DOB Dilema

OK, so what's the "ideal" and practical Dob size? I'm struggling with what I
want to seek out as far as a Dob goes...yes, I can try them all at a star
party....

OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2" The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is it
better to have another 2" of aperture or is it better to be a bit "slower"?

I want a good quality truss Dob around $2200-$2500

How much better is a 1/10 wave or better mirror compared to a mass
produced---Orion? 12.5" mirror?

I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?

Doink

Check out some of the ones listed in the FAQ, some good top end scope makers
are listed. You could also check out astromart too.


--


"And for the second time in four million years, the monolith awoke."
Arthur C.Clarke 2062dyssey three

SIAR
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http://home.inreach.com/starlord
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"Doink" wrote in message
...
OK, so what's the "ideal" and practical Dob size? I'm struggling with what
I
want to seek out as far as a Dob goes...yes, I can try them all at a star
party....

OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2" The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is
it
better to have another 2" of aperture or is it better to be a bit
"slower"?

I want a good quality truss Dob around $2200-$2500

How much better is a 1/10 wave or better mirror compared to a mass
produced---Orion? 12.5" mirror?

I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?

Doink




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On Tue, 26 Oct 2004 21:43:05 -0700, Doink wrote:


I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?


Go to: www.excelsis.com and click on the little box at the top-right
margin which contains the word astronomy reviews. You'll find more
telescope reviews on all manufacturers and their telescopes written by
amateur astronomers then you could imagine.


--
Martin
"Photographs From the Universe of Amateur Astronomy"
http://home.earthlink.net/~martinhowell

OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2"

I'm 6 ft 1". I prefer to view seated. What is your preference?

The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is it

The vast majority of the time the views will not be limited by your optics but
rather by the seeing. Also, consider that only the center of the FOV is
important for planetary viewing...

A 15 incher at F5 will have about the same eyepiece height as a 12.5 incher at
F6. A 16 incher would be F4.7.

A fast scope with a Paracorr will give pretty good wide field views when used
with good quality eyepieces and it is the aperture you are after here. A 16
inch scope will have 63% more mirror area than a 12.5 incher.

I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?

There is not a great deal of difference between a 12.5 inch F5 scope and a 12.5
inch F6 scope optically, there is a great deal of difference from a
useability/transportability standpoint.

My 12.5 inch scope has an OTA that is 52 inches long and fits in my Ford
Escort. With a Paracorr, it gives very nice views even widefield.

From my point of view, if I am going to get a scope that has the hassle of
dealing with a 75 inch long focal length, I am going to make sure that it has
enough aperture that it will be a significant improvement over my current
scope.

Also, the difference between a 10 incher and a 12.5 incher is noticeable but
not earth shattering, so if your current scope is in the 10 inch range, you
probably want to go significantly bigger.

I think a good choice would be a Truss DOB with optics from a 16 inch Meade
DOB.

Bottom line is that its about getting the most aperture into the most useable
package. IMHO, the gains above F5 are not worth the added OTA length.

jon

An article on issues relating to f ratio & mirrors that you might find
interesting:
http://www.rfroyce.com/mirror_performance.htm

An article on why 14.5 inches is better than 12 or 16 for various deepsky
things that you can extrapolate to planets too, perhaps:
http://www.tscopes.com/T14/AboutT14.html

Regards, Mike
--
Non, Je Ne Regrette Rien. E.P.
"Jon Isaacs" wrote in message
...
OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2"

I'm 6 ft 1". I prefer to view seated. What is your preference?

The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is
it

The vast majority of the time the views will not be limited by your optics
but
rather by the seeing. Also, consider that only the center of the FOV is
important for planetary viewing...

A 15 incher at F5 will have about the same eyepiece height as a 12.5
incher at
F6. A 16 incher would be F4.7.

A fast scope with a Paracorr will give pretty good wide field views when
used
with good quality eyepieces and it is the aperture you are after here. A
16
inch scope will have 63% more mirror area than a 12.5 incher.

I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?

There is not a great deal of difference between a 12.5 inch F5 scope and a
12.5
inch F6 scope optically, there is a great deal of difference from a
useability/transportability standpoint.

My 12.5 inch scope has an OTA that is 52 inches long and fits in my Ford
Escort. With a Paracorr, it gives very nice views even widefield.

From my point of view, if I am going to get a scope that has the hassle of
dealing with a 75 inch long focal length, I am going to make sure that it
has
enough aperture that it will be a significant improvement over my current
scope.

Also, the difference between a 10 incher and a 12.5 incher is noticeable
but
not earth shattering, so if your current scope is in the 10 inch range,
you
probably want to go significantly bigger.

I think a good choice would be a Truss DOB with optics from a 16 inch
Meade
DOB.

Bottom line is that its about getting the most aperture into the most
useable
package. IMHO, the gains above F5 are not worth the added OTA length.

jon

The other thing I wanted to say (hit send too fast there :-) was: if you
haven't already got a copy, you should think about grabbing the program Newt
off the net, to play around with different secondary size/focuser size/f
ratio combinations and to check out vignetting, illuminated area and such
like. Even if you're not thinking of building the scope you can decide
which combination of compromises you want to go ahead and buy.
Regards (again), Mike
--
Non, Je Ne Regrette Rien. E.P.
"Michael Kreuzer" wrote in message
...
An article on issues relating to f ratio & mirrors that you might find
interesting:
http://www.rfroyce.com/mirror_performance.htm

An article on why 14.5 inches is better than 12 or 16 for various deepsky
things that you can extrapolate to planets too, perhaps:
http://www.tscopes.com/T14/AboutT14.html

Regards, Mike
--
Non, Je Ne Regrette Rien. E.P.
"Jon Isaacs" wrote in message
...
OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2"

I'm 6 ft 1". I prefer to view seated. What is your preference?

The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up
here...is
it

The vast majority of the time the views will not be limited by your
optics
but
rather by the seeing. Also, consider that only the center of the FOV is
important for planetary viewing...

A 15 incher at F5 will have about the same eyepiece height as a 12.5
incher at
F6. A 16 incher would be F4.7.

A fast scope with a Paracorr will give pretty good wide field views when
used
with good quality eyepieces and it is the aperture you are after here.
A
16
inch scope will have 63% more mirror area than a 12.5 incher.

I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?

There is not a great deal of difference between a 12.5 inch F5 scope and
a
12.5
inch F6 scope optically, there is a great deal of difference from a
useability/transportability standpoint.

My 12.5 inch scope has an OTA that is 52 inches long and fits in my Ford
Escort. With a Paracorr, it gives very nice views even widefield.

From my point of view, if I am going to get a scope that has the hassle
of
dealing with a 75 inch long focal length, I am going to make sure that
it
has
enough aperture that it will be a significant improvement over my
current
scope.

Also, the difference between a 10 incher and a 12.5 incher is noticeable
but
not earth shattering, so if your current scope is in the 10 inch range,
you
probably want to go significantly bigger.

I think a good choice would be a Truss DOB with optics from a 16 inch
Meade
DOB.

Bottom line is that its about getting the most aperture into the most
useable
package. IMHO, the gains above F5 are not worth the added OTA length.

jon

"Doink" wrote in message
...
I want a good quality truss Dob around $2200-$2500

Primary issues:
Eyepiece height
Portability
Cooldown

Secondary issues:
Size of the secondary
Maximum field of view

Discussion:
Eyepiece height needs to be around 9 inches lower than your total body
length, in order to stand comfortably and view at the zenith. I'm 5' 7" and
through experimentation I've discovered that 58" is the most comfortable. I
can go to 60", but it requires too much stretching for best comfort (I have
a 12.5" F4.8 sonotube Dob).

Portability means a lot of different things to different people. I only need
to move a scope from the garage to the backyard, so it's possible to have
any size scope I like.. that is, portability isn't really an issue, given a
little ingenuity, and wheels. Having a truss dob isn't important for this,
although it would take up less room in the garage. The difference in
portability between a 12" truss and an 16" truss isn't likely something to
even think about, unless you have a really small car, or don't want to use
ramps and wheel barrow handles to roll it up into the back of the mini-van.
I do know that a 16" Tscope base with mirror installed can be lifted by one
reasonably strong-backed individual, but if lifting it a lot is in the plan,
smaller is better appreciated. I'm guessing the 14" Tscope (in particular)
is pretty easy to handle this way.

Cooldown, in my mind, is the biggest of all issues when it comes to a
reflector. The difference in cooldown times between mirrors with a
difference of 4" in aperture isn't insignificant.There are ways to mitigate
this, and the level of thermal management one requires is based a lot on
geographic location. Here in Massachusetts, temperature drops occur rapidly
in the early evening, and can continue to do so through midnight, especially
during Spring and Fall. Once you achieve sufficient cooldown, you may have
to constantly play catch up... a fan is really a good idea, otherwise, a
smaller mirror that tracks changes more rapidly.

Size of the secondary (minor axis) is a design parameter in a large
reflector that one must take into consideration when planning the OTA, focal
length, illuminated field of view, and any back-focus requirements.
Generally you want to keep it at 20% of the diameter of the primary mirror
for a good trade-off between high contrast and shortened focal length
(shorter tube length, and lower zenith eyepiece height).

That said, the XT10 I once owned was 25% obstructed and it did a good job on
planets, as well as deep sky under dark conditions. My F4.8 12.5" Dob has a
2.6" (20%) secondary and it performs better than the XT10 on planets (and
everything else too). Difficult to claim it's the difference in CO, since
there is also the additional resolution of a 2.5" more aperture. However, as
a reference point, the 12.5" with 2.6" Secondary, performs better than the
10" with 2.5" secondary on every object, but the 10" F5 was easier to
transport (again not an issue for me really).

Maximum field of view is determined solely by focal length of the telescope
and the field stop of your largest usable eyepiece. One _must_ keep in mind
however, that an obstructed scope has an upper limit on the usable focal
length of an eyepiece, because as you increase the exit pupil (ep focal
length/ primary mirror focal ratio), you also increase the percentage of the
exit pupil that is occupied by the secondary obstruction, which then blocks
light from entering the eye.

An 8mm exit pupil might seriously degrade image brightness in an obstructed
scope. 7mm is considered a maximum, for example a 35mm eyepiece in an F5
obstructed scope or a 32mm eyepiece in an F4.5. These would then also be
wide field designs that use a 2" barrell for maximum field stop. (A 1.25"
barrel has a max field stop of just 27mm, where a 2" can go up to around
46mm field stop.)

Hope that helps, and gives you plenty to think about.

Alternatively, you could let someone else do all the thinking and planning.
If I had the cash you are talking about here, I'd go right ahead and order
the 14" Tscope from Ed Taychert. I wouldn't give it a second thought. His
14" scope has a good eyepiece height for me. I'd get the fan option though.
Maybe even two fans, even if it requires a little custom work on his part.

-Stephen

Doink wrote:
OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2" The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is it
better to have another 2" of aperture or is it better to be a bit "slower"?

12.5" is an excellent size. I think 15" is ideal, but 12.5" is excellent.

Jane's Litebox 12.5" goes with us pretty much everywhere.
http://www.litebox-telescopes.com

Mojo
--
Morris Jones *
Monrovia, CA

http://www.whiteoaks.com

On Tue, 26 Oct 2004 21:43:05 -0700, "Doink"
wrote:

OK, so what's the "ideal" and practical Dob size? I'm struggling with what I
want to seek out as far as a Dob goes...yes, I can try them all at a star
party....

Ed Taychert makes an argument for 14" being the ideal size on this
web page: http://www.tscopes.com/T14/AboutT14.html

Note: I have no connection to Taychert Telescopes, other than being a
satisfied owner of one of his scopes.

-Paul W.

----------
Remove 'Z' to reply by email.

"Doink" wrote in message ...
OK, so what's the "ideal" and practical Dob size? I'm struggling with what I
want to seek out as far as a Dob goes...yes, I can try them all at a star
party....

OK, I've arrived at 12.5" F/6 with a good 1/10 wave or better pyrex
mirror...I'm 6'2" The f/6 should be a bit better on planets than a f/4.5
right? Is 12.5" enough---silly, it's never enough---but is it? Is it
better to go 14 or 15 " and say f/4.5 or f/4? When you get up here...is it
better to have another 2" of aperture or is it better to be a bit "slower"?


Well for planets, unless you enjoy truely extraordinary seeing, you
probably will not gain a huge amount of planetary detail by moving to
a larger scope; if you want to view deepsky though bigger is better;
though even 12.5" will bring thousands upon thousands of clusters,
nebulas and galaxies into reach.

Regarding focal length. Longer focal lengths do have some advantages.
1. In general you can have a smaller secondary to achieve a reasonable
fully illuminated field of view; though in larger scopes even fairly
fast ones can keep the secondary reasonably small. Also of course the
disadvantage of slow scopes in this regard is that once you move out
of the 100% illumination area, illumination drops off rapidly. A
faster scope you can cheat a bit knowing that you really won't notice
if illumination drops off to 70% or so at the edge of the field.

2. Collimation is easier in slower scopes, which is definitely an
issue in Truss scopes that are broken down after use. On the flip
side with a good mirror cell and a laser collimator you can probably
get collimation adjusted in 5-10 minutes.

3. Mirror Quality is an issue. If you are ordering from Discovery or
some other similar mid-priced mirror producer, slow mirrors will tend
to be of higher quality. If you are getting a Zambuto, Spooner, Royce
or other similar high end maker they will give you an excellent
product regardless the f-ratio.

One consideration regarding focal length is that a 12.5" f/6 is
probably going to place the eyepiece a little above your eye-level at
Zenith. If possible I would want to do a 12.5" f5.5. Granted having
to use a step stool or small ladder is not the end of the world, but
in general I think the smaller scopes are easier (and thus more likely
to be used) to use when you can keep both feet on the ground.

Regarding size versus focal length; well in general assuming
everything else is equal, a 15" f/5 Dob is going to be a better scope
than a 12.5" f/6 Dob. The key of course is everything else being
equal. I think going for a slightly smaller scope like the 12.5" is a
good idea if it means you can afford a much better mirror and
mechanicals. A pristine 12.5" mirror will be better than a mediocre
15" (Though theorretically you might still see more in the 15", the
views through the 12.5" will be more pleasing).

I want a good quality truss Dob around $2200-$2500


Well the more you limit your budget, the more sense if makes to go
with the smaller scope if you want to stay within the high quality
range.

How much better is a 1/10 wave or better mirror compared to a mass
produced---Orion? 12.5" mirror?

Well 1/10 wave mirrors is kind of a fuzzey definition of mirrors.
That being said, Orion's reflectors have been rather good as of late;
I haven't seen the 12.5 but an 8" and 10" I looked through were both
good scopes; real bargins for their price. I would guess that they
would give you about 90% of the pristine view that a high end optic
installed in a similar telescope would give. Over all I would guess
that the overall experience would be about 80-85% of the Custom Dobs..
but at half the price or less. Of course the 12.5" in a solid tube is
going to be a real beast to transport particularly if you lack an SUV
or pick-up.


I need a discussion. Cloydy nights doesn't have many reviews in this
range...why?


Because people have a tendency to go rather big when they make the
jump to truss-tube Dobsonians. I have seen more 17.5"-20" Dobs than
15" Dobs and 12.5" truss tubes are rather rare (at least where I do
most of my observing). (Though 10-11" starmasters seem reasonably
common . Its a shame really I think of the 15" f/4.5 as something
close to the idea size in reflectors; small enough to observe with
feet planted firmly on the ground (well at least if your 6'0 tall )
but big enough to give some impressive views of the deep skies.

--
Bill