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Why the Need to Collimate Newts in the First Place

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#1 FirstSight

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Posted 06 February 2013 - 01:49 PM

WELL, DUH! this has a blindingly obvious practical answer: because unless you collimate a Newtonian reflector, the images will be too degraded by aberrations and failure to come to sharp focus for us to enjoy an acceptable view of the night sky. An out-of-collimation reflector is useless; it doesn't even make a very good doorstop or impromptieu coat rack.

HOWEVER, WHAT I'M RAISING IS A QUITE DIFFERENT ANGLE TO THIS QUESTION, ONE THAT DOESN'T HAVE NEARLY SO OBVIOUS AN ANSWER. Consider a decent-quality refractor, which usually comes well-collimated from the maker, usually holds good collimation indefinitely (unless perhaps it takes a hard enough impact to jar it out of collimation), and if it eventually does require recollimation, it isn't very often. MY QUESTION IS THIS: Why cannot reflectors be made such that once collimated, they generally hold good collimation indefinitely, similar to refractors? This isn't a complaint about reflectors, but rather a general engineering, design, and materials question about them. Although obviously the issue of what modifications and tweaks can be made to reflectors that improve their ability to hold collimation is relevant, nevertheless that isn't really the basic thrust of my question; rather the angle I'm interested in is why hasn't it seemed feasible (at least so far) to improve the overall structural design of reflectors to mostly eliminate their vulnerability to miscollimation for an indefinite duration, once it's been initially set?

#2 csrlice12

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Posted 06 February 2013 - 02:15 PM

Good question. You'd think the technology would be there. Could have something to do with the length of the dob OTA along with the weight allows things to flex more than a refractor OTA. Remember the frac is a small diameter tube with supports at the focuser and Objective ends. The Dob only really has support at the mirror end, the rest is pretty wide open. Also, you have a much larger area that is subject to thermal expansion/contraction in the reflector for both the mirror and the OTA itself.

#3 ed_turco

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Posted 06 February 2013 - 02:51 PM

There is a low tech way to hold collimation, by using push pull bolts. One set aligns; the other are screwed in to hold that collimation. Works pretty well at f/6 and above, but below that, everything gets so touchy.

#4 beatlejuice

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Posted 06 February 2013 - 03:14 PM

I am no expert, but I believe as this thread develops it will be found that the inability of the secondary holder to hold position to within the extreme tolerances necessary to achieve perpetual collimation will be the weak link.

Eric

#5 dan_h

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Posted 06 February 2013 - 03:21 PM

The typical refractor has a lens assembly that generates very good images over a much wider angle than the typical fast Newt. A tenth of a degree or even a quarter would hardly be noticeable at the eyepiece. Don't try that with an f4 Newt. The off axis images in the Newt degrade much faster than in a refractor. Tolerances for fine images are tighter.

Large aperature Newts are typically truss assemblies designed to come apart for transport. They don't go back together exactly the same as they were.

A smaller Newt can be built to hold alignment. I can't remember the last time I tweaked my Tal-1. Once you open up the aperature you start dealing with a lot more weight and flexture. You could build it stiff enough to hold but it's going to be massive to handle.

Folks have developed an aversion to anything that has the possibility of image degradation no matter how small that degradation is. Hence spider assemblies are as thin as possible and although useable, aren't exactly ridgid. Nobody is willing to accept a truly robust spider/secondary support with negative impact to the views.

It is difficult and expensive to make it big, keep it light, keep it portable, and keep it precise. So build it with the needed adjustments to allow the precision to be there as needed.

Beside, folks like to have things to tweak. Most refractors don't need adjustments to be provided but folks won't buy them without.

dan

#6 Jon Isaacs

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Posted 06 February 2013 - 03:48 PM

MY QUESTION IS THIS: Why cannot reflectors be made such that once collimated, they generally hold good collimation indefinitely, similar to refractors?



My thinking is like this:

Newtonians are built to be lightweight and reasonable in cost. One of the reasons this is possible is that Newtonians are the simplest of designs, only one curved surface and one flat surface. This means that the collimation requirements are quite lax and so the Newtonian structure can be flimsy in comparison to other designs.

Consider the apo triplet. They stay in collimation because they have to, i.e., they are not user collimateable. I have seen comments by both Roland Christen and Vic Maris that lenses must be centered to within a few microns.

Take a look at your typical truss style Ritchey–Chrétien. R-Cs have much tighter collimation tolerances than a Newtonian so the structure must be much more carefully designed and more robust. Look at a 20 inch Truss Dob and compare it to a 20 inch R-C...

I think the answer to the question really is that the reason we have to collimate our Newtonians is that it is something that by comparison is easily done and it results in a significant cost and weight reduction.

Jon

#7 csrlice12

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Posted 06 February 2013 - 03:56 PM

You can have one today! How much money you got????

Think of big dobs like cars, Big Dobs are like cars from the 50s and 60s, lots of horsepower, but needing lots of maintenance.....and what's really good, is YOU can do the maintenance. I'm sure a "non-collimatealbe" dob would be possible, maybe even somewhat portable; but the cost would be outrageous.......Maybe Scotty can give us his formula for "Transparent Aluminum".

#8 KerryR

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Posted 06 February 2013 - 04:28 PM

I wish all my refractors had (easily) collimateable cells and focusers. It's a pain if it's off and you can't easily and precisely adjust it. In most refractors, admitedly, a little miscollimation isn't harmful. But, on some, it is. I seem to recall my Orion ED100 showing coma a slight lateral color spectrum if the collimation gets off. It's a pain to fix without the handy push-pull thing.

Televue refractors are another example. While they have collimateable cells, they're not easy to collimate without building a jig. The Petzvals, at least the older ones, are or were really sensitive to miscollimation. Point is, difficulties are not limited to the low end, like my Orion ED100.

#9 csrlice12

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Posted 06 February 2013 - 04:37 PM

As a rule, I'd NEVER try to collimate a refractor; that's a little beyond the norm as it requires specific knowledge, skills, abilities, and tools I Don't have. I'd just take it to a scope shop (of course if you don't have one, many just send their scopes in). Collimating a refractor is not for the lighthearted.....

#10 Dick Jacobson

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Posted 06 February 2013 - 04:41 PM

I own a 6" Maksutov-Newtonian which I have never collimated. The primary is adjusted using push-pull bolts if I remember right. The secondary is mounted on a glass disk, not a flimsy spider. The tube appears to be made from heavy gauge aluminum. The whole thing is very solidly constructed. As long as I handle it with reasonable care, I don't believe that I'll ever need to adjust it.

Scaling this scope up to say, 12 or 16 inches would make for a very expensive and very heavy instrument. It could be done but I expect that it would attract few buyers.

#11 MDB

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Posted 06 February 2013 - 05:14 PM

Because I have a truss tube dob which can be easily disassembled for transport or storage I like being able to quickly and easily collimate it. Maybe I am strange but I actually enjoy collimating my scope and knowing it is at it's best before each viewing session.

Mike

#12 dan777

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Posted 06 February 2013 - 08:11 PM

Why cannot reflectors be made such that once collimated, they generally hold good collimation indefinitely, similar to refractors?


Actually, they can be and indeed are made. I've owned two of them - both Orion 8" f/6. I check collimation but unless I monkey with the mirrors or focuser I never have to tweak it.

#13 csrlice12

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Posted 06 February 2013 - 08:37 PM

Yea, John Dobson used to just kick the end of his scope to align the mirror.....and a lot of those earlier scopes you adjusted the secondary by reaching in and grabbing it and twisting it till it was in place (bet they went thru a lot of those single vanes, don't know, but probably bendable tin and removable OTA screws?)

#14 Jerry-rigged

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Posted 07 February 2013 - 02:28 PM

Folks have developed an aversion to anything that has the possibility of image degradation no matter how small that degradation is. Hence spider assemblies are as thin as possible and although useable, aren't exactly ridgid. Nobody is willing to accept a truly robust spider/secondary support with negative impact to the views.

dan


Good example - the "lawnmower blade" spider on my Coulter. If you ever get it collimated right, it will never move short of dropping the scope off your house. But - it does smear Jupiter pretty good with the thick diffraction spikes...

#15 Alterf

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Posted 07 February 2013 - 03:31 PM

I never collimated a Vixen R135S (made in Japan) that I owned. It stayed dead-on. It was a fairly small scope. But my Orion 254mm f/4.7 Newt needs a tweak every time I take it out. Part of the problem is the cell, which does not hold the mirror's position absolutely, but the tube itself is weak enough that it flexes as the scope moves about the sky. As a result, I am unable to guide well with a guidescope for long exposures; an off-axis guider is required. But I did not pay a lot for the telescope, and I've been very happy with it.

I also had an Orion 8" f/6 dob that held collimation very well, as another reported above.

Val

#16 csrlice12

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Posted 07 February 2013 - 03:47 PM

and the sweet spot on them would be like comparing a BB to a golf ball, the faster the mirror, the smaller the sweet spot. It's why faster scopes require more frequent and critical collimation.

#17 Galicapernistein

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Posted 07 February 2013 - 04:23 PM

Why cannot reflectors be made such that once collimated, they generally hold good collimation indefinitely, similar to refractors?


Actually, they can be and indeed are made. I've owned two of them - both Orion 8" f/6. I check collimation but unless I monkey with the mirrors or focuser I never have to tweak it.


I second that. My Orion 8" has never required more than a quarter turn of a knob to be collimated. A small mirror and a metal tube make for one tough telescope.

#18 rmollise

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Posted 07 February 2013 - 05:02 PM

The other side is that a Newtonian that must be collimated is simple to make. AND collimation is a five minute process at worst unless you are a collimatoholic. :lol:

#19 FirstSight

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Posted 07 February 2013 - 05:24 PM

The other side is that a Newtonian that must be collimated is simple to make. AND collimation is a five minute process at worst unless you are a collimatoholic. :lol:


Collimation of a reflector with a good toolset (e.g. Glatter, or Catseye especially) is actually a nerdy bit of good fun, so long as it doesn't drag so overly long that it infringes on observing time or (if it isn't quite dark yet) relaxing and chatting time with your nearby compadres. That said, few of us would be heartbroken if the state of the art did advance (whether through clever mechanical ingenuity or new cost-effective structural materials) such that collimation of even 10"-20" size reflectors only seldom needed tweaking once set. Even so, I would be dubious about having such an improved design unless the design also remained easily accessibility for users to tweak collimation if and when needed. No burying the collimation screws in epoxy for stability, please.

#20 GeneT

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Posted 07 February 2013 - 10:54 PM

I am no expert, but I believe as this thread develops it will be found that the inability of the secondary holder to hold position to within the extreme tolerances necessary to achieve perpetual collimation will be the weak link. Eric


Also, truss tube Dobs have slight difference in truss alignment every time they are set up and taken down. There are several other factors as well that require that a Dob be collimated with every use. My friends who have solid tube Dobs find that the alignment holds very well with very few tweaks in collimation needed--compared to a truss Dob. To your point--those who have their telescopes permanently located in an observatory will probably never have to collimate--unless there was an accident.

#21 GeneT

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Posted 07 February 2013 - 10:58 PM

The other side is that a Newtonian that must be collimated is simple to make. AND collimation is a five minute process at worst unless you are a collimatoholic. :lol:


This is true. I just make collimaiton part of my set up. It is easy and I now do it without thinking (my wife says I do a lot of things without thinking. :grin:)

#22 Atl

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Posted 08 February 2013 - 12:04 AM

Use concrete, rebar, and 6 inch thick mirrors...maybe a titanium spider and the need to collimate vanishes.

#23 beatlejuice

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Posted 08 February 2013 - 04:01 AM

The OP's original question:

rather the angle I'm interested in is why hasn't it seemed feasible (at least so far) to improve the overall structural design of reflectors to mostly eliminate their vulnerability to miscollimation for an indefinite duration, once it's been initially set?



I was sure that the engineers amongst us would have come up with somewhat of a more technical answer than those that have appeared so far. Perhaps it's just that talking about and tinkering with collimation is something that we really don't want to do without and the loss of this cerebral process to a permanantly collimated reflector would be devastating to many of us. (If it can be done, I don't want to know about it!) :grin:

Eric

#24 jpcannavo

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Posted 08 February 2013 - 06:44 AM

Another - and clearly there are many - issue that arises is the mirror cell. Refractor objectives are typically circumferentially "locked" into their cells. Consider instead a large newtonian that is "floating" on a complex layered structure. This introduces a number of potential mechanical variables. So, for example, merely unloading and loading the mirror into its cell will often create the need for re-collimation.

#25 Starman1

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Posted 08 February 2013 - 12:03 PM

It's mostly a matter of weight. If I scaled the 4" refractor here up to the 12.5" size, my scope would weigh around 300+ lbs because the entire structure would have to be beefed up to handle the weight of the tube, etc.
Yet, my scope is 80 lbs total and can be broken down for transport, with no part over 40 lbs.
When I carefully assemble it at a dark site, it's out of collimation because even though I put everything together in exactly the same way, even tiny changes make a difference to collimation.
I can envision having a permanently-mounted scope that never went out of collimation. It wouldn't be light, nor would it be cheap.
I can envision having a portable scope that never went out of collimation. It wouldn't be light for its size, and it would be small in size.

But, alas, we usually want to move them, which puts an upper limit on the weight. So we take a disassembled scope to the field and assemble it there. In the 20-30 minutes it may take to put it together and connect everything, if we spend 5 minutes collimating and observe for, say, eight hours, is it not a 5 minutes well-spent?






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