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Equipment Discussions >> Cats & Casses

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Itz marcus
Pooh-Bah
*****

Reged: 02/12/06

Loc: Brooklyn NY
Why can't sct collimation be set permanently?
      #6175375 - 11/04/13 12:38 PM

Hi,
Why don't scope makes make sct scopes with collimation permanent just as many refractors and mct are. The same question for reflectors. My refractor has remained in perfect collimation so why not these scopes?
Clear Skies
Itz


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GlennLeDrew
Postmaster
*****

Reged: 06/18/08

Loc: Ottawa, Ontario, Canada
Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6175487 - 11/04/13 01:37 PM

Permanent collimation would be feasible only with sufficient accuracy of dimensioning of all parts, assured rigidity and assurance that if disassembled will go back together exactly as before. This would drive up cost *very* significantly!

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Joe Aguiar
Carpal Tunnel
*****

Reged: 02/10/07

Loc: none of your buss
Re: Why can't sct collimation be set permanently? new [Re: GlennLeDrew]
      #6175654 - 11/04/13 02:47 PM

really they cant do it without haveing to raise costs, we put a man on the moon i agree with poster why not, calling all scope makers ok just do it

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Cliff Hipsher
Pooh-Bah
***

Reged: 12/31/08

Loc: North Chesterfield, VA
Re: Why can't sct collimation be set permanently? new [Re: Joe Aguiar]
      #6175703 - 11/04/13 03:12 PM

Its all a slick government plot to keep high quality telescopes out of the hands of the unwashed masses which will keep them from seeing what is really happening on the Moon. Don't forget to wear your aluminum foil hat....

Seriously, don't you think that if it were possible, and if there was money to be made selling such an instrument, it wouldn't be available already?


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pfile
Post Laureate


Reged: 06/14/09

Re: Why can't sct collimation be set permanently? new [Re: Cliff Hipsher]
      #6175708 - 11/04/13 03:13 PM

the apollo program cost about $110 Billion (in 2010 dollars)

so if you want this, as Glenn says, it's going to cost you.

rob


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Ed Holland
Pooh-Bah


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Why can't sct collimation be set permanently? new [Re: pfile]
      #6175744 - 11/04/13 03:36 PM

Interesting question. I'd like to venture a couple of ideas here:

First of all, what are the major contributions to changes in the collimation of an SCT?

I'd answer mirror shifts, and creep in the (usually plastic) secondary mirror tilt mechanism. Presumably the rest of the assembly is sufficiently rigid

Second, do all SCT's shift collimation to the same degree?

Probably not. I've heard that alignment is more stable when the collimation screws are kept tightened to an appropriate degree. Also I've read the opinion that "thumbscrew" type collimation screws make it more difficult to achieve sufficient torque (no personal experience here).

Could a better secondary holder improve stability? Even so, there would still be the potential for mirror shifts (for which I think there is possibly an inexpensive solution).

Did I miss something?


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freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6175748 - 11/04/13 03:39 PM

Refractors use lenses and sct's use two mirrors. If you tilt a lens a bit, the light is only slightly deflected, but if you tilt a mirror, the change in angle of the reflection is twice as large. In addition, refractors tend to be smaller aperture and shorter focal length, so the imperfections are less of a factor.

Spot Maksutov's have a thick, heavy corrector and the secondary is integral. Rumak's can have a separate, adjustable secondary and I haven't used one but I imagine they need recollimation occasionally.

SCT's are designed to be collimated just using slight tilt of the secondary. If the secondary is slightly off center or the primary tilts a bit, the system can be well compensated by tilt of the secondary. The alignment system on sct secondaries isn't very fancy, and I imagine it could be improved upon - but the existing simple system works well.

For something like Edge, the critical elements are the centering of the sct corrector, and the axial alignment of the flattening lenses. Those things are set permanently at the factory - not just in a "collimator" - but an autocollimator, which uses a full aperture precision flat reference in a double pass mode. Those permanently aligned components are rigidly attached, and then the user does periodic tweaking of the collimation with three simple screws on a single mirror.

Large observatory systems are designed around the fact that collimation will not hold as they move around, so they have active optics making slow adjustments to keep the system aligned.

For a 10" mirror, if the reflected beam moves 1' it means the mirror tilted 0.5' or 30 arc-seconds. Over a diameter of 250mm that corresponds to a tilt of only 36 microns. So - sct's are sensitive to tilt and any other changes in the system, but they can be corrected with small turns of 3 screws. It works well and it's cost effective.

Frank


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Sorny
professor emeritus


Reged: 03/15/12

Loc: Southern MN
Re: Why can't sct collimation be set permanently? new [Re: freestar8n]
      #6175793 - 11/04/13 04:08 PM

I've not adjusted collimation on my CPC1100 since I got it a year and a half ago. It is close enough that I'd need a perfect night of seeing to get the final tweak. Haven't had seeing good enough to do that.

My NexStar 5 SE has been adjusted twice in something like 4 years.

Keep the tension high (aka no knurled knobs), and your collimation will stay good for a long time.


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Bob Griffiths
Getting Grouchy
*****

Reged: 10/10/05

Loc: Frederick Maryland
Re: Why can't sct collimation be set permanently? new [Re: Sorny]
      #6175811 - 11/04/13 04:19 PM

Well I own a Meade AR-5 refractor thatr does NOT have a push pull cell to allow me to collimate it...And guess what it is slightly out of collimattion any

Stop complaining about the need to collimate a SCT even if they hold collimation real well...at least you can do it without sending the scope back to the manufacturer

My other refractor (cheap Orion) can be collimated


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why can't sct collimation be set permanently? new [Re: Bob Griffiths]
      #6175824 - 11/04/13 04:24 PM

If properly collimated, the average SCT can go YEARS without needing more attention. And MCTs and refractors are not immune to mis-collimation, either.

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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why can't sct collimation be set permanently? new [Re: Bob Griffiths]
      #6175832 - 11/04/13 04:26 PM

Quote:

Well I own a Meade AR-5 refractor thatr does NOT have a push pull cell to allow me to collimate it...And guess what it is slightly out of collimattion any

Stop complaining about the need to collimate a SCT even if they hold collimation real well...at least you can do it without sending the scope back to the manufacturer

My other refractor (cheap Orion) can be collimated




Even if the cell of your refractor is not collimatable, it's often possible to fix minor errors by loosening the screws that hold the tailpiece in place and shifting it slightly...


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dpippel
scholastic sledgehammer
*****

Reged: 04/05/13

Loc: Desierto de Sonora
Re: Why can't sct collimation be set permanently? new [Re: Ed Holland]
      #6175835 - 11/04/13 04:28 PM

Quote:

Second, do all SCT's shift collimation to the same degree?




I think there's quite a bit of variation within practical limits. For example, I just bought a squeaky-clean 1981 orange tube C8. Took it out for first light last night and checked the collimation - it was spot-on. I got the scope from the original owner and he never touched the screws. Didn't even know what they were for. So here's a SCT that, even though it's been very lightly used, has been on the planet for more than 30 years and the collimation doesn't need to be touched. My practically brand-new 6SE on the other hand needed some nuanced tweaking to get lined up.


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Jon_Doh
professor emeritus


Reged: 09/16/11

Loc: On a receiver's back
Re: Why can't sct collimation be set permanently? new [Re: dpippel]
      #6175883 - 11/04/13 04:53 PM

Uncle Rod just said what I was going to say. But, once you collimate your SCT it will likely remain in collimation. I collimated mine two years ago and boy what a difference it made. And two years later despite hauling it around it had remained collimated. Collimation isn't what I worry about with my SCT. Getting it cooled down is the issue.

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David PavlichAdministrator
Transmographied
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Reged: 05/18/05

Loc: Mandeville, LA USA
Re: Why can't sct collimation be set permanently? new [Re: Jon_Doh]
      #6175888 - 11/04/13 04:57 PM

Once collimated, my obs mounted SCs stay in collimation. However, if you transport and bump the thing around too much, collimation can go south.

David


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Why can't sct collimation be set permanently? new [Re: David Pavlich]
      #6176029 - 11/04/13 06:35 PM

It can and will go south, but it will take a long time to do so if you've done the job correctly, even if you drive over roads like _I_ do to get to observing sites.

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6176038 - 11/04/13 06:40 PM

I believe it is better to have a telescope where collimation can be adjusted. If it was 'locked', there could be a small shift in the optical train that could not be smoothed out.

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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Why can't sct collimation be set permanently? new [Re: GeneT]
      #6176385 - 11/04/13 09:50 PM

I don't understand the question.

Why would you want a scope that could not be user collimated? That would mean that if (when) collimation is required, the scope would have to go in for service. And then the system would have to be robust enough that it could survive shipping without disrupting collimation.

Collimating an SCT is really easy, compared to newts, RCs, etc. Every SCT owner could learn to do it. The biggest challenge is that the best way to learn is to have someone show you how under the stars. It's not as complicated as it would seem, once you've seen it.

To me, this question would be the same as asking why they don't make a guitar that's permanently tuned?


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Ed Holland
Pooh-Bah


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Why can't sct collimation be set permanently? new [Re: WadeH237]
      #6176594 - 11/04/13 11:37 PM

Respectfully, I think the OP's question is in regard to a perception that SCT's require collimation more frequently than some other designs.

I have not noticed this in my scope (a used C8) but have been firm with the collimation screws, and do not travel with my gear. That said, it is necessary to be able to collimate these multi component optical trains, since relatively small mechanical misalignments can have a big impact on image quality.


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WadeH237
Pooh-Bah


Reged: 02/24/07

Loc: Snohomish, WA
Re: Why can't sct collimation be set permanently? new [Re: Ed Holland]
      #6176705 - 11/05/13 01:44 AM

Ahh. I think that the persistence of collimation on an SCT has been covered pretty well on the thread. For my own scopes, I agree that they don't need to be collimated often. My 14" truss newt needs to be collimated every time I set it up, and I have to adjust two mirrors on it.

I just think that many people don't realize how easy it is to collimate an SCT. And there would be serious downsides to having one that could not be easily user collimated.


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seawolfe
sage


Reged: 06/26/13

Loc: N 47.11.23 W 122.20.18
Re: Why can't sct collimation be set permanently? new [Re: rmollise]
      #6176797 - 11/05/13 05:09 AM

Quote:

It can and will go south, but it will take a long time to do so if you've done the job correctly, even if you drive over roads like _I_ do to get to observing sites.




My mind can't help but go back to the scene in the old movie, Mr. Myjestic where that old beat up Ford pickup goes bouncing around on the back roads and even goes flying across ditches so that the hero can get ahead of the baddies.

Driving like that might just get your scope out of collimation....

Edited by seawolfe (11/05/13 05:10 AM)


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