Post by carlos on Mar 26, 2008 19:07:27 GMT
Collimation isn't the nightmare it is often made out to be - after doing it a few times, it becomes very easy, and pretty quick.
You soon pick up what you are actually DOING, and it becomes second nature - and it can be broken down into two basic steps:-
First step is to align the secondary mirror, so that your line of sight from the eyepiece is reflected EXACTLY into the centre of your primary mirror.
Then you need to align the primary, so that it reflects EXACTLY off the secondary, and back through the centre of the eyepiece.
Here's how to do it with a sighting cap and/or 'Cheshire eyepiece.
(Laser method to follow shortly)
To begin with, check you have the allen keys, screwdrivers etc that you will need - often they are supplied with the scope – but you may need to add one or two basic tools of your own. You will need tools to fit the 3 screws/bolts positioned around the front of the secondary – and the one in the centre, as well as the collimating bolts/screws an the bottom of the telescope tube.
Depending on the model of scope, some of these may be simple thumbscrews, some will require a tool.
Also – some manufacturers fix a protective cap over the collimating bolts on the bottom of the scope (I know for sure that SkyWatcher and Orion do this) you’ll need to remove it.
You will also need at least a sighting cap, and preferably a sighting tube with cross-hairs.
The cap is easy enough - I used one off the focuser, simply make a small hole exactly in the centre, and paint the inside silver or white so that it's easy to see reflected in the primary mirror.
I made my own sight-tube out of an old barlow that came with a telescope I've had for years - the barlow was very poor quality and I had NEVER had an acceptable image through it, so I just removed the lenses, and put a cross-hair on the end of it. To make the cross-hairs I printed out a circle the right diameter on a piece of paper, and marked the 4 points around it. Then put the tube on the paper and marked the tube with a fine marker pen, I then cut notches with a fine saw, and stretched black cotton thread across, then stuck it in place with clear sticky-tape.
You can see the cap, and just about make out the cross-hairs in this pic:-
If you don’t want to make your own cap or tube (or you don’t have the bits and pieces lying around) most dealers sell caps, sight-tubes, and ‘Cheshire’ eyepieces (a purpose made tool, similar to a sight-tube)
The main mirror should also have a marker in the centre – usually this is a small ring, a bit like those hole-strengthener rings you get for ring-binders – if the mirror doesn’t have a centre marker, you can make one – you guessed it – by using a hole-strengthener ring.
You’ll need to ‘CAREFULLY’ remove the mirror to do this – measure across the mirror in two directions, at 90 degrees to each other, and make a tiny cross in the centre with a fine marker pen – Don’t worry about marking the mirror in the centre inch or so, this part of the mirror is shadowed by the secondary, and isn’t used when the telescope is operating. Then simply (BUT CAREFULLY) stick a ring EXACTLY in the middle of the mirror.
OK - down to business.
I'll assume that the focuser is square, and firmly fixed on the outside of the tube, and cut straight to the optical alignment, and it is usually easier, if you point the telescope out of the window to let light into the tube so you can see what you're doing.
You should see 3 holes in the end of the secondary, arranged around another screw in the centre. There should be a small screw or allen-bolt in each hole. You may have been supplied with the necessary allen-wrench to fit them.
When I did my first full collimation on my Skywatcher 150, I loosened those allen bolts a fair bit. Then used the central screw to get the secondary at the right position in & out of the tube (left & right through the focuser) I then held it as near to correct as I could by hand, while tightening the 3 allen bolts up to the point where they just touch the secondary's mount, holding it in position, but allowing me to rotate it. I rotated it to the correct alignment, so the primary's centre marker (a ring) appeared central, on the cross-hairs of my sight tube, as seen through the focuser. The spider vanes may also need adjustment, by using the bolt-heads around the outside, in order to get the secondary to the right position up & down as seen through the focuser.
I then used the 3 allen bolts to 'fine-tune' the secondary while looking through my home-made cheshire.
Basically, you should be able to see the secondary is concentric inside your view through the focuser. The 3 clips around the edge of the primary mirror should appear even around the inner edge of the secondary, and the central marker ring of the primary should be central within the secondary (if you have a Cheshire, or tube with cross-hairs, it is a big help at this point) Ignore the reflection of the focuser that you can see in the primary, until you get the secondary mirror right.
Last of all, when the secondary is correct, I put my sighting cap on the focuser, without the sight-tube, and set about lining up the primary. Most people point the telescope out the window to let light into the tube - but I have my own method, I have a torch (flashlight) that fits exactly into the smaller cap that is on the main cap
This pic shows my scope with the sight tube inserted, and the torch wedged into the cap:-
The torch lights up the centre ring on the mirror, and makes it easy to line up the reflection of the small hole in the sighting cap, precisely central within the marker ring. Pointing the scope out the window (careful not to point it at the Sun) has the same effect – the torch allows me to do it when it isn’t daylight.
To align the primary, first you need to remove the cover from the rear-end of the tube (if it has one)
Down here, you'll see the actual collimating screws, almost all scopes will have screws (or bolts) that adjust collimation, paired with a locking nut (or screw)
You should see three pairs, positioned equally around the base of the scope
First undo the lock-nuts a bit, then turn the alignment screw (or bolt) while looking through the sighting cap, you will be able to see the reflection of the cap's central hole moving as you turn the screws. From this, it's easy to work out which screw (or screws) to turn, and which way to turn them, in order to get the reflection of the cap's centre hole aligned exactly in the centre of the primary mirror's marker ring. Try to leave one of the screws alone, and only use the other two to align the mirror - this ensures that over time, after subsequent collimations, that the mirror doesn't gradually 'wander' up or down the tube, and reach the end of the travel on the collimating screws.
When you get the sight-hole lined up exactly central in the marker ring, it's simply a matter of tightening the lock-nuts - but be careful while doing this, because they move the collimation very slightly when they tighten up, so try to get them all to a similar tightness, and keep watching the collimation as you do so.
Above all DO NOT over-tighten them, this may cause the mirror to warp very slightly, causing an optical aberration
Last of all do a star test to check the precision, slightly de-focus a bright star at about 100x - 150x, you should see faint diffraction rings around the star, if they are concentric, the collimation is good, if they are uneven, further minor tweaking of the primary may be needed to get it exact.
You will only have to do the full collimation sequence every once in a while. Once you get the secondary set up right, and tightened, it takes a fair old clout to knock it out of alignment - and all that is necessary after that is occasional slight tweaking of the primary.
Hope some of that may have helped
carlos
also check out this great instructional video:-
www.andysshotglass.com/Collimating.html
Laser collimation is much more convenient, AND can be done on-site, in the dark - but not as accurate as doing it by sight.
As I said further up - I'll add a bit on using a laser collimator soon.
And also, how to check and tweak by doing a star test to get the laser collimation as accurate as the daylight 'sight' method.
You soon pick up what you are actually DOING, and it becomes second nature - and it can be broken down into two basic steps:-
First step is to align the secondary mirror, so that your line of sight from the eyepiece is reflected EXACTLY into the centre of your primary mirror.
Then you need to align the primary, so that it reflects EXACTLY off the secondary, and back through the centre of the eyepiece.
Here's how to do it with a sighting cap and/or 'Cheshire eyepiece.
(Laser method to follow shortly)
To begin with, check you have the allen keys, screwdrivers etc that you will need - often they are supplied with the scope – but you may need to add one or two basic tools of your own. You will need tools to fit the 3 screws/bolts positioned around the front of the secondary – and the one in the centre, as well as the collimating bolts/screws an the bottom of the telescope tube.
Depending on the model of scope, some of these may be simple thumbscrews, some will require a tool.
Also – some manufacturers fix a protective cap over the collimating bolts on the bottom of the scope (I know for sure that SkyWatcher and Orion do this) you’ll need to remove it.
You will also need at least a sighting cap, and preferably a sighting tube with cross-hairs.
The cap is easy enough - I used one off the focuser, simply make a small hole exactly in the centre, and paint the inside silver or white so that it's easy to see reflected in the primary mirror.
I made my own sight-tube out of an old barlow that came with a telescope I've had for years - the barlow was very poor quality and I had NEVER had an acceptable image through it, so I just removed the lenses, and put a cross-hair on the end of it. To make the cross-hairs I printed out a circle the right diameter on a piece of paper, and marked the 4 points around it. Then put the tube on the paper and marked the tube with a fine marker pen, I then cut notches with a fine saw, and stretched black cotton thread across, then stuck it in place with clear sticky-tape.
You can see the cap, and just about make out the cross-hairs in this pic:-
If you don’t want to make your own cap or tube (or you don’t have the bits and pieces lying around) most dealers sell caps, sight-tubes, and ‘Cheshire’ eyepieces (a purpose made tool, similar to a sight-tube)
The main mirror should also have a marker in the centre – usually this is a small ring, a bit like those hole-strengthener rings you get for ring-binders – if the mirror doesn’t have a centre marker, you can make one – you guessed it – by using a hole-strengthener ring.
You’ll need to ‘CAREFULLY’ remove the mirror to do this – measure across the mirror in two directions, at 90 degrees to each other, and make a tiny cross in the centre with a fine marker pen – Don’t worry about marking the mirror in the centre inch or so, this part of the mirror is shadowed by the secondary, and isn’t used when the telescope is operating. Then simply (BUT CAREFULLY) stick a ring EXACTLY in the middle of the mirror.
OK - down to business.
I'll assume that the focuser is square, and firmly fixed on the outside of the tube, and cut straight to the optical alignment, and it is usually easier, if you point the telescope out of the window to let light into the tube so you can see what you're doing.
You should see 3 holes in the end of the secondary, arranged around another screw in the centre. There should be a small screw or allen-bolt in each hole. You may have been supplied with the necessary allen-wrench to fit them.
When I did my first full collimation on my Skywatcher 150, I loosened those allen bolts a fair bit. Then used the central screw to get the secondary at the right position in & out of the tube (left & right through the focuser) I then held it as near to correct as I could by hand, while tightening the 3 allen bolts up to the point where they just touch the secondary's mount, holding it in position, but allowing me to rotate it. I rotated it to the correct alignment, so the primary's centre marker (a ring) appeared central, on the cross-hairs of my sight tube, as seen through the focuser. The spider vanes may also need adjustment, by using the bolt-heads around the outside, in order to get the secondary to the right position up & down as seen through the focuser.
I then used the 3 allen bolts to 'fine-tune' the secondary while looking through my home-made cheshire.
Basically, you should be able to see the secondary is concentric inside your view through the focuser. The 3 clips around the edge of the primary mirror should appear even around the inner edge of the secondary, and the central marker ring of the primary should be central within the secondary (if you have a Cheshire, or tube with cross-hairs, it is a big help at this point) Ignore the reflection of the focuser that you can see in the primary, until you get the secondary mirror right.
Last of all, when the secondary is correct, I put my sighting cap on the focuser, without the sight-tube, and set about lining up the primary. Most people point the telescope out the window to let light into the tube - but I have my own method, I have a torch (flashlight) that fits exactly into the smaller cap that is on the main cap
This pic shows my scope with the sight tube inserted, and the torch wedged into the cap:-
The torch lights up the centre ring on the mirror, and makes it easy to line up the reflection of the small hole in the sighting cap, precisely central within the marker ring. Pointing the scope out the window (careful not to point it at the Sun) has the same effect – the torch allows me to do it when it isn’t daylight.
To align the primary, first you need to remove the cover from the rear-end of the tube (if it has one)
Down here, you'll see the actual collimating screws, almost all scopes will have screws (or bolts) that adjust collimation, paired with a locking nut (or screw)
You should see three pairs, positioned equally around the base of the scope
First undo the lock-nuts a bit, then turn the alignment screw (or bolt) while looking through the sighting cap, you will be able to see the reflection of the cap's central hole moving as you turn the screws. From this, it's easy to work out which screw (or screws) to turn, and which way to turn them, in order to get the reflection of the cap's centre hole aligned exactly in the centre of the primary mirror's marker ring. Try to leave one of the screws alone, and only use the other two to align the mirror - this ensures that over time, after subsequent collimations, that the mirror doesn't gradually 'wander' up or down the tube, and reach the end of the travel on the collimating screws.
When you get the sight-hole lined up exactly central in the marker ring, it's simply a matter of tightening the lock-nuts - but be careful while doing this, because they move the collimation very slightly when they tighten up, so try to get them all to a similar tightness, and keep watching the collimation as you do so.
Above all DO NOT over-tighten them, this may cause the mirror to warp very slightly, causing an optical aberration
Last of all do a star test to check the precision, slightly de-focus a bright star at about 100x - 150x, you should see faint diffraction rings around the star, if they are concentric, the collimation is good, if they are uneven, further minor tweaking of the primary may be needed to get it exact.
You will only have to do the full collimation sequence every once in a while. Once you get the secondary set up right, and tightened, it takes a fair old clout to knock it out of alignment - and all that is necessary after that is occasional slight tweaking of the primary.
Hope some of that may have helped
carlos
also check out this great instructional video:-
www.andysshotglass.com/Collimating.html
Laser collimation is much more convenient, AND can be done on-site, in the dark - but not as accurate as doing it by sight.
As I said further up - I'll add a bit on using a laser collimator soon.
And also, how to check and tweak by doing a star test to get the laser collimation as accurate as the daylight 'sight' method.