Go Back   IceInSpace > Equipment > ATM and DIY Projects

Reply
 
Thread Tools Rate Thread
  #41  
Old 14-11-2011, 03:52 PM
clive milne
Registered User

clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
One other point with wire spiders that I think deserves mentioning, you only need to have just enough tension to put them under tension, they do not need to be torqued up like a guitar... all you achieve by adding tension is a reduction in their load carrying capacity, meaning you would be better of reducing the tension which will let you get away with thinner wires.

2c
Reply With Quote
  #42  
Old 14-11-2011, 07:09 PM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
I definately agree with every word your saying clive but under the given loads if your axially deforming your truss system... wow.. unless your using ultra thin walled sections (>1mm) which most don't so ide say unless your trying for the most ultra light sections this is almost a moot point in my opinion. I have been working on a truss fašade on a 27 story apartment tower that is apart of a steel framed roof, where in the loads are massively bigger than that of a telescope and axial deformation is negligble even under ultimate wind conditions of close to 70m/sec (just to put some perspective on the situation)

So while FEA is very valid we might be talking single digit micron values and this i challange even the most experienced telescope builders to measure during a imaging session and quantify. The main reason why i would go 4 truss's as apposed to 3 is for the principle of resolution and stability of the top OTA section, it forms to the circular tube better and not cut into the actual tube and support on nodal points is better than three why else do cars have 4 wheels instead of 3? (unless the cat burgler swindled your 4th rim... little bugger).

For instance my SW 10" has a OD of 298mm and the mirror is 254mm giving a clearance of 22mm around the outside so the boundary layers of those truss tubes that only clear the actual mirror by 5-10mm will start effecting the defraction and give you spiroulus artifacts being thrown around. You could just get smart and pull cold air down the tube like most high end imaging set ups already do this prevents dew on even a moderately dewy night, effectively the draw down of the tube mixes the are and doesn't allow the ambient air to cool down sufficiently enough when in the local environment of the vane. Job sorted.

I think at the end of the day we are looking for stability and with the loads in the rhelm of Kg not kN and seeing being the limiting value 99% of the time we are well under that cap there for it doesn't matter.

As for the spider vanes yeah they increase the diffraction and to say that the thickness isn't a key player i think is odd. the boundary layer is only so thick. A key point noted by VC 200 L owners is the spider vanes create diamond stars. I have modified about 9 sets of these vanes to take them from 4mm down to 1.5mm and hence deleated the diamond stars back to round stars.

Further more I love my diffraction spikes on the stars when you get it right the rainbow effect is just stunning IMO. So when I upgraded my vanes from 0.8mm flimsy SW vanes to 1.5mm SS my diffraction spikes increased dramatically I don't believe that the boundary layer has done this. With the paint and what not it semi insulates it but i do agree that the thermal loss they will be cooler and hence change the refractive index locally around them though once again, my point before about drawing the air and not allowing it to be cooled moots that point. One other point about the spiders being supported by wires instead of a solid vane while this is fine for an observatory mounted telescope, a telescope like mine that rides shotgun on the back seat on trips out into the yonda down dirt roads and what not I feel far more comfortable with a solid set of vanes holding it all together.

Another point is the actual depth of the vane and this in my opinion is the real winner coming to wire V solid is off axis light rays where they impact the primary mirror the actual depth of the vane as a projected width is huge compared to a slice of wire.

I guess to sumerise what im saying is people are gunning for this level of perfection and cost benifit becomes more cost than benifit and this is by far the most fundamental engineering principle wouldn't you agree?

Brendan

PS. one day I may be lucky enough to have my setup where the skies no longer become the limiting resolution cap and I may start pushing for the extra pfffft but upto now my experiences have lead to my conclusions above. I am no a expert in the physics of optical design but I am a practical person with the ability to see things, learn new techniques rapidly and fix things i have no knowledge about so don't let my little avatar fool you into a young bull headed fella with no idea on the world.

Last edited by bmitchell82; 14-11-2011 at 07:26 PM.
Reply With Quote
  #43  
Old 14-11-2011, 11:03 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
Clive and Brendan,

Interesting points. I presume if there is sufficient clearance, a 3 sided design is possible? it would benefit portability and ease of assembing in remote sites, but I'll have to see just how much clearance I'd need.

clive, good point about the dec joining the ota at the nodes. will keep that in mind, though may not work for the 3 sided design.
as for the spider wire vs solid vane, I want to try the wire, but brendan's point of strength when being lugged around is valid.
aren't curved vane spiders more effective in nullifying artefacts than using thinner vanes or strings?
there's a whole complex explanation of how this works with some actual images.

I've been looking at alternatives for aluminium, came across an engineering plastic PEEK, (Young's modulus is 3.6 GPa and its tensile strength 90 to 100 MPa), might prove useful for parts of my fork if its cost effective, or other engineering plastics.

came across this fork mounted truss ota.
http://www.astroworks.com/c28/info.html
are there any advantages to this truss arrangement as opposed to the RC truss designs?
Reply With Quote
  #44  
Old 15-11-2011, 12:45 AM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
Plastics are really bad mate, they are strong but have shocking Coefficient of thermal expansion you will litterally see it moving!

If you sealed the tube up some how and drew the air though it then as long as it doesn't actually cut the mirror you should be right. but really the difference between 3 and 4 parts to the truss will be about 3 minutes of set up. Ally and Carbon fibre tubes are light so it wont take much wieght

As for nodal points you make a common bracket give the joints a close tolerence eg use a set of verniers to measure the bolt then find a corrosponding drill bit to clear it just you wont have any issues I do this for some of the things i build when i need the tolorence. I am a ATMer like yourself its just i have very little time presently to get into it!

Brendan
Reply With Quote
  #45  
Old 15-11-2011, 10:02 AM
SkyViking's Avatar
SkyViking (Rolf)
Registered User

SkyViking is offline
 
Join Date: Aug 2009
Location: Waitakere Ranges, New Zealand
Posts: 2,260
Quote:
Originally Posted by clive milne View Post
On the subject of wire spiders, they can be made to be sufficiently rigid if you apply the offset design which in effect translates the forces into 1st order instead of second order... a very important engineering principle.
Exactly.
It's the angle of the wires that hold the secondary rock solidly in place. Mine is not under much tension at all, but yes it's a common belief that the tension is what keeps a wire spider in place.
Reply With Quote
  #46  
Old 15-11-2011, 10:57 AM
clive milne
Registered User

clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
Alistair, the Centurion is an excellent design.... you could do a lot worse.
I suppose it is functionally equivalent to more traditional offerings (such as RCOS) the difference being it relies on geometry instead of material mass to achieve stability.

Rolf, nice work on that Newtonian.
If I could make a suggestion; have you considered turning the dovetail plate 90 degrees? It should add a bit of stability if you do.
http://thump01.pbase.com/t4/03/95110...8.WHCINg6K.jpg
Reply With Quote
  #47  
Old 15-11-2011, 11:16 AM
clive milne
Registered User

clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
fwiw) I'm not sure if this is in your budget or not, but Dan Gray has been developing a direct drive telescope system.
The advantage is that you do not need any gears whatsoever, so the mechanics are as simple as it gets.
I suggested to him a while back to try and see if fisher and paykel washing machine motors work... and they do.
http://www.youtube.com/watch?v=9-bVYbQwEUg

The downside is that the shaft encoders required will cost you one of your internal organs, but in return you get zero backlash
(because there is no gear train) perfect pointing accuracy and sub-arc second tracking (zero periodic error)
It is not surprising that this is the preferred drive system for modern professional observatory telescopes.

This can be applied very easily to a split ring equatorial made out of plywood or a simple Alt-Az with field de-rotation.
Reply With Quote
  #48  
Old 15-11-2011, 11:42 AM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
Hi Clive,

I've come across direct drive mounts and motors, but don't think I can delve into them.
I have been looking at harmonic gearboxes though. http://www.acrodyne.com.au/products/...harmonic-drive

for now, my approach is not to use worm gears, but either timing belts or friction drive if I can get the surface machined well enough and reduction ratios reasonable as I'm using 1/32 microstepping. so reasonably high torque at the last stage with relatively low overall gear reduction.

I'm using ovoid gearboxes like the ones in the losmandy mounts, and timing pulleys.
from what i've read, pec and backlash can be adjusted to within 5 arc secs and since we have limitations in viewing, that should be ample.

Last edited by alistairsam; 15-11-2011 at 12:06 PM.
Reply With Quote
  #49  
Old 15-11-2011, 11:53 AM
clive milne
Registered User

clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
Cool... sounds like you are on to it.
Which coma corrector are you using by the way?
Reply With Quote
  #50  
Old 15-11-2011, 12:08 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
hi,
the baader mpcc is on my shopping kit list which is growing btw.
Reply With Quote
  #51  
Old 15-11-2011, 12:38 PM
clive milne
Registered User

clive milne is offline
 
Join Date: May 2011
Location: Freo WA
Posts: 1,443
The Baader RCC1 is supposed to be an improvement over MPCC.
Reply With Quote
  #52  
Old 15-11-2011, 01:37 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
Hi Clive,

Not sure if anyone else has had any success, but Gary in this thread mentions issues with collimation with the RCC1.
http://www.iceinspace.com.au/forum/s...ad.php?t=73293
Reply With Quote
  #53  
Old 15-11-2011, 01:56 PM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
Im not sure if its an improvement but it sure has alot more back focus! instead of the collar back to chip being 55mm i think they move out to 110mm but the trade off is the RCC1 is a whole lot bigger!

The wynne corrector would be the ducks perverbial when it comes to newt correctors for AP though.

91.5mm not 110. Do you need the extra 35mm of back focus?

Last edited by bmitchell82; 15-11-2011 at 02:12 PM.
Reply With Quote
  #54  
Old 15-11-2011, 04:45 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
I'm not too sure if I'll need the extra back focus, I'm using a dslr now but will be moving to a ccd soon, so not sure what's the best combo for a ccd, oag, coma corrector, filter wheel.
but all that's still for next year once I get this up and running.
I want the mount and tracking to be stable and consistent first and get the OTA built as well. still heaps to do.
Reply With Quote
  #55  
Old 15-11-2011, 04:59 PM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
http://brendanmitchell.net/?page_id=...1&wppa-occur=1

thats with the mpcc

Though some people rather the OAG behind the corrector. It doesn't really matter as i guide on bannanas and get round stars! happy days
Reply With Quote
  #56  
Old 15-11-2011, 05:29 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
thanks brendan, 400mm is a lot off the tube. did you not notice any flexure at all?
I remember james did a test on a 12" F4 by adding and removing a ccd.
http://www.youtube.com/watch?v=TTGCu...layer_embedded
Reply With Quote
  #57  
Old 15-11-2011, 05:54 PM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
There may be flexure but with the front of the OTA within 25mm of a solid ally ring and the tube ring about the same possibly only a little bit more and a steel tube (the skywatcher are better built than GSO even though nobody believes me) and the fact its running OAG, you don't see the flexure. Further to this the footprint of the 3" focuser i think is 130mm^2 so it pushes that loading over a decent area.

Balancing the scope I generally have 23kgs thats even with the focuser on the underside of the OTA.

The stars show no flexure in 15 and 20min sub exposures so im a happy trooper.
Reply With Quote
  #58  
Old 15-11-2011, 09:32 PM
BEVAN5433 (BEVAN)
Registered User

BEVAN5433 is offline
 
Join Date: Aug 2010
Location: MELBOURNE AUST
Posts: 19
If a f6 12.5 INCH about 4inch thick mirror will do. I am looking for a good home for .Pyrex of course great thermal prop. Has a mirror cell originated from original owner AOS .Recently Alum coated has secondary with it. I am interested more than price in some one having it whom appreciates what they are getting FIGURED FOR IMAGING .BEVAN IVANHOE 94592939
Reply With Quote
  #59  
Old 15-11-2011, 09:40 PM
alistairsam's Avatar
alistairsam
Registered User

alistairsam is offline
 
Join Date: Nov 2009
Location: Box Hill North, Vic
Posts: 1,832
Thanks for the offer Bevan,
I'm really looking for an F4.
A 12" F6 FL would be 1.9m. would make the ota longer than I'd like and less portable and require longer exposure.
If I haven't found alternatives in a few weeks, I'll probably order from Orion optics.

Brendan, have you found significant difference in stability between the skywatcher tripodsand the meade heavy duty tripod? would you know what the spread of the meade is?
Reply With Quote
  #60  
Old 16-11-2011, 03:39 AM
bmitchell82's Avatar
bmitchell82 (Brendan)
Newtonian power! Love it!

bmitchell82 is offline
 
Join Date: Jul 2008
Location: Mandurah
Posts: 2,597
the only difference is if your a bit of a cluts and bump it the meade one doesnt move. on the negitive its physically BIG. and thats coming from me. If you where setting up for a week or two at a time the meade might be nicer but hey. like i said it works the way it is.

On the other side i finally got around to taking the PE of my polished white turd, Un corrected after 500 points, 4 worm cycles PEMpro recorded my poultry loaded EQ6 (25 ish kg on a 20kg rated mount) at +6.5 - 5.3 arc seconds or 11.8 acr seconds peak to peak. Thats good enough for me in bog stock standard gise!
Reply With Quote
Reply

Bookmarks

Tags
12" f4, ota

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 05:42 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement
NexDome Observatories
Advertisement
Limpet Controller
Advertisement
Astronomy and Electronics Centre
Advertisement