtonians. And of course there is always Mike
Spooner
with his 18 incher up the Page. AZ where he likes to view Saturn at
something
around 860X.

My personal experience is that on-axis, F4 works amazingly well, it splits
doubles and provides good planetary detail as well as contrast. Add a
paracorr
and it provides nice views all around.

I think F4.5 or F5 is a better choice but I choose F4 because it makes an
OTA
that fits in a small car.

So, I think SCTs do provide nice high power views and since they most all
have
tracking, so they are well suited for this. But optically I think there
are
good reasons to think a fast newtonian can do as well and probably better.

jon

All but one I've
looked through was miscollimated to some extent. I believe folks just want
to set up and observe quickly and maybe aren't as picky. My N11 holds
collimation unless I four wheel with it to get to a dark sky/camping site
(which I've done) or tried to tweak it a bit more sometimes not improving it
(which I've also done.).

No arguments here. But of course I will point out that your Nextstar 11 holds
its compromised collimation well. If it had the capability to optimally adjust
the collimation like a Newtonian, it might not hold it quite so easily.

But I agree, there are advantages and disadvantages to all designs, we pays our
money and we takes our choice...

Jon

Oops! Believe it or not, I hadn't intended to say
that an SCT was a flat-out better choice than a
newt. I personally love newts. I also love fast
newts. Heck! I even like the StarBlast!

My point, admittedly OT, was simply that SCT's
aren't all bad. I mentioned it in this thread, only
because the thread brought the point to mind.

Fact is, I'd have a 10 or 12 inch dob right now
if my wife would abide another large piece of
astronomical hardware in the house.

Peace With Clear Skies,
-Larry Curcio

Jon Isaacs wrote:


Yes. Not as good as an f4 with a reducer, but a lot better than
an f10 there. In addition, and this was my point, albeit poorly
expressed, an SCT tends to give better views, at f10, than an
f4 newtonian barlowed up.

Not to start a war but....

Some things to consider:

1. A Newtonian is a simple device, on flat mirror and one parabolic mirror.
Both can and are routinely made to close tolerances. Basic Asian DOBs are
spec'd as to having a minimum of an 1/8 mirror and premium mirrors are much
better. Probably likely that commercial SCTs are not this good.

2. Collimation. A standard Newtonian can be optimally collimated because there
are sufficent adjustments to both the primary and the secondary. The
collimation of a commercial SCT is a compromise because it only has one
adjustment, tilt of the secondary while it has three optical elements that need
alignment.

3. A fast Newtonian can be used with a Paracorr to correct for coma. The
Televue unit has a 15% increase to the focal length. Now add a Barlow as you
suggest and the effective focal ratio will be 9.2.

When it comes to dealing with the aberations in a less than perfect eyepiece,
this will perform like an F9.2 scope which is to say that the long focal length
of the SCT is not advantage in this regard. Also apparently a Newtonian has a
flatter focal plane and there are no chromatic aberations caused by the
corrector plate.
---

So what this tells me is that certainly a fast Newtonian can provide views that
are superior at high magnifications that an normal SCT.

As I understand it, Todd Gross's list of the best planetary scopes he has tried
includes several fast Newtonians. And of course there is always Mike Spooner
with his 18 incher up the Page. AZ where he likes to view Saturn at something
around 860X.

My personal experience is that on-axis, F4 works amazingly well, it splits
doubles and provides good planetary detail as well as contrast. Add a paracorr
and it provides nice views all around.

I think F4.5 or F5 is a better choice but I choose F4 because it makes an OTA
that fits in a small car.

So, I think SCTs do provide nice high power views and since they most all have
tracking, so they are well suited for this. But optically I think there are
good reasons to think a fast newtonian can do as well and probably better.

jon

On 16 Nov 2003 11:58:05 GMT, (Jon Isaacs) wrote:

Of all the portable Newtonians on the market, I gravitate towards the
'Portaball' with its Zambuto primary, or any other design that allows the
rotating of the tube.

Alan:

I would have privately Emailed this to you but in the past you have not
responded.

I rarely access my Hotmail account; I apologise.

It has taken the good part of an hour to write this post so take
some time reading it and thinking about it before you blow it off.

It has been suggested to me that you are a troll.

...a convenient term to throw out when disagreeing with someone.

I'm not so bad, really. I simply have opinions is all, and a need to
express them.

I am not sure that this is the case but it is clear to me that you are more
interested in expounding upon some limited knowledge and experience
than in listening to what others who have more knowledge and experience
than you.

Rather, I elicit that knowledge and experience out into the open
while admittedly so doing, and for all to disseminate and absorb; to get
whatever they may get out of it, therefore what's so bad about that?

Rather than Plonk you, I have decided to explain my perspective.

You're most gracious, truly.

Have I not been so as well, for the most part anyway?

You posted something that your Klee is a magic Coma Corrector. This is a myth
that still exists. David Knisely responded with a carefully written post and
you replied with "I knew that."

This is just rude and uncalled for.

Like his "ad hominem" in inferring that I was a snob, and while
discussing the process by which the erroneous term "Dobsonian" came
into being as applied to a Newtonian in addition to its mounting?

I agree.

So far, he's inferred one name and called me another, and all because he
disagrees with me. Mind you, I've not called him a single name.

He and others simply can't stand to have their preconceived notions of what
is right and wrong questioned, much less shaken.

People are taking their time to try to help you understand this hobby and expand
your understanding.

And I hereby sincerely thank each and every one.

This is not some Yahoo forum where people like you and me trade in myth and
ignorance. This is the big league and you need to respect that fact. Myself
I consider it an honor to be able to have such an intimate contact with these
folks and I have learned a great deal from them. There is much to learn here
if you listen.

I do read, but I like to write, too; and yes, I recognise this group as being
worthwhile, otherwise I wouldn't want to be a part of it, albeit a very miniscule
part.

I myself have spent a fair amount of time responding to your posts...

I appreciate that, truly, but if you consider it an investment with the
expectation of some sort of return(s), then I'm afraid that I may have to
disappoint, depending.

...only to have you respond with something similar to your response to David.
In this case it was the 300 lb 12.5 inch Parks which then transformed into the
"Portaball."

I don't see the connection at all. Of course a 10" or 12.5" Portaball would
be much easier for me to tote out the door and into the yard, and over a Parks
12.5" 'Superior.'

What could've possibly rattled your chain within that?

Unless you have actually seen and used various designs like the Portaballs and
Starmasters then you have no real idea what you will choose when the time
comes. I could go on about the advantages and disadvantges of Portaballs and
Starmasters but I have decided not to.

Have you yourself owned one?

---------

So I say this:

I don't know if you are a troll or not. I have given you the benefit of my
doubt.

But if you don't clean up your act, show some respect, listen to what other
people are saying and only post when you have actual experience and knowledge,
then I, for one am joining David Knisely and several others who will simply not
respond to your posts.

You do precisely what you feel you need to do, Mr. Issacs, and no, Mr. Knisely
owes me a public apology instead, hence...

snip

I think you're just tired of seeing all the bickering and fighting that goes on
within this group, and with just the argument between Mr. Knisely and myself being
the last straw.

In toto, I am here primarily to express my views, mostly upon things astronomical,
but also on other things as they arise, and just as others have done, do, and will do
still. There are no rigid set of rules that govern how discussions are to be carried out
and what they are to contain. May I remind you, sir, that this is an unmoderated group,
and while I agree that that should not give license to one's conscience to run over
everyone and everything, at the same time it is not to constrict said freedom of
expression, either.

I find it rather humorous to see so many running about within this group attempting
to tidy it up and sweep it clean, but only to have it get all dirty again. In vain they
do so, I'm afraid, in vain.

But, if it'll make you feel better, and since you did sincerely and patiently compose
your call to sanity, I will take some of your points into serious consideration. That's
the best I may do.

Toodles.

Alan

t (Ratboy99) wrote in message ...
About 25 years ago, I had the pleasure of meeting John Dobson.
I spent most of
an evening with him up at Mt. Rainier. He mentioned that he thought
parabolas
of short focal length were very lacking visually. At that time, he felt
anything less than f/6 was questionable.
I don't know how he feels now. But I
do believe he was (and is) an authority on Dobs!


Sigh.

Probably a bit different now if he's looked through any Zambutto equipped
Starmaster. Mine's an 18" F4.3, and I'm not the expert John Dobson is, but the
scope is not "lacking visually" regardless of who you ask.
rat
~( );

email: remove 'et' from .com(et) in above email address

Mine's a 22" f/4. It's not lacking anything visually...
Clear skies,
Shneor Sherman

Jon was TOTALLY right, you know.

And you just confirmed it to him, and to everyone else here on SAA, just
like you did with David Knisely, and Brian Tung.

I had been hoping you could stop spewing your mindless trolls long enough
to give us a little data on your new Parks OTA, but William Mattil has
more than filled the bill, where Parks is concerned, today, with an
excellent post and photos.

And so now you are history.

PLONK!

--
To reply, remove the "z" if one appears in my address
"Alan W. Craft" wrote in message
...
On 16 Nov 2003 11:58:05 GMT, (Jon Isaacs) wrote:

Of all the portable Newtonians on the market, I gravitate towards
the
'Portaball' with its Zambuto primary, or any other design that allows
the
rotating of the tube.

Alan:

I would have privately Emailed this to you but in the past you have not
responded.

I rarely access my Hotmail account; I apologise.

It has taken the good part of an hour to write this post so take
some time reading it and thinking about it before you blow it off.

It has been suggested to me that you are a troll.

...a convenient term to throw out when disagreeing with someone.

I'm not so bad, really. I simply have opinions is all, and a
need to
express them.

I am not sure that this is the case but it is clear to me that you are
more
interested in expounding upon some limited knowledge and experience
than in listening to what others who have more knowledge and experience
than you.

Rather, I elicit that knowledge and experience out into the
open
while admittedly so doing, and for all to disseminate and absorb; to get
whatever they may get out of it, therefore what's so bad about that?

Rather than Plonk you, I have decided to explain my perspective.

You're most gracious, truly.

Have I not been so as well, for the most part anyway?

You posted something that your Klee is a magic Coma Corrector. This is
a myth
that still exists. David Knisely responded with a carefully written
post and
you replied with "I knew that."

This is just rude and uncalled for.

Like his "ad hominem" in inferring that I was a snob, and while
discussing the process by which the erroneous term "Dobsonian" came
into being as applied to a Newtonian in addition to its mounting?

I agree.

So far, he's inferred one name and called me another, and all because he
disagrees with me. Mind you, I've not called him a single name.

He and others simply can't stand to have their preconceived notions of
what
is right and wrong questioned, much less shaken.

People are taking their time to try to help you understand this hobby
and expand
your understanding.

And I hereby sincerely thank each and every one.

This is not some Yahoo forum where people like you and me trade in myth
and
ignorance. This is the big league and you need to respect that fact.
Myself
I consider it an honor to be able to have such an intimate contact with
these
folks and I have learned a great deal from them. There is much to
learn here
if you listen.

I do read, but I like to write, too; and yes, I recognise this
group as being
worthwhile, otherwise I wouldn't want to be a part of it, albeit a very
miniscule
part.

I myself have spent a fair amount of time responding to your posts...

I appreciate that, truly, but if you consider it an investment
with the
expectation of some sort of return(s), then I'm afraid that I may have
to
disappoint, depending.

...only to have you respond with something similar to your response to
David.
In this case it was the 300 lb 12.5 inch Parks which then transformed
into the
"Portaball."

I don't see the connection at all. Of course a 10" or 12.5"
Portaball would
be much easier for me to tote out the door and into the yard, and over a
Parks
12.5" 'Superior.'

What could've possibly rattled your chain within that?

Unless you have actually seen and used various designs like the
Portaballs and
Starmasters then you have no real idea what you will choose when the
time
comes. I could go on about the advantages and disadvantges of
Portaballs and
Starmasters but I have decided not to.

Have you yourself owned one?

---------

So I say this:

I don't know if you are a troll or not. I have given you the benefit
of my
doubt.

But if you don't clean up your act, show some respect, listen to what
other
people are saying and only post when you have actual experience and
knowledge,
then I, for one am joining David Knisely and several others who will
simply not
respond to your posts.

You do precisely what you feel you need to do, Mr. Issacs, and
no, Mr. Knisely
owes me a public apology instead, hence...

snip

I think you're just tired of seeing all the bickering and
fighting that goes on
within this group, and with just the argument between Mr. Knisely and
myself being
the last straw.

In toto, I am here primarily to express my views, mostly upon
things astronomical,
but also on other things as they arise, and just as others have done,
do, and will do
still. There are no rigid set of rules that govern how discussions are
to be carried out
and what they are to contain. May I remind you, sir, that this is an
unmoderated group,
and while I agree that that should not give license to one's conscience
to run over
everyone and everything, at the same time it is not to constrict said
freedom of
expression, either.

I find it rather humorous to see so many running about within this group
attempting
to tidy it up and sweep it clean, but only to have it get all dirty
again. In vain they
do so, I'm afraid, in vain.

But, if it'll make you feel better, and since you did sincerely and
patiently compose
your call to sanity, I will take some of your points into serious
consideration. That's
the best I may do.

Toodles.

Alan

On Sun, 16 Nov 2003 16:20:11 -0700, "Jan Owen" wrote:

Jon was TOTALLY right, you know.

I know nothing of the sort. I do know, however, that he made
a mountain out of a mole-hill, and just as you have done.

And you just confirmed it to him, and to everyone else here on SAA, just
like you did with David Knisely, and Brian Tung.

You mustn't worship people, no matter how much they think
of themselves, including myself.

I had been hoping you could stop spewing your mindless trolls long enough
to give us a little data on your new Parks OTA, but William Mattil has
more than filled the bill, where Parks is concerned, today, with an
excellent post and photos.

Nonetheless, I'll be posting my experiences as promised.

And so now you are history.

PLONK!

Alas, no more lazy summer afternoons by the swimming-hole;
no more cake-and-cookie birthday parties replete with astronomically-
oriented favors; no longer the scents of Royal Copenhagen and Aqua
Velva to fill the night air...

8^)

TO TEARS THOU HAST BROUGHT ME, SIR, TO TEARS!!!

Alan

Uh... scuse me. Aren't we over-reacting here
just a smidge? Everybody who sets up a little
dramatic tension on a list isn't a troll, after all.

I've said it before and I'll say it again. Trolls are
attracted to goats - fat old goats in particular.
There are two defenses against trolls:

1) A sense of humor; and

2) A bunch of old goats bigger than the troll.

Seems ta me this list has a tradition of choosing
option 2. That sorta works, only the effect on
the list gets worse than the effect of the trolls was.

I humbly suggest we stop buckin' around and
lighten up.

-Larry (Eyes as big as saucers) C.



Jan Owen wrote:

Jon was TOTALLY right, you know.

And you just confirmed it to him, and to everyone else here on SAA, just
like you did with David Knisely, and Brian Tung.

I had been hoping you could stop spewing your mindless trolls long enough
to give us a little data on your new Parks OTA, but William Mattil has
more than filled the bill, where Parks is concerned, today, with an
excellent post and photos.

And so now you are history.

PLONK!

--
To reply, remove the "z" if one appears in my address
"Alan W. Craft" wrote in message
...
On 16 Nov 2003 11:58:05 GMT, (Jon Isaacs) wrote:

Of all the portable Newtonians on the market, I gravitate towards
the
'Portaball' with its Zambuto primary, or any other design that allows
the
rotating of the tube.

Alan:

I would have privately Emailed this to you but in the past you have not
responded.

I rarely access my Hotmail account; I apologise.

It has taken the good part of an hour to write this post so take
some time reading it and thinking about it before you blow it off.

It has been suggested to me that you are a troll.

...a convenient term to throw out when disagreeing with someone.

I'm not so bad, really. I simply have opinions is all, and a
need to
express them.

I am not sure that this is the case but it is clear to me that you are
more
interested in expounding upon some limited knowledge and experience
than in listening to what others who have more knowledge and experience
than you.

Rather, I elicit that knowledge and experience out into the
open
while admittedly so doing, and for all to disseminate and absorb; to get
whatever they may get out of it, therefore what's so bad about that?

Rather than Plonk you, I have decided to explain my perspective.

You're most gracious, truly.

Have I not been so as well, for the most part anyway?

You posted something that your Klee is a magic Coma Corrector. This is
a myth
that still exists. David Knisely responded with a carefully written
post and
you replied with "I knew that."

This is just rude and uncalled for.

Like his "ad hominem" in inferring that I was a snob, and while
discussing the process by which the erroneous term "Dobsonian" came
into being as applied to a Newtonian in addition to its mounting?

I agree.

So far, he's inferred one name and called me another, and all because he
disagrees with me. Mind you, I've not called him a single name.

He and others simply can't stand to have their preconceived notions of
what
is right and wrong questioned, much less shaken.

People are taking their time to try to help you understand this hobby
and expand
your understanding.

And I hereby sincerely thank each and every one.

This is not some Yahoo forum where people like you and me trade in myth
and
ignorance. This is the big league and you need to respect that fact.
Myself
I consider it an honor to be able to have such an intimate contact with
these
folks and I have learned a great deal from them. There is much to
learn here
if you listen.

I do read, but I like to write, too; and yes, I recognise this
group as being
worthwhile, otherwise I wouldn't want to be a part of it, albeit a very
miniscule
part.

I myself have spent a fair amount of time responding to your posts...

I appreciate that, truly, but if you consider it an investment
with the
expectation of some sort of return(s), then I'm afraid that I may have
to
disappoint, depending.

...only to have you respond with something similar to your response to
David.
In this case it was the 300 lb 12.5 inch Parks which then transformed
into the
"Portaball."

I don't see the connection at all. Of course a 10" or 12.5"
Portaball would
be much easier for me to tote out the door and into the yard, and over a
Parks
12.5" 'Superior.'

What could've possibly rattled your chain within that?

Unless you have actually seen and used various designs like the
Portaballs and
Starmasters then you have no real idea what you will choose when the
time
comes. I could go on about the advantages and disadvantges of
Portaballs and
Starmasters but I have decided not to.

Have you yourself owned one?

---------

So I say this:

I don't know if you are a troll or not. I have given you the benefit
of my
doubt.

But if you don't clean up your act, show some respect, listen to what
other
people are saying and only post when you have actual experience and
knowledge,
then I, for one am joining David Knisely and several others who will
simply not
respond to your posts.

You do precisely what you feel you need to do, Mr. Issacs, and
no, Mr. Knisely
owes me a public apology instead, hence...

snip

I think you're just tired of seeing all the bickering and
fighting that goes on
within this group, and with just the argument between Mr. Knisely and
myself being
the last straw.

In toto, I am here primarily to express my views, mostly upon
things astronomical,
but also on other things as they arise, and just as others have done,
do, and will do
still. There are no rigid set of rules that govern how discussions are
to be carried out
and what they are to contain. May I remind you, sir, that this is an
unmoderated group,
and while I agree that that should not give license to one's conscience
to run over
everyone and everything, at the same time it is not to constrict said
freedom of
expression, either.

I find it rather humorous to see so many running about within this group
attempting
to tidy it up and sweep it clean, but only to have it get all dirty
again. In vain they
do so, I'm afraid, in vain.

But, if it'll make you feel better, and since you did sincerely and
patiently compose
your call to sanity, I will take some of your points into serious
consideration. That's
the best I may do.

Toodles.

Alan

I'm sure you're right, I've looked through a proprerly collimated 14.5 and
it was gorgeous. I'm willing to bet that if an f5 or f6 mirror by Mr.
Zambuto was available in 20" (and you had a cherry picker to lift you so you
could look through it) it would look a smidge better.

"Ratboy99" wrote in message
...
About 25 years ago, I had the pleasure of meeting John Dobson.
I spent most of
an evening with him up at Mt. Rainier. He mentioned that he thought
parabolas
of short focal length were very lacking visually. At that time, he
felt
anything less than f/6 was questionable.
I don't know how he feels now. But I
do believe he was (and is) an authority on Dobs!


Sigh.

Probably a bit different now if he's looked through any Zambutto equipped
Starmaster. Mine's an 18" F4.3, and I'm not the expert John Dobson is, but
the
scope is not "lacking visually" regardless of who you ask.
rat
~( );

email: remove 'et' from .com(et) in above email address