Jump to content


Photo

Dumb Newb Question

  • Please log in to reply
17 replies to this topic

#1 Mxplx2

Mxplx2

    Mariner 2

  • -----
  • Posts: 249
  • Joined: 12 Sep 2012
  • Loc: PA USA

Posted 18 June 2013 - 01:41 PM

So why all this collimation? If components are rigidly mounted, aligned, and then locked in place, what's going to change? Is it necessary, or do amateur astronomers just like to fiddle with stuff?

#2 hawk

hawk

    Mariner 2

  • -----
  • Posts: 239
  • Joined: 12 Mar 2013

Posted 18 June 2013 - 01:50 PM

Alas, if you want to focus there's gotta be *something* that's not locked in place. In SCTs, when you twist that focus knob you're actually sliding the primary mirror forward/back in the tube, for example.

#3 Seldom

Seldom

    Viking 1

  • -----
  • Posts: 841
  • Joined: 05 Aug 2012
  • Loc: N of Cedar City Light Dome

Posted 18 June 2013 - 01:55 PM

May not be necessary if you have a refractor, but with a Newt the primary may flop around (say +- 1/8") when moved, and the secondary may get knocked. I need to collimate my scope every time it's moved.

#4 DavidC

DavidC

    Apollo

  • -----
  • Posts: 1393
  • Joined: 24 Nov 2005
  • Loc: Mesa, Arizona

Posted 18 June 2013 - 02:59 PM

Whey collimation? I can't say for sure on sct's, but for the best views on reflectors you need to collimate every time you set up. Moving a reflector can move the mirrors around, even slightly enough to make a difference. If your driving anywhere, the movements and vibration will throw everything off. And if you have a truss type, your're assembling every time you go out.
David

#5 SeattleScott

SeattleScott

    Apollo

  • -----
  • Posts: 1414
  • Joined: 14 Oct 2011

Posted 18 June 2013 - 03:42 PM

One extreme example is Televue. Their refractors are not designed to be collimated, because they feel they build them well enough that they won't lose collimation. But if you drop it off your balcony the collimation can get messed up, and now you can't fix it. Not very easily anyway. Mere temperature changes can affect collimation in a reflector, so they make it easy to adjust. The other optical designs do not permit them to hold collimation like a refractor, so they need to be easy to adjust. I am waiting for someone to build a scope that can collimate itself in 3minutes or less with the touch of a button, but haven't seen one yet.

#6 jerwin

jerwin

    Viking 1

  • *****
  • Posts: 955
  • Joined: 16 May 2012
  • Loc: Romeoville IL

Posted 18 June 2013 - 03:44 PM

I think typically this is more of an issue with dobsonians. I travel with mine and don't slam it around when loading it in my truck or anything like that but I think just the longer tube and somewhat flexible spider veins have a little more movement than an SCT.

Ignorance is bliss somewhat. If the views look good, then I think most would agree don't fiddle with it. One night I decided to fiddle with my SCT and upon completion I could actually resolve Jupiter moons to a small disk rather than a star. Seeing might have been a big part of that too, but it didn't pop out until I collimated it.

Jim

#7 BigC

BigC

    Gemini

  • *****
  • Posts: 3333
  • Joined: 29 Sep 2010
  • Loc: SE Indiana

Posted 19 June 2013 - 08:46 AM

I have found the collimation to be seriously out of adjustment in about half the used reflectors personally encountered!A few minutes of work restored the telescope to usefulness.

Some of them probably suffer from people "tightening loose screws" ,a problem once commonly found in repairing consumer electronics.Some people don't know the difference between an adjustment screw and a fastening screw, and so tighten all they see.

And the more a scope is transported ,the more likely it will get out of adjustment.

#8 csrlice12

csrlice12

    Voyager 1

  • *****
  • Posts: 11482
  • Joined: 22 May 2012
  • Loc: Denver, CO

Posted 19 June 2013 - 08:58 AM

When you drive a car, you have to gas it up to make it go; no difference for a reflector, you gotta collimate it to make it work.....luckily, both are easy to do. It's like a Homer Simpson "Doh" moment.

#9 Midnight Dan

Midnight Dan

    Voyager 1

  • *****
  • Posts: 11516
  • Joined: 23 Jan 2008
  • Loc: Hilton, NY, Yellow Zone (Bortle 4.5)

Posted 19 June 2013 - 08:58 AM

Refractors almost never need collimating. SCTs require it very infrequently, unless handled roughly. Reflectors, especially the larger ones, and especially if moved and assembled, need it very frequently because the primary mirror has some freedom of motion and because the disassembly/assembly process doesn't always create a precisely repeatable structure.

-Dan

#10 Kfrank

Kfrank

    Surveyor 1

  • -----
  • Posts: 1833
  • Joined: 20 Dec 2008
  • Loc: Northern Colorado

Posted 19 June 2013 - 11:16 AM

Alas, if you want to focus there's gotta be *something* that's not locked in place. In SCTs, when you twist that focus knob you're actually sliding the primary mirror forward/back in the tube, for example.


But note that you have no way to adjust the primary on an SCT without major surgery.

#11 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44743
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 19 June 2013 - 11:50 AM

So why all this collimation? If components are rigidly mounted, aligned, and then locked in place, what's going to change? Is it necessary, or do amateur astronomers just like to fiddle with stuff?


As noted, Newtonians require attention to collimation.. There are couple of reasons for this...

- Mirrors cannot be held tightly, they deform under even slight pressure, they need to be free to move.

- Of all the designs, Newtonians are the simplest, they have only 1 curved surface and 1 flat surface. This means that collimation tolerances are greater because there are not two or more curved elements that must be precisely aligned. One could build a Newtonian that is sufficiently robust, sufficiently stiff, that it would not require frequent collimation but it would be far heavier and far more expensive... The need to collimate is a small price to pay for the portability.

Other scopes do require collimation but generally the end user is only required to do the final step because of the precision required for a full alignment. When one collimates an SCT, the assumption is that the corrector is centered and in the proper rotation. A refractor that does go out of collimation, may require a trip back to the factory. The fact that the owner cannot fully collimate a scope means that it has to be much more robust so that it will remain collimated under severe conditions.

Jon

#12 Tony Flanders

Tony Flanders

    Voyager 1

  • *****
  • Posts: 11301
  • Joined: 18 May 2006
  • Loc: Cambridge, MA, USA

Posted 19 June 2013 - 12:21 PM

Newtonians require attention to collimation .. There are couple of reasons for this...

- Mirrors cannot be held tightly, they deform under even slight pressure, they need to be free to move.


Yeah, but ...

If that's true for Newtonians, why isn't it true for SCTs of the same aperture? After all, the primary mirror of an SCT is roughly f/2, so its collimation tolerances are truly critical, much worse than for an f/5 Newt. Yet as noted, SCTs go out of collimation less fequently than Newts.

#13 brianb11213

brianb11213

    Cosmos

  • *****
  • Posts: 9047
  • Joined: 25 Feb 2009
  • Loc: 55.215N 6.554W

Posted 19 June 2013 - 01:11 PM

If that's true for Newtonians, why isn't it true for SCTs of the same aperture? After all, the primary mirror of an SCT is roughly f/2, so its collimation tolerances are truly critical, much worse than for an f/5 Newt. Yet as noted, SCTs go out of collimation less fequently than Newts.

Perhaps the fact that SCTs have spherical primary mirrors has something to do with it. This also explains why the only collimation provision in SCTs is the tilt of the secondary. (The corrector plate is so thin that a small tilt is of little consequence.)

#14 BlackBirdCD

BlackBirdCD

    Explorer 1

  • -----
  • Posts: 60
  • Joined: 19 Aug 2012
  • Loc: Pacific Northwest

Posted 19 June 2013 - 01:30 PM

Consider collimation to be similar to tuning a musical instrument before playing. With practice it's not difficult and requires little time.

#15 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 12839
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 19 June 2013 - 05:02 PM

Per many others, there are no dumb questions. We do give some dumb answers on Cloudy Nights, however. :grin:

Optics are ground to very close tolerances. To get the best performance, they must be collimated accurately. Reflectors go out the soonest, while refractors and SCTs hold their collimation a little better. I own a 12.5 inch Dob and I would not even think of viewing without collimating first. The telescopes' various components do shift, even if slightly, need to be collimated to ensure you get the best views possible.

#16 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 44743
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 19 June 2013 - 08:02 PM

Yeah, but ...

If that's true for Newtonians, why isn't it true for SCTs of the same aperture? After all, the primary mirror of an SCT is roughly f/2, so its collimation tolerances are truly critical, much worse than for an f/5 Newt. Yet as noted, SCTs go out of collimation less fequently than Newts.



SCT mirrors are center mounted rather than held in a cell. They are also spherical...

One advantage that SCTs and refractors have over Newtonians is that the optical axes of the components are concentric.

Jon

#17 CosmoSat

CosmoSat

    Apollo

  • -----
  • Posts: 1273
  • Joined: 24 Jul 2009
  • Loc: India

Posted 19 June 2013 - 11:40 PM

So why all this collimation? If components are rigidly mounted, aligned, and then locked in place, what's going to change? Is it necessary, or do amateur astronomers just like to fiddle with stuff?


Speaking of newtonian reflectors, Guess if the manufacturer decides to mount the optical components rigidly leaving no provision for collimation, he himself would find it difficult to get the optics aligned in the first place. Imagine doing it at a large scale where even slight mechanical errors would render the telescope useless...

Thats how the newtonian design must have evolved, earlier when there were no manufacturers and amateurs used to make them, rigidly mounted scopes must have caused problems and later with trial and error the present design has come in use and has been passed on which we still follow today.

Clear Skies!

#18 izar187

izar187

    Surveyor 1

  • -----
  • Posts: 1724
  • Joined: 02 Sep 2006
  • Loc: 43N

Posted 20 June 2013 - 12:24 AM

What they all said.
The example of fueling your vehicle is a good one.
You may or may not need to do it for each outing. But it is simply best to check it so you know.

Same for scopes. Whether or not it needs to be re-collimated every time out depends upon the individual scope. But best performance is possible if one checks.
As a step by step process to do it, it is no more complex than fueling a vehicle.
And far. far less dangerous.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics