Building first dob - questions

Hi folks,

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.

- 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?

- 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).

Thanks in advance,

Nate Perkins
Ft Collins, CO

- 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.

You don't want to ever use a step stool/ladder if you can help it, so try
and build it so that you (and those most likely to observe with you) can
comfortably look through the eyepiece standing when the scope is pointed
nearly straight up. Then, you can use an adjustable observers chair
(commercial, homemade Denver, etc) to support your backside when you are
observing lower.


- 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?

We like 45degrees. A great feature is a rotating upper assembly, but this
is hard to build in such a way that collimation is preserved.


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

No magic here. Baltic birch/appleply is much better than regular plywood,
but there have been plenty of scopes made out of other materials.

Most important is to get the bearings right, i.e. no stiction. I would
stick with Ebony Star and Teflon unless I was willing to research/experiment
and make modifications.

Dennis

In article ,
says...

A couple of questions for you more experienced ATM builders:

....can't guarantee I fully qualify there, but a few comments.


- 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.

The eyepiece height will be mostly dictated by the focal length, with
some variance available in the height of the rocker box, ground
board, etc. I suggest making them overall as short as possible for
stability.


- 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?

This is apparently a personal preference. On the 10" I built back in
'91, I initially had it 90 degrees (flat), but later changed it to
45. On that scope, it worked great. On my current scope (14.5"), it's
about 20 degrees above horizontal. In general, you're trading off
ease at low altitudes against difficulty at higher ones. Given your
size scope, I personally would err towards making it easier at low
altitudes (i.e., I'd probably make it about 30 degrees above
horizontal). Just keep in mind that the "higher" it is, the more it
ends up "behind" the tube at high altitudes. With a 12.x" scope, the
mirror/rocker box combo probably won't be so large as to get in the
way of observing at that position, even if the eyepiece is pointing
relatively backward.


- 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

True. However, stability and flatness are desireable attributes
pretty much /everywhere/ on a Dob.


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).

I understand your desire for a nice looking wood - Me too! If a 15-
ply 3/4" oak plywood was available, I'd use it (mayb it is...)!
However, BB and AP /are/ nice looking woods, too, IMO. Also, I'd
strongly recommend keeping the rocker box walls double thick to
prevent flexure. That's also the reason to keep them as short as
possible. In my old Sky Designs Dob, you could pull on the end cage
and see the single-thickness rocker box walls flex and bend before
any of the bearings gave way and moved (and they weren't that stiff,
either). Bottom line is that they were too tall and too thin to be
very stable. That translated to a spongy feel with jumpy images at
the eyepiece.

If you haven't already, I strongly suggest reading "The Dobsonian
Telescope" by Dave Kriege and Richard Berry. LOTS of good info in
there. It's published by Willmann-Bell and is available from their
website (www.willbell.com). The cost of the book will be repaid many
times over the course of designing and building a kit. Plus, it's
just fun to read if you like to build telescopes.

There are also various companies out there that sell telescope
components that could be useful (e.g., AstroSystems, etc.).

Have fun though, at any rate!

--

-- Len Philpot - --
------ ----- http://philpot.org/ --

This will take a bit of work -- but -- go to Google and search for such
terms as:
-- dobsonian
-- building a dobsonian
-- my homebuilt dob
-- etc., etc.

I did that one night and found several sites where folks who built their own
Dobs posted photos and descriptions of what they did, how, and why. Some
beautiful scopes out there, quality woodworking.

--

-----
Joe S.



"Nate Perkins" wrote in message
om...
Hi folks,

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.

- 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?

- 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).

Thanks in advance,

Nate Perkins
Ft Collins, CO

"Nate Perkins" wrote in message
om...

- 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.

58" is ideal for a 5' 7" person. Lower is better if you like to sit all the
way through the range. 60" is the maximum for standing at zenith.


- 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. This helps
with how close you have to stand to the scope when aimed a few degrees below
zenith. Not sure I can explain that well, but I found that with the eyepiece
aimed at 45 degrees upward, there was a range of motion where getting to the
eyepiece was more difficult (my toes would be crammed up against the base).
I sacrificed a little bit of horizon comfort, since I rarely observe
anything that low. Going full on parallel, makes the loss of horizon even
greater, so you want to avoid that as much as possible.

That's my nickel,
Stephen Paul
(My solid tube Dob has been modified several times to make it more comfy.)

"Joe S." wrote in message ...
This will take a bit of work -- but -- go to Google and search for such
terms as:
-- dobsonian
-- building a dobsonian
-- my homebuilt dob
-- etc., etc.

I did that one night and found several sites where folks who built their own
Dobs posted photos and descriptions of what they did, how, and why. Some
beautiful scopes out there, quality woodworking.

Yes, of course. Naturally I did a Google search, and also looked
through lots of the sci.astro.amateur archives. I've also seen dozens
of those pages, but you may notice that the questions that I asked in
particular are rarely if ever addressed on them.

Uncle Bob wrote in message ...
Hi Nate!

I'm finishing up a 12.5" f/7 mirror, and starting to think about
dobsonian design for this project. I have already converted a Sky
Designs 18" Dob to an "Obsession Clone" using Kreig&Berry's book.
The conversion went well pics he
http://www.bogusnet.net/gallery/album04

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. ;-)

Hi Uncle Bob,

I had seen your website when I was doing some initial looking. It's a
great website! Terrific setup, pictures, and description.

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.

Ultimately, I'd like to motorize and field-derotate both scopes,
starting with the 12.5 (it's going to be a planetary scope, primarily).
Mel Bartels has released a new product that seems to to just that (goto
and field derotation) for a modest cost. More he

http://www.bbastrodesigns.com/NewPro...ouncement.html

I had seen a lot of Mel's designs, and at some point I might try to
build a little equatorial platform. I am mostly doing visual at this
point, and so field rotation is not much of a problem for me. His
stuff is very neat, though.

Good luck with your telescope. It's a wonderful feeling to go out with
a scope you built and know intimately. Keep us posted about your project.

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

CDSTY!
Uncle Bob
Marin County, CA

Len Philpot wrote in message ...
....
- 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?

This is apparently a personal preference. On the 10" I built back in
'91, I initially had it 90 degrees (flat), but later changed it to
45. On that scope, it worked great. On my current scope (14.5"), it's
about 20 degrees above horizontal. In general, you're trading off
ease at low altitudes against difficulty at higher ones. Given your
size scope, I personally would err towards making it easier at low
altitudes (i.e., I'd probably make it about 30 degrees above
horizontal). Just keep in mind that the "higher" it is, the more it
ends up "behind" the tube at high altitudes. With a 12.x" scope, the
mirror/rocker box combo probably won't be so large as to get in the
way of observing at that position, even if the eyepiece is pointing
relatively backward.

Terrific, Len. Just the kind of experienced input I was looking for.
Most of my previous observing is on an SCT or a Mak-Newt, so these
comments are very useful to me. Many thanks.

If you haven't already, I strongly suggest reading "The Dobsonian
Telescope" by Dave Kriege and Richard Berry. LOTS of good info in
there. It's published by Willmann-Bell and is available from their
website (www.willbell.com). The cost of the book will be repaid many
times over the course of designing and building a kit. Plus, it's
just fun to read if you like to build telescopes.

You are right. I've been an amateur long enough to hear a lot about
the book, but I've never got a copy of it. I will grab one through
interlibrary loan this weekend.

Hi Stephen,

Your comments were just what I needed. Many thanks to you and the
other folks who posted replies.

I'll put some pics online in a couple of months when the project's
finished. My mirror is on backorder, with an estimated 2 month
delivery time. That's no problem since it will probably take me two
months to build the rest of the scope anyway.

Cheers,
Nate Perkins
Fort Collins, CO
www.ncastro.org (club website)
home.earthlink.net/~nateperkins1 (personal website)

"Stephen Paul" wrote in message
...

"Nate Perkins" wrote in message
om...

- 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.

58" is ideal for a 5' 7" person. Lower is better if you like to sit all
the
way through the range. 60" is the maximum for standing at zenith.


- 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. This
helps
with how close you have to stand to the scope when aimed a few degrees
below
zenith. Not sure I can explain that well, but I found that with the
eyepiece
aimed at 45 degrees upward, there was a range of motion where getting to
the
eyepiece was more difficult (my toes would be crammed up against the
base).
I sacrificed a little bit of horizon comfort, since I rarely observe
anything that low. Going full on parallel, makes the loss of horizon
even
greater, so you want to avoid that as much as possible.

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. Always easily accessible, and straight
ahead at all altitude angles, without having to look up, down, or wrap
around the tube and look to one side or the other... I find it much
easier to remain relaxed and can hold myself much more steady at the
eyepiece than while standing. This lets me be more focused on the image,
and helps me squeeze out more subtle details in images, that I might have
missed otherwise. Perhaps I have ants in my pants, as my Dad used to say,
so standing at the eyepiece doesn't work well for me... But, on the other
hand, if your preference is to view standing, the angled approach often
works best, if not angled too far...

Since I'm 6'4", this straight-out-the-side orientation works much better
for me, even with larger scopes; perhaps less so, though, for others not
be as tall. I also have an observing chair that lets me go pretty high
before I have to stand up (but that wouldn't happen if I were using a 12"
f/5)...

--
Jan Owen

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