Short Focal Ratio Dobs

Plan on having a 2" focuser and a Paracorr.
Should be great then....
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Tony Flanders wrote:

Also worth remembering that most pro scopes these days are F/2
or thereabouts.

But it's also worth noting that they're not for visual use. Also, they
often use only a very small portion of the center of the field -- star
images, for example -- for science. And many have optics after the
primary (far more elaborate and expensive than a Paracorr) that increase
the effective focal length of the telescope by as much as 25x.

Mike Simmons

On Fri, 14 Nov 2003 09:57:56 -0500, "Stephen Paul" wrote:

"Alan W. Craft" wrote in message
.. .
Such a fast focal ratio is best if not only suited for
astrophotography,
an activity for which Dobsonian-mounted Newtonians are ill-advised

Using an EQ Platform drive, a Dob is suitable for astrophotography.

http://astronomy-mall.com/regular/pr...ms/images.html

"For a price, Ugati, for a price." - Humphrey Bogart to Peter Lorre
in "Casablanca"

Which, of course, would negate one of the Dob-mounted Newt's
primary attractions: cost, or rather the lack thereof.

Alan

On 14 Nov 2003 17:33:10 GMT, (Don Martin fan) wrote:

Plan on having a 2" focuser and a Paracorr.

My new Klee barlow is a magic coma eraser!

Should be great then....
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Alan

Alan W. Craft wrote in message . ..
On 14 Nov 2003 17:33:10 GMT, (Don Martin fan) wrote:

Plan on having a 2" focuser and a Paracorr.

My new Klee barlow is a magic coma eraser!

Should be great then....
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Alan

I'm sorry, but your Klee will have little if any effect on coma
created by a paraboloidal primary mirror. If it did, a Klee Barlow
would cause a reverse sort of coma on telescopes which do not have
much coma, such as some refractors. The Klee increases the effective
focal length of the instrument and thus can sometimes reduce the
problems which some less-expensive eyepieces show when used at very
short f/ratios. The most notable of these problems is off-axis
astigmatism. It is produced by less complex eyepieces being used in
shorter f/ratio (under f/7) telescopes, and is often mistaken for coma
by inexperienced amateurs. The only practical way of dealing with
off-axis coma in a Newtonian is via the use of coma corrector (such as
the TeleVue Paracorr) positioned ahead of the eyepiece. This will
reduce the coma, but will do little for any astigmatism produced by
the eyepiece. To deal with off-axis astigmatism, you either need a
longer focal length instrument or a more complex eyepiece design such
as the Panoptics or Naglers. Clear skies to you.

David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

One of the problems would be the size of the diffraction limited field. Even
in an f/5, this area is very small. The paracorrs correct a lot of this but
not all. If you look in a f/4 you will see a very comatic field using standard
oculars and no coma corrector. A bigger scope (by virture of its size) shows a
smaller true field of view. If you want good definition and contrast on an
extended object, that will be very hard to accomplish. Also, the bigger
aperture, the harder it would be to meet its resolution criterion unless you
have a fabulous figure...and, again, the size of that field is extremely small.
Many years ago I spent some time panning with a 10" f/4 Newtonian (expensive)
richest field telescope. I used several eyepieces and could not believe the
extremely comatic images and light scatter. This may have been a poor example
but I was shocked.

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!

Maybe the thing to do is talk to someone about making a Ross corrector
(paracorr type) specific to an f/3. Richard Suiter could probably point you in
the right direction regarding ray tracing. I think best case you would end up
with a lot of optical aberrations...and that secondary mirror minor axis would
be pretty darn big-and the mirror expensive to boot!

Which, of course, would negate one of the Dob-mounted Newt's
primary attractions: cost, or rather the lack thereof.

Alan

For some that might be the primary attraction, and for me, it is part of it.
But the cost is only the iceing on the cake.

The compactness, ease of setup and tear down, freedom from vibration,
comfortable viewing postion, intuitive star hopping, these are some of the
attractions.

The most often cited drawback is the lack of tracking. An EQ platform will
allow you to retain the other advantages while adding tracking capability.

While for an 8 inch scope, an EQ mount is quite doable, though one substantial
enough to handle an 8 inch F6 scope with the same stability and freedom from
vibration as a DOB mount with be quite substantial. And for a Newtonian 12.5
inches the EQ mounts are getting into the huge range, and of course for large
scopes like being discussed here, they are essentially impossible for a scope
that is portable.

On the other hand, the equatorial platform retains the low profile compactness
and portability of the Dob design while providing good quality tracking. And
when the scopes get to the sizes being addressed here, it is really the only
option. It is nice that they are also affordable but that is of less
importance to the owner of a 20 inch DOB than the fact that the mount can be
lifted easily by one person and does not require a huge counterweight.

So, yes it is nice that DOBs are inexpensive, but it that is the primary
attraction, then one will probably soon move on. On the otherhand, if the
attractions are the elegent simplicity and ability to work as a close knit
team, then it is likely that one would choose a DOB even if it were the more
expensive route.

And of course sometimes it is. A ELT 12.5 inch Starmaster will run about $3800
without Goto and another $2300 for the GOTO+ tracking, around $6000.

jon

Jon Isaacs wrote:

Hi, what's the shortest focal length Dob that's practical? As the aperture
gets larger (greater than 20") what is the downside of making an F3 dob
rather than an F4.5 (more common). Thanks ...

I own a 12.5 inch F4.06 DOB. It does work and it works quite nicely with a
Paracorr, it is also very touchy about collimation and it is hard on eyepieces.


Slightly OT:
With all the mud slung at SCT's for their central obstructions, this is
certainly a point in their favor when it comes to wide aperture scopes.
You get the aperture without an impractically long tube, and you can
use a focal reducer to get low power. With Dobs, wide aperture means
long tubes or fast optics.

-Larry Curcio.

Slightly OT:
With all the mud slung at SCT's for their central obstructions, this is
certainly a point in their favor when it comes to wide aperture scopes.
You get the aperture without an impractically long tube, and you can
use a focal reducer to get low power. With Dobs, wide aperture means
long tubes or fast optics.

-Larry Curcio.

Of course while my Newtonian is F4.06, the primary mirror in an SCT is normally
about F2, so you are in reality not getting away from fast optics, what you
have avoided is the need to parabolize the mirror but the optics themselves are
faster.

The central obstruction issue is still there, the Newtonian being 25%, the SCT
being 35%.

And at F10, with a focal reducer you will bring that scope down to F6.3 which
means you will not have as wide a field of view as is possible with an F4
scope.

jon isaacs

On 14 Nov 2003 22:52:33 -0800, (David Knisely) wrote:

Alan W. Craft wrote in message . ..
On 14 Nov 2003 17:33:10 GMT, (Don Martin fan) wrote:

Plan on having a 2" focuser and a Paracorr.

My new Klee barlow is a magic coma eraser!

Should be great then....
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

Alan

I'm sorry, but your Klee will have little if any effect on coma
created by a paraboloidal primary mirror.

I know that, I KNOW THAT.

I just wanted to say it, and for its shock value.

It appears to have worked...

blows upon fingertips

Though not without realising that I did so irresponsibly,
as I do have a conscience, albeit mischievous. I'll
try to restrain myself in future.

If it did, a Klee Barlow
would cause a reverse sort of coma on telescopes which do not have
much coma, such as some refractors. The Klee increases the effective
focal length of the instrument and thus can sometimes reduce the
problems which some less-expensive eyepieces show when used at very
short f/ratios. The most notable of these problems is off-axis
astigmatism. It is produced by less complex eyepieces being used in
shorter f/ratio (under f/7) telescopes, and is often mistaken for coma
by inexperienced amateurs. The only practical way of dealing with
off-axis coma in a Newtonian is via the use of coma corrector (such as
the TeleVue Paracorr) positioned ahead of the eyepiece. This will
reduce the coma, but will do little for any astigmatism produced by
the eyepiece. To deal with off-axis astigmatism, you either need a
longer focal length instrument or a more complex eyepiece design such
as the Panoptics or Naglers. Clear skies to you.

Well, there goes my idea for a wide range of Kellners.

I don't think it'll be as bad as that would seem to suggest, however.

David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

eyes the bulk of David's post, hungrily

Very well, I won't snip it, lest a newbie were to be misinformed.

Alan