How to Collimate your Newtonian
Posted 15 February 2008 - 08:25 AM
I created this procedure for users with CatsEye tools and Glatter lasers, although it should work well for most scopes with a simple combo tool (and a properly aligned economy laser that has consistent registration with the focuser drawtube if one's available.) The final autocollimator procedures are useful for resolving any residual axial errors--but more importantly, the autocollimator teaches the end user to be more deliberate when completing each collimation step.
1. Center the spider.
2. Check the focuser for perpendicularity to the tube axis.
3. Center spot the primary mirror (or verify the accuracy of the existing center spot.)
SIGHT TUBE AND LASER COLLIMATION
(I usually go back and forth with the TeleCat and the Glatter laser with the 1mm aperture stop for best precision.)
1. Position the sight tube in the focuser until the perimeter of the bottom edge appears slightly larger than the outer edge of the diagonal holder. Or, with a holographic laser, position the diagonal under the focuser until it appears centered in the holographic display.
2. Collimate the outer perimeter of the diagonal mirror with the bottom edge of the sight tube (or holographic pattern) by adjusting the axial and rotational alignment of the diagonal holder (and the focuser tilt, if necessary.)
3. Collimate the primary mirror spot with the intersection of the sight tube crosshairs by adjusting the tilt of the diagonal mirror. Or, with a simple point source laser or holographic laser, adjust the diagonal tilt to align the laser with the primary mirror center spot.
4. Check the tilt and coverage of the diagonal mirror with the reflected edge of the primary mirror. The primary mirror reflection appears concentric with the actual edge of the diagonal mirror and the mirror retaining clips are visible. Verify the rotation and angle alignment. Repeat steps 2 and 3 if necessary.
5. Collimate the reflection of the inside of the focuser (seen in the silhouetted reflection of the diagonal mirror) with the intersection of the sight tube crosshairs by adjusting the tilt of the primary mirror. Or, use the simple point source laser or holographic laser and collimate the return beam to the laser aperture.
(Or, if you're using the TeleCat, make sure that the primary mirror alignment is corrected to the precision of the calibrated Cheshire ring and triangular primary mirror center spot.)
(6. Fine tune the diagonal mirror tilt (focuser axial alignment) during regular observing sessions with the Glatter laser and 1mm aperture stop. The laser beam and its diffraction pattern facilitate the alignment with the triangular center spot perforation on the primary mirror.)
Proceed to the Cheshire or Barlowed laser next.
THE CHESHIRE EYEPIECE AND THE BARLOWED LASER
1. Fine collimate the primary mirror with the Cheshire eyepiece by adjusting the tilt of the primary mirror until the reflection of the primary mirror spot is observed centered in the reflected image of the Cheshire eyepiece. Or collimate the primary mirror to align the silhouetted center spot on the face of the Barlowed laser.
(2. Fine tune the primary mirror collimation during regular observing sessions with a diffuse red light to illuminate the Cheshire or use the Barlowed laser. Follow the procedure above.)
(Note that the Barlowed laser procedure is parallax free. This means the position of your eye relative to the optical axis has no impact on the read. Also note that if you're using the 1mm aperture stop with the Glatter laser, you can often see the silhouette of the triangular primary mirror center spot on the white target on the face of the laser, similar to a Barlowed laser protocol. This makes the Glatter laser with the 1mm aperture stop an after dark high precision combo tool of sorts, capable of assessing and correcting both axes!)
1. Begin by accurately collimating the telescope with the other tools.
2. Set up the telescope in a brightly lit room or outside environment or illuminate the primary center spot with a red flashlight at night.
3. With the autocollimator in the focuser, observe the reflected images of the primary center spot and using the iterative method, stack the reflections into a closer jumble (by adjusting the diagonal or the focuser), recollimate the primary mirror with the Cheshire or Barlowed laser, restack the reflections in the autocollimator by adjusting the diagonal or the focuser, recollimate the primary with the Cheshire... Continue to reiterate the procedure until the reflections are within tolerance or they disappear behind the primary mirror center spot.
3b. Carefully decollimate the primary mirror. Adjust the diagonal or focuser to align the fainter, second inverted reflection with the primary mirror spot. Recollimate the primary mirror by stacking the remaining reflections until they disappear behind the primary mirror center spot.
4. Verify the primary mirror collimation with the Cheshire eyepiece or Barlowed laser and the focuser with a simple point source or holographic laser. All tools should agree.
Posted 15 February 2008 - 09:19 AM
I've already posted my "procedure" before, but I'll be happy to do it again.
Posted 15 February 2008 - 11:15 AM
I just wanted to express my thanks for this thread. I've studied Vic's book, Don Pensack's articles, and Alexis' posts about collimation, and still consider myself somewhat inexperienced in that department.
Extensive detail, semantics, or analysis of fairly obscure problems can still mystify me, leaving me temporarily dazed and confused - because of my relative inexperience compared to the experts. Nevertheless, my understanding has improved during this discussion.
Posted 15 February 2008 - 03:18 PM
Extensive detail, semantics, or analysis of fairly obscure problems can still mystify me, leaving me temporarily dazed and confused - because of my relative inexperience compared to the experts.
You can always ask questions. Of course, that has a tendency to derail the thread if it's someone else's (as should be clear from this thread), so you could post another thread. But then the original poster might not see something that is relevant, so choosing between posting it in an existing thread or in a new one is a call you can only make perfectly with hindsight .
Posted 15 February 2008 - 04:29 PM
Posted 16 February 2008 - 04:06 AM
1. Move the outline of the secondary¹ to be centred under the focuser (you can either physically move the secondary or tilt the focuser towards it; to pick a choice, see how centred the secondary looks from the front of the tube and see how easy it is to adjust the tilt on the focuser)
2. Rotate the secondary to make it appear as circular as possible
3. Tilt it to centre the primary centre spot reflection under the focuser, or to make a laser collimator's forward beam hit the primary's centre spot.
4. Go back to 1, until 1-3 no longer require you to do anything². Don't obsess - good enough is, so stop when you're fed up. 3. is most critical, and what follows is more critical.
5. Change the tilt on the primary.
5a. If you have a tool in the focuser with a centred pupil, make the reference (the pupil itself, or a bright Cheshire ring concentric with it) concentric with the primary centre spot reflection³.
5b. If you have a barlowed laser, centre the silhouette of the primary's centre spot in the returning light cylinder in the focuser opening.
6. Go back to 3. or 1., and repeat until you're fed up with it or no further adjustments are needed. Again, 5. is critical if you're getting too bogged down in the early steps; move along to 5. if you think the rest is good enough, but don't skip that step.
¹Don't confuse the outline of the secondary itself with the primary's reflection in it. Use coloured paper behind the secondary or between the secondary and the primary to make things more clear if necessary.
²The astute reader will have noticed that 1. and 3. together mean the reflection of the primary and the outline of the secondary will be concentric.
³The astute observer will see that the silhouette of the reflection of the secondary is actually not concentric, but offset along an imaginary line towards the primary. If the offset isn't on that line, you may have a small residual error in what you've done in steps 1. and 2., but it's your choice whether to ignore it or start again.
Posted 17 February 2008 - 01:39 PM
Posted 17 February 2008 - 01:42 PM
2. Check the focuser for perpendicularity to the tube axis.
What is the best way to do the above?
Posted 17 February 2008 - 01:56 PM
I posted this quite a bit earlier (7/23/04 "Secondary Mirror Alignment", now in "The Best of Reflectors" thread.) There are, of course, other ways to align the mechanical focuser axis. And even though the focuser axis is the reference axis for optical collimation, this "mechanical" adjustment is really just a good starting point...What is the best way to do the above?
2. Check the focuser for perpendicularity to the tube axis.
Step one. Measure the spider vanes from edge to center and from the four (or three) mounting points to the front edge of the tube. If the spider isn't properly centered or is mounted incorrectly and not parallel to the front of the OTA, make the necessary adjustments to make it all "textbook".
Step two. Remove the diagonal. Using a laser (or sight tube--but a laser is easier!), rack the focuser out far enough so that the focuser drawtube or the bottom of the laser doesn't protrude into the interior of the tube (front cage assy). Rotate the laser to verify that the laser spot on the opposite side of the tube from the focuser does not move. Insert a ruler from the front edge of the tube (cage) until it moves under the focuser and occults the laser beam. Take a measurement. Go to the opposite side of the tube (cage) and insert the ruler until the laser spot just moves entirely onto the ruler. This measurement should equal the previous measurement and indicates the focuser axis is correct longitudinally. If it's off more than about 3 to 5% of the minor axis of the diagonal, you should shim (or adjust) the focuser. Next, with the laser still in the focuser and the primary mirror uncovered, stand in front of the tube assembly and line up the actual spider with its reflection (in the primary mirror). Hold this position and place your hand with your palm facing you between the spider and the primary mirror. Look down the hole of the spider (where the diagonal holder is normally mounted) and notice the position of the laser spot. If it is high or low (about half the earlier tolerance), the focuser axis will need to be adjusted. This preliminary alignment procedure uses the optical/mechanical axis as the alignment guide for setting the pitch angle (as opposed to the intercept angle) of the focuser. This is only a starting point for focuser collimation and should not be considered "final" collimation for the focuser. A “squared focuser” is a good place to start--but other considerations may require additional adjustments.
Step three. Reset the diagonal adjustment screws to the "start" position (all screwed in equal amounts) so the rotational and angular adjustments of the diagonal can be set without additional skew errors. Visual inspection of the the diagonal back plate (push plate) and the adjusting screw plate should reveal roughly parallel surfaces. I've seen these get really screwed up when collimation is performed exclusively with a laser. If they are out of alignment, make the necessary adjustments to correct the mechanical condition. I also inspect the barrel that retains the diagonal mirror to verify that it is mounted flush and level to the back plate. Again, all “textbook”.
Reinstall the diagonal. Using a sight tube or Cheshire eyepiece (for shorter focal ratios), adjust the longitudinal positioning and rotation to center the diagonal under the focuser. If the diagonal can’t be properly centered without adjusting the three (or four) angle adjustment screws, the focuser should be shimmed to accomodate the centering process. Then you can set the diagonal angle adjustment with the laser or sight tube. Procede with primary mirror collimation (Cheshire or Barlowed laser) and fine tune as needed...
Posted 18 February 2008 - 02:54 PM
Posted 20 July 2008 - 09:20 AM
Posted 20 July 2008 - 10:39 AM
Posted 20 July 2008 - 08:42 PM
You'll spend more time reading about it than you'll spend doing it (except perhaps the first time, when you place the secondary under the focuser).
Wow, I hope this is a much easier process in practice than it is reading how! When I get my dob, I want to spend my time looking at the sky, not collimating the darned thing!
Posted 21 July 2008 - 07:07 AM
Am I missing something or over thinking this?
Posted 21 July 2008 - 09:45 AM
When you collimate a Newtonian you are basically aligning three things (pretty much in this order):
...My question, does that sound right, shouldn't the secondary be dead center or as long as I can get the laser dead center both on the primary and on the return beam good? Am I missing something or over thinking this?
First--the secondary mirror
Second--the focuser axis
Third--the primary mirror axis
Note that the focuser and primary mirror have axes, and the secondary mirror does not. The laser beam points wherever the focuser drawtube points, so it defines the focuser axis. Once the focuser axis is precisely aimed at the center of the primary mirror (usually accomplished by adjusting the secondary mirror tilt), the return beam can be used to align the primary mirror axis.
But wait--we forgot the first step, aligning, or centering the secondary mirror under the focuser. OK, so you use the collimation cap to center the actual edge of the secondary mirror relative to the bottom edge of the focuser (notice we're aligning circles, not axes). But then you adjust the secondary mirror again when you aim the laser at the center of the primary mirror (focuser axis alignment)--which messes up the secondary mirror alignment you just did with the collimation cap!
The goal, of course, is to get both alignments corrected simultaneously. Unfortunately, most folks use secondary mirror tilt to fix both, and end up going back and forth undoing the alignment of the other! The secret is to use secondary mirror tilt to fix one (laser), and rotation and/or movement toward or away from the primary mirror to fix the other (collimation cap.) This way, after a couple of tries, you will soon resolve both alignment errors.
Re: secondary mirror appearing "dead center". If the actual edge of the secondary mirror is centered relative to the bottom edge of the focuser drawtube, the dark silhouette reflection of the secondary mirror will NOT appear centered in the reflection of the primary mirror, but will instead appear offset towards the primary mirror end of the OTA. This is how the finished secondary mirror alignment should look. (If the dark silhouette reflection of the secondary mirror appears centered in the reflection of the primary mirror--the actual edge of the secondary mirror will NOT appear centered relative to the bottom edge of the focuser drawtube, but will instead appear offset away from the primary mirror end of the OTA. This secondary mirror alignment still works, but it doesn't provide optimal illumination at the focal plane.)
Posted 21 July 2008 - 11:56 AM
Thanks for the reply. My secondary mirror appears round when looking through the collimation cap, the mirror is centered in the focuser tube. I can see the secondary holder also, but the mirror itself appears round and centered. I thought that was the goal. I then used the tilt feature so that the primary was centered and I could see all three clips and centered it. After that I used the laser and made slight adjustments. So your saying I may have it off? Should I start from scratch. I wasn't comfortable moving the spider adjustments, but tried rotating the secondary holder and raising /lowering it, but could not keep everything centered as I thought that was the goal. I'll keep reading, maybe purchase the hologram attachment for the laser would help with the secondary placement?
Posted 23 July 2008 - 06:22 PM
Posted 23 July 2008 - 11:04 PM
Posted 25 July 2008 - 07:14 AM
Posted 30 July 2008 - 11:53 AM
:mrevil: You folks are so tempting
Posted 30 July 2008 - 12:47 PM
It does help (if you are using a triangular reflective center spot) to set the mirror in its cell such that either the points of the triangle, or else the centerpoint of each triangle edge, are aligned with each respective collimation knob. This gives you a much better sense of cause-and-effect of turning each knob than if the triangle alignment had no easily visible correspondence with the collimation knobs.
Posted 26 September 2008 - 03:17 PM
Thank you in advance
Posted 26 September 2008 - 08:47 PM
I bought an 6.5" Maksutov-Newtonian and I would like to know if the instructions by Vic about collimation can be applied for my telescope in a general way.
Have you tried to evaluate the optical performance with a star test yet? If you suspect that the collimation is incorrect, do you have access to any collimation tools?
In fact I want to know first if my telescope is collimated or not.