First Light For Stringscope

[stringscope]

Well my stringscope achieved 1st light tonight (2nd light for mirror). I had to recheck focal plane position before fabricating the "string" truss assembly. What better way than starlight from Alpa Centauri.

Jobs completed:

Upper tube assembly.
Mirror box.
Electrical panel.

Major jobs outstanding:

1. "String" truss assembly.
2. Altitude bearings
3. Recoat primary mirror.
4. Rocker box.
5. Lots and lots of little jobs.

Cheers,

[astro_south]

Nice one Ian

Must be satisfying to reach this stage of the construction (must have been fun aligning alpha Centaurii that way too!!)

Can't wait to see the finished product.

[[1ponders]]

Wow :eek: :jawdrop: Its amazing what you can do with a bit of string Ian

[mch62]

Great job and excellent to see another ATM on the forum.
Have you seen Mel Barlels and Dan Grey from the states string scopes and spiders?

http://www.bbastrodesigns.com/tm/tm.html
http://www.siderealtechnology.com/28inch/

Mark

[Orion]

Hello Ian, Great stuff! Love to see home built telescopes.
Are you trying to determine the truss assembly length in that photo?

[mch62]

Orion that's the fun part i remember from making my truss scope.
As a double truss with a mid mounting point getting the balance made it even more fun:P
There are programs around that have been written to calculate the lengths and balance points for truss scopes but i found them even harder to work out .
I used Newt 2.5 first to get the overall length and then using a weight set set up a balance beam and mounted components on the beam except for the mirror and just adjusted every thing untill I got the balance while keeping the over all length the same.
This workd out for me.
But every one has there idea that they hope will work for them.
Good luck with scope.
Mark

[stringscope]

Thanks for your words of encouragement guys.

I first became aware of the "stringscope" concept from Dan Grays web site. There are quite a few of them out there now from Graig Colvin (6") to Dan Gray (28").

By conducting a star test, I was working on the basis of measure twice and cut once, at least thats my excuse. In fact the truss length is not super critical as I have 60mm of available mirror cell travel on the collimation rods. The reality was I couldn't wait, I just had put it together and see if it all worked.

I have attaced an image looking down into the mirror box.

Cheers,

[Striker]

Nice one Ian....

OMG your telescope is invisible...hehe

[mch62]

That look like you can adjust your collimation from the top?
That would make the task easier.

[mch62]

Why go any further .
Who needs a tube when a big beam will do
This looks like your scope:D

[Orion]

That's the way!
A couple of mirrors and a pole and you have a telescope.

[stringscope]

Mark,

Wow...... is it a "beamscope"?? what is it??

yes, front collimation access. This design feature allowed me to locate the batteries and electrics at the very rear of the mirror box to provide greatest counterweight effect. They are thermally isolated from the mirror by a sheet of plywood and there are 2 80mm fans for mirror ventilation.

Some of the inspiration for this design came from the Portaball and Teleport scopes; front collimation access, single ring UTA, accessories drawer in rocker box and carry bag.

Photo of mirror box internals. Electrics next post.

Cheers,

[stringscope]

Rear electrics panel.

[ving]

man that looks weird!

great job mister! :)

[iceman]

Awesome Ian, top stuff! Looks great.

Aside from portability and the "made it yourself" kick, what are the other benefits of that design? Wouldn't you get a lot of off-axis light bouncing off the mirror?

[mch62]

Hay ice I might be able to shed some light on that question:D

You have seen my avatar and see that my mirror is even more exposed , no box at all.
I don't have a problem with stray light , even from the street light near my home.
The only time it becomes a problem is when the light is coming from directly behind or the front of the scope.
It then will get onto the secondary or primary and cause problems and for that I have a black felt shroud that can go around the scope once it has cooled down real quick.
The most important part that need to be protected all the time is directly behind the focuser .
You need to have some thing to block the light going directly into the focuser , either a full tube for the end or a baffle opposite the focuser.
I can shine a torch beam straight through the side and it has no effect or little.
You have to remember your physics and the angle of incident and reflection stuff to relize what it will take for light to get into the focuser and eyepiece.
If you have light coming from the side of a scope onto the mirror it is going to bounce off out the other side depending on angles , and for it to make it's way to a focuser at right angles to the mirror ---well
You don't see many research scopes with a full tube , not even shrouds just a darkened area.
A full tube will of coarse be better if you surronded by city street lights.
Hope this helps.

Go Trusses:cool:

[Starkler]

I imagine that you would have to keep that mirror scrupulously clean though, being able to scatter light from most any direction back toward the focuser.

[ving]

just one word.... dust particles!

well, ok 2 words. you would have to clean more often wouldnt you?

[mch62]

What don't you guys keep your mirrors clean:P
It's very easy to clean with out having to remove the mirror and possible loss collimation.:D

[ving]

yup a bit of steelwool and industrial cleaning agent 'll do the trick hey! ;)

(this is a joke to all newbies out ther)

[stringscope]

The stringscope will have a shroud made of black ripstop nylon. This will assist with stray light control onto the primary mirror dust particles, stray light entering the focuser and help keep dew off the mirrors. There will also be a removable (400mm) dew shield attached to the UTA, this will complete the dew protection for the secondary and stray light control for the focuser.

Cheers,

[stringscope]

The "string" truss is now complete. Fabricated from 12mm Al tube and "Plus 450" bow string. This photo was taken after initial assembly.

Next job is altitude bearings.

Cheers,

[Orion]

Hey Ian, thats looking great! Are you going to put something on the other side of the focuser to stop stray light coming into the eyepiece?

[seeker372011]

lookin good....

[stringscope]

Hi Ed,

Yes I will be using a black ripstop nylon shroud over the outside of the truss (classic Dob). I am planning on a 400mm dew shield attached to the front of the UTA ring. I will probably use 5mm black closed cell foam for the dew shield.

Cheers,

[iceman]

That's looking great Ian! I love the compact/light look of it.. would be excellent for travel.

[stringscope]

I have made some progress with the stringscope. All components (except shroud and carry bag) now complete. All have been checked for fit and function and I have just carried out a final check laying it all out on the garage floor.

Remaining tasks:

All wood components to be painted.
Primary Mirror bonded to cell.
Secondary mirror bonded to cell.
Teflon pads drilled and countersunk for fixing screws.
Accessories drawer foam insert cutouts for eyepieces etc.
Then.......put it all together......

Photo's following:

Cheers,

[stringscope]

Lower tube assembly, rocker box and ground board side on

[stringscope]

While I had the camera out .......... a photo of one of the Canberra Astronomical Society hire scopes. This scope has been allocated to the introductory group (I am one of the coordinators) it has been out on hire for the lst 3 months but I couldn't convince anyone to hire it at the last meeting.......oh well I will just have to use it myself for the month. It is a great scope, 4" F10 and really sharp. Comes with a set of orthoscopic eyepieces and is great for the moon and planets. Perfect height to use seated and the whole thing can be picked up and carried one handed.

ps. It also carries John Dobson's signature 1995.

Cheers,

[seeker372011]

those perfect circles and rings--are you going to let us know the secret-how do you do it?

love the little 4 inch dob btw

[Orion]

The wood work on this telescope looks like a work of art. Who built it?

seeker372011, The circles I cut out of wood are made with a router and a jig and they come out as perfect circles.

[stringscope]

seeker372011, yep Ed is correct.....router with circle cutting radius arm. Makes cutting circles really easy. The only circles I have cut with the router to date are the ones in the photos. The ID for the altitude bearings was too small for the radius arm so I cut them free hand........not quite so good.

Ed, the little 4" dob is one of my favourite scopes. I believe it was built by a past CAS member who was also a musical instrument maker. I think the tube in particular is a work of art. Very light weight. Sides are 2mm ply wood with an internal frame including baffles and some sort of textured fabric lining glued to the inside. This scope exhibits excellent contrast.

I made some additions and matched the staining as closely as possible:

Holder for star atlas and instruction manual.
Holder for tube end caps.
Eyepiece drawer (extended in the photo).
Small accessories box (opposite side of mount to eyepiece drawer and out of photo).
Steering handle under front of tube.

I also added a Rigel QuikFinder.

Cheers,

[stringscope]

All stringscope components have now been painted/coated as required.

Outstanding tasks:

A very minor balance issue to resolve. I can either move the altitude bearings 5mm or I could use a light virtual spring counter weight. If I use a virtual counter weight, then I could also fit a 50mm finderscope!!!!!

Azumuth bearing a little too firm.....still thinking on this one. I might replace one of the teflon pads with a roler bearing. I thought I might have some issues with this and the ground board it only temporary pending final design.

Protrctive covers for both mirrors.

I decided not to have the mirror recoated at this stage pending some detailed star testing. (I was never able to do a proper star test in the original Celestron Starhopper due to secondary mount and focuser issues. Star test last night between cloud bands and in pretty ordinary seeing indicated the mirror is probably worth keeping.

Shroud and dew/light shield to be fabricated.

Equipment drawer foam liner to be cut out for EP's, filters etc.

Protective Carry Bag.

Probably another couple of weeks to go.

Cheers,

[Starkler]

Looks great !
How long do you think setup/collimation and pack up times would be in the field ?

[stringscope]

Geoff,

Erecting and collapsing the truss takes about 30sec each way in daylight and taking some care not to drop one of the segmented tension tubes onto the primary mirror.

It appears to hold perfect collimation between assemblies, bearing in mind this is an F6 system. I have only run it through 3 assembly cycles and a collimation check using a cheshire collimator indicated it returned to the previous collimation setting each time.

When complete assembly steps will be:

1. Removal from car.
2. Remove scope from carry bag.
3. Extend truss.
4. Attach finderscope and eyepiece.
5. Collimation check.
6. Attach shroud and dew shield.
7. Place carry bag back in car.
8. View.

I guess from removal out of the car to viewing should be no more than several minutes.

[iceman]

Looks fantastic Ian! Will be a great travel scope, and the fact you made it yourself will be so much more pleasing.

[Orion]

:work: The craftsmanship on the telescope seems to be first class. Well done!

I like the blue and the black those colors always work well together. Great paint job. I am just curious on the preparation of the wood before the paint, how many coats does it have, it looks like it's been sprayed on .Very nice indeed. :thumbsup:

[atalas]

You know Ian, you guy's are fantastic! you build these dobs and they end up looking better and working better than bought ones! Im talking about you too ED!

Louie :2thumbs:

[stringscope]

Thanks for the comments guys.

The wood is Hoop pine ply wood. Interier flat black acrylic. Exterior sanded down to 240 grit with an orbital sander. One undercoat followed by 4 coats of "marine blue" epoxy enamel. "Sanded" between coats with a kitchen scourer. Trouble is, I didn't count on the mirror finish showing ALL (and I mean all!!) the blemishes.........sigh.......I haven't done any thing like this before. I will know better next time!!

Elapsed time was about one week.

The painting is all brushed. I do have a spray gun but the thought of setting up a spray booth in the garage was all too much.

I am just doing a few minor mods, should have its first serious observing session next weekend.

Cheers,

[Orion]

Thanks Louie,
Telescope making for most of us ATM's is a work of love. Last night George came over and we were star testing a new secondary for my telescope till 1pm now thats dedication.

[stringscope]

Ed,

What was the issue with your secondary. I assume this was for 18" F4.5?

Cheers,

[Orion]

Well Ian my secondary needs recoating so we decided to make another secondary with a slightly better figure than the last and yeh,we like playing around with class.

I cut a few secondary mirrors in one go. Here is a pic of the trepanning process

[stringscope]

Hi Ed,

Wow! cutting your own secondary. That not bad. While I am happy to have a go at building a scope using commercial optics, I don"t think I would have go at a primary let alone cutting and grinding a secondary.

I presume you achieve a flat surface by gringing and polishing between 3 surfaces??

How flat do you think you have got it.

Cheers,

[Orion]

I might be getting of the topic here but anyway cutting 6 to 8 secondary in one go allows me to pick the one with the closest flat, we test this with an optical flat and then proceed with the figuring there is no grinding unless the class has scratches on it.
I am not sure at what wave length it is at the moment but I am not in a hurry for it.

[stringscope]

Well after 12 months of planning and construction my string scope Dob is finally in a useable form. Still a couple of jobs to do:

1. Padded canvas cary bag.
2. Some form of lid, I am still thinking about this.
3. New shroud and dew shield once I have used the temporary ones for a little while.

My observing buddy (17 yo son) and I had both scopes out for a decent observing session last night. Seeing was quite good although transparency was down a little on the previous night.

I am quite pleased with the stringscope.

The good news:

Jupiter @ 280X quite spectacular showing cloud swirls in the large equatorial bands and quite a number of additional bands during moments of steady seeing. Split Antares at 100X, very clear at 200X. The homunculus in Eta Carina showed signs of structure at 100X and up.
I was able to do some star testing. There is no sign of turned edge.

The not so good news:

While star testing I noticed what appears to be a minor zone within the inner third of the mirror radius. I guess this is probably OK as the images look OK to me and the intra and extra focus patterns look fairly closely matched.
There is some noticeable glare around bright stars which is less noticabe in the other scope with the same EP. I suspect this might be the degraded mirror coating. I need to look at this further.

Attached photo's show the completed scope side on plus equipment drawer and electrical panel.

Cheers,

[iceman]

Great looking bit of gear Ian, a real work of art!

Congrats!

[stringscope]

Thanks Mike.

I have got to build another one now. Mmmmm ....... I think I might be hooked!! As might be expected my observing buddy is of the view the next one should be 16" with a full set on Naglers. I am much more inclined to rebuild our existing solid tube dob to gain additional experience before attempting a larger one, 12.5" perhaps. As for the full set of Naglers, he has my full support to go and get a job for that one!! Of course, I will happily use them once he has bought them.

Cheers,

[h0ughy]

Your scope should win you a prize if you the it to SPSP next year. It is a great personal achievement :2thumbs:

[Orion]

I agree Ian it is a work of art! How is the balancing with the light shroud and dew shield on?

[stringscope]

Thanks guys.

Balance with heaviest EP, shroud and 400mm dew shield has worked out just about right. This was after I moved the after I moved the altitude bearings 5 mm about a week ago. Holds any position down to horizontal, ever so slightly front heavy if anything. If I want to fit a 30 or 50mm finderscope or indulge in a 24 Panoptic, I will have to use a spring "virtual" counterweight.

Cheers,

[FNQ_View]

Ian,
Great Scope!!
I've not seen this " String Scope" design before. It could be the perfect solution for my 10" that stalled a couple of years ago. I notice you haven't fitted a backboard on the upper assembly. Has this been a problem ?

What are your trusses made of?

[stringscope]

Mick,

Sorry about delay in reply......I have been away.

I am not sure what you mean by a "backboard". The cloth shroud and foam dew shield prevent stray light from entering the focuser. If I operate without the shroud and/or dew shield then I would have to fit a 200mm diameter baffle on the UTA opposite the focuser. I have found the black rip stop nylon shroud wooks well except when a car drives close by and a little glare is then noticeable.

The "truss" is made of "Plus 450" archery bowstring triangles, held under 10kg tension by 16mm aluminium tubes in spring loaded sockets.

Cheers,

[wavelandscott]

Stringscope,

I'm guessing that it is your article on Cloudy Nights...

If so, congratulations...great write up!

[stringscope]

Yes, I have to confess it was me.

Thanks for the comments.

The chance to be in the running for a TeleVue-anything was too much to let pass.

Cheers,