Collimation confusion

[leinad]

After receiving my collimating laser tool today I placed it in the focuser to see how far off the collimation was.

The laser reflecting back off the pri. mirror was clearly missing the sec. mirror and hitting the wall.

Holding the sec. mirror spider and twisting with my hand clockwise sowed where I needed to adjust it, to be closer to the ring circle on the pri. mirror.
However Im a little confused about whether the sec.mirror can be rotated clockwise or not.
After looking at the construction diagram here:
http://www.fpi-protostar.com/ftp/instman16.pdf

it appears that the 3 collimation screws/bolts on the sec. mirror do not allow the unit to be rotated without the whole assembly being rotated also. As this cant be done without the entire holder being rotated, am I wrong in thinking that the mirror cannot be rotated slightly clockwise ?

Tommorow night, Im going to attempt adjusting the sec. mirror to properly collimate the scope. A little hesitant in adjusting the sec. mirror so I thought Id check first before applying any kind of pressure on twisting the sec. mirror clockwise.

I believe tilting wont solve the collimation but only twisting the mirror slightly clockwise will allow the reflection of the focuser to appear centered in the reflection of the sec. mirror when looking down the focuser tube.

Maybe I worry too much .:doh:

As you can see in the pic(ignoring the arrows) all is not exactly centered.

[skwinty]

Hi Leinad
Loosen all three screws and the diagonal will rotate.
You can then adjust (tighten) the three screws to align an hold the diagonal firmly.
Regards
Steve:)

[leinad]

Must they loosened all the way so that they actually can be pulled out?

[mrsnipey]

Definitely not. :eyepop: You only want to loosen them.
If you take them out and you're not holding onto the
secondary, it could fall onto your primary. :scared:

My secondary tilts (a bit) and twists once I've loosened the
bolts. The hard part is getting it to stay in the same (correct)
position as you tighten the screws back up.

[rmcpb]

Some steps for you:

Loosen the three secondary screws enough to allow the secondary to pivot on the central bolt.

Turn it till the image of the secondary seen through the focuser is circular.

This circular image should be centred in the focuser by adjusting the central bolt up or down till it has an even margin all the way around the secondary. It may help to put some paper in the tube between the secondary and primary to stop the image of the primary confusing you at this step.

Adjust the three secondary screws till you can see the primary centred, if you use your laser it should hit the primary right in the middle.

Check the shape of the secondary image again and adjust if necessary. This will mean adjusting the secondary tilt.

Repeat this till the image of the secondary is centred in the focuser (this is where a Cheshire eyepiece is really handy), the image is round and the beam of your laser hits the primary in the centre.

Adjust the primary to get the return beam from the laser to retrace itself back to the laser hole.



It takes practice and time. This is not the "best" way to collimate but it is a definite step up from the hole in the film canister trick.

Cheers

[Glenhuon]

If you want to rotate the secondary a bit, slacken off the centre screw a very small amount, just enough to let you turn the secondary holder, but not so its loose. Best not to touch the other 3 until you have it centred. A cheshire is best for it but you can substitute a black camera film case with a pin hole in the centre of the base as a peep hole. Adjust it by small amounts until the secondary mirror looks round looking through the peep hole and gently tighten the centre screw.
If you find the secondary is too far up or down the tube slacken the centre screw by 1/4 turn (Max), adjust the other 3 equally in to go down tube or out to come up tube, gently tighten the centre screw and check through the peep hole.
Put the laser colimator back in and adjust the other 3 screws by tiny amounts in or out to swing the lazer beam on to the centre spot on the primary mirror. You'l find the adjustments are very small for quite a large movement of the beam on the primary.
When that is done you can begin adjusting the Primary colimating screws to get the return beam into the correct spot in the colimator.
Its a bit fiddly, but once you have done it a couple of times it doesn't seem so bad and secondary adjustment is usually just a small tweak after it is set up initially.

When that is done you can begin adjusting the Primary colimating screws to get the return beam into the correct spot in the colimator.

You might find the beam has moved off the centre spot a fraction after primary adjustment, tweak it back on and adjust the primary again to get the return beam back on target.

Thats my method and it seems to work pretty well.
I've found that it seldom moves on my 150 newt and the only adjustments required now are an occasional tweak of the primary.

Cheers
Bill

[Kokatha man]

[quote=rmcpb;303963]Some steps for you:

Loosen the three secondary screws enough to allow the secondary to pivot on the central bolt.

As squinty, mr snipey and rmcpb (above quote) say; just loosen the 3 secondary screws very slightly - this takes their "holding pressure" off of the secondary holder and allows the secondary to be rotated.Sorry Bill, but you don't turn the centre bolt/screw to rotate the sec mirror: this seems to be an ongoing confusion for a number of people re its' functions. Whilst your caution to leinad about only "loosening" this centre bolt a bit to turn the sec does work, you are actually interfering with the sec's alignment under the focusser by doing so, and it is not the proper procedure.

This centre bolt/screw does not, in the ordinary sense of the word, "tighten" or "loosen" anything - unless you take it to the extremes of its travel either way - it is a spring tensioned "toggling" device that relies on the pressures of the 3 setscrews to "set" its tilt and to hold it in said position.

Its other function is to centre the secondary mirror (and its holder) under and concentric within the focusser tube view/position. If this is required you will need to unscrew those aformentioned 3 screws - but only if you need to "pull" the sec mirror and holder "up" the scope away from the primary mirror to centre it under the focusser.

To accomplish this (if necessary) you screw the centre bolt/screw clockwise: if you needed to move the sec mirror etc towards the primary you would turn the centre bolt anticlockwise and its spring tension would move the sec towards the primary. This of course would not impinge upon the 3 screws so they would not need slackening off initially.

If you need to perform this operation, as said, you would still need to pinch up the 3 screws to set and hold the sec's tilt afterwards.

Cheers, Darryl.

[leinad]

Thanks again Darryl.
So if Ive got this right, only the 3 screws need be loosened to rotate the mirror, then are to be tightened and then final adjustments are to alter the tilt of the mirror.

The central bolt will only loosen/tighten the mirror in order to either move it towards or away from the pri.mirror.

[Kokatha man]

Hi leinad - as rmcpb says in the above.

Your: "The central bolt will only loosen/tighten the mirror in order to either move it towards or away from the pri.mirror." is correct but as I pedantically keep saying' you should substitute "loosen/tighten" in your statement with "shift." Because this central bolt is a spring-loaded job, if you keep "tightening it" as in screwing it clockwise its spring will eventually become squished up on itself and no more turning will be possible - conversely, if you keep on "loosening it" as in turning it anti-clockwise the sec mirror and holder will fall off the spider with (possibly) disastrous consequences.

As rmcpb says, turning the central bolt/screw is for getting the mirror "centred" under the focusser tube: this means that when you look at it down the focusser tube the oval sec mirror will "appear" as if it is circular (round) and there will be a uniform gap all around it relative to the focusser tube's bottom end (ie appearing like a circle within the focusser tube's circle)

As rmcpb also says, neither a pinhole in the cap of a film canister nor a laser collimator are the proper tools for this procedure, and I suggest he is endorsing the "sight-tube/chesire" as a fitting device for this procedure just as I do.

Regards, Darryl.

[Glenhuon]

Centre bolt is not spring loaded on mine, it works against the other 3. Slackening it requires the 3 tilting screws to be screwed in to compensate and vice versa for tightening. But other scopes may be different, I've only ever had this one newt.

[Kokatha man]

Hi Bill - I do apologise to you: there are most certainly sec mirror holders that do not use the spring system - in fact the first 2 scopes I made (a 6" & 8" newt) did not employ them; though I should say I constructed the whole sec assembly myself.

Notwithstanding the above, I'm surprised that many/any nowadays don't use the spring tension system as it adds a degree of stability not inherent in the other set-up. Of course, I should say to you also leinad that you'd best check which system you have before accepting my procedures' description - but'd be surprised if it isn't spring-loaded!

Regards, Darryl.

[leinad]

Firstly, Thanks all for your input. :thumbsup:

Collimated the scope today and I must say it was waay off the previous few weekends viewing.
At low magnification this wasn't really noticeable, but I can see now that at higher magnification the view was blurry and now I think it would have been a lot better(clearer at high mag.) if the scope was properly collimated.

By loosening the secondary mirrors 3 screws I was able to rotate /adjust the mirror so that through th focuser I could see the primary mirrors three clips. Then by using my Skywatcher collimating piece I was able to fine tune this by slowly tensioning each screw.

then using the laser collimator I fine tuned the screws again to line the laser with the ring on pri mirror, then adjusted the pri mirror.

Notes on collimation process with laser and skywatcher collimator tool.

1) The laser had quite a bit of wobble in the focuser, and after wobbling rotating the laser in the focuser I was able to ensure that the laser dot was as close to center rotating around the pri. mirror ring circle.

2) the Skwatcher collimation tool was much better in viewing the collimation alignment of mirrors. bit of a disappointment with the wobble of the laser in the focuser as it could never be perfectly held in the focuser.

3) Tried the barlow laser trick. am i supposed to check this reflection by looking at the pri mirror(this is where I could see th reflection.
the focuser tube in the scope didnt allow me to see the barlow end with the paper circle attached to the bottom.

I believe I've got the collimation as good as I can get, but I did notice that in order to see all 3 clips of pri. mirror in the focuser that when the collimation tool was in the focuser and looking through the pin hole, then reflection of the tool/diagonal looked offset slightly towards the open end of the scope.

Is this acceptable as long as the collimation looks all good?
Do Skywatcher 8" dobs have a slight offset like this?

Hoping it will be clear tonight to test the scope again, and see how much clearly Saturn is at higher magnification as well as other open/globular clusters. :D

see pic below of offset

[RAJAH235]

Yep, that looks a whole lot better than previous image.
Still just a tad off with the primary.
There's one thing that no-one mentioned. That was to check that the laser collimator beam is centred & not skewed to one side.
Easy test bench is a lump of 4" x 3" bout 8" long with 2 x 3" nails at 45 degs/crossing each other at one end & another 2 x 3" nails at 45 degs/crossing each other at the opposite end. Length between nails < the laser length.
Photo/s on site somewhere.
Edit] Found pic...
> http://www.iceinspace.com.au/forum/attachment.php?attachmentid=5713&d=1130763353
Sit the laser in the "V's" so formed & rotate the laser while watching the beam against the wall. If it moves in a circle, it needs adjusting so it don't. :D

A star test will probably be all that is req'd to do a final adjustment to the collimation.

HTH.
Regs, L..

ps. Just use the Cheshire. Better unit all round.

pps. A piece of white paper taped or bluetacked to the inside of the tube, opposite the focuser will enable you to 'see the 2ndary' better.
All 'rough' collimation is best done in daylight. (In the house is good). Not night time.

[Glenhuon]

Quote
1) The laser had quite a bit of wobble in the focuser, and after wobbling rotating the laser in the focuser I was able to ensure that the laser dot was as close to center rotating around the pri. mirror ring circle.

I am assuming you have a Rack and Pinion focuser. I could never get really good collimation with my 150 newt because of the slop in mine, even after shimming the inside of the focuser body.
Just fitted a Crayford and the difference is wonderful, no movement of the laser spot on the primary from full in to full out on the draw tube.

Bill

[leinad]

I have the Crayford also on my 8" Skywatcher Dobsonian.
The laser collimator is a GSO from Andrews Comm.

There is absolutely a bit of wobble when its in the focuser, bit of a disappointment, but I can gauge where the center laser point should be by rotating the laser in the focuser.

[skwinty]

I also bought a GSO laser collimator.
Very disappointed. The collimator is not collimatted or focussed properly.
Waste of money. Get better results from the cheshire.
Regards
Steve:(

[Kokatha man]

Hi leinad and skwinty - I dunno 'bout dem laser thingies: wuz waitin fer one that never came, so's I bought the Orion sight tube/chesire combo tool and nedder looked back......stills, I would'n mind tryin' one jist to see if'n they's as good as some reckons and as bad as t'others says...

[leinad]

With regards to the pic in previous post, the offset in the reflection has been driving me mad, do other with skywatcher 8" dob owners also have this , or can this be fixed by moving the mirror by adjusting the central bolt, or adjusting tilt.
If I rotate the mirror to correct it, it throws the alignment of seeing the three pri. mirror clips when collimating..

Reading the following, seems that this is nothing to worry about, but the reflection of the focuser upwards, whereas my offset id to the left. Some assurance that others have this would put my mind at rest.

http://www.cameraconcepts.com/Collimating%20Newtonian%20Optics.ht m

:|

[leinad]

*bump*

[RAJAH235]

leinad,
The last diagram in your link shows how the final collimation should be.
If you can achieve that, then all is good.
Can do a star test to tweak it, if req'd.
Offset > http://www.skyandtelescope.com/howto/diy/3306996.html
> http://www.skyandtelescope.com/howto/diy/3306876.html
Oldies but goodies.

[Kokatha man]

Hi leinad - yes, it does become extremely confusing when one tries to fathom the absolute depths of collimation!

All the links suggested (including your own) are good to digest - but it can also cause added confusion which is often semantics as much as specifics. I've read (and heard) some heated arguement on these matters over the years: with people taking up positions that ultimately betray their own lack of understanding of certain aspects.

Which is to say pointedly that I'm no expert and many things still have me scratching my noodle: but I reckon that if you can get your scope to perform well with no (or very little of the) nasties like coma etc, and star-testing gives good diffraction ring images you are fairing way better than 90 odd percent of AA's!

Just a couple of things I've gleaned by re-reading the links provided in this thread and bouncing them off my own thoughts/experiences:

Bartels (imho) seems to be giving the methodology for "full" offset collimation whereas Carlin is opting for "partial" offset.

There "final View" diagrams are different, Bartels one being what you seem to be getting frustrated about you not achieving; whereas Carlin's equates with your attached image - figure where you stand there!

I repeat, these are non-expert opinions of mine but I've been led to believe that partial offset is preferable to full offset - the reason I thought was something to do with the amount of impedence the secondary creates for the primary (my presumption!)

In Bartels article there is the confusion of him stating that his "controversial advice to many amateurs" is not to square the focusser with the tube (ie mechanical collimation) giving one of his feasons as the difficulty of adjusting the focusser alignment - but in his "how-to" launched into shimming the focusser!

It would seem axiomatic to me, that Bartels' method of mechanically adjusting those 2 spider arms he mentions in his article, creates the full offset of the secondary and (possibly) creates the situation in his final collimation diagram wherein the reflection of the focusser tube is moved "up" towards the reflection of the secondary mirror (the thing you're getting frustrated about.)

Conversely, in Carlin's article (the first link Rajah's given you) he describes the partial offset methodology - in particular re-read paragraphs 4 & 5. This I presume produces the scenario in his "final collimation" diagram, which is similar to your own!

Finally, do what Rajah suggests and just get out and scope it out, if you're getting coma "comets" you'll know it and dong a star-test on a clear still night will give you more info.

I hope this hasn't been too long-winded: your problems are, I assure you, the same for many of us no matter how long we have been "at it" - it's just that most won't admit that collimation confuses them, and near-enough is good-enough.

Hopefully none of the above has made you more confused: I think it has helped me by articulating it - as I said, rajah's last bit is the most important

[ngcles]

Hi Leinad, Daniel & All,

Daniel wrote:

"yes, it does become extremely confusing when one tries to fathom the absolute depths of collimation!"

Too true, too true ...

If you want to observe the degree of angst the subject of collimation method (laser/cheshire etc etc) and offset (partial/full), why not join the Bigdob group of the Yahoo groups, pose a question on partial/full offset ... and stand well back -- ithe effect is akin to stirring a hornets nest with a large stick.

Both the partial and full offset camps have their devotees and for what it is worth, I don't think either of them are wrong per se -- they are just different methods for producing collimated optics with a centred and circular 100% field.

I have a different opinion. It is just my opinion and not an "answer".

Here it is: I don't do either. (Cries of heretic echo around).

Why?

Why collimate?

The purpose of collimation is to align the optical axis of the telescope's optics, with the optical axis of the eyepiece. If you don't do that, image quality will suffer.



Why offset?


To borrow a little from Nils site here (so you can read in full):

http://web.telia.com/~u41105032/kolli/kolli.html#Err2

But the salient part is reproduced here:

The secondary mirror has an elliptical surface with a major to minor axis ratio equal to the square root of 2, for 90 degree deflection. Depending on its size, it lets some of the focal plane be fully illuminated, that is any point within the area of full illumination sees the whole primary mirror reflected in the secondary. Outside of this, some light is lost.
Due to the 45 degree tilt, the elliptic surface appears circular when you see it with your eye centered on the optical axis near the focus. However, due to the perspective, the center of the circle you see is offset from the geometric center of the ellipse, towards the edge nearest to the focuser. To be optically centered, the secondary mirror must be offset both in the direction away from the focuser and towards the primary mirror. The offset in each direction can be calculated with very complex formulae, but the formula offset=minor axis/(4*focal ratio) is accurate enough for practical purposes (it is exact if just the center is fully illuminated - with a larger fully illuminated field, the error is insignificant anyway). The distance along the mirror face from the center of the ellipse to the optical center is the offset multiplied by 1.414 (the square root of 2).

And

An error of type 2 causes the fully illuminated field to be offset relative to focus, and will cause an uneven light loss near the edge of the low power field. For wide-field photo, the secondary mirror should be large enough to let the whole film frame be fully illuminated, but for visual use, a secondary size of no more than 20-25 % of the primary mirror diameter is commonly preferred, in order to minimize unwanted diffraction effects. This means there is usually some light loss by the edge of the field, but at least the focus should always be fully illuminated - the tolerance should not be larger than the radius of the fully illuminated field. At least for short focus instruments, light loss is very gradual outside the fully illuminated field, and an offset of a few millimeters should have little effect visually. Sufficient accuracy is easily achieved with suitable tools.

So, we offset the secondary in order to circularise and perfectly centre the 100% illuminated within field within the eyepiece field. If you don't offset, you get a 100% field that is ever so slightly offset from the centre of the eyepiece FOV and is also slightly elliptical. In a visual only newtonian, is this so very, very objectionable?

Personally, I don't think so. I think it hardly matters in a visual use newtonian that has a good sized secondary because you will never notice its practical effect. As an example, my old 31cm f/5.3 Newtonian with a 56mm secondary has a 100%illuminated field 24 arc-mins diameter and a 75% field 69 arc-mins diameter. With my Plossls, the largest low power field was 57 arc-mins diameter whcih was about 84% illuminated at the very edge. Could I (visually) see a difference in brightness of a star from the centre to the edge?

Nope.

In practice a 20% drop-off at the edge can't be visually noticed. If it can't be noticed visually, what is the point in making it circular? None that I can see. If you (anyone) can, let me know please!

The alternative then is to do away with offsets and centre everything -- which is what I do. The stars are just a sharp, the 'scope is just as well collimated it is just that the 100% field is slightly oval.

Centration has the decided advantage of simplicity and ease of collimation. I've been through the whole offset deal and find it offers no practical advantage for a visual use newtonain with a good-sized secondary.

Things change however for photographic/imaging applications -- particularly wide field imaging where any vignetting toward the edges of the frame will not be centred within the frame. Then, by all means offset ...

Use either method (partial/full) and I think that for most practical purposes they will produce a similar result.

Only my view for what it is worth.

Best,

Les D
Contributing Editor
AS&T

[ngcles]

Hi All,

Oh, I forgot. laser -v- cheshire/sight tube?

Both have advantages and disadvantages. I purchased a laser about 6 years ago when they first became very popular. I liked it and it produced perfectly good results. A friend of mine on his 20" truss dob uses a barlowed laser and it too works fine and is a pretty simple method.

However, about 12 months ago I went back to a cheshire/sight tube. The laser I had was a pain in the neck to collimate itself and had a habit of falling very slightly out of collimation itself when I when through the operation of changing its batteries. Also the batteries unless very fresh were unable to produce an easily seen (bright enough) dot for daytime setup/collimation. One time I found the batteries were dead out in the field at night and it was totally useless (so I collimated on a star image)

The old-fashioned cheshire never needs batteries, unless seriously abused does not fall out of collimation itself and can be used day or night (provided you have a weak light source at night) and makes errors in secondary colimation more easily seen and fixed.

So, that is my own personal preferrence -- my 2c worth.

Best,

Les D

[leinad]

Thanks heaps guys. :)

Whilst I'm not entirely up to speed with the technicalities etc, I think for the meantime I can live with this until I learn a bit more.

Viewing seems fine at the moment, and until I get more hours under my belt Im pretty satisfied with the collimating so far and its results.

I guess as they say you can only learn from experience, unfortunately it often comes the hard way.

I thank you all again for going out of your way to assist a beginner and sharing your knowledge.

:thumbsup:

[ngcles]

Hi Leinad,

Good on you -- enjoy the 'scope which seems to be producing good images and don't obsess over it till later (much) on. Observing (particularly for the beginner) should be a joy and not filled with worrying over whether the 'scope is adjusted so perfectly that you spend no time actuallly enjoying it. In fact that is one of the reasons I like to keep my collmation aims as simple as possible.

Best,

Les D