Advice needed for a new (well, first time actually) ATM project

[shahgazer]

Hi All,

For 2008, I'm planning for a telescope making project. Despite not having ANY ATM experience , and almost non-existence ATM community here, I'm totally depending on my readings on the internet. :scared:

My current scope a Meade 8" LX90 (using since 2001), and the old Meade's 10" f/4.5 Starfinder Dob (using since 1995). Both have kept me busy all this while and has been a wonderfull companion.

My next scope should be somewhat a significant jump from the 10". I was aiming for perhaps a 14" or 16" f/4 ? A 12" would a kinda small step from the 10 inch, me think.

As for height, 2 or 3 step ladder should be forgiving for me (I guess!) :shrug:

I'm actively doing videoastronomy using Mallincam, so I'll sure need some kind of tracking for the Dob. Should I go a Mel Bartel's design or build an Equatorial Platform? My aim is to minimize the dependency on batteries!

Aiming for objects, I'll be happy to use a DSC, perhaps use the design from Dave Ek's DSC. I'll love to have Argo Navis as part of the setup but I think it'll increase the overall cost of the budget!

Using David Kriege's Dobsonian Book as my reference to get hold on the fundamental concept of telescope making, I'm inclined towards a minimalist design. Mainly due to the fact that I'm using a Toyota Avanza for travelling around, and with 3 children running around, I have little space to put anything bulky and or have to end up using more than 1 passanger space.

Material. This is a bit complicated. In Malaysia, humidity is very high. The scope tends to get really soaked up after midnight. My Meade's 10" Dob sonotube is really suffering from that, and starts to sagged a bit. And I think wood as the main material would have some significant effect on the overall weight of the scope if I planned to travel around in flight. Would an aluminium-dominated design be a good option?

The new 18" UC Obsessions seems to be a good design to follow. I'm also very interested to follow Allan's "Deep Space Scope" DS4 (http://www.xmission.com/~alanne/DS4Main.html (http://www.xmission.com/%7Ealanne/DS4Main.html)) design. And I am very inspired reading Tim Nott's article on his 16" Tri-Dob.

Primary mirror? I have my eyes on the GSO mirrors.

As most would agree, the first thing I should do before jumping into ATM is to go and test myself on other scopes and star parties. Unfortunately I don't have that luxury here. Almost 99% of astronomy enthusiast here buys telescope (mainly Meade's, Celetron, Orion etc). So I am really limited in having the first hand experience on big Dobs.

This is a big step for me. Any comments or word of wisdom is most welcome! :help:

[Rod]

Hi Shahgazer,

Sounds like a great project and you are looking up some good references regarding the design. I really like the minimalist designs too. I built a 12 inch flex rocker design after finding the conventional truss style too hard on my back.

I think a predominantly aluminium design would be quite feasible. Then use marine ply for structures like the ground ring (if you go for for the flex rocker).

The thing I learnt from mine is that getting good motion is a little trickier with a flex rocker. The traditional ebony star and teflon did not work well - it became very stiff. I think this is due to the very large size of the bearings. I replaced three of the teflon pads on each axis with roller bearings (and left one teflon pad on each) and replaced the ebony star with aluminium on on the alt axis and smooth formica on the azimuth. Now the motion is very smooth, except in dusty environments when dust gets into the teflon block on the azimuth axis.

I found it easier to use a central pivot for the flex rocker than use a hollow ring, like with the tri-dob design. It also makes it much simpler to mount encoders later. Dale Eason's design shows what I mean:

http://home.comcast.net/~doeason/16inchscope.html (http://home.comcast.net/%7Edoeason/16inchscope.html)

I used 6 truss tubes on mine and it works well but they are much less forgiving of errors (than the usual 8). You need to close the truss triangles at the mirror box and the upper ring carefully or the truss arrangement flexes as it moves.

If I were you I would go for the 16 inch mirror and I would prefer an F5 to an F4 (better images and better chance of good optics). With a minimalist design you can keep the eyepiece height relatively low and should be able to avoid too many steps.

I have attached some photos that I hope will give a few ideas.

Hope that helps and good Luck!

Rod.

[GrahamL]

great scope and wise words rod ..best wishes for your project sha

[shahgazer]

Hi Rod,

Thanks for the advice! Gave me some points to ponder, for things like the flex rocker and the number of truss tubes.

I wasn't show sure if how will a 6 trusses perform as opposed to the recommended 8. Is there a logic behind it?

And what about the single upper ring. Will it sway or stressed under the tension from the truss tube? Design wise, it sure save a lot of space, but performance wise?

I came across Bruce Mills 20" F4 telescope at http://www.freewebs.com/brucemills/20inchf4telescope.htm

seems like a great design... no wood! :-P

[cristian abarca]

Hi Shahgazer I have seen through this scope and I can tell you that there is no difference to that of a normal truss design, except for the weight factor. It is just as solid and now that Rod modified the flex rocker it moves extremely smooth. The next telescope I make will be of the same design as Rods specially as the bigger the mirror the bigger the box to carry it is. Have a look here http://www.bbastrodesigns.com/index.html



Regards Cristian

[netwolf]

Hi Shahgazer,
The trilateral design is one of the best options for portability.

Ever since i saw this design on Mel's website I have thought this would be the one i would make if i every i did.

Another few links.
http://home.wanadoo.nl/jhm.vangastel/Astronomy/tridob/tridob.htm
http://www.bbastrodesigns.com/trilateral.html

Some day with much help perhaps i will do it.

Regards
Fahim

[GrahamL]

hi sha :)

Just spotted this one over cloudy nights
GSO is worth an email as I believe they may sell you a kit direct which I guess isn't to far from you freight wise ?
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2089159/page/0/view/collapsed/sb/5/o/all/fpart/1

[Rod]

Hi Shah,

The only reason for 6 tubes is to save a little weight and save on making extra hardware you don't need (truss fixtures). As Cristian says it is just as stable as 8 tubes. Incidentally you may notice the piece of aluminium angle on the mirror box. By attaching the truss split blocks to it, I was able to do away with any shims - which can otherwise be difficult with six tubes.

I did originally have a single upper ring. It worked fine - no stress or tension. The way to avoid this is to close the triangle at the top. I use a modified version of Krieg and Berry's offset bracket design crudely based on this example:

http://www.wcc.net/%7Emyastronomy/ATM/B3/Upper%20Cage_truss%20tubes.htm

The only problem with the single upper ring was that I became nervous about the truss tubes hitting the secondary mirror during assembly. It was also harder to store it in the car safely so I compromised with a very squat secondary cage which protects the optics better and can sit on top of the mirror box for transport.

I agree Bruce Mills' scope looks great. It's way beyond my skill level though. You would need to add light baffles - Mel Bartels has some excellent advice on this. A baffle just below the focuser is very useful on a single ring as is a baffle on the ring opposite the focuser. Under the focuser I use a camera iris I bought from surplus shed. It is helpful to also have some plastic or black fabric cover the back of the mirror cell to stop any light being reflected up through the the bottom of the tube.

If you build flex rocker, try to minimise the size of the base ring if you want to build an equatorial table. The base ring on mine is over 20 inches across, so an equatorial table to suit it would be very large. Consequently I am going down the Bartels stepper drive route - more work but it is an interesting project and there is lots of help available on the net.

Sorry for the long ramble, hope some of it's helpful,

Rod.

[tnott]

Hi Shagazer,

The 16" Tridob I built works well but was more complex than some other designs around. The main advantage of the design is the ability to fold the altitude bearings over the mirror box but others have just made the two side bearings completely removable.

The first issue with having 3 altitude fins instead of 2 is that they must be exactly concentric or it will not move properly in Alt. Second, I discovered that because the altitude fins have three contact points instead of four you don't get the same horizontal lever action when you move the scope in azimuth, so there is a tiny amount of slop in this axis. This may have been exacerbated in my case by putting the Alt. support points at 70 degrees rather than the 90 degrees used by Mel's original design. It is very minor, however, and I can still track easily by hand at 500X. It also is not a problem if the scope has a drive (I'm in the process of doing that now).

Both axes are very light and smooth using the teflon/ebony star combination - a fact commented on by many others that have used the scope. I did make sure the pads had the recommended psi from the Kriege/Berry book and renew the turtle wax on it every 6mths or so which helps a lot. I also need to clean the AZ. pads and laminate regularly to keep the motion smooth as the ground ring bearing surface is uncovered and sometimes collects dust.

If you want to add DSC to the scope then not having pivot points on which to mount encoders is a problem, but if fitting a drive system like SiTech or Mel's system as well and your drive system doesn't slip then you don't really need the extra encoders anyway.

I originally had a problem with the secondary not holding collimation but solved this by reinforcing the top ring with aluminium tubing bent into a circle and using slightly thicker wire for the spider. I think if you are going to use a single ring at the top end then it is best if the support points for the spider are where the poles fix to the ring for maximum rigidity. Note that this is what happens in the Obsession UC and Mel's original Tridob.

Hope this helps with your design. I am currently working on a 22'' version of the Obsession UC for my next project.

Good luck,

Tim.

[Alan Scott]

Shahgazer,
I though I would add a few thoughts on the DS-4 scope.

The goals of DS-4 was to not compromise quality of a large scope, to have a scope that is easy to lift, to have a scope that has the smallest possible size, to have the lowest possible eyepiece height, and to have an easy to build design.

I went with a conventional design, instead of a Mel type Tridob for a few reasons.

* The Tridob looks like it is more complex to create. (Since I have not made one, I don't know this for a fact.)
* The Tridob, if it follows Mel's design where the mirror box can pass through the center of the ground ring, will probably have a larger footprint than a conventional design. Otherwise, it won't have as low of an eyepiece height.
* The Tridob will have more trouble with friction and dirt in the azimuth direction, when used on a dirt field.
* The Tridob will have a lower eyepiece height by about 4 cm than a well designed conventional scope. This was a design compromise.
* The Tridob will have smaller, lower, more compact bearings. I don't consider this an advantage, since bearings are so easy to remove.

As far as 6 vs 8 trusses, DS-3 (12" ultralight) uses 6 trusses. DS-4 (16" ultralight) uses 8. Both are totally rigid, and have no vibration or bending when in use. 8 trusses do have a few advantages. 8 trusses will hold a shroud out of the light path, 6 will not (assuming that the mirror box is small in design, and assuming you don't use other tricks). Another advantage is that 8 trusses fit in the corners of the mirror box well. 6 do not, thus requiring you to either increase the size of the mirror box, attach the trusses on the outside of the mirror box, or place one set of trusses on a cross brace between the bearings. Yuck.


Alan

http://www.xmission.com/~alanne/DS4Main.html