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 .
As you can see in the pic(ignoring the arrows) all is not exactly centered.
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
Definitely not. 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.
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.
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.
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.
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.
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.
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
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.
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.
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!
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.
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/a...3&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.
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.
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.
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.
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
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...
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.
You only want to loosen them.
