Collimation issues......

[Ande]

Well, it was short and sweet, but it was nice knowing Dobbie.  Alas, I decided to try my hand at collimation, and subsequently killed it

I've been following Astrobaby's guide, and am about as confused as I could possibly be.  Peered through the collimation cap, as suggested, and managed to convince myself that the secondary mirror was elliptical.  Was it?  I can't say for sure.  The mind plays funny games when being overly critical.  Anyhow, the screwdriver, and allen key came out, and I proceeded to adjust it.  I placed a piece of white card to block out the primary mirror, and quickly discovered that the inside of a dob scratches so easily, even with cardboard.  Anyhow, that's no biggie, just cosmetic.  Got the secondary looking perfectly round, and central and proceeded to the next stage - bringing the retaining clips of the primary mirror into shot........  Trouble is, every time I attempt this, the mirror goes elliptical, and off-centre again.  So I just seem to be going round in circles now.  And to add to my confusion even further, is this image towards the end of Astrobaby's guide:

 

 

"This is an actual photograph taken through a Cheshire and shows a near perfect collimation pattern for a fast (f5) Newtonian. It is in fact my own Sky-Watcher 200P

Note that the centremost circle is made up of the primary mirrors centre ring and its associated reflection. 
The cross hairs exactly intersect the centre circle.

Note also the offset typical of a fast Newtonian."

 

In that image, the huge, offset reflection of the focussing tube looks worse than what I started with, but is, apparently, near perfect collimation.  One circle inside another when offset, gives a bloody good optical illusion of being elliptical.  My head is spinning, lol.

[wookie1965]

Take your time and do each step one at a time, it is like riding a bike once you have done it  it gets easier. 

I used Astrobaby`s and this one you may find it useful.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

[Trikeflyer]

Have a cup of tea and relax a minute. If you have the secondary mirror as round central to the focuser as it can be, the three adjusting screws in front of the secondary should now be slightly adjusted to get the primary clip holders into view. This should not really alter the shape of the secondary too much as it appears in the focusing tube and should not take too much adjusting. If it does, maybe the secondary retaining centre screw needs a bit of loosening or tightening but don't do that until you are sure it needs it. Have a go, send a pic if you need more help. 

Steve 

[Ande]

33 minutes ago, wookie1965 said:

Take your time and do each step one at a time, it is like riding a bike once you have done it  it gets easier. 

I used Astrobaby`s and this one you may find it useful.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

Thanks for the link.  That looks a lot less confusing than the Astrobaby guide.  I shall start from scratch, and operate somewhere between the two.

[Ande]

59 minutes ago, Trikeflyer said:

Have a cup of tea and relax a minute. If you have the secondary mirror as round central to the focuser as it can be, the three adjusting screws in front of the secondary should now be slightly adjusted to get the primary clip holders into view. This should not really alter the shape of the secondary too much as it appears in the focusing tube and should not take too much adjusting. If it does, maybe the secondary retaining centre screw needs a bit of loosening or tightening but don't do that until you are sure it needs it. Have a go, send a pic if you need more help. 

Steve 

Much obliged, but I think I need something a bit stronger than tea, lol.  I'm going to have another crack following your helpful advice.  If it all goes south then I'll post some pics.  Probably of me stood over a white, crumpled tube, lump hammer in hand

[Chefgage]

When i first did mine i used a combination of the astrobaby method and the link posted above.  It seemed an overly complicated thing to do this collimation magic, but after taking my time with each step it went ok.  I am sure i saw a definate inprovement after collimating but of cause that could have just been the seeing conditions

It was the adjustment of the secondary that proved most difficult. When having the secondary where i wanted it and then tightning up it would move.  I had to compensate for this by haveing the secondary slightly out of place so that when it was tightened it moved to where i wanted it.

 

[Ande]

Okay, thanks both.  That other guide was a big help.  I think I've got it collimated now.  Although the proof will very much be in the pudding.  Guess I need a star to pop it's head above the parapet, so I can test things.  Fingers crossed

[Ande]

4 minutes ago, Chefgage said:

When i first did mine i used a combination of the astrobaby method and the link posted above.  It seemed an overly complicated thing to do this collimation magic, but after taking my time with each step it went ok.  I am sure i saw a definate inprovement after collimating but of cause that could have just been the seeing conditions

It was the adjustment of the secondary that proved most difficult. When having the secondary where i wanted it and then tightning up it would move.  I had to compensate for this by haveing the secondary slightly out of place so that when it was tightened it moved to where i wanted it.

 

Yes, I was having similar issues.  The primary mirror was an absolute joy to adjust, in comparison

[Chefgage]

You have got me thinking now.  Its been a awile since i have checked to see if mine is still collimated.  It never gets bumped/knocked so should be ok but i will make a point of checking this week. The one thing that never works for me is the star test.  I just never see the rings or possibly i am and i expect to see them more clearer??

[Trikeflyer]

16 minutes ago, Chefgage said:

You have got me thinking now.  Its been a awile since i have checked to see if mine is still collimated.  It never gets bumped/knocked so should be ok but i will make a point of checking this week. The one thing that never works for me is the star test.  I just never see the rings or possibly i am and i expect to see them more clearer??

You need really high mag to see the rings and good seeing is a help! If your stars are clear and pretty round you are collimated! 

[25585]

When I have unboxed the new 12 F5, mirror check & collimation will be #1 task. 

Always use Cheshires so get there eventually. I usually over adjust, then go back slowly. But a laser is tempting, see how it goes. 

[xvariablestarx]

I use a laser, since I don't own a Cheshire, It seems to work well enough. Collimation can be intimidating, but once you get it right the results are worth it.

[Trikeflyer]

I have both and they both work well. Laser is quicker and easier in the field.  The thing about the laser is that it too must be collimated fairly well and if it is, then all good, if not, could be a nightmare. Moral, if you buy a cheap laser collimator and it's well collimated, you are a lucky astronomer. Even some expensive lasers might not be collimated! 

Steve 

[25585]

14 minutes ago, Trikeflyer said:

I have both and they both work well. Laser is quicker and easier in the field.  The thing about the laser is that it too must be collimated fairly well and if it is, then all good, if not, could be a nightmare. Moral, if you buy a cheap laser collimator and it's well collimated, you are a lucky astronomer. Even some expensive lasers might not be collimated! 

Steve 

Howie Glatter any good? I like the Tublug idea whatever laser used. My Newts are solid tube. 

What are the best "perfect" star eye piece types & magnifications to use?

[wookie1965]

5 hours ago, Chefgage said:

When i first did mine i used a combination of the astrobaby method and the link posted above.  It seemed an overly complicated thing to do this collimation magic, but after taking my time with each step it went ok.  I am sure i saw a definate inprovement after collimating but of cause that could have just been the seeing conditions

It was the adjustment of the secondary that proved most difficult. When having the secondary where i wanted it and then tightning up it would move.  I had to compensate for this by haveing the secondary slightly out of place so that when it was tightened it moved to where i wanted it.

 

My 8" Skywatcher kept slipping back from where I had set it found out the collimation screws had put marks in the secondary holder so I used the astronomy shed secondary mod. 

 

https://m.youtube.com/watch?v=MUE8fxqH3Qc

 

 

 

[xvariablestarx]

2 hours ago, Trikeflyer said:

I have both and they both work well. Laser is quicker and easier in the field.  The thing about the laser is that it too must be collimated fairly well and if it is, then all good, if not, could be a nightmare. Moral, if you buy a cheap laser collimator and it's well collimated, you are a lucky astronomer. Even some expensive lasers might not be collimated! 

Steve 

How do you tell if your laser collimator is collimated correctly?

[Chefgage]

10 minutes ago, xvariablestarx said:

How do you tell if your laser collimator is collimated correctly?

With another laser collimator of course

 

[wookie1965]

If you use a V block spin the laser and check where the red spot traces if it is not in the same spot it needs collimating. I have had two both needed to be collimated. 

I use a Cheshire and even at night you can just use a red light torch shine it on the reflected surface and look through it adjust accordingly. 

[RT65CB-SWL]

13 hours ago, xvariablestarx said:

How do you tell if your laser collimator is collimated correctly?

Make a 'V' block. Dion on Astronomy Shed makes one form 3/4" plastic pipe and explains how to use it. You can also make one from a block of wood and 4x long nails too.

[billyharris72]

Here's another way to look at it. The secondary mirror is optically neutral - all it is there for is to divert the image into the focuser. The truth is that minor misalignment of the secondary won't have a meaningful effect unless you are dealing with fast (considerably faster than f5) optics.

If the secondary is not pointing down the focuser (so it appears more or less circular) then the sweet spot of the image could be slightly misaligned with the centre of the eyepiece. Not good but not the end of the world. Get it as circular as you can (if you're in doubt, it's fine).

If you are not able to see all the mirror clips you might lose a small portion of the light from your primary - it is worth spending time to get this right, but you do have a bit of leeway. 

Basically, get it as good as you reasonably can (it is fiddly so make a cuppa, get comfortable and block out a couple of hours) and then leave it alone. The bit to concentrate on is the primary. That is important, and worth doing every time, but much less fiddly.

Billy.

[25585]

Do all these collimators & techniques rely on a central spot on the primary? Could be worth checking the spot is at dead centre if so. 

[Tim]

On 17/04/2018 at 17:48, 25585 said:

Do all these collimators & techniques rely on a central spot on the primary? Could be worth checking the spot is at dead centre if so. 

Yes they do.

At the moment I only have a couple of dobs that need collimating, but have owned and used several fast Newtonians over the years.

Without hesitation I can recommend the Cats Eye tools by Jim Fly. They are super accurate, far more so than a standard chesire, due to the unique design. They are expensive, yes, however they are a one time purchase.

The problem with using a laser, even a perfectly collimated one, and even assuming your focuser will hold the laser perfectly accurately in place, is that it is possible to introduce rotational errors in the secondary, which are then sometimes incorrectly adjusted for, and the error can increase incrementally each time, leading to all sorts of weird and wonderful light paths.

For visual use only, probably the Cats eye gear is a bit too good, the eyes wont appreciate the level of accuracy. But pop a camera in there, or just be slightly OCD about collimation, and they are just the thing. Used in conjunction with Vic Menard's book you have a winning combination.

https://www.firstlightoptics.com/catseye-collimation-tools.html

https://www.firstlightoptics.com/books/new-perspectives-on-newtonian-collimation-vic-menard.html

[Buzzard75]

11 hours ago, Tim said:

...

The problem with using a laser, even a perfectly collimated one, and even assuming your focuser will hold the laser perfectly accurately in place, is that it is possible to introduce rotational errors in the secondary, which are then sometimes incorrectly adjusted for, and the error can increase incrementally each time, leading to all sorts of weird and wonderful light paths.

...

This is the reason I always recommend the use of either a collimation cap or a cheshire in addition to a laser. A collimation cap or a cheshire is the only way to ensure there are no (minimal) rotational errors in your secondary/focuser alignment. I always start with a collimation cap or a cheshire to check the secondary and focuser alignment, which rarely needs adjusting once it's set, and then use the laser to align everything else. After that I follow-up with the collimation cap or cheshire again just to verify everything is aligned. I only do visual observing with my dob so my method may be a bit overkill, but it works for me and I have piece of mind knowing it's right.

[Ally8446]

6 minutes ago, Buzzard75 said:

This is the reason I always recommend the use of either a collimation cap or a cheshire in addition to a laser. A collimation cap or a cheshire is the only way to ensure there are no (minimal) rotational errors in your secondary/focuser alignment. I always start with a collimation cap or a cheshire to check the secondary and focuser alignment, which rarely needs adjusting once it's set, and then use the laser to align everything else. After that I follow-up with the collimation cap or cheshire again just to verify everything is aligned. I only do visual observing with my dob so my method may be a bit overkill, but it works for me and I have piece of mind knowing it's right.

Definitely a +1 for your method @Buzzard75, that's exactly how I carry out my collimation. But then I enjoy collimating.........am I ill on some level ? 

[Geoff Lister]

I get most enjoyment in checking collimation, and finding it spot-on.

Geoff