Article: Passive Tool Collimation and the Newtonian

[iceman]

Hi all

Don Pensack has written a fantastic article titled Collimation and the Newtonian. It's a must-read for every newtonian owner.

You can read the article on the IceInSpace Projects and Articles (http://www.iceinspace.com.au/index.php?projects) page, or directly by clicking the link below:

Collimation and the Newtonian (http://www.iceinspace.com.au/index.php?id=63,390,0,0,1,0)

Thanks to Don for contributing the article to IceInSpace. If you'd like to submit a review, article or other content to IceInSpace, please contact me (http://www.iceinspace.com.au/index.php?contact).

[iceman]

Article uploaded.

[davewaldo]

mmm a good read, I look forward to giving it a try when I am reassembling my Dob this week.

Thanks Don!

[davidpretorius]

thanks mike and don!

[ausastronomer]

Thats a very good article.

It covers all the important BASIC issues very well. It does neglect one step which I consider important. Particularly, if you use the scope for long exposure photography, or require "PINPOINT" pointing accuracy from your digital setting circles. This involves making the focuser axis coincident with the mechanical axis of the telescope, prior to centering the secondary mirror under the focuser.

This is what you need to do:-

Part 1:

Firstly remove the focuser completely off the tube. Lay the tube at 45 degrees or lower, so if you drop a screw, nut or spanner, it slides down the inside edge of the tube and doesn't hit the mirror. Measure around the outside of the telescope tube from the edge of the focuser hole on each side. Add the radius of the focuser hole to this number. Write the number down. Measure this distance around the tube. Mark the tube with a line, lengthwise along the tube. This line should be directly opposite the focuser and coincident with the mechanical axis lengthwise down the tube. The line needs to be about 70 to 100 millimetres long, use a steel rule and a whiteboard marker so you can remove it later.

Part 2:

Measure from the top of the tube on the focuser side, to the closest point of the focuser hole, along the "approximate" central axis of the tube. Add the radius of the focuser hole. Write this measurement down. Measure "down the line" on the opposite side of the tube this distance. Put a little Mark across the line at 90 degrees. The intersection should now be a point directly opposite the central axis of the focuser. At this point 1mm accuracy is fine. Now drill a 1mm or 1/32nd hole in the tube to "permanently" mark this point on the telescope tube. Yeah Yeah!!! I can hear you all cringeing. It won't hurt anything. I remove all the optics from the scope whenever I drill it and hold a magnet at the exit point of the drill, to catch any filings. It has to be a very small hole that you can see on close inspection, but not big enough to let any stray light in. If your tube is "flocked" all the better, as the flocking material "conceals the hole and stops stray light. My tube is flocked and I find the hole and insert a 1" x 1mm nail through it to "mark" the hole, when I need to reposition the focuser after removal.

We now have a small hole marking a point directly opposite the central axis of the focuser, which is also coincident with the mechanical axis of the tube.

Part 3:

Remove the secondary mirror and holder. Remount the focuser without overtightening the screws. Move the focuser to the "centre" of its travel position. Put a laser into the focuser and then "wiggle" the focuser to get the laser aligned on the hole. Once you get it right start to tighten up the screws a little more. Now move the focuser the full length of its travel and the laser "SHOULD" stay centered on the hole in the tube (Ha!! Ha!! bet it doesn't). You then need to work out which corners of the focuser are high and low and shim the focuser near the appropriate bolt with small thin pieces of cardboard or plastic. Thin plastic is the best because it is fairly stiff and you can force it into position without having to loosen all the focuser mounting bolts. After a lot of "trial and error" the focuser should be able to be moved over its full travel and the laser will remain fixed on the hole on the opposite side of the tube. This means the central axis of the focuser travel is perfectly aligned to the mechanical axis of the telescope. You now tighten the focuser mounting bolts right up.

Part 4:

Put the secondary back in the telescope. Using a sight tube centre it perfectly under the focuser. Two things to adjust. The secondary distance from the primary so it sits right under the focuser and also the "rotation" so it's face is at 90 degrees to the focuser central axis.


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Alternatively you can follow the instructions at the beginning of the article provided with the cats-eye collimator. It is the very 1st section titled "The focuser should be square to the tube axis" and the second section titled "the diagonal should be centered on the tube axis".

http://www.catseyecollimation.com/Collimating%20Procedure%20-%20R1.pdf

Whilst these initial mechanical alignments don't have an effect on the optical performance of the telescope, they can significantly affect it's pointing accuracy when using digital setting circles and a high power eyepiece. Whilst time consuming, these things normally only have to be done once, if you don't continually pull your telescope apart or remove the focuser. This is the reason many manufacturers of high quality focusers, also produce a collimateable base plate.

CS-John B

[Geoff45]

Good article. I'm glad Don points out that laser collimators are pretty useless for accurate collimation of the primary, except when a barlowed laser is used. This is my preferred method. Don doesn't go into the details, but if you are interested in this method, you can find them at
http://www.obsessiontelescopes.com/learning_center/index.html#Collimation
Geoff

[circumpolar]

This is an excellent article. Every newt owner should read it!

<----- :thumbsup:

Also these collimation tools [catseye] look really good.

[iceman]

I've got the catseye set now too, just haven't installed them.

[Orion]

Very nice article....Does anyone know how to collimator a Nasmyth three mirror system. I don't think a laser collimator will work on this type of set up.

[bird]

This article is excellent, and basically mimics my collimation process pretty much exactly, except that I do rely on a barlowed laser to re-check the collimation after repointing the scope at night since its much easier to see and doesn't require me to hold up any extra light.

I use the red triangle on my primary, as Don shows in the article, and to get maximum accuracy I printed a black outline on my barlowed laser paper target that is exactly the same size as the reflected shadow. This makes it even easier to judge the precise alignment - you rotate the barlowed laser until the drawn outline is aligned with the reflected shadow and then adjust the primary until all 3 sides of the reflected shadow are precisely aligned with the 3 drawn lines.

One more thing that Don might have mentioned is the idea of pointing the red triangle on the primary so that each of the 3 points are aligned with the collimation screws. If you do that then it's easy to figure out which collimation screws need to be adjusted.

I also check that all tools agree, as a double-check against doing something stupid - ie the Catseye blackcat and barlow laser must both tell me that Ive got the primary pointed correctly, and the autocollimator and telecat must both tell me that I have the secondary pointed correctly.

One more note - for planetary use where only a couple of mm at the centre of the field are used, precise centering of the secondary under focusser is not required, but you want to be reasonably close. I do check this every time I set up, but normally it doesn't need to be changed.

regards, Bird

[Garyh]

I agree with all, A good article on collimation...well worth reading!!! and thanks John for adding your bit on getting the focuser lined up with the scope axis..I shall follow the article next time I collimate..
Cheers

[iceman]

This article has been updated to the latest version as submitted by Don.

It really is the definitive guide to Newtonian collimation and every Newtonian owner should read and understand it.

Passive Tool Collimation and the Newtonian (http://www.iceinspace.com.au/index.php?id=63,390,0,0,1,0)

Thanks for the updated article, Don.

[Don Pensack]

Thanks for the compliments, guys.

John: Yes, I neglected the instructions to do Classical offset, which makes the optical and mechanical axes coincident. For visual use it isn't critical, and, at f/5 (my chosen f/ratio), the offset is too small to have a dramatic effect on DSC accuracy (especially with the Argo Navis), but at shorter f/ratios you are correct that perhaps making the two axes coincident would be of more value.

I have found that the tilt of the optical axis away from the mechanical axis is, more or less, equal to the offset required in Classical offset, so I might add a caveat that if the offset is less than or equal to 1/8", the likelihood is that the New Model collimation is unlikely to affect your DSC accuracy.

But if offset calculations indicate a greater offset is required, it might be worthwhile to build in the Classical offset movement of having the secondary positioned slightly away from the focuser by the calculated amount (there are on-line calculators for offset).

The beauty of the New Model is that offset is automatically built in when collimating, whether the focuser is "square" to the tube or not. As such, it is easier for the user. But I agree that there are circumstances where perfect coincidence of optical and mechanical axes would be called for.

Perhaps I will incorporate those instructions in version 4.

Don Pensack
Los Angeles.

[iceman]

This article has been updated and is now at version 4.

It's worth a read for anyone struggling with how to do collimation, or why in fact you need to collimate and what you're trying to achieve.

Great for newbies and experienced alike.

Collimation and the Newtonian Telescope (http://www.iceinspace.com.au/63-390-0-0-1-0.html)

[Don Pensack]

In this latest version, I also show that bi-directional offset (aligning the optical axis with the tube's mechanical axis) can be done two ways--by offsetting the secondary mirror from the centerline of the secondary holder, or offsetting the entire secondary holder from the center of the spider.
The latter technique is employed by at least one commercial maker--Starstructure telescopes.
I did not go deeply into the mathematics of collimation tolerances, but, for those interested, the collimation tolerances for the primary mirror (primary mirror axis error, or PAE) are (tight tolerances) .0036mm x f/ratio cubed and (loose tolerances) .0088mm x f/ratio cubed.
A (loose) tolerance for the secondary (focuser axis error, or FAE) is approximately .005 x mirror diameter in mm.
As you can see, secondary tolerances are a lot looser than primary tolerances.

Some factors to note:
--an off-center center marker will impact the primary mirror's collimation error and tolerances.
--the use of a Paracorr will tighten tolerances by a factor of 6

Don Pensack
Los Angeles

[fauxpas]

Do 4" tabletop beginner dobs need to be collimated?

And do larger (8-12") collapsible dobs need collimating after each setup/reassembly?

Thanks...

[Don Pensack]

Yes and yes.
By the way, an updated version of the collimatiuon article is here at:
http://www.cloudynights.com/item.php?item_id=2677

[fauxpas]

Okay, cheers for that...

/me goes off to buy collimator... :)

[astro_nutt]

Excellent article Don.
I agree with Matt. Every Newt owner should have a read and while is does look complicated, diagrams do makes it easier to understand.
I will add that a laser can be used to check the movement of the focuser. With the focuser all the way in, observe the laser dot on the centre mark on the primary. As you wind the focuser out, see if the laser dot moves across the centre mark.
After using a cheshire sight tube, I will use a laser to check the primary alignment by observing the return beam to the exit hole. I place a small pocket mirror behind the secondary mirror.
What I look for is a red "donut" around the exit hole and the return beam centreing over the exit beam. (I do this by touching one of the spider vanes)
I will then wind the focuser in and out fully to check any movement.
If both the Cheshire sight tube and the laser agree, it's a fair bet that the mirrors are pretty well collimated.

[Don Pensack]

IF the secondary/focuser axis is perfectly aligned, and IF the primary is perfectly aligned with a Cheshire (reflective circle with dark center), THEN the return beam of a perfectly collimated laser can be used to confirm the alignment of a primary mirror.
But, the barlowed laser technique is more accurate for aligning the primary, and it doesn't even matter if the laser is collimated.
Here is an explanation:
http://www.cameraconcepts.com/barlowed%20laser%20collimation.pdf

[astro_nutt]

Many thanks to you Don...I'm in the process of making a barlowed laser.
Cheers!

[gasman1]

I just collimated my 130mm f/5 Newtonian and found that I can just see the tip of the Laser Pro at the bottom of the focuser. It was a snap to set to laser dot in the center of the mirror and then confirm the return beam landed less than 1/4" from the small laser beam exit hole. I may tweak the primary later, but for now that'll do it for me.