Collimation & Engineering

[TopHouse]

I only got my first scope last week, an explorer 200p on EQ5, also got an antares laser collimater and some other bits & bats, because it's been either foggy or cloudy since I got it, I've been doing a LOT of tinkering and reading and thinking, which brings me to this:-

Is collimation as important as some would have you believe? Considering this, you have either a 2" or 1.25" focuser, into that you slide an eyepiece and fasten it with 2 screws, now, prior to fastening the 2 screws there's a degree of play in there, now each time you change eyepiece, surely there's variance on the exact positioning of each eyepiece because of this 'play'. So, even if you use a 'self centering' "hotech" collimater, it's all well and good, except that your eyepieces 'aren't' self centering, the only way that they would be is if the focusers were precisely engineered so that the eyepieces slid in like a piston in a cylinder!

Now the degree of play might only be 1 mm, but over a distance of only a few inches, a variance in angle by only a fraction can end up meaning the line of sight is out by quite a bit.

This has led me to look at the Orion Precision Centering Eyepiece Adapter :-

Orion Precision Eyepiece Centering Adaptor

Now surely, if you use one of these, and a normal bog standard laser collimater like the antares, the collimater and and eyepieces should in theory be held in a much more accurate central position, in fact surely this device must actually give your standard eyepieces the same level of precision as the marketing says the Hotech colimater has? If this is the case, and the engineering of this adapter is so simple and cheap to manufacture, why do telescope manufactureres not use this method instead of the 2 screws, which HAS to be rather inacurate for something that so depends on accuracy of allignment of mirrors and lenses.

As I'm a total newbie my theory could be absolutely full of holes but what do you think?

[Merlin66]

After designing and building telescopes for over 40 years I can't disagree with your logic!!

You're 100% right!

Ken

[The Warthog]

OK, so this device centres the 1.25" ep in a 2" focuser. What centres this device in the 2" focuser? Isn't it as likely to be off-centre in the 2" focuser? I don't think the play in my focusers is as much as a millimetre, and I don't think the little bit of play there is makes a lot of difference to the collimation. I can still get a nice, round Airy disk with the ep in the focuser, and nice tight stars in the eyepiece. That's good enough for me.

And I use a Cheshire collimator.

[yeti monster]

When I use my cheshire collimator, I rotate it to ensure that it is central. The holding screws I barely nip up, allowing the rotation. Collimation sets the mirrors to their optimum (or as near as in practicable), EP position thereafter will be dependant upon the final positioning of the holding screws, which does allow for a fair degree of play. Bearing in mind that tolorances of EP shank diameters is highly variable, unless you go to a lot of effort to ensure that all you EPs are exactly the same diameter, and a good sliding fit into the focuser, you are always going to suffer some degree of misalignment in the final optics.

[brianb]

You're 100% right!

Indeed, this is the reason there is no substitute for fine tuning the collimation using a natural star in good (steady) seeing. Or an artificial star if a natural one isn't available.

Collimation tools are in any case no more accurate than the errors built into them, and many laser collimators are miles out. I've found, with just a collimation cap (a hole in the bottom of a 35mm canister), I've been able to collimate a f/8 Newtonian accurately enough to get excellent views ... a colli cap alone on an f/6 scope, let alone f/5 or faster, is not good enough for anything other than a starting point for star collimation. If you have to do that anyway, why bother forking out for a tool?

[TopHouse]

@Warthog What holds the device is irrelevant, once it's held, it's held there semi permanently, ie you don't take it off after use, so therefore it's position remains the same.

It isn't the play in the focuser itself, it's the play in the eyepiece when you insert it, then tighten 2 screws, meaning that there's a level of variance every time you change eyepieces.

No, I don't work for the company than makes them, honestly.

[Merlin66]

Warthog,

I think the idea is to fit the centring device and leave it there ALL the time.

So if the collimation was done with it in place ie cheshire etc then the outome would be improved using normal 1.25" eyepieces.

Yes ... No???

Ken

[TopHouse]

@brianb, again, you can collimate on a star, but as soon as you change eyepieces, you introduce a variable, because the new eyepiece will in all probablity not be in exactly the same position as the one you collimated with.

[TopHouse]

Warthog,

I think the idea is to fit the centring device and leave it there ALL the time.

So if the collimation was done with it in place ie cheshire etc then the outome would be improved using normal 1.25" eyepieces.

Yes ... No???

Ken

EXACTLY

As i said, i have no links with the manufacturer or anything, just during my reading, and tinkering did I formulate this theory

[The Warthog]

OK, and I'm not trying to be difficult, but when I put my cheshire ep in the focuser and tighten the screw (I lost the other one!) it is in the same position my ep will be. I see the advantage of the central position of the ep in this device, I just dont' think it's that big a deal.

And if I put a 2" ep in, the 2" ep is subject to misalignment, too, but still not enough, I think to really mess with the view.

[yeti monster]

Perhaps we need an auto collimator, a device with which we can recollimate whilst viewing?

[TopHouse]

@Warthog

Leaving the cheshire aside for a moment, although it would still be relevant to my theory, a laser beam can be seen (well, the red dot can) and if you place a standard (not hotech) laser collimater into position, tighten up your screw, note the position on your mirror of the red dot, loosen your screw, remove the laser, reinsert it, tighten your screw again, nine times out of ten it wont be hitting the same spot on the mirror. In fact this is the reason 'why' they invented the hotech and what it's marketing points at.

The reason i discounted the cheshire is because for illustrative purposes, the laser dot helps build a picture of what i'm explaining.

[Merlin66]

TopHouse,

As I said before I agree....

BUT in all honesty, based on 40 years of experience .... it really doesn't matter.....

To get the best performance out of your scope sure it has to be well collimated, but the only 100% positive method of doing this is with a real star. Full stop.

Living with compromise is what we do all every day of our lives; get the best you can and work from there. There are many, many other variables which will affect the image quality... collimation is only one of them.

Ken

[The Warthog]

@Warthog

Leaving the cheshire aside for a moment, although it would still be relevant to my theory, a laser beam can be seen (well, the red dot can) and if you place a standard (not hotech) laser collimater into position, tighten up your screw, note the position on your mirror of the red dot, loosen your screw, remove the laser, reinsert it, tighten your screw again, nine times out of ten it wont be hitting the same spot on the mirror. In fact this is the reason 'why' they invented the hotech and what it's marketing points at.

The reason i discounted the cheshire is because for illustrative purposes, the laser dot helps build a picture of what i'm explaining.

OK, but what I am asking is, once you have collimated your mirror first time with the laser, or in my case the cheshire, will the fact that a small misalignment occurs throw your collimation out so seriously that the Airy disk will be distorted, or your view of the stars degraded?

That being said, I kinda like this device anyway.

[TopHouse]

I have no idea by how much any of it would be out, the fact is that there would be 'some' difference, whether or not it would make any real world difference i don't know, I've only had my scope a week and it's been cloudy or foggy all the time since! So, basically I've just been reading and tinkering. All this stuff occured to me whilst playing around, for instance, for a scientific instrument that you would want to be as accurate as possible (within reason) the sloppy eyepiece fit 2 fixing screws method for eyepiece fitting is bad.

When tinkering with the laser collimater, I saw that only a slight movement, or removing the collimater and replacing it made a difference, which caused me to read up on collimating, hence i came across the Hotech which is a self centering collimater, using the same method as the adapter I found, which i actually found as a result of reading on here about a guy who just bought a 150p and he had one fitted.

So, it does look like a much better option than the two screws thing, even if there's actually no visible difference in collimation variance from using the two screws method, it still seems a simpler and more accurate method of fitting eyepieces, and i bet it's a darn site easier for cold fingers, I ordered one anyways lol

Bottom line, the marketting and hype and even user experience of the way the hotech colllimater centers in the holder can't be any bad thing to transfer to eyepiece holding as well!

[FLO]

... in all honesty, based on 40 years of experience .... it really doesn't matter.....

Bear in mind that you can use the Hotech laser collimator whichever way you like. If you are concerned that your focuser is pushing the eyepiece off-centre then you can bypass the self-centering system and clamp it using the focuser's thumbscrew. If you are confident that your focuser is well engineered with minimal play then you can take full advantage of the self-centering mechanism. Either way you will have a superbly engineered laser that will never need re-alignment.

Also, its accuracy and repeatability means it can even be used to return a SCT to a known collimation. And when used together with a Hotech SCA T Adapter for imaging it really is unbeatable.

HTH

[TopHouse]

Steve, you may have missed the point a little, it's not play in the actual focuser i describe but play in the eyepiece fit INTO the focuser.

[FLO]

Nope, I understood

[Astro_Baby]

Of course all that asummes a focuser with non-compression fittings.

I can put a Cheshire into my Moonlite focuser get collimation accurate with the Cheshire then take it out, rack the focuser in and out, wiggle everything then pop the Cheshire back in at any angle I like and - yippeee all is still as it was.

Same for the eyepieces - thats the benefit of a serious focuser - makes collimation a breeze with little or no uncertainty. I can collimate with a Cheshire and and KNOW that the scope will pass a star test everytime.

[dark knight]

Spot on AB, you beat me to it. I only use a focuser with a compression ring fittings...

[Tim]

Very interesting thread this, and perhaps a lesson for focusser makers.

It would be great to think that once you drop anything into your focusser, be it laser, eyepiece or camera, it would be instantly properly centred and square, maybe something for Moonlite to look at! EDIT: Looking at the posts above it seems they DO do one

For those that havent seen it, the Cats eye method of collimation is supposed to be very good, as is the software CCD inspectors collimation routine for use with a camera.

CATSEYE Collimation Main Page Frameset File

I suppose at the end of the day, really high quality engineering adds $$$ to the prices.

[TopHouse]

Being a newbie, I wasn't aware of compression ring focusers, my only experence is of the two screws holding the eyepiece thingamybob, so this moonlite focusser, does that work like the thing I linked to earlier in the thread?

Is this it :-

http://www.firstlightoptics.com/proddetail.php?prod=mcf2-sfocuser

[John]

If you don't want to change the whole focusser you can get compression ring adapters which are an improvement over the thumbscrew only approach. The Orion centering adapter is a step forward again and would appear to use a 360 degree compression ring approach which holds whatever is placed in it centrally by applying equal pressure all around it. I think Takahashi have had a similar fitting on their diagonals for a while now. Of course you have to get the centering adapter properly centered itself otherwise it holds all your accessories very precisly .... just off centre

[TopHouse]

I ordered the Orion thing and i'll report on my findings when I've played a little, think first test will be to simply try putting in a laser collimater and putting it in and out a few times (without doing the hokey cokey) and checking if there's any difference in dot position.

@Jahmanson even if the centering adapter isn't perfectly aligned, because the idea is that it doesn't get removed or even moved, if you collimate with it in postion, it 'should' then stay precise as first collimation with it in place 'should' correct any very slight misalignment.

Have to say, that moonlite focusser looks a very tasty piece of kit, would be even more tasty if it wasn't more expensive than the rest of the OTA though

[Astro_Baby]

Tophouse - yes - the Moonlites all have compression fitting. As you tighten down the thre screws there is a metal band which grips the eyepiece all around and holds it tight. You dont really need much pressure at all. The upshot is the focuser tends to hold the EP as close to centre each time as its really possible.

Moonlites also have very precision engineering so theres no 'slop' in the focuser (ie the drawtube donest wobble from side to side or up and down).

There is of course a downside - its the cost. But in fairness anyone who sees a Moonlite would tend to garee the price is pretty reasonable considering the quality of the product. I saw Astronuts Moonlite on his dob a couple of years ago and immediately threw myself on the floor and cried until I was bought one for Christmas