Just when I thought I'd sussed it...

[Andrew*]

I had an infuriating few hours over the past days trying to sort my collimation.

Firstly I used my Baader laser to centre the dot in the centre spot. No probs.

I checked this with my film canister collimating cap. Way off. I could only see half the primary mirror. So I corrected it with the colli cap and put the laser back in. Way off.

Assuming the laser was uncollimated itself, I continued collimation with the cap by centring the image of my own eye over the centre spot.

However, the centre spot is not in the middle of the shadow/reflection of the secondary mirror. As I understand it, it is normal that this shadow is offset away from the focuser, but is is also off-centre. Here's a pic. Strangely, the rings appear to be concentric when looking in the colli cap...

(click to enlarge)

I tried to centre the secondary over the centre spot by fiddling with the spider support knobs, but this made no appreciable difference.

I can only assume my focuser needs adjustment, or the centre spot in in fact not central

Andrew

[barkis]

Andrew, I used to use a film cannister with a small hole in the dead centre, I also put a hole in the centre of the film cannisters cap and put it back on the cannister, thus ensuring you are looking right down the centre of the drawtube when it is in place. I also always had a small mark on the flat bang on the optical centre. Not everyones cup of tea, but that mark won't affect anything, as the shadow of the flat, although reduced in size, will fall over the spot.

The alignment of the focuser is no less important than the rest of the optical train, and ought to be checked for accuracy. Even a millimetre out is not acceptable.

Ron.

[themos]

I'm not sure if this is helpful but I have been checking collimation with a camera on a tripod positioned a few feet away from the focuser. I figured that the extra distance will magnify any out-of-true of the focuser alignment but maybe I'm imagining things.

[beamer3.6m]

Why not just use the laser and then do a star test to see if it is accurate.

If not, use the other method and then do a start test.

You will know which is correct straight away.

[Andrew*]

The alignment of the focuser is no less important than the rest of the optical train, and ought to be checked for accuracy. Even a millimetre out is not acceptable

Hi Ron,

how on earth do you align the focuser?? Sadly there are no hex nuts for that...

Why not just use the laser and then do a star test to see if it is accurate.

If not, use the other method and then do a start test.

As I said, collimation with the laser puts the secondary way off.

Also, I don't really know how to do a star test - any guides?

Andrew

[barkis]

Andrew, if your scope is out of collimation, it will manifest in poor star images, mostly coma as a result of the misaligned optics. Checking has to be done when the scope has had sufficient time to reach ambient temperature. Just place a star in the field of view of a medium power eyepiece IE 18mm-20mm and focus carefully stopping just before a hard focus. Look for any coma like shape in the star image, If there is one, then the brighter part of it is pointing to the optical axis.

Put your hand in front of the scope, and watch it's shadow as you move it around to locate the position of the coma's bright part. Turn the adjustment screws of your mirror to tilt the cell towards the position where the silhouette your hand was, a little at a time otherwise the image may leave the field of view. You may see diffraction rings and the should be perfectly symmetrical.

If they are bunched up at one edge, then the previous procedure applies.

Once you have completed the job with the medium power, do it again with a high power, until you have it perfect. This is not a thankless task, because the results when spot on will reward you well for your effort. There is bad collimation, not bad collimation and perfect collimation. Only the latter is acceptable. It is worth it, and the more you do it, the easier it gets.

Ron.

PS check on both sides of the focus Andrew. Inside and outside. You can discount astigmatism then.

[Kaptain Klevtsov]

Looks like the secondary mirror might be rotated about the OTA axis Andrew. If it s a bit out you can correct for the angle by adjusting the tilt, but the rotation manifests itself as the secondary mirror being somewhat wonky.

Looking from outside the OTA, position your eye so that you are looking across the surface of the secondary mirror and the front edge and back edge coincide. You will then see the secondary mirror as a line, and that line needs to be at 90 degrees to the laser line exiting the focus tube.

HTH

Kaptain Klevtsov

[supernova]

Andrew this is much more accurat

Have a go.

http://astro.neutral.org/eq/col.html

Bernie.

[Astro_Baby]

I'm hardly expert at this but I have had to do it a few times. Once when my youngest decided to unscrew the spider vanes on my 130 PM and a fair few times with the new SW 200 until I realised why it wouldnt collimate relaibly ( primary mirror was loose but thats another story ) in the interim I have collimated every night for the past 10 days just about.

Initially I found on my 130 PM that the visual collimation with a collimation cap was always different to when it was aligned with a laser. Someone told me that it could be the laser itself is out of collimation but I suspect a more likley explanation is that as you adjust the collimation on the secondary for tilt you introduce rotation errors into the secondary mirror.

ie you get good alignment visually - then you add the laser and while your centring that in tilt you slowly introduce rotation errors.

What I did to get it perfect on my 130 was this. Start with the collimation cap ( or Cheshire ) I personally find a collimation cap easier myself. Once its perfect put the laser in and align the secondary Check with the collimation cap afterwards. If its different readjust for visual and then go back to the laser. Its a kind of do one then the other and kind of split any difference. I do it more by fee than by science Eventually you reach a point where both will agree or be as close as you could ever hope to get. Once thats done do the whole process again but this time aligning the primary and the laser return path to the secondary.

Its a long slow process - to get it aboslutely perfect on the 130 took me about 4 hours but it now aligns perfectly by whatever method you use. Visual without a cap, with a cap, with a Cheshire or with a laser.

Something else that may be introducing errors is a difference in fit between the laser and the collimation cap. My laser is relatively loose in the focuser whereas the collimation cap is pretty tight. To compensate for this I recently bought a Cheshire which has the same amount of slack in its fit as the laser. What I do is fit them exactly as I fit an eyepiece with respect to the pressure on the screws that hold an eyepiece in.

From my limited experience to date there is ALWAYS some slack in the mechanism and I accept a small error between the two. Star testing it shows it to be accurate to the limits of my eyeballs and thats good enough for me.

[Ajohn]

Try this link. I posted a bit of information and a link to another site http://stargazerslounge.com/index.php/topic,26383.0.html

Your's is the perfect example of how lasers mess up peoples scopes - no matter how much you spend. Your lucky the error is a bad as it is. Some aren't so lucky. There's a few more comments in another thread in the same section too.

John

[Andrew*]

Ron, thank you for your in-depth guide to star testing - I'll remember that for when I've almost sussed it!

Looks like the secondary mirror might be rotated about the OTA axis Andrew. If it s a bit out you can correct for the angle by adjusting the tilt, but the rotation manifests itself as the secondary mirror being somewhat wonky.

I've tried rotating the 2ndary so that when viewing with the film canister, the entire primary is visible. The laser beam is about an inch off. I have collimated my collimator.

Looking from outside the OTA, position your eye so that you are looking across the surface of the secondary mirror and the front edge and back edge coincide. You will then see the secondary mirror as a line, and that line needs to be at 90 degrees to the laser line exiting the focus tube.

Sounds great, but a) I can't see across the secondary surface - it is too far down the tube, and I can't see the laser line unless I put some dust in the way for it to reflect off.

Andrew this is much more accurat

I'll give it a go when I've got the secondary correct...

AstroBaby, thanks for your words. I've tried to minimise any possible difference from focuser slop. I don't want any difference between colli cap and laser.

Try this link. I posted a bit of information and a link to another site http://stargazerslounge.com/index.php/topic,26383.0.html

Your's is the perfect example of how lasers mess up peoples scopes

THanks John. Actually, I think in this case the laser has saved me from living with poor collimation. It's made me realise that I need to adjust something else. Possibly the focuser...

The problem is that the secondary is at the wrong angle. When viewing through the film canister, I can see the entire primary, but clearly that's not all there is to it, because the secondary does not appear round. This is confirmed by looking from behind the secondary, so that the primary centre is obscured.

I think this is further confirmed using the laser - the secondary is not angled at exactly 45 degrees, even though I can see the whole primary.

If I rotate the secondary so that it appears round from both POVs, I cannot see the entire primary, even after fine adjustment with the allan key.

I could correct this if I moved the secondary within the tube, but I have run out of travel in the spider adjustment knobs. I don't believe that the secondary is SO incorrectly positioned! It must be the focuser.

Removing the colli cap, I can see everything correctly aligned if I offset my eye about 1cm from the centre of the focuser tube.

Okay, that's the entire situation explained and I'm not even onto the primary mirror yet! I'm a bit confused about what to do now. I'm almost certain I need to align something other than the usual suspects. Do I need to remove the secondary and align the focuser correctly?

I would appreciate your help so much! This is driving me a bit nuts. I can only be thankful that this happens in summer when I can be out with the scope in the warm and light, and when I rarely use it anyway...

Andrew

[Ajohn]

All I can say is oh dear. You have a scope that's intended to be used with an offset secondary mirror and that makes life difficult. I found a reprint of the sky and telescope article that sent newts that way recently. The mirror is moved off center and effectively the center of the secondary is also down towards the mirror. Makes life difficult and that's why I don't think they should do it. There's some maths to calculate the offset but they include the size of the field that the scope is supposed to cover rather than just where the center of the 2ndry should be. It states that the offset of an 8ins F6 scope is 0.056ins and isn't worth worrying about but that it's nearly 0.15ins on a 12ins F4. This is all from the early 60's and was supposedly missed by earlier telescope makers as they didn't make fast mirrors - me thinks things aren't as simple as that.. Coma spoils off axis images in newts and gets worse as the diameter goes up and the F ratio goes down. Loosing some of the light in off axis images from the edge of the mirror could even be beneficial + earlier telescope makers were not stupid.

Give me a while and I will try and make some sense of the maths and if I can't I'll post the lot. It was published in sky and telescope in Dec 1962 and then corrected in Feb 1963. Seems the "inventor" didn't get it right 1st time. I'm interested because I have a scope like that too. I don't use it because I've moved on but won't sell it because it has the same problem. Be warned the 2ndry isn't at 45 degrees on that one either and from memory that's where the corrections article comes in. No mention of that in the reprint I have.

John

[Andrew*]

Hmmm...

The strange thing about what you say is that the scope I have must be the single most popular scope on this forum! I haven't read a single similar complaint from anyone else...

Andrew

[barkis]

Andrew, I have posted this diagram taken from Jean Texereaux's Telescope Making Book, which depicts the Offset of the diagonal in a Fast Newtonian. It may help, or it may not. You know what they say about a picture.

Ron.

(click to enlarge)

[Ajohn]

Barkis :x why not post the next page with the formula on it? That should still be ok if d (that's little d) is set to 0. It says exact only it isn't but there' no real reason to worry about that. Given the formula it can be sorted out. I've had the same book since 84 but have only recently looked at the appendix No idea why not.

John

[barkis]

OK. The formula as requested.

(click to enlarge)

[Ajohn]

( Mirror Diameter / 2 ) ^2

e is the depth of the mirror which = ---------------------------

4 * The focal length

Need to find out what L is. (Distance from the axis of the mirror to the focal plain)

That can be done in 2 ways. Plossl eyepieces focus on images just in front of the field lens so

focus on something about 1 to 200 mtrs/yards away or more. Measure the over all length of the focuser, add

half of the tube diameter and subtract the appropriate length taken from the eyepiece.

Or focus the scope with something translucent held over the end of the focuser. Best to try and get this right to

to say 4 or 5 mms or so.

Now do 2 calculations M= 2 * ( Focal length - Diameter of the miror - e) and

N= 2 * ( Focal length + Diameter of the miror - e )

L/M - L/N

The offset then = ------------

2

I cant comment on the maths because I did start looking at this and came to the conclusion

its a sin A / A thing plus similar triangles et. Decided to remachine the 2ndry holder

instead as having to do it this way is a pain. The offset arrangement should be stamped on

the scopex. Given that the tube can be distorted the whole thing is bit much and doesn't really

gain that much in any case. It would be very interesting to compare the lost mirror area as a

% on an 8ins F6 against a 12ins F4

Elsewhere some one has achieved the offset by placing the diagonal central in the tube and

then tilting the miror " to get the diffraction patterns correct ". Problem doing it that way

is that the mirror is being used off axis all of the time. Not a good idea and the tube isn't in

line with the axis if the scope

People who don't know can't complain.

Going back to the other posts on collimation having got the offset right the same sort of process can

be used but this time things still have to be central except that the main mirror and the bottom of the

tube will not look central across the long axis of the 2ndry. There may be another problem caused by the

the other factor that was originally mentioned. That's the 2ndry not being at 45 degrees. Pass at the

moment in that case.

When you use a laser collimator (if you must) it's important that the beam strikes the center of the main

mirror and is reflected back. Spots need a clear hole in them. Just tweaking the 2ndry so that the laser hits

the spot is pointless. It doesn't mean anything. The reflected beam has to go back exactly where it came

from. That means that the 2ndry is square and at 45 degrees to the focuser and the bit of the main mirror

that is generating the reflection. One way of doing that is to place a card inside the tube with a suitably

small hole in it. The laser light goes through the hole and the returned reflection can be seen on the card.

The 2ndry can then be tweaked to send the return beam back up it's own orifice.

Only problem then is that bit of main mirror that's reflecting the beam back up the tube. It's curved and may not be

square to the axis that's just been set up. That miss alignment will have been tweaked out when the 2ndry

was adjusted. Some lasers project a square grid so if the scope is pointed at a wall and the axis is square to it this

sort of miss alignment will show up as distortion in the grid that can be removed by tweaking the main mirror. The grid

will be distorted if the axis isn't square to the wall too. (not good)

It's then a case of going back to the 1st step of tweaking the beam back up it's own orifice. Repeat the grid alignment and so

on. It's also possible to look at the beam coming off the 2ndry and the one coming back from the main mirror by blowing smoke

down it (not popular these days) or by sticking something fairly transparent in the beams. The error will be largest up by the 2ndry

and the 2 beam will form an oval or be split. Personally I think that the eyeball reflections are more accurate and quicker to do

especially on the main mirror adjustment.

Maybe some will now see why lasers aren't exactly as good as some seem to think.

This can all go wrong if the beam isn't hitting the main mirror more or less dead centrally for

some reason. The mirror is curved and slopes away in all directions. This isn't a problem

when collimation is eyeballed. A laser may have some advantages when the 2ndry is set.

John

[Andrew*]

oh my :shock:

Do I REALLY need to do this offset malarky? :oops:

I think I'll mull over this tomorrow some time. Maybe next year...

Thanks anyway guys.

Andrew

[Ajohn]

That just about sums up why I think the whole thing is a very bad idea for the vast majority of people Andrew.

You could try forgetting the offset and use the laser as I've outlined with the 2ndry holder central in the tube. If you can't see all of the mirror you MAY have one like the one I have which is even worse news. BUT the sums are easy and the measurement only needs to be done once.

There is one good point. Once you have set up the 2ndry other than major bumps it shouldn't need touching again. The main mirror may need some slight tweaks from time to time. Depends on how well the scope is built. If the scopes on it's side the mirror may slide a bit because it shouldn't be held tightly. The fix is simple point the scope up and shake it about slightly to make sure the mirror is settled. On an 8ins scope bearing that in mind collimation can be something that only has to be done once. I would do it this year if I were you and then be pleased when you get it out next year. All that would be needed then is a bit of a tweak against a star.

What scope is it anyway?

John

[tony4563]

Andrew.

Surely the laser method is the easiest way to collimate the scope? First make sure the collimator is snug fitting in the focusser. If it's loose then wrap one even layer of insulating tape around the collimator. That normally does the trick. Gently loosen your secondary and direct the beam of light until it hits the centre mark on your primary mirror, then gently tighten up the secondary. Now adjust the primary screws to direct the beam back up the tube until it hits the centre of the "bullseye" on the collimator. Easy.

That should be job done. Do a star test and check for concentric circles at high mag during focussing. If the star test fails then it may be that your primary mirror is not centred correctly. A few millimetres out and collimation won't be correct. You may need to remove the primary at some stage and check if you still aren't happy.

[Kaptain Klevtsov]

oh my :shock:

Do I REALLY need to do this offset malarky? :oops:

Andrew

No mate, not at all. This offset secondary malarky is a total red herring here, as is the discussion around it.

As the secondary is eliptical, you have to look at it at an angle to see it as a circle. Just as, if you look at a car wheel it appears to be elliptical unless its square on to your face. At dinner time, look at your plate, and it will appear elliptical because it isn't square to your line of sight. This is totally normal and we are used to it.

This effect was harnessed by the telescope pioneers as they understood that, to appear circular, a mirror at an angle of 45 degrees has to be made as an ellipse. They also worked out that the centre of the apparent circle isn't the centre of the ellipse (but only by a little bit), so the offset is necessary.

The easy bit is, if you use a sight tube, you just have to centre the circle as you see it and forget the hard sums. The angle is not calculated, its done by sight, so that it bends the light from the primary mirror onto the secondary into the focus tube. All dead easy stuff.

Get the secondary centred in the focus tube by eye, reflecting the whole of the primary, and all this clever stuff happens automagically. Use a laser and you'll not be far out either, probably within 1/10th of a degree of the absolute correct secondary angle if the laser dot is even close to the primary centre.

My suspicion is that your secondary mirror is rotated along the 'scope axis by a tiny amount, so try turning it by a tiny amount and see what happens.

Forget the theory, thats only needed to design the thing, not to collimate it, but it helps if you know what the offset is there for, so you can understand why it looks lopsided from certain angles. If the design is correct, like in the SW 200mm Newt., you don't have to reshape the secondary to get it to look round, just move it until it looks round, end of.

HTH

Kaptain Klevtsov

[Andrew*]

Andrew.

Surely the laser method is the easiest way to collimate the scope? First make sure the collimator is snug fitting in the focusser. If it's loose then wrap one even layer of insulating tape around the collimator. That normally does the trick. Gently loosen your secondary and direct the beam of light until it hits the centre mark on your primary mirror, then gently tighten up the secondary. Now adjust the primary screws to direct the beam back up the tube until it hits the centre of the "bullseye" on the collimator. Easy.

Tony, I appreciate your help, but you seem to be misunderstanding me! The fact is when the laser dot is on the centre spot, the primary is not fully visible by sight. When I adjust it so it is, the laser dot goes off by in excess of an inch.

I'll come back to the other replies. Thanks everyone for your help.

Andrew

[FLO]

Andrew, everything you need to know is in Tony4563 and KK's last posts. I also suspect that your secondary mirror has swivelled a bit on its axis, if that doesn't resolve things check that the circle/marker in the centre of your primary is actually central. It is rare but occasionally they can be placed a few millimetres off-centre during manufacture.

HTH

[daz]

Andrew

Can you take a picture of the 2ndry and post it?

[Ajohn]

Maybe you should search kaptains posts - you'll find one showing a picture with an offset 2ndry etc - including one of his own scopes from the way he was discussing it.

On the tape round it to pack it. The nose of the collimator should be the same size as an eyepiece. The whole idea is to square it all up to the eyepiece so just clamp it in place just as you would clamp an eyepiece. Ideally with the focuser were it will be in normal use.

My collimator came free as it had wrecked 2 scopes and the person who owned it didn't want to risk it again. I know for a fact that the 2nd one was down to 2ndry offset being incorrectly set and the 2ndry not meant to be at 45 degrees. He was only using 2/3rds of the mirror - sounds familiar doesn't it.

This collimator projects a grid and a dot and cost rather a lot of money. The grid when it's reflected back of the 2ndry can be centered on the focuser but I'll leave it to the pundits to tell everybody why I suggested a piece of card with hole in it. Easy to do and can be held in place with one of the wife's clothes pegs. Well that's what I use and well making a roundish hole might be a bit of a pain but not much.

Because the main mirror isn't flat it's possible to get a double whammy. One miss alignment correcting another. That's why I suggested using something translucent to look at the beam under the 2nry - it's too late after it.

All pointless? Only giraffes can look at a star and adjust the main mirror easily. Fact is if Andrew has the sort of telescope he appears to have he will only get it set up correctly until the offset is set. He might also find that he has to move the 2ndry down or up as well as adjusting the tilt. Most likely down as he is correct that it won't look round through the focuser anymore. The 2ndry isn't a dinner plate and he isn't sitting at a table either.

John