View Full Version here: : collimation free dob?
Hi All,
Something to ponder about and whether its doable. If you think I'm mad, I apologise now.
Do you think its possible/feasible to design a DOB such that it never requires to be collimated. I understand that from time to time the primary may need a clean, but can it be replaced back into its cell such that its aligned.
Or are there too many variables for it to be doable (like the glass cooling/expanding).
Better still what about a self collimating dob ? (I dont mean collimating it yourSELF either). :innocent:
Something to think about !!!:doh:
mickoking
30-09-2005, 05:20 PM
It would be nice, self collimating Dob.
asimov
30-09-2005, 06:22 PM
I've often thought of hooking up 2 stepper motors to the X and Y axis on the mirror, & collimating at the EP with a hand-controller...
gaa_ian
30-09-2005, 07:17 PM
The Edmund Scientific "Astroscan" has fixed mirrors & does not require collimation !
It is a "Rich Field" scope though rather than a full focal length.
mickoking
30-09-2005, 07:19 PM
I dont think the astroscan is designed for anything above moderate magnification. Isn't its focal length less than f5?
Starkler
30-09-2005, 08:07 PM
Dont you love the marketing term "focus free" used for cheapy cameras? What it means is that it cannot be adjusted.
A newt thats "collimation free" would have me running away just as fast.
It isn't impossible to get close enough to perfect permanent collimation. But it would be a bit silly due to the expense of producing the precision locating parts.
The silliest way would be to make everything perfect without collimation. This requires great accuracy and repeatability in all components. The less silly way is to design a method of assembly that is very repeatable and do a single collimation that will be permanent.
Possible yes, practical no.
gaa_ian
30-09-2005, 09:03 PM
The astroscan is a 445mm FL & 108mm aperture giving a focal ratio of F4.2.
It would require both very tight tolerances in construction, and materials that remained in an identical state at all temperatures - I'm sure these are around, but suspect they are prohibitively expensive. Then it would also need to built like the proverbial out house to avoid vibrations from transport altering it - I think a "collimation free" Newt would be a dud.....
janoskiss
30-09-2005, 09:48 PM
Possible, but it would be way too heavy and way too expensive.
ballaratdragons
30-09-2005, 11:59 PM
. . . and what would us dobbers have to do on cloudy nights?
Ya gotta fiddle with collimation, it's half the hobby!
wavelandscott
01-10-2005, 09:22 AM
In the latest issue of the US version of Sky and Telescope, there is an article of someone who has "motorized" his dob collimation...
I've not read the article in detail yet but it looks like he has put a motor on each collimation screw via a system of pulleys and gears/steps with the controls near his eyepiece...
While complicated to "two left thumbs" me it at least looks like it eases the completion of a star test and collimation is a "one man job"...
too right ken!
if we cant fiddle with our scope then what? :confused:
You don't need a zero coefficient of thermal expansion. You just need the angles to stay the same which means that your expansion/contraction just has to be even. Remember that collimation doesn't have the tiny tolerances that the optical surfaces do, and yet contracting materials are used for the optics. And you don't need components built like outhouses for vibration resistance, otherwise aircraft would never get off the ground. But you do need cleverness and expensive precision and different construction techniques like the use of dowels or rivets and tight fits. Definitely no locating of parts with screws. So it would be a dud anyway.
fringe_dweller
02-10-2005, 01:23 PM
my two dobs are virtually collimation free - maybe needs doing once evry two years, if that. but they are long focal lengths f7 10" f6.3 8" in steel tubes - makes a big diff
Kearn
netwolf
02-10-2005, 06:13 PM
I think it would be possible, and i agree with Mig that the process could be a last step in the production post assembly. Nothing will ever get invented if we belive it to be impossible. I would guess an initial solution may come from ATM'rs.
slice of heaven
02-10-2005, 08:43 PM
Collimation free newt....hmmm... just need an optically perfect .5mm-40mm zoom ep to go with it.
mickoking
02-10-2005, 10:20 PM
If you had a long focal length newtonian (eg f15) you could probably get away with out collomating.:D
asimov
03-10-2005, 12:42 AM
I eventually did the 'double valve spring' trick on my 3 collimation screws. 75x6 steel strengthening clamp around the 2mm aluminium OTA where the cell is, & the spider end as well. :thumbsup: No wonder the OTA weighs 80 Lbs! :scared2:
ballaratdragons
03-10-2005, 12:47 AM
It sounds like it will hardly ever move John! Where did you get springs small enough for 'double springing' the 2ndry?
asimov
03-10-2005, 01:02 AM
No, the double springs are on the primary mirror cell Ken. The secondary should never move on you anyway unless you give the OTA a hard knock or something..
ballaratdragons
03-10-2005, 01:09 AM
Oops, sorry.
I agree, I have never had any problems with my 2ndry moving. I would love to change this wierd 6 point system the GS 12" has and make it a simple 3 point collimation set-up like my old 4 1/4" had.
asimov
03-10-2005, 01:17 AM
Oh? A 6 point? That's interesting! Would that make it harder to collimate?? I would imagine so. Wonder why they added that 'feature'....Probably for added strength to stop it going out of collimation. How often do you have to collimate your scope?
ballaratdragons
03-10-2005, 01:29 AM
Not often. The 6 point system works on 3 pulling and 3 pushing. the 3 pulling screws are spring loaded. The idea behind it is to collimate with the 3 sprung ones and then lock them with the 3 pushing ones. Problem is if you do up the pushing ones just a smidge too much it changes the collimation.
The principle is to lightly finger tighten the lock screws but I found it kept going out of alignment too often so now I get it as close as possible with the sprung ones and then collimate it perectly by doing up the lock screws tight and collimate using them. I know it's the wrong method for this design but hey, it works and hardly ever goes out of collimation any more.
But it was all so much easier on my old 3 screw reflector!
mch62
03-10-2005, 06:58 AM
Collimation free mirror is possible .
My Conical mirror has not been touched for three years since it was built except for when I made the focuser cage a rotating cage.
It has been over some rough dirt roads in the back of a ute.
The truss assemble does not disassemble thou so that is a concideration.
The conical mirror is located on a central stud and held in place by the stud with 4 O rings on the central hole, and a front retainer cap.
I have removed the mirror for various reasons and as long as it is replaced in the same orintation , does not require re collimation.
The Cell assemble is a 3 arm spider with push pull screws .
No floatation points , mirror straps or mirror retaining clips.
No springs here , once adjusted and locked thats where it stays , full stop end of story.
Very easy to adjust.
The black lock bolt is wound out a little to allow the silver adjuster to be free.
The adjuster is held in place by the nylock nut and is tightened just enough to allow the bolt to turn .
Simple but very very effective.
Once adjusted do up the lock bolts.
Actually you don't even need the lock bolts , but are there just to be sure.
Mark
asimov
03-10-2005, 02:05 PM
Now that is very interesting Mark. Great system & thanks for posting.
Do you by any chance also have an optically perfect 0.5 mm - 40 mm zoom eyepiece to go with it?
Apparently it is as common as collimation free Newtonians. :D
mch62
04-10-2005, 07:44 AM
No , not off hand but give some time and i will see what i can come up with .
Now do you want that with 82 or 90degs AFOV and how's about 0.5-50mm instead :poke: :rofl:
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