Collimating a Newtonian...

[The Warthog]

I forgot that point, but I always do my collimation with the scope level or pointed slightly down. I adjust only two of the three screws on the back of the scope, unless I run out of adjustment on one of the screws, or get really frustrated, angry and stupid.

Leave the vanes alone. The longitudinal or lateral position of the secondary is the least likely thing to be giving you problems, unless you have been playing with it, or are building a scope. You may need to turn the secondary towards the focuser, and adjust it so the entire primary is visible and centred in the secondary. With most scopes, you should only mess with the position of the secondary in the tube if you look through the scope and see a blue moon.

Then adjust the Primary so the image of your eye, and the central dot in the mirror line up. If you are using a Cheshire collimator, line up the cross hairs, too, so they are superimposed. You will get decent views if you are even close to perfectly collimated, so if you are getting frustrated but are close, quit, have a cup of tea, and use the scope for the night. Go for perfect later.

[RichieJarvis]

Another tip I learnt from this - not sure how much impact it had.

The Primary cell has a certain amount of movement inside the tube in mine - about 3mm all around. To get it bang in the centre of the tube whilst it was on its side was impossible. I put the tube on its 'head' with the focuser nearest the ground, and then was able to tighten the sprung adjusters with the mirror cell in the centre.

Worked for me - don't know how critical it was though!

Cheers,

Richie

[Astro_Baby]

My 'long time in the making' collimation guide is now online. If anyone wants to take a look and feedback I'd be glad to get some responses.

This isn't a techy guide - its geared for novices, no math but lots of pics.

Its online HERE

[Astronut]

My 'long time in the making' collimation guide is now online. If anyone wants to take a look and feedback I'd be glad to get some responses.

This isn't a techy guide - its geared for novices, no math but lots of pics.

Its online HERE

That's a great tutorial - easy on the eye and well formatted. I've read dozens of collimation guides and IMHO a novice would be hard pressed to find a better guide than this. If you have a Skywatcher newt then look no further.

Mel - you've covered all the bases and the diagrams and photos are excellent. You can't beat piccies - they speak a thousand words

Ian

ps. I've not seen the photo of the airy disc with tube currents before - interesting. Come to think of it I've not seen an airy disc yet - the chances of doing a star test under these awful UK skies are remote to say the least, also I haven't got a short FL eyepiece to do one.

Next star party maybe I'll borrow one.

[Astro_Baby]

Thanks Ian, coming from a collimation ace like you thats a real compliment.

I just took all I learned and worked at with practice and slapped it down as a guide. If it spares another newbie pain and grief its been worth it

[wingman]

Great work Mel, it looks so straightforward I might even have a go myself, if it makes sense to me you've REALLY done a good job!!

[vlebo]

Thanks Mel

That was really helpful. I spent the best part of a day trying to get my collimation right ,I even dropped an allen key down the tube,but luckily it did not touch the mirror,just the side ,which I touched up with matt paint.

Mine is a Flextube 300 which is F5 and I was concerned about still having the offset collimation pattern after spending hours trying to get it right but after reading your article and seeing the pics of the offset effect I may actually have got mine right. And whatsmore tonite is a vey clear night so cancel everything and I am getting out there.

Have a great weekend.

Vlebo

[RichieJarvis]

That guide is awesome Mel.... I've only skimmed it so far, and it covers pretty much everything anyone's ever told me... Suggest SGL puts a sticky post with just the URL in for reference - cos that way, it'll google beautifully

I plan to put it into action next time I get chance... One thing that reassures me about this is the very handy images at the bottom showing the patterns you will see for each symptom. I can't be that far away from perfection, because the only thing I can see is the doughnut slightly (and I mean slightly!) off to one side.

I'll be bringing it down for the next but one Adur meeting anyway, so I might just have to get you to cast your expert eye over it then! (if you don't mind...)

Quick Q for you though - just been reading some more.... This image:

Are my eyes wrong, but the secondary looks slightly to the right. Its supposed to be bang in the centre, right?

Oh, and love the different shades of nail polish in each explanation pic Its nice to see a feminine touch to Astronomy! (thats meant in a genuine and nice way btw - before anyone takes offence....)

Thanks for all your help!

Richie

[Astro_Baby]

Richie - to me it looks slightly to the left. Its our eyeballs I fear ( in my case old age ). It may well be that its just because the pics down the Cheshire and collicap were done with a happy snappy cheap cam. Oddly it was the ONLY cam that could get any kind of a pic. But is wasnt perfectly centred for sure.

That probably accounts for the slight difference. But yes it is supposed to be bang in the centre. My own feeling is that on mass produced scopes you'll prpbably never get everything exactly as it looks in a textbook.

Your slight 'off' centre star patterns may well be down to unavoidable stuff like the tube flexing etc.

I know my collimation wasnt exactly bang on at Salisbury cos Ian put the Catseye collimator on mine and it showed collimation errors though the Cheshire was bang on - that doesnt surpise me - the Catseye system is a much more rigourous system but the fact is that star testing showed flawless collimation.

Theres always some error - for one thing your eye is never going to be in exactly the same place each time and on most scopes neither is the collimation tool because theres slack in the focusers.

One thing with the moonlite focuser is the collimation tool is rock solid every time.

Vlebo - the offset thing had me pulling my hair out at one time and for a while I just assumed either a) I was dead thick or the scope was the spawn of Satan

I am hardly expert by the way, just very persistent and very obsessive. Once I get into something I will read up everything I can lay hands on that covers the topic.

Anyway glad people like the guide - very happy to take suggestions on improvements because I only do these to help other newbies get to grips and I am not precious about stuff.

[Astronut]

The secondary offset to the right that's visible in the photo is normal. Remember this is a relatively 'fast' F/5 'scope so the secondary is offset slightly away from the focuser than it would be in a slower, say F/8, 'scope. Secondary offset is necessary (but not critical) to achieve full field illumination at the eyepiece (no light loss at the edge of the field of view).

Skywatcher 'scope secondaries are glued to their mounts at the factory with this transverse offset built in so as long as the spider vanes are of equal length (as detailed in Mel's tutorial), you'll automatically have the correct transverse offset (away from the focuser). Longitudinal offset (up/down the tube centre) is taken care of during collimation ie. lining up the secondary to the focuser.

If you look at the back of your secondary and its mount from the front of the 'scope you'll notice that the secondary isn't mounted exactly centrally on its mount. The distance from the mount to the secondary edge furthest from the focuser is more than the same distance to the edge nearest the focuser.

It's only a difference of a few mm or less (even less on a slower focal ratio 'scope) depending on the size of the secondary so if it's not obvious by sight, careful measurement from each edge to the mount side will confirm it.

The end result is, after successful collimation, the reflection of the secondary within the primary reflection looks offset from centre - perfectly normal.

....

The Primary cell has a certain amount of movement inside the tube in mine - about 3mm all around. To get it bang in the centre of the tube whilst it was on its side was impossible. I put the tube on its 'head' with the focuser nearest the ground, and then was able to tighten the sprung adjusters with the mirror cell in the centre.

Worked for me - don't know how critical it was though!

Cheers,

Richie

A bit worrying :? Is this movement of the cell itself or the mirror within the cell? The cell shouldn't move at all in relation to the tube.

If it's the mirror moving in the cell then removing the cell from the tube may be needed to check the mirror clips and retainers.

Not sure if your 'scope's mirror cell is the same but on my Skywatcher, the mirror is held centrally in the cell by 3 plastic or nylon screws at the side of the main mirror clip mounts. These small screws touch the side of the mirror to prevent any side to side movement.

As with the main mirror clips (the one's touching the mirror face) which hold the mirror in the cell, these shouldn't be tight but they should be screwed up to the mirror side just enough to hold the mirror centrally and prevent it sliding around in the cell.

The way you've collimated ('scope upended to compensate for the movement) isn't going to achieve much because as soon as the 'scope is back in its normal position, the collimation will be thrown out again. The primary mirror has to be held firmly enough (but not tightly) in its cell to prevent it flopping or sliding about.

[Astro_Baby]

I missed that post - yes Astronut is bang on there. Mine also when new had a certain amount of 'flop' in the mirror. On mine this was due to the mirror clip retaining screws being ever so slightly too long - about 1mm. I ground the ends down and replaced them. The clips should hold the mirror but not press on it. Because the screws were too long the mirror had too much play in it and it was noticeable when you rotated the scope tube - you could hear a dull thump coming from within which was the mirror shifting in its cell :shock:

A well known issue ( so I am told ) with Sky-Watchers and Orion US scopes ( same thing really ) is that they wont hold collimation when the tube is moved about. Personally I am a bit sceptical of this as after I tightened the mirror clips I find mine holds collimation perfectly well when being rotated and at Salisbusry the scope was in transit, in variable weather ranging from very hot to wet and cold and was taken downstairs, loaded into a car, transit to Salisbury over some bumpy roads, then bought back and through it all collimation was spot on - at least to the limits of what I can see with a Cheshire and thats as good as you can get really.

I read in my 'education' that the optimum position to collimate is with the scope laying flat on the floor and with the focsuer point up at 90'. To me that seems a bit pointless as the scope will have to move at some point unless you only intend observing your skirting board

It might be that the lock bolts arent set tight ( though they shouldnt be overdone ) if thats ok then it may be worth you taking the mirror cell out and seeing if the clips are loose.

ps the pic of the mirror being off on that post up there a bit - I think I misunderstood. THe central spot shows the mirror being offset but on that pic the only element which is being demonstrated is the position of the secondary under the focuser and the fact that all three mirror clips are shoing at equal positions around the secondary. I think I picked up wrong.

[narrowbandpaul]

hopefully we will be acquiring a new newt soon..a fast one too,

is it best to collimate with the camera in...perhaps use the laser, to get it close and do fine adjustment with the CCD in place.

The scope should come witha rock solid 3" focusser, so there shouldnt be an ounce of flexure. The laser should work well for this right?

The camera has small pixels..so the resolution should be good enough for collimation?

Dont like the idea of the human eye for collimation...too many variables, plus its not the eye thats being used here, its the CCD.

Cheers guys

Paul

[Astro_Baby]

Cant advise you - dont do imaging so really dont know - Steppenwolf would know I am sure as imaging with Newts is very much his speciality.

[narrowbandpaul]

its a picostar we have...presumably centre it, defocus it, and adjust airy disk to ensure concentric rings, if the quick colli with the laser isnt good enough?

[vlebo]

On a slightly different point I think I may have to get a better crayford focuser for my Flextube as there is too much slop in the one that comes as standard because whenever I put my Baader laser collimator in place it sights up differently every time unless you tighten it up the same way every time.

I was led to believe a laser collimator was the best way to go but maybe now I will get a cheshire and try that.

Vlebo

[glennbech]

I put my Baader laser collimator in place it sights up differently every time unless you tighten it up the same way every time.

I have both a chesshire and a laser. I am much more pleased with the chesshire. One disadvantage of the baader laser collimator is that the tube that fits in the focuser is very short compared to the full length of the chesshire. This makes it possible to move the laser enough to make a huge impact on the return beam. You can fiddle with it for hours without getting coherent results. (It was probably okay in the first place...)

In short I'd say that the laser gives a false impression of high precession. (At least that is my humble opinion on the Baader)

[narrowbandpaul]

but if the focusser hold the laser rock steady, then the laser offers better precision than the human eye presumably

[glennbech]

but if the focusser hold the laser rock steady, then the laser offers better precision than the human eye presumably

The question is (i guess); is it the focuser's fault or the collimation device?

A design similar to the chesshire would have helped a lot (it fits very well, and are inserted maybe 10 centimeters into the focuser).

On my 8" Skywatcher a small turn of the screw that holds the EP in place is enough to make the reading of the return beam useless.

[narrowbandpaul]

i know what youre saying...i suppose we will have to wait and see if the focusser is strong and the optical path of the laser is steady.

[vlebo]

That is the problem I have, unless you put the laser collimator in the exact position and tighten it the same way everytime the return beam is in a different place on the plate. Even just touching the baader makes the return beam move. Maybe putting some tape around the neck to tighten it in the focuser would help but methinks a new crayford may be on the cards but I fear funds are running low. Donations gratefully accepted

Vlebo

[Astro_Baby]

My own experience of a laser was that a) it didnt hold collimation well itself ( ie the beam would move in the device which meant before using the laser I had to collimate it each time) and the slop in the focuser meant it was always gave slightly different results.

The upgraded focuser has cured the slop but to be be honest I found the laser a pain. I'm told the top end of laser collimators gives more reliable results but if I wanted to go further with collimation I'd go for the Catseye system which collimates the scope over 7 times its focal length and is as accurate as your ever going to get. I was also mightlily impressed with the quality of the Catseye system having had a look at Astronuts.

It seemed to me to be better quality and more precision based than anyting else I have seen.

[narrowbandpaul]

is the catseye not really difficult to use

[Astronut]

re. Baader laser collimator. It's recomended (in Baader's instructions) to line up the on off screw of the laser with the focuser's lockscrew before tightening it in the focuser. The problem with this is that if your focuser's lockscrew is not facing the primary mirror it's going to make it difficult to see the return beam hitting the reticule while you're fiddling with the primary collimation screws.

re. Catseye system. IMHO this has to be the best collimator on the market for 2" focusers. What appealed to me about it is it eliminates the 1.25" adaptor from the equation. If the 1.25" adaptor is a sloppy fit in the 2" focuser it's just going to introduce more errors. It would be difficult to use in a poor quality focuser or a sloppy rack and pinion focuser as the autocollimator is extremely sensitive to movement of the focuser.

Even with the Moonlite focuser, the multiple reflections of the centre spot triangle in the autocollimator dance about slightly as the focuser is racked in and out - it's THAT sensitive. It's for this reason that I always use the autocollimator with the focuser racked to my usual focus position.

However when using the Blackcat cheshire it's not possible to detect any movement of the triangle as the focuser racks in and out. It's extremely easy to collimate the primary with the Blackcat. The reflective triangle (with each point pointing to a primary collimation bolt) makes it very precise for eyeballing concentricity. The autocollimator makes final collimation even more precise.

The most difficult part of using the Catseye system is applying the triangle to the mirror centre (a template is supplied). If it's off by just a nat's then the collimation is going to be off slightly.

The only worry for me now is this: Is the triangle applied EXACTLY at the OPTICAL centre of the primary - only a star test will confirm this.

It's the GEOMETRIC centre that the triangle is applied (using the centreing template) but what if the optical centre of the parabolic curve is slightly offset from the geometric centre? I've read that this occurence has never been found with modern mirrors but again only a star test would confirm an offset (if any).

I seem to be getting good images at the eyepiece though so I'm not too worried about it - big thumbs up for Catseye

[The Warthog]

I have both a chesshire and a laser. I am much more pleased with the chesshire.

Snap! A lot of people who have used both claim that the Cheshire is more accurate than the laser. I have never used a laser collimator, but I find the Cheshire so simple to usse that I can't see spending a couple of hundred dollars to get an uncertain increase in accuracy. My Cheshire is well-made, and cost me $27.

If you are having trouble with collimation because of a sloppy fit in the focuser, perhaps you need to fix that problem first. I would suggest making a shim out of a beer can to tighten things up. Anyway, if your collimator is off centre by a few thousandths of an inch, then various eps will also be off centre. Considering that you can get good results with slightly off centre collimation, how much is the extra accuracy worth? In any case, you can get great collimation with a relatively inexpensive Cheshire, or, for that matter, a film can with a hole drilled in the top.

Stop fiddling with things and get out and observe, eh?

[Astronut]

...Stop fiddling with things and get out and observe, eh? :D

just waiting for the endless Pennines Lancashire cloud cover to go away... and waiting, and waiting....