8" SCT v. 11" SCT

Stephen Paul wrote:

When I had my 12.5" F4.8 Dobsonian, I never could tolerate the views in a
larger field eyepiece than the 24mm Panoptic (1 degree field at 67x). I just
don't like overly large exit pupils. In my eyes, the stars are all
distorted.

Have you considered eyeglasses or an astigmatism-correcting eyepiece
like the TV dioptrix?

I have a similar problem, yet one of my most recent purchases was
a 40mm eyepiece, which I use quite often even on my 12.5" f/5 Dob.
I don't think that ultrawide (a.k.a. rich-field, a.k.a. wide-exit-
pupil) views are the be-all and end-all of deep-sky observing --
not even close. But I do find that they're an important dimension.

I'll give two examples, illustrating slightly different aspects.

The North America Nebula is one of the few objects where field of
view is genuinely critical. I'm quite happy to view M31 or the
Veil piecewise, but the North America simply disappears if I
can't fit at least a big chunk of it into a single field. The
40mm eyepiece gives me a field a bit over 1.5 degrees on my
12.5" f/5 Dob, which is just (barely) big enough to do the job.

Now it's true that I'm "wasting aperture" at an 8mm exit pupil,
and that I would get an equally good view using my 27mm eyepiece
on a 9-inch telescope. But I don't own a 9-inch telescope --
and if I did, I wouldn't be inclined to lug it out just to view
one object. It's far more practical to carry around a 40mm
eyepiece than a 9-inch telescope.

As for the true maximum exit pupil -- 5.5mm, which just matches
my own eyes' exit pupil -- I get that using my 27mm eyepiece
on my 12.5-inch Dob, and it happens to yield a field of view
*just* big enough to fit both halves of the Double Cluster.
It's a stupendous view; the large aperture brings out the
magnificent contrasting colors of the two clusters as no
smaller scope could.

I have both hefty myopia and fairly strong astigmatism, so I
need glasses to see stars naked-eye. But I normally take them
off when I'm working at the telescope. In fact, I keep the
focus of my finderscope adjusted for glasses-off rather than
glasses on. And when I'm just using my 27mm or 40mm eyepiece
for finding things, I also usually keep my glasses off; who
cares if the stars are all smeary and spiky? But when I'm
viewing the Double Cluster at a 5.5mm exit pupil, I put my
glasses on to correct for astigmatism, and the view improves
dramatically.

Mind you, even with glasses on, my vision is pretty bad at
large exit pupils; no doubt that's why I have so much trouble
seeing more than 6 stars in the Pleiades naked-eye. In other
words, my eyes have plenty of higher-order aberrations. So
stars never look as tight and pleasing at a 5.5mm exit pupil
as they do at a 2.5mm exit pupil -- just as for you. But
to view the whole Double Cluster through large apertures,
or to view a dark sky naked-eye, it's worth putting up with
some spikes.

So, getting back to the original question, have you considered
figuring out some way to correct your astigmatism rather than
forswearing wide-field views altogether? As Josh Roth commented
in his "stargazing eyeglasses" article, it's amazing how many
people balk at spending $100 on a pair of glasses but happily
spend four times that much on a single eyepiece -- which is
much less versatile.

- Tony Flanders

wrote in message
oups.com...
So, getting back to the original question, have you considered
figuring out some way to correct your astigmatism rather than
forswearing wide-field views altogether?

Well, I didn't really forswear wide-field views, I just made a choice to use
smaller exit pupil.

Ultimately all of this is moot for me though. I'm not a Dob kind of guy. I
_enjoy_ owning and using a high quality GEM, and that makes the SCT a better
choice for large aperture, despite the wide field performance limitation.

Folks who ask questions like, 8" SCT vs. 11" SCT, and in that same context
indicate the consideration of a GEM vs. a Fork, get my attention because I'm
a GEM/SCT kind of guy. ;-)

The whole Dob thing came into this thread in an unsolicited fashion.

As far as I'm concerned, the entire experience of trying just about every
solution imagineable, has only proven that my initial instincts on how to
enjoy this hobby were right, for me. I'm glad to be back doing this hobby on
my own terms.

I assume that others have either gotten their on their own, or that they
will.

As far as I'm concerned, the entire experience of trying just about every
solution imagineable, has only proven that my initial instincts on how
to
enjoy this hobby were right, for me. I'm glad to be back doing this
hobby on
my own terms.

---------------

Stephen: You and I have had some interesting discussions and you have
had some good times with a great variety of scopes. Learning and
enjoying, that is what is important. As I remember it, your initial
instinct was for a 4 inch MAK on a GOTO alt-az mount and then on to a
SCT on an EQ Fork mount, there was the DOBs and the refractors... Now
it seems you are happy with SCT's and EQ mounts. Is this the final
configuration, true happiness, only time can really tell.

--------
I assume that others have either gotten their on their own, or that they
will.

--------------

I think you have gotten where you currently are with a lot of
discussion and interaction with others, myself included. I know I
have gained a lot of insight and understanding from this group that has
helped me end up where I am these days.

If one assumes that we all get "there" on our own, then interacting
with this group would seem meaningless.

I see no reason to keep a thread limited to initial options as long as
the intent is to provide meaningful help to the original poster.

Jon

wrote in message
oups.com...
That potential wider field of view of a faster scope is an advantage,
something that has to be considered when making a choice.



Between an 8" SCT and an 11" SCT?? :-)

Fortunately none of us, you, me, the original poster, are limited to
making a choice between an 8 inch SCT and an 11 inch SCT.

That is why myself and others have brought up the possibility of
choosing another design. Sometimes people get stuck with a specific
type scope because it was the first thing they had and have not had
experience with other designs.

While it seems unfortunate that your eyes don't accomodate larger exit
pupils, I find exit pupils of 5mm quite acceptable as do many others.
I like those big fields of view, particularly when viewing targets like
the Veil. A current favorite is IC4756. While this OC is only about 1
degree in diameter, it looks best to my eye when framed by a 1.4 degree
FOV.

I might point out too that you are comparing a 9.25 inch scope with a
12.5 inch scope. It is generally easy to get those wider fields of
view with a smaller scope. A more realistic comparison would be a 12
inch F10 SCT.


You're lost in the weeds here Jon. The OP asked about upgrading from an 8"
SCT to an 11" SCT. Someone then said it "made more sense" to get a 10" to
12" Dob, which is more than just suggesting it as "an option".

To which I responded "To you maybe" [it makes "more sense"].

Whether or not something "makes sense", turns out to be subjective as hell.
I'm not without experience here. I know what "makes sense" to me, and what
doesn't. While it's true that I'm not a Dob kind of guy, I do not "dislike"
them, nor do I try to pursuade others to like or dislike them. Nor do I say
positive things about one, and negative things about the other, outside the
context of voicing my own equpment preferences.

In fact, I even apologized for having a non-Dob preference!! It turns out
that the SCT makes more sense for me.

I don't know how it is that "Dob lovers" generally manage to turn such a
simple thread into an apologetic for other scope designs. There is "sense"
in the preference, whether it be for the SCT, the refractor, the Dobsonian,
or the EQ-Newtonian. No one makes "more" sense than the other, outside of
the context of the user's needs, goals, and preferences.

wrote:

I guess if you need me to point out that the vast majority of
Newtonians are F6 and below.... My own 12.5 inch is F4.06 which with a
2 inch eyepiece is capable of a tad bit over 2 degree FOV.

We could digress into a discussion of what the edge of field looks like
in an f/4 Newtonian but this isn't the place to do so. Suffice to say
that the FOV that you rave on and on (and on) about is not as ideal as
you would have others believe.


As far as reading the subject line... Possible you have noticed that
myself and others have suggested to the original poster that other
options are available to him and that he ought to consider them before
making a purchase. My first scope too was an 8 inch SCT, I am
certainly glad I found other designs.

You need to reread the original post again. You are using a request from
someone regarding the advantages of upgrading from an 8 inch SCT to an
11 inch. Clearly to anyone listening he wasn't seeking information
regarding how terrific you thought a Newtonian was. Stephen Paul
noticed this as well. You seem to use any means possible to tout the
greatness of the telescope you prefer. I find that curious.




How many galaxies can you see in the Virgo Cluster with your naked
eye.....

Rhetorical - you did understand my point yes ?


I have no problem admitting that I enjoy DOBs. I also own two SCTs.
However the point here is not to play favorites, rather it is to help
the original poster make a choice that he can be happy with.

No - you are confusing the original question. Please re-read it.


By
letting him know what the advantages are of other designs, it just may
be that he will spend a bit of time investigating those choices.

I didn't see you exspousing on the merits of an 8inch APO ?!?! Does that
make you a DOB bigot ?

It is relatively easy to get the idea that s.a.a is the home of the Dob.
And it only takes two of three *dedicated* posters with far too much
time on the hands for this to occur. Unlike yourself Jon I actually
don't have a dog in this hunt. I can say this because I probably have
more Newtonians than you do. And SCT's and a Classical Cassegrain and
even a refractor. And every one of these OTA's has value in certain
situations. Even the SCT. And to denigrate it as you are doing rankles
me and at least one other individual. SCT's make perfect sense for many,
many people. Are they the best choice for them ??? Can't be answered by
anyone reading this except the person who gets to make the choice.


Think about this. It's not that your points are wrong ... but that they
skew the facts. But now this has drifted way off-topic and if you want
to discuss this further it would be better served to do it via email.


Regards

Bill

--

William R. Mattil : http://www.celestial-images.com

The original poster asked for experienced opinions and sadly he is missing
in action here.

Since he only asked about purchasing the 8" in Jan 2005., it's likely he is
limited by his urban skies and limited observing experience.

My "recommendation" for a dob was not meant to quirk any SCT fans though I
was not the first to suggest it - remember the 18" dob?

One thing for sure thought, a 8" under dark skies will show more faint
objects than a 11" SCT under urban skies. M101 is invisible in any sized
scope from my location.

I'll leave it to the OP to tell us what makes sense to him.

JD

wrote:

Yes, Jon ignored the opposite -- and with good reason. The situation
is not (alas) symmetric.

In some sense, a telescope's focal ration is irrelevant. It can be
decreased with a focal reducer or increased with a Barlow lens.

Not without a certain penalty.

In
fact, that's the heart and essence of the Cassegrain design.

I'll beg to differ. A Cassegrain has only reflective surfaces.

An SCT's
primary mirror has a focal ratio of f/2 or thereabouts, but the SCT
uses the hyperbolic secondary mirror as a focal extender -- very much
like using a Barlow on a Newtonian.

No it is reflective in nature and because of the design the performance
doesn't necessarily suffer from it's inclusion. This isn't necessarily
true when using a Barlow or focal reducer using glass. Most barlows will
also vignette depending on the other optical paramters.


So it is an absolute physical fact that the maximum practical
true field of view of an 11-inch f/5 Newtonian is *inherently*
wider than the maximum practical field of view of a standard
commercial 11-inch f/10 SCT.

Doh ? Did you somehow get the impression that I didn't know this ? Oh,
perhaps you interpreted my rhetorical statement as a legitimate question.


Whether you actually care about
the wide field of view is a whole 'nother matter; that's a
subjective judgment, and I can't argue about it.

Here we are then ... discussing it laughs


But it is an objective fact that assuming equally good optical
construction, everything that you can see in an 11-inch f/10
SCT will show equally well or better in an 11-inch f/5 Newtonian,
while the reverse is clearly not true.

Clearly then in your perfect world there is no reason to own anything
other than the Dob eh ? g


Mind you, SCTs have other advantages. Most obviously, an 11-inch
SCT tube is much more compact than an 11-inch Newtonian tube.
Which means, in turn, that a high-quality tracking drive for
an 11-inch SCT is cheaper, smaller, and lighter than a high-
quality tracking drive for an 11-inch Newtonian.

Particularly so since the original poster already had such a mount ?!?


Thanks for enlightening me Tony.


Regards

Bill


--

William R. Mattil : http://www.celestial-images.com

You're lost in the weeds here Jon. The OP asked about upgrading from an 8"
SCT to an 11" SCT. Someone then said it "made more sense" to get a 10"
to
12" Dob, which is more than just suggesting it as "an option".
----

Sorry Stephen, NO weeds in San Diego, I am not lost, I know where I am.


I cannot say whether it makes more sense for the original poster to buy
a 10 or 12 inch DOB rather than a 11 inch SCT but it is certainly
something worth considering. There are advantages and there are
disadvantages to each. One of those advantages of the DOB is a wider
possible field of view. Whether that is of any interest to the
original poster, well, only he can decide that.

The fact that a DOB doesn't not make sense for your own situation or
that it does make sense for my situation is irrelevent. What is
relevent is that it is a choice and one that ought to be at least
mentioned.

I hope that this discussion has caused the original poster to at least
investigate a bit if he had not previously. Other choices like a
smaller refractor might also be interesting but given that he seemed
interested in a larger scope that would seem unlikely.

I think we are all in agreement on somethings. One of them is that
there is a reasonable amount of WOW when switching from an 8 inch scope
to an 11 inch scope.

Best wishes to all....

Jon Isaacs

You need to reread the original post again. You are using a request from
someone regarding the advantages of upgrading from an 8 inch SCT to an
11 inch.
====

Good old Bill, it's fun having your around. Keep up the good work.

Oh yes, if someone came and asked me whether a Roast Beef Sandwich was
better than a Spam Sandwich, I would tell them that in my experience
indeed the Roast Beef Sandwich was better than the Spam Sandwich. I
might also point out that I found the "fat free" Turkey Sandwich to be
quite tasty as well and to have less cholesterol.

Jon Isaacs

wrote in message
oups.com...
If you want to see more deep sky objects a 10" or 12" dob makes more
sense.

To you maybe

Three simple words..... who knew?