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How many members don't collimate their newtonians

spaceboy

By spaceboy
in The Astro Lounge

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Macavity

Macavity

Posted
Posted

I think it is anecdotal that a bloke once turned up for guitar lessons... The tutor suggested he might usefully start by tuning his guitar. Bloke was completely taken aback: "But they told me it was in-tune, when I *bought* the thing"! :)

Seriously though, I do wonder about some (innocent) telescope owners too! Mind you, with an F4 Newt to "worry about", I have been blithely ignoring the <ahem> slippage in my "collimation free" MAK150 for some time now... :(

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John

John

Posted
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I always give my 10" F/4.8 newtonian a collimation check followed by a star test each time I use it. It rarely needs more than a small touch on one or other of the collimation nuts but I like to keep it as "on song" as possible.

With scopes with proportionately large central obstructions I reckon keeping accurate collimation is important - loss of contrast and resolution kicks in fast as things get out of shape.

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Codmate

Codmate

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I have a Hotech, so checking and fixing collimation takes between 30 seconds to 2 mins before every obseving session.

I will *never* spend hours squinting down a cheshire with and without my glasses EVER again; and I completly refuse to do the primary collimation 'dance' between the primary screws and the EP!

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Dazzyd

Dazzyd

Posted
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I've only done a 35mm film canister with a hole collimating so far but I'll be ordering a Cheshire this week to do it properly. Looking on FLO, is there any difference between the normal Cheshire and the premium except the red anodising?

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Rihard

Rihard

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Hi All

I just wondered due to the concerns some people have about collimation how many members don't collimate their scopes on a regular basis ? and are there member who don't collimate there newtonians at all through worry of causing more harm than good?

As a kid I bought a small newtonian and had no clue about the need of collimation, so it got no collimation for about 14 years :( :(

Then I bought a larger 200P and I collimated it after 1 day :)

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8kids

8kids

Posted
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Even though I'm new to astronomy I collimate or check collimation every time I observe. I figure I can't actually break it so I might as well have a go. I purchased a used farpoint which makes it a doddle.

Also I have stripped my scope down completely so I have a better understanding of what goes where and what each thing does, I have found this helps when twiddling my knobs (ooh er):(

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chazzersdad

chazzersdad

Posted
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Had my 150P for 5 or 6 years and still haven't done it. Unless I've got it wrong, an un-focussed star looks like a "fuzzy donut" with the hole perfectly centred. Please correct me if I'm missing something.

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umadog

umadog

Posted
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If you're seeing a hole then you've defocused too much for the "star test" which is commonly used to check primary tilt. Scroll down to the images. You're looking for the diffraction rings, not just a hole.

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RikM

RikM

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Had my 150P for 5 or 6 years and still haven't done it. Unless I've got it wrong, an un-focussed star looks like a "fuzzy donut" with the hole perfectly centred. Please correct me if I'm missing something.

As Umadog says, if you can see the dark hole you are too far out of focus. You need to use a very high magnification, make sure the star is in the exact centre of the view and just de-focus a little bit so you can see if the rings are concentric. The seeing is rarely steady enough for a useful star test in the UK.

It is not worth loosing sleep over though. If you are happy with the views you get, then that is collimated close enough for you.

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neil phillips

neil phillips

Posted
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Some memebers scopes must be working at less than optimal. Despite what most think. The secondary and primary will shift around quite a lot, if the scope is moved often. Ive seen my laser go way off the centre spot probably just by the secondary screws being lossend a little too much from prior collimation. The report of not collimating for TWO YEARS. Damm that scares the hell out of me. Unless the scope is built like a tank. Surely its shifted a bit in that time. I collimate each and everytime i image. Each and every time its moved at least a bit, not often to the level were visual will be badly affected. But possibly for hi res imaging. Get collimating guys. you may be surprised once learned, your scopes performance suddenly jumps at you

a bit too much.

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Moonshane

Moonshane

Posted
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basic collimation which is good enough to gain maximum visual performance is no more difficult than an etch-a-sketch or (coincidentally) using an alt-az mount. if you can understand that turning a little screw or bolt one way and it moves the mirror left (or right) and that turning it the other way moves it right (or left) then you'll be able to do it with some basic and relatively inexpensive tools. unless you drop something, anything you do can be put right so there's no danger of ever making the scope unusable.

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cotterless45

cotterless45

Posted
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I just do a check using a Barlow with the collimated laser in the end. Normally it's spot on. I've put the scopes in the back of the car and they've kept collimation .If you number trusses and always rebuild the same way, it'll hold collimation well.

I've never had to adjust the 130 Heritage or the 150pl. They just keep it.

It is amazing how much difference spot on collimation makes .

Newts are planetary killers. If it's badly out I phone up and use a LeeB.

Nick.

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squeaky

squeaky

Posted
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I have to move my beast in the car to get to any decent viewing site because my horizons are rubbish and LP is dire - so I check collimation every time. Usually it just needs a minor tweak.

Not really a problem when viewing if I don't do it - but it can make a noticeable difference when imaging.

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661-pete

661-pete

Posted
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At first I was terrified of touching anything: I assumed that if I were to touch one of the screws by so much as a hairs-breadth, I'd never be able to straighten it out again! Once I'd taken the plunge and disassembled it in order to centre-spot the primary, and bought myself a laser, I grew a bit bolder. Now I find, I don't need to collimate for every session, but after every few nights or so. I find that a combination of laser, for the secondary, and cheshire, for the primary, works best.

Talking about what people find acceptable in collimation, I remember, in the early days, going round a shop and looking over a few dobs, I asked a casual question about 'how easy is it to collimate'? The chap gave me a quick demo, he got me to look down the focusser at one of the display models, and remarked "as you can see, that one's already pretty spot-on". Well, even with my inexpert eyes, and with no sight tube to assist, I could see that it was far from it. But I held my peace...:( Maybe Dob users are less stringent?

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cs1cjc

cs1cjc

Posted
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It might be worth putting some figures on this.

Even a perfect newtonian mirror suffers from coma which softens the image off the optical axis. Collimating is the business of getting the optical axis and centre of the focus tube lined up. The diameter of the coma free area (where the coma lies inside the airy disc) at the focal plane is 0.022*f*f*f mm, (where f is the focal ratio of the mirror) which for some popular f/ ratios gives the following diameters for the sharp area at the focal plane:

f/4 1.4mm

f/4.5 2mm

f/5 2.75mm

f/6 4.75mm

f/8 11mm

This can be compared with the field stop for a 9mm orthoscopic (40 degree AFOV) eyepiece of about 6mm. So in such an eyepiece, a perfectly collimated f/4.5 telescope will only show the central 1/3 of the view as sharply as it can be. If the collimation is out by 4mm or more then the sharp area will actually be outside the field stop of the eyepiece, so not visible.

It is reasonable to work to about 1/4 of the diameter as a tolerance for collimation so that the coma free area will overlap with the centre of the field of view, so for example for an f/4.5, 0.5mm.

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Macavity

Macavity

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At first I was terrified of touching anything: I assumed that if I were to touch one of the screws by so much as a hairs-breadth, I'd never be able to straighten it out again!
Heheh. After hours(?) of grim-faced knob twiddling, I'd probably steamed up the mirrors and the tiny batteries of my laser collimator finally gave out! But then I discovered the merits of rough VISUAL checking for gross misalignments. I'd been trying to correct secondary "tilt", when it was clearly (visibly!) rotated. :(

Later, I *too* discovered the merits of totally stripping down the OTA (Take it slowly and think?). But you learn a lot about exactly what is happening while you're twiddling away - A great thing for the confidence! And you can add a judicious amount of grease, maybe file a few burrs of the ends of a screws etc. These days, I just "do my best" with collimation? I doubt I can match the theoretical needs of an 8" / F4 Newt - Moreover, I sense the "mechanics" are not sufficiently capable anyway... :)

As a slightly heretical thought, I feel that my "collimation errors" are often within the limits of my EYES. I know(?) that the primary is still "a bit off", but the Newt works well enough as a "light bucket" for VIDEO (small chip) Astronomy. Upto about 120x it even gives reasonable images visually. And maybe, with a bit more twiddling... :(

If I wanted REAL precision, I'd buy an APO Refractor, a posh MAK, a more expensive Newt, but sadly...

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