Best tube material for a planetary Newtonian?

In the spirit of Sketcher's open appeal: Let's see how we get on: :-)

The arrival of Carbon Fiber in the amateur telescope market offers a new option to the four main types: Metal tube, wooden tube, cardboard tube and all combinations of open truss.[Possibly with a cloth shroud]

CF tubing can be made as a solid mass or a laminated sandwich. Or even a combination of CF and GRP for local impact resistance. CF remains very expensive in comparison to all other options. Claimed advantages for CF are low self-expansion and greater stiffness for less weight.

Problems with existing [solid] tube materials:

Internal thermal currents clinging to the inner tube wall often affect the image due to variations of index with air temperature.

Super-cooling of metal tubes to the cold night sky increases the thermal differential between the cold air and the mirror. Wooden and cardboard tubes are almost immune to super-cooling but are very heavy when suitably stiff.

Heavy materials greatly increase the tube's moment: Making them far more difficult to mount well equatorially. Local movement and setting up of long tubes is increasingly difficult as tube's weight grows very quickly indeed.

The Dobsonian is usually considered ill-suited to high powers due to rapid object movement across the field of view.

The primary mirror is usually the greatest heat source in our [normally] cold air system. Thermal lag from full thickness, glass mirrors has led to the adoption of small electric fans and even conical mirror blanks.

Do the members have an opinion of which material and construction is most suitable? Bearing in mind that high magnification observation of fine, low contrast, planetary and lunar detail are the ONLY priority in this exercise.

A long tube is assumed with apertures below 12" to maximize the chance of reasonable seeing for most observers in our often difficult atmosphere.

Do try and stay on topic. ;-)

On Sunday, February 1, 2015 at 5:32:11 AM UTC-5, Chris.B wrote:



The Dobsonian is usually considered ill-suited to high powers due to rapid object movement across the field of view.

That is far from a universal opinion.

Anyway, there are equatorial tables as well as dual axis drives that might address your concerns.

There is also an obscure method of using a string or wire connected the upper end of the tube and the ground, which will impart "equatorial" tracking to an alt-azimuth mount.

On Sun, 1 Feb 2015 02:32:09 -0800 (PST), "Chris.B"
wrote:

Do the members have an opinion of which material and construction is most suitable? Bearing in mind that high magnification observation of fine, low contrast, planetary and lunar detail are the ONLY priority in this exercise.

My preference is for some type of truss design, because the light
weight and low moment make tracking easier (and I consider automated
tracking very important), and because good truss designs are very
mechanically and thermally stable.

A long tube is assumed with apertures below 12" to maximize the chance of reasonable seeing for most observers in our often difficult atmosphere.

For many reasons, this is a practical aperture limit. But planetary
viewing is one of the few areas where I think there's a real advantage
to even larger apertures. Large apertures don't degrade the view of
planets. The best views of planets I ever had were with huge apertures
at very high magnification. There are always moments of good seeing,
and a skilled observer learns to take advantage of them.

Of course, if we allow apertures to get larger than 12", truss type
tubes become even more attractive, ultimately becoming nearly the only
practical approach.

On Sunday, February 1, 2015 at 9:09:37 AM UTC-8, Lord Androcles wrote:

... And why not a CF mirror, or a CF or resin/fibreglass reinforced thin mirror
if CF is the way to go for lightweight rigidity?

See this page for a roundup of various lightweight mirror ideas that are being developed, including CF...

http://www.altazinitiative.org/AA%20Tech%20&%20Demo.htm

.... scroll about 1/3 of the way down the page. There is a photo there of 2 young girls holding a 40" CF mirror blank.

\Paul A

"Chris.B" wrote:
In the spirit of Sketcher's open appeal: Let's see how we get on: :-)

The arrival of Carbon Fiber in the amateur telescope market offers a new
option to the four main types: Metal tube, wooden tube, cardboard tube
and all combinations of open truss.[Possibly with a cloth shroud]

CF tubing can be made as a solid mass or a laminated sandwich. Or even a
combination of CF and GRP for local impact resistance. CF remains very
expensive in comparison to all other options. Claimed advantages for CF
are low self-expansion and greater stiffness for less weight.

Problems with existing [solid] tube materials:

Internal thermal currents clinging to the inner tube wall often affect
the image due to variations of index with air temperature.

Super-cooling of metal tubes to the cold night sky increases the thermal
differential between the cold air and the mirror. Wooden and cardboard
tubes are almost immune to super-cooling but are very heavy when suitably stiff.

Heavy materials greatly increase the tube's moment: Making them far more
difficult to mount well equatorially. Local movement and setting up of
long tubes is increasingly difficult as tube's weight grows very quickly indeed.

The Dobsonian is usually considered ill-suited to high powers due to
rapid object movement across the field of view.

The primary mirror is usually the greatest heat source in our [normally]
cold air system. Thermal lag from full thickness, glass mirrors has led
to the adoption of small electric fans and even conical mirror blanks.

Do the members have an opinion of which material and construction is most
suitable? Bearing in mind that high magnification observation of fine,
low contrast, planetary and lunar detail are the ONLY priority in this exercise.

A long tube is assumed with apertures below 12" to maximize the chance of
reasonable seeing for most observers in our often difficult atmosphere.

Do try and stay on topic. ;-)

Although the telescope in this video:

http://youtu.be/Up4frF8k2gU

has fast objects I have wanted to build something like this (an enlarged
Astroscan) for some time. A larger ball with a CF tube would be a
possibility.
The drive on the featured telescope seems a marvel of simplicity. His
website:

https://www.sff.net/people/j.oltion/trackball.htm

has more details.

On Sunday, 1 February 2015 05:32:11 UTC-5, Chris.B wrote:

Anything but aluminum carries with it too many negatives to be positive. Cardboard in an insulator and prevents hot air from leaving tubes. Carbon fibre expands and contracts too much with temperature and doesn't conduct heat. Fibreglass is too heavy. Aluminum is still, IMO, the best overall tube material.

For some, cost and ease of construction can be important factors. In which case a Dobsonian with a wood tube might be a reasonable choice. With practice (especially if one has no alternative) one's arms and hands eventually learn to make the necessary adjustments (even at planetary magnifications) with hardly any conscious effort. I have a 10-inch Newtonian with a long, hexagonal, cedar tube - a Dobsonian, that's been used (among other uses) as a planetary telescope. One of the odd things about this scope is that it has four instead of the usual three collimation screws. There was insufficient space within the OTA for three screws 120 degrees apart. I have to be careful when collimating to not induce astigmatism! On the other hand, once the scope is collimated the four screws effectively 'lock' the collimation in place.

The long, solid tube extending 18 inches beyond the focuser makes the optics impervious to dew and frost and helps shield the tube's interior from the observer's body heat.

Sketcher,
To sketch is to see.

On Sunday, February 1, 2015 at 3:32:11 AM UTC-7, Chris.B wrote:
In the spirit of Sketcher's open appeal: Let's see how we get on: :-)

The arrival of Carbon Fiber in the amateur telescope market offers a new option to the four main types: Metal tube, wooden tube, cardboard tube and all combinations of open truss.[Possibly with a cloth shroud]

CF tubing can be made as a solid mass or a laminated sandwich. Or even a combination of CF and GRP for local impact resistance. CF remains very expensive in comparison to all other options. Claimed advantages for CF are low self-expansion and greater stiffness for less weight.

Problems with existing [solid] tube materials:

Internal thermal currents clinging to the inner tube wall often affect the image due to variations of index with air temperature.

Super-cooling of metal tubes to the cold night sky increases the thermal differential between the cold air and the mirror. Wooden and cardboard tubes are almost immune to super-cooling but are very heavy when suitably stiff..

Heavy materials greatly increase the tube's moment: Making them far more difficult to mount well equatorially. Local movement and setting up of long tubes is increasingly difficult as tube's weight grows very quickly indeed..

The Dobsonian is usually considered ill-suited to high powers due to rapid object movement across the field of view.

The primary mirror is usually the greatest heat source in our [normally] cold air system. Thermal lag from full thickness, glass mirrors has led to the adoption of small electric fans and even conical mirror blanks.

Do the members have an opinion of which material and construction is most suitable? Bearing in mind that high magnification observation of fine, low contrast, planetary and lunar detail are the ONLY priority in this exercise.

A long tube is assumed with apertures below 12" to maximize the chance of reasonable seeing for most observers in our often difficult atmosphere.

Do try and stay on topic. ;-)

"palsing" wrote in message
...

On Sunday, February 1, 2015 at 9:09:37 AM UTC-8, Lord Androcles wrote:

... And why not a CF mirror, or a CF or resin/fibreglass reinforced thin
mirror
if CF is the way to go for lightweight rigidity?

See this page for a roundup of various lightweight mirror ideas that are
being developed, including CF...

http://www.altazinitiative.org/AA%20Tech%20&%20Demo.htm

.... scroll about 1/3 of the way down the page. There is a photo there of 2
young girls holding a 40" CF mirror blank.

\Paul A
================================================== =
Very good. Great minds think alike.
The Axial Flux Direct Drive Motor on that page was my idea for a bicycle
drive.
Embed permanent magnets in a solid bicycle tyre, alternating N and S poles,
and put a horseshoe electro-magnet over the tyre where the calliper brake
goes.
Feed that with a variable frequency oscillator (say a modified battery
backup uninterruptable power supply for a computer) and you have a low cost
electric bike with high torque, the bigger the wheel the better. And it can
be used for a computer controlled very smooth turning portable Dob running
off your car battery, not that keeping in synch with the rotation of the
Earth interests you, we already had that discussion.

-- The Reverend Lord Androcles.
Je suis Charlie.

On Monday, 2 February 2015 01:23:29 UTC+1, Sketcher wrote:
For some, cost and ease of construction can be important factors. In which case a Dobsonian with a wood tube might be a reasonable choice. With practice (especially if one has no alternative) one's arms and hands eventually learn to make the necessary adjustments (even at planetary magnifications) with hardly any conscious effort. I have a 10-inch Newtonian with a long, hexagonal, cedar tube - a Dobsonian, that's been used (among other uses) as a planetary telescope. One of the odd things about this scope is that it has four instead of the usual three collimation screws. There was insufficient space within the OTA for three screws 120 degrees apart. I have to be careful when collimating to not induce astigmatism! On the other hand, once the scope is collimated the four screws effectively 'lock' the collimation in place.

The long, solid tube extending 18 inches beyond the focuser makes the optics impervious to dew and frost and helps shield the tube's interior from the observer's body heat.

Sketcher,
To sketch is to see.

Thank you all for your responses. 'Solid' aluminium tubes may have some advantages but quickly become heavy and unwieldy in longer [suitably stiff wall thickness] lengths.

So far I have tried cardboard form tubes but found them much too heavy for comfort. I ended up with an ultra-lightweight, aluminium, twin beam effort. But still have some local stiffness problems to overcome. Hopefully without increasing the moment too much for comfortable mounting.

CF seemed to offer the lightness I desperately need for my 10" F:8 optics but is still very costly. Not something I want to experiment with unless it stood [literally] head and shoulders over traditional alternatives.

I am determined to use my vintage, driven equatorial, if possible, before giving up and building a Dobsonian. I have built Dobs up to 12" and they do handle very nicely. But, they just don't offer the freedom to take extra-focal 'snaps' with a digital compact camera. Nor webcam video frame stacking which I want to try.

I've also built tracking platforms before but with the eyepiece already 7' off the ground I have serious reservations about increased height.

I'd still be very interested to hear from anybody who has direct experience of using a long, CF tubed Newt.

Thanks

"Chris.B" wrote in message
...

On Monday, 2 February 2015 01:23:29 UTC+1, Sketcher wrote:
For some, cost and ease of construction can be important factors. In
which case a Dobsonian with a wood tube might be a reasonable choice.
With practice (especially if one has no alternative) one's arms and hands
eventually learn to make the necessary adjustments (even at planetary
magnifications) with hardly any conscious effort. I have a 10-inch
Newtonian with a long, hexagonal, cedar tube - a Dobsonian, that's been
used (among other uses) as a planetary telescope. One of the odd things
about this scope is that it has four instead of the usual three
collimation screws. There was insufficient space within the OTA for three
screws 120 degrees apart. I have to be careful when collimating to not
induce astigmatism! On the other hand, once the scope is collimated the
four screws effectively 'lock' the collimation in place.

The long, solid tube extending 18 inches beyond the focuser makes the
optics impervious to dew and frost and helps shield the tube's interior
from the observer's body heat.

Sketcher,
To sketch is to see.

Thank you all for your responses. 'Solid' aluminium tubes may have some
advantages but quickly become heavy and unwieldy in longer [suitably stiff
wall thickness] lengths.

So far I have tried cardboard form tubes but found them much too heavy for
comfort. I ended up with an ultra-lightweight, aluminium, twin beam effort.
But still have some local stiffness problems to overcome. Hopefully without
increasing the moment too much for comfortable mounting.

CF seemed to offer the lightness I desperately need for my 10" F:8 optics
but is still very costly. Not something I want to experiment with unless it
stood [literally] head and shoulders over traditional alternatives.

I am determined to use my vintage, driven equatorial, if possible, before
giving up and building a Dobsonian. I have built Dobs up to 12" and they do
handle very nicely. But, they just don't offer the freedom to take
extra-focal 'snaps' with a digital compact camera. Nor webcam video frame
stacking which I want to try.

I've also built tracking platforms before but with the eyepiece already 7'
off the ground I have serious reservations about increased height.

I'd still be very interested to hear from anybody who has direct experience
of using a long, CF tubed Newt.

Thanks

================================================== ===
What is the best material for a bicycle?

Thank you all for your responses. 'Solid' steel tubes may have some
advantages but quickly become heavy and unwieldy in longer [suitably stiff
wall thickness] lengths.

I am determined to use my vintage penny-farthing before giving up and
building an electric tricycle with a steering wheel. Not something I want to
experiment with unless it stood [literally] head and shoulders over
traditional alternatives.

So far I have tried cardboard cores from toilet rolls but found them much
too short for comfort. I've also built oak and elm boneshakers before but
with the saddle already 7' off the ground I have serious reservations about
increased height.

I have built tricycles with up to 12" wheels and they do handle very nicely.
But, they just don't offer the freedom to lean over when cornering at speed.

I'd still be very interested to hear from anybody who has direct experience
of buying a long, CF tubed Harley-Davidson.

-- The Reverend Lord Androcles.
Je suis Charlie.