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Old 17-06-2007, 04:51 PM
a1120028
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Already going mad

Firstly though Hi to all, I'm totally new to telescopes and on a whim decided to go buy one after doing a filler astronomy course at uni. My main area of study has always been theoretical physics never "hands on".
I must say I'm hooked, have aperture fever already and am going mad trying to work out how to properly collimate my 8" light-bridge. I must of read 10 articles on collimating all say different things so I really need help. Especially on how the offset of fast focal length scopes effects collimation. HELP!!!!
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Old 17-06-2007, 08:01 PM
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Collimation and impact of secondary offset in 8" f6

Hi, welcome to the group and to amateur astronomy.

Short Answer: Not in any practical or meaningful way, most would reckon.

Longest Answer: For an f6 8" dob you can get perfectly fine and usable collimation using the non-offset, partial offset, or full offset techniques. (An f6 is probably not in the “fast” category for Newts, by the way. In comparison to SCTs, yes, but not compared to the now common fast Newt Dobs with f ratios <4.5 out there in abundance)

All the offset is doing is trying to make sure the optical axis of the telescope coincides with the physical centerline of the tube itself (mechanical center). The ideal tradition is “fully offset”, but even "non-offset" won't cause any meaningful image impairment at the eyepiece if you have the primary and secondary collimated otherwise. You could suffer some vignetting of the image if the tube diameter is small and/or if the disparity between the optical and mechanical axes is grossly abnormal in the non-offset situation. But that wouldn't affect image quality at the eyepiece for all practical concerns.

For some computer controlled equatorial or fork-mounted Newts, having the optical and mechanical axes coincide is more important. Critical in some cases. But that just falls away when talking about a visual-use Dob, really.

Depending on the tools you have (sight-tube, Chesire or laser +/- Barlow attachment, autocollimator, etc) you may gravitate toward full offset, partial offset, or non-offset. Full offset requires you to move your secondary mirror by a very small amount away from your focuser and toward your primary mirror: "down and away". The distance down and away, actually, for your 8" f6 is going to be a small fraction of a centimeter, like 1.5-2mm or something. If you secondary is about 1.5” (?) or so, that’d be right…Kinda hard to get excited about a 1.5 mm shift, eh? In the case of a very large very fast dob, that offset might approach 1 cm.

see http://www.skyandtelescope.com/howto/diy/3306996.html

Even most fussy folks with fast dobs are happy with “partial offset” of the secondary. This is moving the secondary "down" but not "away". Off-set toward the primary ("down") is easily done with a sighttube...you move the secondary down until the sighttube shows it to be centered under the focuser tube in the peep hole of the sighttube. You need a sight-tube to do it well. Really, even if you don’t care whether you are off-set or not, a sight tube is invaluable because often the secondary isn’t placed correctly in the first place. It might be centimeters off depending on the QA policy of the manufacturer. I had a cheap newt delivereed with the secondary installed 3 cm too high and backwards many years ago.

A crude sight-tube can be made with a 35 mm film canister in a 1.25” focuser tube with a hole poked in the dead center of the film cannister. Or you can buy a nice expensive commercial one. Chesires and/or laser collimators you are probably going to need to purchase unless you're an engineering wiz with a workshop. Ditto with an autocollimator. You can buy all of them if you want. The collimat-aholics out there (I think I’m one, now, sadly) would say you should.

A purist will also tell you to square your focuser, such that the focuser tube central axis points straight and also straight “at” the secondary, and not angled off to a side. (and so, it follows that squaring the focuser should really be your very first step in this whole process, even before doing the secondary). The sighttube can help you do this, also, as can a laser collimator (without the Barlow attached). It’s worth checking in your LB…..focuser squaring problems are a common unrecognized source of image degradation…often noticed with changes in eyepieces as the focuser racks farther in and out of collimation.

For the best compromise in your case, a sight-tube and Chesire could get you close enough. In truth, there are users who just use a sight-tube and can eyeball the primary collimation (provided they have a mirror center spot) close enough with that, and forego the Chesire step. That’d be fine with an f8 or f10 newt…

One can argue the practical value of going the last step and using an autocollimator with an f6 scope. It will be better if you finish with the AC but it won’t be as obvious as it would be in an f4 or something, where the coma-free field (sweet spot) is much smaller.

If funds are tight (and aren’t they for all of us?) I’d say get a sight-tube and a Chesire, find a helpful veteran in Adelaide to guide you, center the secondary mirror under the focuser using the sight tube and thereby accomplish your partial offset, dial in your primary with the Chesire, get that routine down on your own (since you’ve a truss and will need to do this often) and start observing!

You're a physicist, so you'd pick up this concept far faster than most of us. Join the Yahoo group Collimate_your_telescope, and you can "hear" the gurus of collimation (Nils olof Carlin, Vic Menard, etc) discuss and spar about the general concepts and the bewilderingly complex nuances. You can search the old threads to answer questions. And ask questions for free, and get very erudite and practical answers in return.

Scott

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Originally Posted by a1120028 View Post
Firstly though Hi to all, I'm totally new to telescopes and on a whim decided to go buy one after doing a filler astronomy course at uni. My main area of study has always been theoretical physics never "hands on".
I must say I'm hooked, have aperture fever already and am going mad trying to work out how to properly collimate my 8" light-bridge. I must of read 10 articles on collimating all say different things so I really need help. Especially on how the offset of fast focal length scopes effects collimation. HELP!!!!
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Old 18-06-2007, 05:12 AM
a1120028
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Thanks for the info, I've got an idea it's gonna be like most of the mathematics i struggled with for so long, you keep going and one day in a moment of clarity it all makes sense. My biggest problem is that I would of thought a laser collimater would of done the trick well as it sends a return beam back to the focuser. As a laser does not spread as it travels I the optical centers would be aligned. This does not seem to be the case however.
There's a public open night at the Hights observatory here in Adelaide on the 22nd so I'll try get along to that and pick a few brains. Thanks again for your help.
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Old 18-06-2007, 07:45 AM
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Far out

Well after watching the tutorial on andyshotglass page I gave it a go and boy was it out a mile. It is all centered and aligned now, the articles are right insofar as actually doing it a few times makes the whole thing more comfortable. I can not believe I sent it back to the shop to be collimated and just by moving it to my house it went out by so much. It was that bad that a return laser did not even hit the secondary. I suspect the shop played me for a fool and they would not be far off as I know next to nothing about telescopes atm.
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Old 18-06-2007, 07:59 AM
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rmcpb (Rob)
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Collimation is one of those things you get used to. The first time its a horror, then it progressively gets easier till it just becomes part of the maintenance of your gear.

BTW welcome to IIS.
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Old 18-06-2007, 11:48 AM
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Collimation

With a truss, it can be way off each time. That's possible and not necessarily a sign of a flaw.

The focuser alignment and secondary "down" offset often don't move since they are fixed in the upper cage. I don't bother checking my secondary offset placement except occasionally, if it hasn't migrated in several sessions. DItto with focuser squaring.

But the relationship between the primary and the upper cage will be off with trusses when you reassemble. A bit of temp change, some wear on a metal fitting, some slight (undetectable) bow in a truss tube, and voila, you are off by half a mirror diameter on the primary collimation.

Checking the tightness of the spider vanes is worth doing, as a stripped or loose screw there can often explain collimation volatility.

And the primary will normally wiggle in the cell with transport. Not a flaw, just reality. But check the tightness of the primary collimation bolts and springs...Some bolts come loose and aren't actually holding the cell together..the mirror can "flop" with changes in tube position. Get down and study the thing and it should make sense after a few minutes.

I've a telescope with an identical cell to the LB, altho mine is a different vendor. Do you have locking bolts in addition to primary collimation bolts? If so, be aware if you collimate, then tighten the LOCKING bolts, that the collimation will CHANGE. I don't recall seeing much on this in print, but it does change. My webcam-collimation experience (see Articles section) proved this to me. I got the colimation with the barlowed laser dead on, then tightened the locking bolts. Even with mild tightening, the center mark migrates a full diameter away. So, either don't use the locking bolts, or recheck after you lock the bolts to fine tune.

But, yes, enlisting the assistance of a veteran Newt user is priceless and very efficient at your phase in learning the process. In truth, we're all learners, all the time....

Regards

Scott



Quote:
Originally Posted by a1120028 View Post
Well after watching the tutorial on andyshotglass page I gave it a go and boy was it out a mile. It is all centered and aligned now, the articles are right insofar as actually doing it a few times makes the whole thing more comfortable. I can not believe I sent it back to the shop to be collimated and just by moving it to my house it went out by so much. It was that bad that a return laser did not even hit the secondary. I suspect the shop played me for a fool and they would not be far off as I know next to nothing about telescopes atm.
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Old 18-06-2007, 12:24 PM
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A friend of mine has the 12" Lightbridge made life easier for himself...after the first spot-on collimation he then labeled the the truss positions against the mounts on the top and bottom sections...then just a matter of a slight tweek after re-assembly..if necessary..
As with any locking screw on either the primary or secondary mirror cell..please be careful not to overtighten..just a slight nip-up will do..
Have fun..and welcome to IIS a1120028!
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Old 18-06-2007, 12:30 PM
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Tannehill
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locking bolts

thank you, yes, excellent point there. A mistake I find myself making if I'm not paying enough attention. The (wrong) instinct is to tighten it until you find yourself grunting with the effort.

Even with the slightest tightening, tho', my donut moves off the center mark of my barlowed laser collimator.

Cheers

Scott



Quote:
Originally Posted by astro_nutt View Post
A friend of mine has the 12" Lightbridge made life easier for himself...after the first spot-on collimation he then labeled the the truss positions against the mounts on the top and bottom sections...then just a matter of a slight tweek after re-assembly..if necessary..
As with any locking screw on either the primary or secondary mirror cell..please be careful not to overtighten..just a slight nip-up will do..
Have fun..and welcome to IIS a1120028!
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Old 18-06-2007, 12:47 PM
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Hi all,
If the donut moves when tightening then dont tighten until donut is adjusted to allow for the movement. That is the donut will move into place when you do tighten. Just a suggestion.
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Old 18-06-2007, 01:39 PM
a1120028
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Thanks for everyone's help and comments. I spoke to my local retail guy and he was very helpful this time. Seems last time I spoke to someone there I caught them on a bad day, happens to all of us I guess. I think I've made headway, the star test will tell me how well I've done tonight if the clouds clear (fingers crossed). Then on the 22nd I'll head up to the viewing night and find a Newt owner to help out with any questions I have.

Thanks again for all the help, I'm really glad I found this site. 10/10 IIS
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Old 18-06-2007, 03:50 PM
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impact of locking bolts on collimation

True. Also, actually, what works is to tighten every locking bolt the exact amount, then the net final donut movement is zero. That's what I noticed. But since I "cheat" and watch my webcam image of the donut as I tighten the bolts (again, just lightly) it's actually pretty easy to zero it in however. But doing an equal amount on each bolt works fine..

My reason for mentioning it was simply to advise new folks that those locking bolts DO have impact on the primary mirror's position, something that isn't intutive to someone who is told those are just locking bolts to "fix" the position.

Cheers

Scott

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Hi all,
If the donut moves when tightening then dont tighten until donut is adjusted to allow for the movement. That is the donut will move into place when you do tighten. Just a suggestion.
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Old 18-06-2007, 04:57 PM
a1120028
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Unlock the locking nuts!!

I noticed or rather, didn't notice anywhere in the meade instruction manual that the lock nuts need to be undone before you adjust the primary. This may seem common knowledge to most but not knowing how the back end of my scope was put together I didn't know if by loosening the lock nuts too much I might make the primary mirror fall out or something drastic like that. I'm a little embarrassed to admit that but hey, if you don't ask you don't learn right?
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Old 18-06-2007, 07:06 PM
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Even after a collimation in my lounge room with a film cannister and eyes only my scopes performance has improved 10 times. WOW I was almost ready to give up and sell it. Not now. I only had a few minutes before the clouds and rain but what I saw was far better and clearer than anything I've seen through it in the past. This was only from my backyard with a football oval a street away with lights blazing. I can't wait to get it out were it's really dark and after a good collimation. Thank God I didn't give up. I would of really missed out.
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Old 18-06-2007, 08:03 PM
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Collimation and Incurable Aperturophilia

Excellent, congratulations. You might consider borrowing or asking for help in the field with a Chesire or laser collimator to see how much benefit you get for the expense, over and above the sighttube technique. Maybe you – like many – have a good enough eye to line it up without those fancy (expensive) tools. Or do you already have a laser?

The mirror can't fall out by removing the locking bolts. The mirror is held by the collimation bolts. But if all the collimation bolts are removed so that they aren't threaded into the upper part of that cell assembly, the mirror and upper cell assembly - the part of the mirror cell holding the mirror with the rubber-padded clips - can clunk down and incur damage. That's a good point if you are swapping out new collimation bolts or springs, for example: replace one collimation bolt/spring assembly at a time, don't try to remove them all at once before putting new ones in. That clunk sound is quite disheartening...

Yes, you'll need to loosen the locking bolts to collimate. The mirror is held by dynamic tension between the springs pushing the mirror "up" and the collimation bolts pulling "back" against the springs by threading into a part of the upper assembly holding the mirror. The springs need a bit of compression so that the mirror is held snug. If a spring is loose, the mirror will wiggle about and not hold collimation. The locking bolts just thread up and push against the back of that upper assembly and blocks the action of those bolts from moving the assembly "back" toward you when you tighten the collimation bolts. But the locking bolts don’t prevent you from loosening the collimation bolts.

If you have time and study the back of the cell and twiddle bolts and watch things move, it should make sense.

Congrats on your success. But do not presume to have attained permanent satisfaction. No. Sadly, peace and happiness are not yours to enjoy. Ever.

Aperture fever implies a temporary and treatable ailment. It is the wrong term, as this is far from some temporary or easily-solved problem.

The better term is Aperturophilia: the permanent obsession for greater greater aperture. It is treatable, mind you, but the treatment is more expensive each year of the disease. It is not curable.

Astrotechnoneurosis: a pernicious attraction to increasingly complicated astronomy technology. Can be diagnosed when one's astronomy email messages contain more acronyms than real words, or when the astro gadgets in one's possession weigh more than your telescope.

Astrophotographemia: a distressing and often socially disabling impulse to capture and process astronomy images yourself. A truly tragic disease. Sadly, but perhaps mercifully, the victims are strangely unaware of the pathologic nature of this condition.

and

Hyperatlasophitis (a costly build-up of both text-based and software-based stellar mapping publications).

There are more, but I don’t want to frighten you. It will, unfortunately, become all to clear in the years to follow….

Regards
Scott
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Old 19-06-2007, 04:31 AM
a1120028
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It's too late, why didn't anybody warn me. My god how many more innocent victims will Astronomy devour?
I'm already steering toward the new 16" Lightbridge and I haven't even got the 8" working properly yet.

To answer you question I don't have any collimation tools but am going to buy a laser when I get the chance. I'm a single Dad of a 10 month old so I don't get out much, that's part of the reason I took up astronomy, something I can do in my backyard while she sleeps.
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Old 19-06-2007, 08:05 AM
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Hi all,
Aperture Fever or more correctly "Aperturophilia" is and always will be totally incurable. You will get the 16" and then a 24" then 36" then Siding Spring then Mt Stromlo then Hubble then,then,then.LOL
Cheers,
Duncan
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Old 19-06-2007, 08:13 AM
a1120028
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Hubble now that gives me an idea.

I heard NASA is going to dock a rocket module into the hubble and send it back to earth when it's no longer needed. Hmmm I wonder......With a real soft matress......and a bit of luck......maybe, just maybe.
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Old 19-06-2007, 08:16 AM
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We need that Branson fellow on our side. Then we can just go up there and pinch it. Put it on the other side of the moon where no-one can find it,LOL
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Old 19-06-2007, 10:22 AM
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I say we pay the Russians to steal if for us. The way their money converts it'll be cheap. Only problem with the other side of the moon is were gonna need to somehow get hold of a com sat to bounce communications off the scope. Pity we didn't think of this before they let Mir come back to earth.

........I really have to much time on my hands.
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