Building first dob - questions

Nate Perkins wrote:
Uncle Bob wrote in message ...

For the 12.5, I'm leaning more towards the Plettstone variety of dob
design, found he
http://plettstone.com/telescopes/con...on_details.htm

This design seems very well thought out in terms of breakdown/setup
simplicity and transportability. The link describes much of their
design philosophy in more detail.


That is a very neat design. I had not seen the Plettstone before.
One thing I especially like is the way that the alt bearings mount
directly to the tube. It seems like you could make the clamps that
hold the tube to the alt bearings adjustable, which might give you
freedom to rebalance the scope depending on what you load on the
observing end.

I saw these scopes at the Shingletown star party in N. California last
summer. The clamps are adjustable, so you have some leeway with
balance. Additionally, the finder scope is attached to the tubes, and
it can be moved up and down to allow some balance correction as well.
What I like is the simplicity of design that allows you to use higher
quality materials, but less of them, so you keep costs down.
Oh, and the truss poles are all different diameters, so they nest inside
each other for transport, and two of them are used as wheelbarrow
handles to move the scope from shed or vehicle to observing location.
Downright clever, I'd say.

Thank you! I'll post some pics online when I'm finished. It's likely
to be about 2-3 months till first light.

Hurry! Jupiter and Saturn are coming (to the evening sky, that is)! ;-)

Please try to take some shots as you design/build your scope, to help
pave the way for the next builder who may be looking for ideas.


CDSTY!
Uncle Bob
Marin County, CA

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(Nate Perkins) wrote in message . com...
Hi folks,

Ok, I will give a shot at answering the questions.


I'm getting ready to build my first dob. I've got a 12" f5 mirror
coming to build the dob around. I'm an experienced woodworker and
have previously observed with either a C8 or (most recently) with an
Intes MN66.

A couple of questions for you more experienced ATM builders:

- What's the optimum eyepiece height to design the dob at? I observe
with my little Mak-Newt using a drummer's stool, but the dob is going
to be much higher. So do you try to design it to do most of the
observing standing or sitting? I'm relatively short at about 5'7"
tall.


Not sure that there is such a thing in Dobsonian Design. Eyepiece
height will be a function of the focal length of the mirror. Longer
focal lengths require higher eyepiece. with an f-ratio of about 60",
you are probably looking at an eyepiece height around 61 or 62". This
should allow you to view standing up when at zenith and sitting when
viewing at lower altitudes. You can of course increase the height of
the eyepiece at low altitudes by increasing the height of the altitude
bearings but that is going to make the rocker box significantly
heavier and probably not as steady either.

- What's the optimum eyepiece angle? Do you normally set the eyepiece
axis to be parallel to the ground, or do you orient the secondary cage
so that it tilts up a few tens of degrees?


Depends on the scope; larger scopes it makes sense to have the
eyepiece parallel to the ground, in smaller scopes the angel makes
more sense. At the size we are talking here we are right about at the
cut off between which makes the most sense. Be prepared to experiment
a little to see what works best for you.


- Why the widespread use of Baltic birch and apple ply? These woods
make sense to me around the ground board and azimuth bearings, for
stability and flatness. But it seems like the secondary cage could be
of other materials, and in particular you could make the primary
mirror box and the base for the alt bearings out of some more
attractive woods (say walnut, maple, or cherry in 1/2" thickness with
a poly varnish finish).


Stability is good in all parts of the telescope. Yeah you could make
the scope out of any wood you want, but baltic birch and appleply have
become popular for their usefulness. Mostly though it is a matter of
practicality. If you are buying a 5'x5' sheet of baltic birch, unless
you scope is going to be rather large a single sheet will take care of
all of the wooden parts of the scope (Out club made a 17.5" with one
sheet of baltic birch). ATMers tend to often be a decidedly
utilitarian lot and will usually trade appearance issues for price or
practicality. Just be glad you don't live in the days when ATMers
thought an engine block of a truck axel made the ideal poloar axis .

--
Bill

"Jan Owen" wrote in message
news:1ERed.78925$cJ3.72795@fed1read06...

"Stephen Paul" wrote in message
...

"Nate Perkins" wrote in message
om...

- What's the optimum eyepiece angle? Do you normally set the eyepiece
axis to be parallel to the ground, or do you orient the secondary cage
so that it tilts up a few tens of degrees?

Tilt it up. If possible, don't go the typical 45 degrees, pick something
that keeps the eyepiece a little lower when aimed at the horizon.

The focuser/eyepiece orientation is a matter of personal preference,
closely related to how you prefer to observe. I always orient my focuser
to be parallel to the altitude bearings, because I prefer to do my
observing seated. This moves the focuser in an arc directly in front of
me, and never at an awkward angle.

Interesting. I didn't find this to be the case with the 12.5" F4.8, which is
setup with a zenith eyepiece height of 58". On the other hand, my old XT10
had a 45 degree focuser position and I had no trouble with that one at all
(don't remember what the zenith eyepiece height was with that scope, but it
worked out fine for seated viewing throughout its range of motion).

I tried both 0 degrees and 45 degrees from horizontal in the 12.5-incher. At
0 degrees, low altitudes meant lowering the Starbound seat so far down as to
be uncomfortable. At 45 degrees, zenith altitudes found my toes against the
base, and even then I'd still have to "wrap" myself around the OTA. And I
_had_ to stand for that pleasure too.

So I pulled the bearings off (yet again), and reoriented the OTA to point
the eyepiece at 22.5 degrees and that made the scope very comfortable
throughout its range.

Although, at 22.5 degrees, the binoviewer turns out to be a little difficult
at lower altitudes. Down that low, I tend to arc my torso to the right a
little bit to look into the eyepiece. With the binoviewer down there, I need
to also turn my neck a little further to get both eyes at the correct
distance from the oculars. With one eyed viewing, I don't need to turn my
head quite so far, and the position is prefectly comfortable.

-Stephen

Hi Nate, I went with 45 deg on my 10" F6, no probs. If you want to get
on with the joinery while waiting for the optics, cast a concrete
mirror blank to your dimensions; it won't be too far off when you come
to balance up.
jc

--
http://mysite.wanadoo-members.co.uk/jc_atm/



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"Stephen Paul" wrote in message
...

"Jan Owen" wrote in message
news:1ERed.78925$cJ3.72795@fed1read06...

"Stephen Paul" wrote in message
...

"Nate Perkins" wrote in message
om...

- What's the optimum eyepiece angle? Do you normally set the
eyepiece
axis to be parallel to the ground, or do you orient the secondary
cage
so that it tilts up a few tens of degrees?

Tilt it up. If possible, don't go the typical 45 degrees, pick
something
that keeps the eyepiece a little lower when aimed at the horizon.

The focuser/eyepiece orientation is a matter of personal preference,
closely related to how you prefer to observe. I always orient my
focuser
to be parallel to the altitude bearings, because I prefer to do my
observing seated. This moves the focuser in an arc directly in front
of
me, and never at an awkward angle.

Interesting. I didn't find this to be the case with the 12.5" F4.8,
which is
setup with a zenith eyepiece height of 58". On the other hand, my old
XT10
had a 45 degree focuser position and I had no trouble with that one at
all
(don't remember what the zenith eyepiece height was with that scope, but
it
worked out fine for seated viewing throughout its range of motion).

I tried both 0 degrees and 45 degrees from horizontal in the
12.5-incher. At
0 degrees, low altitudes meant lowering the Starbound seat so far down
as to
be uncomfortable. At 45 degrees, zenith altitudes found my toes against
the
base, and even then I'd still have to "wrap" myself around the OTA. And
I
_had_ to stand for that pleasure too.

So I pulled the bearings off (yet again), and reoriented the OTA to
point
the eyepiece at 22.5 degrees and that made the scope very comfortable
throughout its range.

Although, at 22.5 degrees, the binoviewer turns out to be a little
difficult
at lower altitudes. Down that low, I tend to arc my torso to the right a
little bit to look into the eyepiece. With the binoviewer down there, I
need
to also turn my neck a little further to get both eyes at the correct
distance from the oculars. With one eyed viewing, I don't need to turn
my
head quite so far, and the position is prefectly comfortable.

-Stephen


Like I said, it's mostly a matter of personal preference... There is no
one correct solution for all. Folks should try and use a number different
scopes and as many different relative focuser positions as possible, then
make up their own minds. Once an observer finds the right combination for
THEM, what Jan Owen or Stephen Paul may have to say on the matter will
only serve as alternative perspectives...

How big are your feet?

--
Jan Owen

To reach me directly, remove the Z, if one appears in my e-mail address...
Latitude: 33.662
Longitude: -112.3272

One thing I learned was to keep the secondary cage as light as possible
and to find the balance point before you build the rocker box. ;-)

Yes! You have it! Kriege and Berry show you some simple math for
figuring the moment arm on both ends before you build. That gets you
close


For the 12.5, I'm leaning more towards the Plettstone variety of dob
design, found he
http://plettstone.com/telescopes/con...on_details.htm

This design seems very well thought out in terms of breakdown/setup
simplicity and transportability. The link describes much of their
design philosophy in more detail.

That is a very neat design. I had not seen the Plettstone before.
One thing I especially like is the way that the alt bearings mount
directly to the tube. It seems like you could make the clamps that
hold the tube to the alt bearings adjustable, which might give you
freedom to rebalance the scope depending on what you load on the
observing end.

Yes, that is one way to do it. But there is another way that is much
more elegant and user friendly.

One of the key features of the Plettstone design (and yes, I build the
Plettstone scopes) is that I can provide an optional counterweight
that rides up and down one of the vertical struts. This means that
you don't have to ugly your scope by adding weights anywhere and it
also means that balancing for your light weight scope to use a 1/2
pound plossl or a heavy bino viewer with panoptics is very easy to
do... typically without moving from your observing position.

BTW, if you are going the ATM route and are looking at the three strut
design that I employ, I'm happy to answer any questions via email. I
love working with other ATM'ers. You can contact me through my
website.

Michelle Stone
Argo Navis and
Custom Telescopes by Plettstone
http://www.plettstone.com/telescopes

(Nate Perkins) wrote in message . com...

A couple of questions for you more experienced ATM builders:

- What's the optimum eyepiece height to design the dob at? I observe
with my little Mak-Newt using a drummer's stool, but the dob is going
to be much higher. So do you try to design it to do most of the
observing standing or sitting? I'm relatively short at about 5'7"
tall.

Nate, the height of the eyepiece is a function of the focal length of
the mirror and how low you can get the mirror to the ground. So, as a
quick response, for a 12.5" F4.2, you'll be sitting all the time. For
a 15" F5 on the other hand, you'll be standing most of the time and
you'll also need a step stool.


- What's the optimum eyepiece angle? Do you normally set the eyepiece
axis to be parallel to the ground, or do you orient the secondary cage
so that it tilts up a few tens of degrees?

For a 12.5" design, I build my scopes so that the focuser is up 45
degrees. This makes it more comfortable when you are trying to hit
lower objects in the sky. For larget scopes, I set the focuser up 5
to 10 degrees. This will give you some advantage for the lower targets
but still be perfectly accessable on the ladder. Other companies
build larger scopes with the focuser at 0 degrees.


- Why the widespread use of Baltic birch and apple ply? These woods
make sense to me around the ground board and azimuth bearings, for
stability and flatness. But it seems like the secondary cage could be
of other materials, and in particular you could make the primary
mirror box and the base for the alt bearings out of some more
attractive woods (say walnut, maple, or cherry in 1/2" thickness with
a poly varnish finish).

Plywoods tend to fair better in the harsh conditions to which
telescopes are subjected. They don't shrink or swell as much. They
don't crack, split, or bend wild on you. And they hold screws very
well.

The Baltic Birch and Apple Ply are superior forms of plywood than what
you get at the local home center. Standard birch and oak offerings
have a few layers of a softer hardwood in the core. Baltic Birch and
Apple Ply have more than double the layers and use harder core
materials. These high density plywoods are superior for making a
telescope strong and reliable.

Now, that doesn't stop you from laminating a beautiful birdseye maple
veneer or walnut crotchwood onto them! (Yes, I love to build nice
furniture as well as scopes... see
http://plettstone.com/woodworking/custom_work.htm (click on images for
a larger version)

Michelle Stone
Argo Navis and
Custom Telescopes by Plettstone
http://www.plettstone.com/telescopes

(Michelle Stone) wrote in message . com...
(Nate Perkins) wrote in message . com...

Hi,

Thanks very much to you and the others for the replies. I am leaning
toward copying some aspects of the designs done by you and Albert
Highe.

One question you may be able to help me with. Here in Colorado, we
often fight frost and dew while observing. I was thinking that a
shroud might be necessary to avoid frosting on the primary (as well as
a heated secondary). I wonder if you have tried a shroud on the
three-pole design, and if it obstructs the lightpath?

I guess the question I'm asking is in general how large a problem is
primary dew and frost in the ultralight designs, where the primary
tends to be very shallow in the mirror box.

- Why the widespread use of Baltic birch and apple ply?
....

Plywoods tend to fair better in the harsh conditions to which
telescopes are subjected. They don't shrink or swell as much. They
don't crack, split, or bend wild on you. And they hold screws very
well.

The Baltic Birch and Apple Ply are superior forms of plywood than what
you get at the local home center. Standard birch and oak offerings
have a few layers of a softer hardwood in the core. Baltic Birch and
Apple Ply have more than double the layers and use harder core
materials. These high density plywoods are superior for making a
telescope strong and reliable.

Now, that doesn't stop you from laminating a beautiful birdseye maple
veneer or walnut crotchwood onto them! (Yes, I love to build nice
furniture as well as scopes... see
http://plettstone.com/woodworking/custom_work.htm (click on images for
a larger version)

Excellent idea! I also do furniture making (as a hobby only), and
I've done veneering and marquetry befo
http://home.earthlink.net/~nateperki...oodworking.htm

(Nate Perkins) wrote in message
Hi,

Thanks very much to you and the others for the replies. I am leaning
toward copying some aspects of the designs done by you and Albert
Highe.

One question you may be able to help me with. Here in Colorado, we
often fight frost and dew while observing. I was thinking that a
shroud might be necessary to avoid frosting on the primary (as well as
a heated secondary). I wonder if you have tried a shroud on the
three-pole design, and if it obstructs the lightpath?

I guess the question I'm asking is in general how large a problem is
primary dew and frost in the ultralight designs, where the primary
tends to be very shallow in the mirror box.

In our experience here in California, the scopes with shrouds dew up
about 15 minutes after the Plettstone scopes. That's with heavy dew
conditions. So, out come the hair dryers for all the scopes. And
it's easier to get to the mirror to dry it off if you don't have to
remove the shroud first Some of the materials used for standard
shrouds (rip stop nylon for example) are not waterproof. On my old
18" scope (made by someone else), if I touched the shroud when wet, it
would drip on my mirror... argh!

Secondary dew heaters are available on our scopes for an additional 45
dollar charge. Run your wires up and clip on to two ends of the spider
assembly and you are set. You can get these spider/diagonal assemblies
for your ATM projects from Protostar.

If you really feel the need to do a shroud on a three pole design,
you'll have to do a collapsing design with hoops in it to keep from
interfering with the light path. Also, you'll lose your sliding
weight/balance option (with the finder scope and/or weights).


- Why the widespread use of Baltic birch and apple ply?

Now, that doesn't stop you from laminating a beautiful birdseye maple
veneer or walnut crotchwood onto them! (Yes, I love to build nice
furniture as well as scopes... see
http://plettstone.com/woodworking/custom_work.htm (click on images for
a larger version)

Excellent idea! I also do furniture making (as a hobby only), and
I've done veneering and marquetry befo
http://home.earthlink.net/~nateperki...oodworking.htm

That's some beautiful work there! Very nice indeed!

Michelle Stone
Argo Navis and
Custom Telescopes by Plettstone
http://www.plettstone.com/telescopes

Hi Michelle,

Thanks for the kind replies. Your scopes are very impressive.

Cheers,
Nate