So this astronomy disease is progressing faster than I expected. With my recent find of a relatively nearby dark site (30 minutes) and fun with my quite recently acquired 114 Newt I am contemplating building something a little bigger.
My germ of an idea is to build a 8" f6 Truss Dob collapsible so it can easy pack in the wagon but I had the notion that if I build the cell big enough I could upgrade the mirror to a 10" f5 later when finances allow or if I just decided to go that way when it all comes togther. (intention is to buy the mirror but price nearly doubles for the 10").
There is only 50 mm difference in the focal length calculated and that would be adjustable in the truss rods or maybe even with an extender.
OTA length would be about 1100mm and collapsed down to 800 or 700. That length would make it an easy fit across the back rather than some awkward diagonal placement it otherwise requires.
Are there any major obstacles or differences in the mirror sizes that might cause problems ?
The only two I can think of is mirror support points and eyepiece depth to the secondary mirror but I can engineer that out with either buying the 10" sized secondary to begin with or an adjustable side plate for the eyepiece.
I have already started some basic designs Cell depth 4-500 mm with 300 mm side width. Mirror plane at about 150 mm from the base with a fan cavity below the mirror.
An 8" only option would be only 50 mm smaller in side width so for the sake of 2" of wood .....
Can anyone give me some average weights for a 8" and 10" mirror so I can do some geometry exercises with the pivot placements.
Why the 10" option ? I was just stunned by the difference in light grasp between the 8" (817) and the 10" (1326).
The idea of going from a 8" to a 10" with accomodation for the increase in size be allowed for sounds good, but I'd advice some caution. You've mentioned some points already to be weary of, but the secondary mirror is something to not forget about. The secondary for an 8" f/6 mirror is small compared to one for a 10" f/5. You would either need to purchase two secondaries, or an oversize one for the 8" to accomodate the 10" in the future. I too would suggest just to make the 10" from the start.
I'd also suggest determining the design of the scope before determining wieghts and geometery blindly. Work out what design is easiest for you to make, use and store. Sounds simple, and it is, but reasearch is needed.
I've included a couple of pics of the 10"/5 scope I made. It uses only two poles and had a pedigree of some 20 years before I built mine 14 years ago. I've seen this design used in 4" through to 17.5" scopes. I made mine as simple in components as I could, but it works much more smoothly than most commercially made dobs, . Other than the optics and the focuser, everything else is made by me. Takes three minutes to set up and start playing.
This link shows a much more 'hotted up' two pole design for comparison, and to show how the design lends itself to hybridising, but it is just a compact as mine:
Here is another ATM'ers site. It goes into some depth in component design and commercially available components. I used this site extensively for inspiration when designing my 17.5" scope:
Have a good look through the DIY forum. There are many finished scopes you can use to help decide on a design for. My 17.5" is there. An advantage of a 10" mirror is that it is much more forgiving in requirements than a larger mirror.
gbeal - thanks for the encouragement, I'm leaning towards that option
Mental4astro - Many thanks for the plethora of links and information and your own picture record. Hadn't thought two strut but it's an idea to consider.
I notice your mirror cell is just open to the rear, no fans or heaters required ? Dew is a problem here even in summer.
How do you ensure you don't ding the mirror when you mount the poles inside the cell ?
What cell design did you use ? I'm favouring a 9 point balanced construction but want to make sure the mirror is well held for transport. It may have to lie down in the car on its side although if the cell is short enough that may not be necessary.
How do you protect the focusing gear in transport, it seems rather exposed on the top board ?
Again, thanks for your input. Looks like I may take some time to get the details right. Plenty of options if you look hard enough.
A 7mm thick perspex sheet covers the mirror. It is the LAST thing to come of when setting up, and is the FIRST thing to go back on when packing.
The focuser is wound in all the way. It is also a 'low profile' style. These have a very low housing, but with some clever designing, have a remarkable amount of draw tube travel. I recently upgraded the 1.25" focuser to a 2" focuser with no increase in the size of the focuser housing. This is the one I got:
JMI also makes there focusers predominantely in the low profile form, though they are pricey. You may be able to find others.
A 10" mirror doesn't need a 9 point cell. It is stable enough to work well with a three point. Much easier to make too. Any bigger and you do begin to need better support for the flex in that amount of glass, regardless of the material being Pyrex or plate glass.
Been playing with guiplop, clever bit of software (thanks for all them links again) and it calculates that a 6 point system has less overall error than a 9 point. Interesting to say the least. More is not always better then.
I'll keep it simple, more important to get the alignment mechanics sorted.
Construction has started. I'll post up pix of build stages. I tend to design as I build according to the materials at hand and already I've modified my original design to eliminate some stability problems in the mirror cell. I should come up with a simplified 6 point suspension cell using very few parts many of which can be recycled from other equipment. This will not be your classical ultra beautiful scope but a simple but functional 4 pole truss collapsible beast for photon collection.
Run into one small problem though. The local astro supplier has not imported any 10" mirrors, low demand he reckons and his next order won't be till Xmas nearly. I'll have to import one probably from Bintel across the ditch. Prices seem comparable even including freight etc.
bummer about no mirrors till x -mass.. Have you tried GSO directly ?
they should be able to land you one direct in a few days .
I know there larger mirrors come with a secondary , unsure with the smaller ones... prices payable in US $ btw.
Hi NightStalker, thanks for the info. I'll check out the GSO possibilty. At present I'm thinking I'll just contact Bintel in Australia. Close and prices seem comparable.
Also you have given me an idea about the focusser. I was leaning towards building my own crayford (I have a reasonably equipped workshop including a small lathe) when I realised I have several old video projectors which have helical adjustments on the lens arrangements and quick levers as well. I might pull one or two apart and see what I can make of it. They have quite large IDs in most cases and I can turn up adapters etc from other resources.
If I can build one I'll document it up. Goes under the 'Frugal Astronomer' category but also fun to build or adapt stuff from scratch.
Excellent !!! more fun in the workshop !!
I've also designed and built a Super Simple 6 Point Suspension System ( try saying that quickly ) for the mirror. Very low profile and quite rigid using rubber (hi temp neoprene) wheels for both the mirror collimation 'springs' and as pivots on the three arms beneath the mirror. I'll post up some pix and details for comment later.
Time for an update;
Progress has been surprisingly good and problems have been overcome. Bottom mirror cell box is built and even spraypainted matt black inside. The mirror mount cell is also built and painted with my 'patented' 6 point mount. The truss poles are all acquired and the clamps that secure them inside the cell box all made and fitted.
Out walking and came across a piece of formica clad custom wood which will be the turntable and the teflon bearings are from a automotive high pressure compressor gasket I acquired on Trademe for $15.00.
Built a helical Crayford using the fine screwed barrel from an old video projector combined with some optical parts from a microfilm reader, the thread of which was perfect match for the eyepiece mount. Has about 20mm of thread depth but I'm sure I can extend that with other ideas if necessary.
Just need to build the upper assembly, the altizimuth mount and import some mirrors. Bintel will have my requirements at end of August.
I'll post up some pix later when I sort them out a bit, there were a few abortive sidetracks which had to be redone.
Best advice to any budding telescope builder. Get a drill press !! Mine is cheap but it has allowed me to set up jigs and drill accurate holes to replicate parts and keep things accurately assembled. My wee lathe has done stirling effort with the optical modifications but that is my penchant for building it all myself and the challenge it presents.
Most of his has been built out of what was lying around or scrounged from various sources. So far I have only really bought aluminium stock, paint, and nuts and bolts.
Shots are pretty explanatory, excuse the mess in the garage workshop. The cell suspension units use clear neoprene copier rollers, no springs. Keeps the cell unit very stable, no sideways movement at all. When the mirror is bonded down the centre pivot will also be filled with RTV Silastic so it sets flat with the mirror back. Supports will all be blackened by then as will the truss clamps in the box of course.
Cheers Peter, bit of encouragement always helps. Working on a novel spider/secondary design at present. Trying to make collimation easier and stable. Trouble is every time I get a 'good' idea another one comes along to trash or modify it. I'll settle on something soon,...I hope !
Built the rocker box this afternoon. Weather was no good for gardening, kept raining so had to spend time in the workshop, what a shame !!
Excuse the mess in the photo and my little project car in the background.
And a question. Haven't built the pivot table yet. What is recommended ?
Teflon on the base facing up or on the underside of the rocker box facing down ?
Ayway heres a pic so far ,...
Azimuth assembly and setting ring built with levellling base. The setting ring is temporary, I'm going to print out one with better quality and accuracy before it gets sealed down onto the plinth. Three teflon pads beneath and teflon to the rocker pivot as well now. It's all had two coats of diluted varnish to seal it and will get full strength coat or two to finish the job later. Azimuth setting to go o the side as well and I'll fit a evel and compass preset on the base at what will be the north point corner of the triangular base. red light mouted on rocker box to illuminate the setting circle, level and the compass. I can confirm true north with the GPS as well.
I'm really waiting now for the mirror so I can set the final height for the UTA and focusser although it will be easily adjusted and I'm building to allow some height drop later for a camera to come into the focal plane.
Current status pic:
I have a question, are the altitube bearing fixed in place now, or temporary? I'm guessing temp.
What shape is the secondary cage going to take, single sheet of plywood, or a totally enclosed cage?
I hadn't seen your previous posting where you asked a question about which way to place the azimuth teflon pads- they should be fixed to the ground board, with the laminate facing down from the rocker box. This orientation is so that dust is reduced from soiling the laminate.
Hi Alex,
Teflon is always temporary, it will peel off anything. A long strip held down by silastic in the bottom of each 'U'. Similar for the pads on the ground board and yes, I got them the right way, facing up. The whole assembly rocks and swivels quite smoothly now.
Secondary UTA is started, just a square box cage form in MDF around the poles at present with alum tubes that fit over the pole tops. I want some light protection. The idea is that the whole scope is easily modified as I come up with new ideas.
I'll add some pics of the base board construction, rocker box removed because it has worked out so well it's worth seeing.
Going to do some weight reduction as well once I've got it working. Cut down the rocker box profile and the UTA will have cutouts. I may have to shift the Altitude bearings up the bottom cell as well but that was designed as an option in construction so easy enough. Better that than adding weight to balance. It is quite heavy compared to other 'travel' scopes I've read about but I have a big wagon and it seperates easily. Fortunately I still have my health, I can swing an axe and weild the chainsaw so it's not a problem.
It will mostly either get pulled out from the garage onto the driveway or up to my sheltered 'HomePad' site in the backyard, about 30 meters away for longer sessions.
Aaah .. misunderstood the question re 'altitube bearing fixed in place now, or temporary?' but answered it later anyway with 'may have to shift the Altitude bearings up the bottom cell '.
I thought you were talking about the teflon. Doh!
Hi Brent,
I used a lazy susan for my azimuth bearing and it is as smooth as!
It only cost me $16 from memory.
Carbatec have a store in Auckland NZ if you are interested. http://www.carbatec.com.au/lazy-susan-bearings_c15410
I like what you have done. Looking forward to some more photos.
Cheers
Hi Brent
the scope looks great....I was looking on the Andrews website and in the 'used and surplus' section they have a new gso 10" f5 mirror (that needs recoating) for $199 might be worth looking into....
have fun building
Phill
Build advice please ..
