collimation free dob?

norm · #1 30-09-2005, 05:13 PM

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).

Something to think about !!!

mickoking · #2 30-09-2005, 05:20 PM

It would be nice, self collimating Dob.

asimov (John) · #3 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 (Ian) · #4 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 · #5 30-09-2005, 07:19 PM

Quote:

Originally Posted by gaa_ian

The Edmund Scientific "Astroscan" has fixed mirrors & does not require collimation !
It is a "Rich Field" scope though rather than a full focal length.

I dont think the astroscan is designed for anything above moderate magnification. Isn't its focal length less than f5?

Starkler (Geoff) · #6 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.

MiG · #7 30-09-2005, 08:54 PM

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 (Ian) · #8 30-09-2005, 09:03 PM

The astroscan is a 445mm FL & 108mm aperture giving a focal ratio of F4.2.

Lee · #9 30-09-2005, 09:33 PM

Quote:

Originally Posted by MiG

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.

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 (Steve H) · #10 30-09-2005, 09:48 PM

Possible, but it would be way too heavy and way too expensive.

ballaratdragons (Ken) · #11 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 (Scott) · #12 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"...

ving (David) · #13 01-10-2005, 09:47 AM

Quote:

Originally Posted by ballaratdragons

. . . and what would us dobbers have to do on cloudy nights?

Ya gotta fiddle with collimation, it's half the hobby!

too right ken!
if we cant fiddle with our scope then what?

MiG · #14 01-10-2005, 05:26 PM

Quote:

Originally Posted by plasmodium

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.....

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 · #15 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 · #16 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 · #17 02-10-2005, 08:43 PM

Collimation free newt....hmmm... just need an optically perfect .5mm-40mm zoom ep to go with it.

mickoking · #18 02-10-2005, 10:20 PM

If you had a long focal length newtonian (eg f15) you could probably get away with out collomating.

asimov (John) · #19 03-10-2005, 12:42 AM

Quote:

Originally Posted by fringe_dweller

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

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.

No wonder the OTA weighs 80 Lbs!

ballaratdragons (Ken) · #20 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?