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Equipment Discussions >> Cats & Casses

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Itz marcus
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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
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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
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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
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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
Carpal Tunnel


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
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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
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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
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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
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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
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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
scholastic sledgehammer


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
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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
Carpal Tunnel


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
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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
professor emeritus


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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Itz marcus
Pooh-Bah
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Reged: 02/12/06

Loc: Brooklyn NY
Re: Why can't sct collimation be set permanently? new [Re: seawolfe]
      #6177136 - 11/05/13 10:52 AM

Hi,
Quote:

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.
Quote:



My question was why can't it be an option. I myself have no problem collimating (anymore) but it is time consuming (20 minutes or so maybe I will get better and faster) and in 30 degree weather a real pain. Refractors with set cells (like my EON) do not cost anymore than collimatable refractors. In fact they many times cost less than an equal aperture and quality scope. Maybe they do lose collimation but that has not been the case with me and many other VERY happy non user collimatable refractors.
Clear Skies
Itz

Edited by Itz marcus (11/05/13 10:55 AM)

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rmollise
Postmaster
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Reged: 07/06/07

Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6177402 - 11/05/13 01:31 PM

Again, if you are doing your SCT's collimation right, you have exactly what you are asking for: a telescope that can be collimated, but which rarely needs it.

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bicparker
Carpal Tunnel


Reged: 02/07/05

Loc: Texas Hill Country
Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6177780 - 11/05/13 04:42 PM

Quote:

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




There is good design sense in making any optical train mechanism adjustable, not only in focus, but in collimation and other features, as well (mounting, rotation of focus, et al). This same logic extends to any mechanical system that is expected to operate within certain tolerances while maintaining a dynamic stance in a variable environment throughout its operations. The system needs to be adjustable so it can operate within its design tolerances under a wide range of circumstances. It would be a bad design decision to not have a collimation adjustment.

Perhaps a better question is, why can't a robust collimation adjustment design be put into place that isn't so easily changed by factors other than temperature, humidity, and atmospheric pressure? It is kind of a pain when the adjustment system is the primary reason the system is out of adjustment. This, of course, is the logic behind any push-pull screw system for collimation, but you generally have a straight set of screws in an SCT, and these simply loosen when shaken enough or bumped the wrong way.

Without respect to how well or not so well it was implemented, I think Meade was really on to a good notion when they introduced the electro-mechanical collimation system in their RCX 400 telescopes. Here was a system that was easily adjustable, and wouldn't (or wasn't supposed to) change due to bumps in the road or a loose screw. Those two criteria should be the fundamental criteria used for designing and making a telescope collimation system.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Why can't sct collimation be set permanently? new [Re: bicparker]
      #6178208 - 11/05/13 09:11 PM

Quote:

It would be a bad design decision to not have a collimation adjustment.



I agree. I hope they don't design scopes than cannot be user collimated in the field.


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Gil V
professor emeritus


Reged: 09/09/12

Re: Why can't sct collimation be set permanently? new [Re: seawolfe]
      #6180941 - 11/07/13 12:23 PM

Backing up Rod 100%. Once collimated, there is virtually no need to re-collimate an SCT. If there is, it's because the scope was not properly collimated by the user (screws with improper tension/tightness). Even when shipped, collimation usually holds.

Best practice is to use an artificial star, IMHO.

They hardly ever go out. I really think part of the problem is people adding after market hardware. Total waste of money.


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why can't sct collimation be set permanently? new [Re: Gil V]
      #6181783 - 11/07/13 08:30 PM

if you dont have the secondary screws tight enough it can
go out, another thing that can trick you is mirror flop, always tweek your focus towards counter clockwise.
my scopes usually stay in for close to a year.


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MitchAlsup
Pooh-Bah


Reged: 08/31/09

Re: Why can't sct collimation be set permanently? new [Re: orion61]
      #6181916 - 11/07/13 10:00 PM

A couple of three points:

A) A refractor only has to hold alignment between the objective and the focal plane; while an SCT has to hold the corrector at the right spot, the primary at the right spot (a significant distance away), and the secondary at the right spot (another significant distance away), all the way to the focal plane. Its this distance which makes the difference.

2) I have a Top-end refractor (AP) that occasionally needs collimation (made in 1989).

3) My SCT came (circa 1990) miscollimated such that I had to change the spacings on the corrector to secondary holder to get the corrector plate and secondary in the right spots. This is WAY harder than aligning a Newtonian (even for the very first time.)


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corpusse
scholastic sledgehammer


Reged: 04/11/10

Loc: Mississauga, Ontario, Canada
Re: Why can't sct collimation be set permanently? new [Re: MitchAlsup]
      #6182060 - 11/07/13 11:46 PM

Not that it is really possible, nor would I even trust Celestron if they could offer it, but if there was an SCT that did not need collimation and I think that would be worth at least an extra $1000.

It's interesting to see how some people say you don't need to collimate for years and others say you should at least check if not adjust daily (mostly high resolution planetary imagers).

If you live somewhere where the seeing is always poor it can take months or longer to get a chance to actually collimate.


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Spacetravelerx
Carpal Tunnel


Reged: 12/23/12

Loc: New Mexico
Re: Why can't sct collimation be set permanently? new [Re: corpusse]
      #6182084 - 11/08/13 12:10 AM

Quote:

Not that it is really possible, nor would I even trust Celestron if they could offer it, but if there was an SCT that did not need collimation and I think that would be worth at least an extra $1000.

It's interesting to see how some people say you don't need to collimate for years and others say you should at least check if not adjust daily (mostly high resolution planetary imagers).

If you live somewhere where the seeing is always poor it can take months or longer to get a chance to actually collimate.





I am one of those who hasn't needed to collimate my telescope for years, and certainly there hasn't been a need to collimate my 14" LX850 since I received it in the spring. Visually the views are stunning in the new unit. Planet views are wonderful too. Of course being in the high desert at over 5,500' helps too on the wonderful views,


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Footbag
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Reged: 04/13/09

Loc: Scranton, PA
Re: Why can't sct collimation be set permanently? new [Re: Spacetravelerx]
      #6182528 - 11/08/13 09:30 AM

I think it would be great if they could make collimation permanent... That is as long as I can adjust it if it goes out.

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Jon Isaacs
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Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Why can't sct collimation be set permanently? new [Re: Spacetravelerx]
      #6182571 - 11/08/13 09:51 AM

Quote:

I am one of those who hasn't needed to collimate my telescope for years, and certainly there hasn't been a need to collimate my 14" LX850 since I received it in the spring. Visually the views are stunning in the new unit. Planet views are wonderful too. Of course being in the high desert at over 5,500' helps too on the wonderful views,




As someone who frequently observes in the high desert, generally the high desert is transparent but seeing is the issue. Also thermal equilibrium is a big issue because the temperature drops so quickly after sundown and continues to drop because of the transparent sky. What does the star test look like at, what do you think, is 600x is appropriate for a 14 inch?

Jon


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vahe
scholastic sledgehammer


Reged: 08/27/05

Loc: Houston, Texas
Re: Why can't sct collimation be set permanently? new [Re: Itz marcus]
      #6182611 - 11/08/13 10:08 AM

Quote:

Hi,
Why don't scope makes make sct scopes with collimation permanent just as many refractors and mct are.





If one detects any amount of image shift while focusing that indicates that the primary position does not remain stable and in alignment as it moves back and forth and if that is the case then I do not see how critical collimation can be achieved in any system that can not maintain prefect alignment of its optics.

Zero image shift focusing mechanisms used in many high end Maksutovs are expensive to implement and will add to the price of instrument and it is just one more reason why quality Maks are generally more expensive than SCT’s, since vast majority of SCT owners are perfectly happy with what they get for the $$$ there is no incentive on the part of manufacturers to go beyond simple and inexpensive focusers supplied with these scopes.

Vahe


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Spacetravelerx
Carpal Tunnel


Reged: 12/23/12

Loc: New Mexico
Re: Why can't sct collimation be set permanently? new [Re: Jon Isaacs]
      #6182692 - 11/08/13 10:50 AM

Quote:

Quote:

I am one of those who hasn't needed to collimate my telescope for years, and certainly there hasn't been a need to collimate my 14" LX850 since I received it in the spring. Visually the views are stunning in the new unit. Planet views are wonderful too. Of course being in the high desert at over 5,500' helps too on the wonderful views,




As someone who frequently observes in the high desert, generally the high desert is transparent but seeing is the issue. Also thermal equilibrium is a big issue because the temperature drops so quickly after sundown and continues to drop because of the transparent sky. What does the star test look like at, what do you think, is 600x is appropriate for a 14 inch?

Jon




Jon,

When you say seeing is the issue, what do you mean exactly?

Yes, where I am the temperature does have a big drop, I want to say up to 30 deg after sunset. However, I have my LX850 under the Telegizmos 365 cover during the daytime, which tends to keep the temperatures cooler inside the cover (I will measure it when I get back home as I am on travel right now). This in turn seems to keep the cool down delta smaller. With a the metal OTA heat is also transmitted out quicker vs. say plastic or CF parts. Thirdly, I really don't start observing sometimes until 2-3 hours after sunset for a variety of reasons; things are a bit more stable at that point.

Besides the altitude (less air to look through), the other advantages in New Mexico: overall lower humidity and many clear nights (310+ a year). I never have to deal with dew, whew. Michigan? Total sogginess! Either way I find the view spectacular regardless if I am using the 14" ACF or my APOs in New Mexico. When I moved from the midwest to New Mexico the change in observing was profound. It was like I got a brand new telescope.

I don't do a whole lot of high magnification stuff, however I did observe the "double double" in Lyra with an HD-60 4.5mm eyepiece (632x). The view was wonderful and emotional - why do some objects invoke emotions when you view them? Jupiter of course looks impressive too. The views through my 8.8mm UWA look fantastic (323x) - I am looking at getting the 5.5 mm UWA when they become available (I think early next month).

I do occasionally check the collimation, but it so far has not been an issue.


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Lew Zealand
Pooh-Bah
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Reged: 06/28/04

Loc: Pasadena, CA
Re: Why can't sct collimation be set permanently? new [Re: Footbag]
      #6185727 - 11/10/13 12:33 AM

I can assure everyone here that you really only need to collimate your SCT twice in the space of 10 years— a touch-up fresh of the box and once again when it takes a digger into the sand at Joshua Tree National Park.


Hey, at least the LX90 was in it's case when gravity got it's grubby mitts on it!


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orion61

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Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Why can't sct collimation be set permanently? new [Re: Lew Zealand]
      #6186290 - 11/10/13 10:43 AM

I always check mine, but something I haven't heard anyone mention is, Always fine focus Counter Clockwise when aligning, and checking optics. Mirror flop can drive you crazy

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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Why can't sct collimation be set permanently? new [Re: orion61]
      #6186702 - 11/10/13 02:15 PM

The counter clockwise rule is a good one.

What I would like to experiment with are some ideas I have to eliminate the mirror flop altogether, whilst retaining the neatness of internal focussing. To my way of thinking, this might (I stress the might) be achieved without costly additions to the design. There are some considerations:

1) The anti-flop system would have to be tolerant of temperature

2) It should not introduce undue resistance in the mirror motion, or shifts in focusing.

I'd retain the principle of two concentic tubes - it is a very good way to provide the starting point for aligning the optical axes of the mirror and image planes. What I am trying to conceive is a method to provide a higher tolerence bearing, or compliant interposer to maintain a more precise relation between the two cylinders than is provided by a grease film. I have some ideas for this.


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Howie Glatter
Vendor


Reged: 07/04/06

Re: Why can't sct collimation be set permanently? new [Re: Ed Holland]
      #6186789 - 11/10/13 03:02 PM

> some ideas . . to eliminate the mirror flop altogether,
> whilst retaining the neatness of internal focussing.

The way to do it is simple in principle, but the mass-producers of SCTs don't seem willing to undergo the extra time, care, and quality control that would be necessary to machine the two linear bearing surfaces (mirror bushing bore and baffle tube exterior) to be true cylinders with a diameter difference of no more than 0.0005". Maybe they consider the cost to do this prohibitive, but it has been done by others, for example TEC on their Maksutovs, which have zero image shift. Even better would be three rows of pre-loaded rolling element bearings in grooves machined on the surfaces, spaced at 120 degrees. No doubt this would be expensive.


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Ed Holland
Carpal Tunnel


Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Why can't sct collimation be set permanently? new [Re: Howie Glatter]
      #6187329 - 11/10/13 06:51 PM

I was musing some simpler ideas:

.. an o ring at each end of the mirror tube, retained in a groove. Designed carefully, this could restrict off axis movement between the tubes. One would need to be sure how this held up to temperature changes.

.. make longitudinal cuts in the mirror tube ends to create compliant fingers - or add a spring collar to give the same function. These could be formed inward so that they are under preload when assembled with the baffle tube. Compromises would likely involve providing the necessary holding force without introducing undue friction.

... Collet type arrangements at each end of the mirror tube to allow adjustment of fit to the baffle.

These are just thoughts, not fully formed ideas, on methods that might be implemented without major cost impacts.

Ed


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