Black tube and eyepieces, two separate questions

Hello everyone,
I'm an astro-newbie so forgive me if these are fundamental questions, I
haven't found the answers in my research.
1. I built a 10" f4.7 scope and, for aesthetic reasons, painted it black.
Now I'm wondering if heat dissipation isn't happening as well as it could.
Star test always seem to be dancing or waving. And, the scope seems to
crap-out at high mags (anything over 150x) especially when viewing planets.
I believe collimation to be right-on, I have the secondary offset, and a
good set of Plossls. I saw a design that used two computer power supply
fans venting air directly over the primary. Does anyone have an opinion
here? I would fear the fans would cover the mirror in dust/
2. Is there any such animal as a short focal length (say 10mm and lower) 2"
eyepiece and would there be any advantage? Any advantage to using a 2"
Barlow to achieve the same?
Thanks for your time,
-Mac

The telescope can be seen at:
www.macs.studio.web1000.com
if interested.

Now I'm wondering if heat dissipation isn't happening as well as it could.
Star test always seem to be dancing or waving.

And, the scope seems to
crap-out at high mags (anything over 150x) especially when viewing planets.

You say you are an astro-newbie so before you get too concerned....

Seeing and thermal equillibrium... If the seeing is not good (turbulence etc)
then nothing you do to your scope will help.

If the image is dancing that is a good sign that either the scope has not
cooled down or that the seeing is bad.

How long are you waiting for it to thermally equillibrate? Where do live? some
parts of the country are famous for bad seeing...

jon

1. I built a 10" f4.7 scope and, for aesthetic reasons, painted it black.
Now I'm wondering if heat dissipation isn't happening as well as it could.
Star test always seem to be dancing or waving. And, the scope seems to
crap-out at high mags (anything over 150x) especially when viewing
planets.
I believe collimation to be right-on, I have the secondary offset, and a
good set of Plossls. I saw a design that used two computer power supply
fans venting air directly over the primary. Does anyone have an opinion
here? I would fear the fans would cover the mirror in dust/
2. Is there any such animal as a short focal length (say 10mm and lower)
2"
eyepiece and would there be any advantage? Any advantage to using a 2"
Barlow to achieve the same?
Thanks for your time,
-Mac



1. Is there a fan blowing on the back of the mirror? How thick is the
mirror, what is it made out of, and how is it mounted? It can be difficult
to tell the difference between bad seeing and tube currents. However, if
you are now blowing on the back of the mirror and up the tube then I think
you should eventually (a couple of hours max) achieve equilibrium, and then
could rule out tube currents. However, seeing isn't always bad. Do other
folks you observe with get good views?

We have a couple of homemade dobs with fan(s) blowing across the primary,
but we had to do this as they are flexed mirrors, and so blowing on the back
is not an option. We get great views with this method, but I know that
there are concerns about vibration if the fans are not properly mounted.

2. There is nothing wrong with good plossls, and you will not gain anything
by replacing them with similar quality short focal length 2" eyepieces.

Do you know the quality of the primary and secondary mirrors?

Dennis

Mac wrote:

1. I built a 10" f4.7 scope and, for aesthetic reasons, painted it black.


There is another reason, and a very good one, for painting the tube black or
some other very dark color. When observing with both eyes opened, a tube of
a brighter hue can be quite distracting to the eye which is over it and not
the eyepiece. I painted my tube black a while back and have never regretted
it. Oh yeah, by the way. . .I also have a fan installed.


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

I did omit some pertinent information, my apologies.
The Primary mirror is from Orion (I planned on upgrading later if necessary)
It's pyrex, diffraction limited, and, if I remember correctly, about 1.5"
thick. The secondary is also Pyrex and 1/10th wave.
I built the cell to accept a fan but haven't installed one yet.
I live in the midwest and, since finishing the scope, it's been constant
thunderstorms and the dew has been frustrating.
So far I've been the soul observer at my dark sky site so I haven't had the
opprtunity to compare to other instraments (but that should change soon).
My question about 2" eyepieces wan't meant as a solution to the problem, I
just like the larger optics.
Thanks for all the input.
-Mac

The Primary mirror is from Orion (I planned on upgrading later if
necessary)
It's pyrex, diffraction limited, and, if I remember correctly, about 1.5"
thick. The secondary is also Pyrex and 1/10th wave.
I built the cell to accept a fan but haven't installed one yet.

The May and June 2004 issues of Sky&Tel contain articles by Brian Greer
titled "Improving the Thermal Properties of Newtonian Reflectors". I think
that you need to add your fan, and you need to compare your performance with
other scopes of known quality.

Dennis

"Mac" wrote in message link.net...

1. I built a 10" f4.7 scope and, for aesthetic reasons, painted it black.
Now I'm wondering if heat dissipation isn't happening as well as it could.
Star test always seem to be dancing or waving. And, the scope seems to
crap-out at high mags (anything over 150x) especially when viewing planets.
I believe collimation to be right-on, I have the secondary offset, and a
good set of Plossls. I saw a design that used two computer power supply
fans venting air directly over the primary. Does anyone have an opinion
here? I would fear the fans would cover the mirror in dust/

Only the inside needs to be black, the outside can be of any color, however,
having the outside a light color makes one less likely to bump into it in
light free conditions.

I suggest that you set up your scope and use it with the fan on for the
first 2 hours one night. Then turn off the fan and see if the stars still
waver around. 2 hours gives the mirror sufficient time to cool down;
and turning the fan off separates the jiggle of a vibrating fan from the
jiggle of an unsteady atmosphere. The jiggle can also be in the hand of
the user. don't touch the scope for 10 seconds while watching the star.
Do the vibrations slowly settle out? Until you master the vibrations
you can't really use higher power than 150.

When you run into a steady night, then do your collimation. F/4.7 is not
a very tollerant F/ratio and you need to get it bang on. Since Polaris
does not move much, you can do colimation on it even on a hand powered
scope, although a laser collimator or Chessire will get you close, a high
power EP will allow you to see the miscolimation easier. After each and
every collimation adjustment recenter the collimation star in the exact
center of the EP.

Ignore the dust on the mirror until you plain just can't stand it any longer.
Then budget 2 hours to clean it with exceeding care.

2. Is there any such animal as a short focal length (say 10mm and lower) 2"
eyepiece and would there be any advantage? Any advantage to using a 2"
Barlow to achieve the same?

When I perform fine collimation on my 20" DOB F/4 I end up using a 2X
barlow and a 7mm EP and still want more power. Since it is hand tracked,
Polaris is the best colimation object.

Thanks for your time,
-Mac

Mitch

The primary will take a couple of hours to cool down if you don't
install the fan. I would recommend that you do so. Then your mirror
should cool down in 1/2 hour or so, depending on the temperature
differential between storage and outside.

It makes a huge difference in seeing if the mirror has not cooled
down.

Mark

On Wed, 02 Jun 2004 12:58:25 GMT, "Mac"
wrote:

I did omit some pertinent information, my apologies.
The Primary mirror is from Orion (I planned on upgrading later if necessary)
It's pyrex, diffraction limited, and, if I remember correctly, about 1.5"
thick. The secondary is also Pyrex and 1/10th wave.
I built the cell to accept a fan but haven't installed one yet.
I live in the midwest and, since finishing the scope, it's been constant
thunderstorms and the dew has been frustrating.
So far I've been the soul observer at my dark sky site so I haven't had the
opprtunity to compare to other instraments (but that should change soon).
My question about 2" eyepieces wan't meant as a solution to the problem, I
just like the larger optics.
Thanks for all the input.
-Mac

Thanks for all the info. I'll get the fan going ASAP. I believe the
collimation is accurate, I use a cheshire/site tube first, then a laser,
then star test at high power on Polaris. Compared to the images in several
books, it looks very close (except for the shimering caused by turbulents).
Does using a fan cause an increase in tube currents?
Thanks again,
-Mac