Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
I've already posted my "procedure" before, but I'll be happy to do it again. Still, I feel it's important to caution the user that it's possible, even with a systematic approach to alignment, to end up on what appears to be a "dead end." I sometimes find it's easier (and quicker!) to re-zero all of the mechanicals and start over...
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.
PRELIMINARY ADJUSTMENTS
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!)
THE AUTOCOLLIMATOR
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.
or,
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.
Edited by Don W (02/15/08 04:27 PM)
|
BozemanWalt
super member
Reged: 01/05/08
Posts: 185
|
|
Thanks for posting this Vic, much appreciated.
--Walt
Quote:
I've already posted my "procedure" before, but I'll be happy to do it again.
|
backwoody
Pooh-Bah
Reged: 01/08/07
Posts: 1114
Loc: Idaho USA
|
|
Don, Jason, Alexis, and others:
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.
--------------------
woody
a parsec farther out...
12.5" f/4.8 custom truss dob, EQ platform
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
|
|
Quote:
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  .
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
Don W
Postmaster
Reged: 05/19/03
Posts: 14615
Loc: Wisconsin, USA
|
|
Thank you very much, Vic. I want to thank Sixela and the others who have contributed to this subject here in the past. Good collimation makes such a great difference in observing.
--------------------
Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
|
|
I'll take a crack at an even shorter text for the procedure (not with the level of detail of Vic's though). To be as short as possible, I won't discuss the tools that let you do this, I'll just discuss what they aim to do.
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.
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
backwoody
Pooh-Bah
Reged: 01/08/07
Posts: 1114
Loc: Idaho USA
|
|
Alexis, nice work. Thank you.
--------------------
woody
a parsec farther out...
12.5" f/4.8 custom truss dob, EQ platform
|
Spaced
Carpal Tunnel
Reged: 03/01/05
Posts: 2176
Loc: Tacoma, Washington, USA
|
|
Vic, thanks for posting the Summary of all Summations. I'm glad it was permanently pinned to the forum.
--------------------
Mike
"The spiral light of Venus,
Rising first and shining best . . ."
_____________________________
Webster 14.5" f/4.5 "Sugaree"
Megrez II 80 ED Triplet APO "Punk"
Siebert Black Night BVs
8 X 42 Celestron Regals
|
Almach
Pooh-Bah
Reged: 02/27/07
Posts: 1185
Loc: Manitoba, Canada
|
|
Quote:
2. Check the focuser for perpendicularity to the tube axis.
What is the best way to do the above?
--------------------
Jim
Optical Star Fleet:
Orion XT10 f/4.7
Orion XT4.5 f/8
Orion 102mm f/7 ED
70mm f/7 Skywatcher Refractor
8x40 Nikon
Ocular Arsenal:
BO/TMB, Orion Sirius Plossl, Pentax, Televue
Ocular Support:
TV Paracorr, 2x & 2.5x Powermate, 2x TV Barlow, WO Binoviewer
Orion Skyview Mount
Observing since 25 December 2006
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Quote:
Quote:
2. Check the focuser for perpendicularity to the tube axis.
What is the best way to do the above?
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...
Procedure--
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...
--------------------
|
Jim Moscheck
Carpal Tunnel
Reged: 01/08/07
Posts: 1619
Loc: Under the SE Michigan lightdom...
|
|
Thanks for the post Vic. I am already doing close to your procedure and it'll now be easy to fine tune it. I was only doing one iteration with the auto collimator and now I'll do a few more. Also, when stacking the triangles I was always confused when the refections disappeared and now I know they are supposed to. I don't have a 1mm aperture stop either and will call HG and see if I can get one.
--------------------
Jim Moscheck
Ford Amateur Astronomy Club
ATM truss dob
Stevens 14.5" f4.3
"A mind is like a parachute. It doesn't work if it's not open." - Frank Zappa
|
GaN
member
Reged: 06/05/05
Posts: 39
Loc: Europe, Czech
|
|
http://www.andysshotglass.com/Collimating.html
|
Tull1996
member
Reged: 07/07/08
Posts: 40
Loc: Plattsburgh NY, USA
|
|
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!
|
Wobrak
super member
Reged: 04/18/08
Posts: 186
Loc: SC, USA
|
|
With practice it is an easier process than it appears. It's the learning curve that makes seem as though you will spend the entire evening collimating instead of viewing.
--------------------
Karl
Zhumell 20x80 Binos
Zhumell 10" Dob
WO 28mm UWAN
TV 13mm Ethos
TV 8mm Ethos
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
|
|
Quote:
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!
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).
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
Mike Conley
sage
Reged: 09/23/07
Posts: 306
Loc: NW Ohio
|
|
Just picked up a custom truss tube.It's been some time since my last reflector ( Orion 8" Dob ). I had no trouble collimating the Orion, but this one is puzzling me. It has a 3 vain spider with bob knobs or something similar. I can place the howie glatter in the moonlight focuser and get a perfect return beam, center on the primary and return to dead center on the face of the laser. Good enough ? Well, when I place the collimation cap in the focuser, the reflection of the primary is off center to one side. I'm having to tilt my secondary for the laser alignment which is making the sighting through the cap out of alignment. Went back and forth about 6 times, same everytime ( I'm a hard head ). Tried realigning the secondary, loosening the center post and slightly turning to square to the face of the focuser, but it actually seems to need to go the opposite of square to show correct in the collimation cap. The only way I've been able to correct it was to tighten one adjutment screw on the spider while loosening the other two. I had to pull the spider to one side about 1/16 of an inch off center. I now have alignment both in the cap and with the laser. 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?
Thanks Mike
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Quote:
...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?
When you collimate a Newtonian you are basically aligning three things (pretty much in this order):
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.)
--------------------
|
Mike Conley
sage
Reged: 09/23/07
Posts: 306
Loc: NW Ohio
|
|
Vic,
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?
Thanks Mike
|
CatseyeMan
Vendor (Cats Eye Collimation)
Reged: 12/16/04
Posts: 295
Loc: Huntsville, AL USA
|
|
Before you get too far down the investigative path with Vic's guidance, let's don't forget validating the internal collimation of the laser by rotating it 360 dgrees in the focuser against the drawtube lip and observe the beam spot on the Primary... the spot must remain stationary through 1 complete rotation in a collimated laser. If it "orbits" significantly, all bets are off.
--------------------
Jim Fly - Manufacturer
|
travvy
member
Reged: 12/13/07
Posts: 29
Loc: Tillsonburg, Ontario
|
|
Mike, I have the same problem with my orion classic 8, but being a newbie. I realized that i never knew the secondary could travel in and out a bit... so this maybe my problem. the andyshotglass is a great movie to watch and learn.
Good luck
--------------------
Orion Starblaster 4.5 inch mini dob
Orion skyquest classic 8 inch dob
i started out with a magnifying glass.... poor ants
|
Mike Conley
sage
Reged: 09/23/07
Posts: 306
Loc: NW Ohio
|
|
Thanks Guys. I ordered a cheshire and positioned the secondary ( it was off ), followed by the single beam laser ( it is collimated, rotated it in the focuser ) . The laser confirmed the chesire and just needs to be star tested now.
Mike
|
DanJ
Pooh-Bah
Reged: 12/01/04
Posts: 1088
Loc: Youngsville, NC
|
|
I "believe" my scope is collimated pretty good, star tests and etc, but this thread is tempting me to go out and see if some of the above is on the mark....HELP 
 You folks are so tempting
Cheers!
--------------------
Dan J.
--------------
XT8i
ETX 90
PST
---------
"A government big enough to give you everything you want, is strong enough to take everything you have."
Gerald Ford
|
FirstSight
Post Laureate
Reged: 12/26/05
Posts: 3870
Loc: Raleigh, NC
|
|
One of the trickier aspects of Newtonian collimation is that turning the adjustment screws for either the secondary or the primary mirror does *not* produce a neat, linear movement of the aligment, but rather somewhat of a curving arc. And so, e.g. when the primary center-spot is off-center directly above where it should be, turning any of the knobs will cause it to move not just up or down, but also cause it to deflect a bit to the right or left as well. Likewise, when the primary center-spot is off-center to the left or right, turning any of the knobs will cause it to move not just left or right, but a bit upward or downward as well. Again, the movement is never along a linear path, but more along the sort of arc that might be produced by drawing with a compass centered where the knobs are, with the pencil where the center spot is. Regardless of the kind of tools you're using to do the collimation with, this inescapable aspect of using a system of three adjustments symmetrically spaced around a circle can be a bit maddeningly nonintuitive, especially as you draw progressively closer, but not yet adequately spot-on during the procedure. Especially those laaaast irresistible attempts to nudge the triangle dead-center from just a small tinch up or down (or right or left).
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.
--------------------
Chris M., aka "First Sight"
Orion XT12i Dob with Moonlite CR-2 focuser
WO Megrez 90 refractor on UniStar Light mount
Nikon 10x50 Binoculars
Edited by FirstSight (07/30/08 02:01 PM)
|
Paulin
newbie
Reged: 05/08/08
Posts: 3
|
|
Hello everybody. 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. In fact I want to know first if my telescope is collimated or not.
Thank you in advance
Regards
Paul
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Quote:
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.
Yes.
Quote:
In fact I want to know first if my telescope is collimated or not.
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?
--------------------
|
Paulin
newbie
Reged: 05/08/08
Posts: 3
|
|
Hello Vic. Thank you for your answer. Here in Mexico City I have had cloudy nights everyday, but I´m sure that this mak-new telescope is discollimated because I can see that the secondary was turned during the transport. I have not tried to evaluate the telescope because I´m searching for a complete way to collimated this kind of optics. With respect a tools collimation I have your book ( fourth edition)with the three basic tools. I have too a barlowed laser. I have collimated my reflectors but this mak is different. I can use the general guidelines to collimate this optics but I don´t have any idea about how to do the primary collimation or how to move the secondary if this is neccesary.
Thanks a lot Vic
Regards
Paul
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Quote:
...Here in Mexico City I have had cloudy nights everyday, but I´m sure that this mak-new telescope is discollimated because I can see that the secondary was turned during the transport...
I've had some experience collimating a similar scope for a friend. His secondary mirror was also rotated out of alignment and routine axial alignment with a simple thin beam laser did not improve the image performance. Like a Schmidt/Newt, this scope is collimated by aligning the corrector axis (the axis of the meniscus lens) to the focuser axis.
Since there's no accommodation for rotating the secondary mirror, we gambled and rotated the meniscus lens (and the secondary mirror with it) in its cell (followed by routine axial alignment to correct the secondary mirror tilt) until the skewed secondary mirror looked "textbook." As I recall, the OTA was pretty long and the secondary mirror appeared to be mounted centered relative to the meniscus, so the final alignment was centered, not offset. The image performance improved dramatically after the secondary mirror alignment was corrected.
As I said--it's a gamble. In this case I don't think you have much to lose, worst-case scenario you send it back for realignment. We got a lot of fingerprints on the meniscus lens making the rotational adjustments, but they cleaned off easily after the alignment was corrected. (You can index the meniscus "starting point" for reference somewhere along the edge with a permanent marker if you decide you would like to undo the meniscus rotation adjustment.)
--------------------
|
Paulin
newbie
Reged: 05/08/08
Posts: 3
|
|
Hello Vic, thank you for the information. I have some questions about this collimation that you did with a similar scope.
Does the secondary mirror can be aligned in axial way but not can be rotated?... You had to move all the meniscus, so I suppose that the secondary does not have independent movement. When you say about aligning the corrector axis to the focuser axis if I understand well, there is only a right position to left the meniscus lens. If only the meniscus lens is turned and the secondary not ( the secondary keeping his right position), this is going to affect the performance on this kind of telescope?..
Sorry for so many questions. The collimation seems to be something harder that in standars newtonians.
Regards
Paul
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Quote:
Does the secondary mirror can be aligned in axial way but not can be rotated?...
Is this the scope you're trying to collimate?
http://www.cloudynights.com/item.php?item_id=1548
Quote:
You had to move all the meniscus, so I suppose that the secondary does not have independent movement.
The reason I asked if you're trying to collimate an Intes Micro MN66 is that I just got back from the Peach State Star Gaze where I was asked to collimate a MN86, which did accommodate secondary mirror rotation. I had to loosen a knurled retaining ring (after removing the screw-on cover to expose the tilt adjustment screws.) The knurled retaining ring had a small set screw that had to be loosened with a jeweler's screwdriver, but once the ring was loosened, secondary mirror rotation and tilt were readily adjustable.
Quote:
When you say about aligning the corrector axis to the focuser axis if I understand well, there is only a right position to left the meniscus lens. If only the meniscus lens is turned and the secondary not ( the secondary keeping his right position), this is going to affect the performance on this kind of telescope?..
Perhaps--but if your scope is an Intes Micro, and it has the same secondary mirror alignment adjustment as the MN86, you won't have to rotate the meniscus.
Quote:
Sorry for so many questions. The collimation seems to be something harder that in standars newtonians.
Although making fine adjustments to accurately align the secondary mirror can be tedious, from what I've read about the optical configuration, the scope is supposedly more tolerant of small axial misalignments when compared to a simple f/6 Newtonian.
FWIW--when I was reading about the scope (in a recent edition of Astronomy Technology Today), I was led to believe that the knurled retaining ring setscrew enabled the user to simply twist the knurled retaining ring to adjust the secondary mirror rotation. On the scope I collimated, the ring needed to be loosened first, as it was tight enough to prevent manual rotation (at the meniscus). I suspect the rotation alignment error resulted after someone accidentally loosened two or more tilt adjustment screws at the same time, allowing the secondary mirror to rotate on the central mounting screw...
--------------------
|
Timber
sage
Reged: 11/08/08
Posts: 394
Loc: SW foothills of Mt. St. Helens
|
|
Vic, I posted in the wrong place, I'll try this. Do you have a link to Tectron, I have been unable to find their website for the Infinity XL Autocollimator and Cheshire. Thanks
|
CatseyeMan
Vendor (Cats Eye Collimation)
Reged: 12/16/04
Posts: 295
Loc: Huntsville, AL USA
|
|
Quote:
Vic, I posted in the wrong place, I'll try this. Do you have a link to Tectron, I have been unable to find their website for the Infinity XL Autocollimator and Cheshire. Thanks
I sent you a PM.
Regards,
--------------------
Jim Fly - Manufacturer
|
Timber
sage
Reged: 11/08/08
Posts: 394
Loc: SW foothills of Mt. St. Helens
|
|
Thanks Jim, I don't know what a PM is but I found your website, will be in touch
|
Don W
Postmaster
Reged: 05/19/03
Posts: 14615
Loc: Wisconsin, USA
|
|
PM is a private message. Look at the top left of your Cloudy Nights screen for a blinking envelope.
--------------------
Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
|
Gianluca67
member
Reged: 11/26/08
Posts: 96
|
|
Thank you all for this interesting thread. I have an f/4 Newtonian so collimation is crucial to get good images. I use a sight tube and fine align on a star whenever the seeing is excellent. The curious thing is that although the in focus star seems to be perfectly circular with concentric rings the shadow of the secondary mirror of the defocused star is shifted. The amount of the shifting is higher if I defocus inward (10-12 wavelenghts). Does it mean that collimation is wrong? I can often use extremely high power if the scope is at thermal equilibrium.
Thanks
Gianluca
|
jayw65
newbie
Reged: 04/02/09
Posts: 3
|
|
Is this confusing or what? I hope when I get my Astro Sky 15" it will not be so perplexing. It has not been a big problem with the 8" Orion I am currently using or maybe ignorance is bliss.
Jay W
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Here are some great animations provided by Jason D if you're sorting out a secondary mirror alignment error: link
--------------------
|
RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1175
Loc: Grand Rapids, Michigan
|
|
Vic, I read this thread the other day and waited for a good clear night for a star test. I've been a fan of "Well, its close enough" for the last few years. I found it very hard to collimate my 10" newt because of the low eyepiece, and having to sit in a chair to view, get up, turn a knob on the mirror cell, then back to the eyepiece and find the star has completely left the field of view. Until last night where I put my scope up on the deck right at the edge of the stairs, where I could stand up and look through the EP, then reach down to twist the knobs vs. having to get up out of the chair, etc. etc.
What I found was that my Primary Mirror center spot is not exactly on center. Very very close, but not centered. The star test revealed a misaligned optical train. I made a drawing to represent what I saw in the Eyepiece,i.e. how the Startest (at 300x) vs. the Centerspot looked visually.
Collimation is easy once it "clicks" in your head. Get it close with your collimation tools, then spend 15 minutes tweaking the collimation screws to get the diffraction rings of a nice Bright defocused star to look concentric.
The short views of the moon I got before the clouds rolled in with the diffraction rings very much concentric was the best views I have ever had!
Thanks, Vic!
--------------------
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Brad, have you considered replacing the triangle and getting the new one accurately centered? Or perhaps "dotting" the existing triangle (on the right side in your graphic) to remind yourself where the optimal Cheshire alignment is with your slightly off-center triangle? It could save you those 15 minutes of tweaking the primary mirror alignment on a star.
Of course, if you prefer fine aligning the primary mirror on a star each time you observe, that's OK too!
--------------------
|
RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1175
Loc: Grand Rapids, Michigan
|
|
Vic, I actually did that centerspot myself! LOL! It can only be off by a little bit, but I think that I'll be doing a star test from now on. Bad seeing was another factor in me not collimating the scope using a star test in the past. Its difficult when the defocused star looks like a flickering candle flame. Last night was very nice until the clouds rolled in. And it was much easier doing a star test with a star closer to the North star with my non-tracking Dob, they don't move as much.
Hopefully my imaging will improve significantly as well. I had a great night with Jupiter about a month ago where I was told by Lunatiki that my collimation was off, badly, because the shadow on the face of Jupiter was elongated.
here is the image, the top dark spot is the moon shadow, the bottom dark spot is Callisto(I believe). You can see the elongation due to bad collimation. The moon shadow should be perfectly circular for those who are wondering what it should look like. 
--------------------
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Interesting.
Here's an image of Saturn taken moments after aligning the primary mirror with a Barlowed Glatter laser: Saturn
How many images did you stack for your Jupiter image?
--------------------
|
RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1175
Loc: Grand Rapids, Michigan
|
|
I do not remember exactly how many frames this was but its over 400 for sure. I usually get a grainy look with anything under 400. ToUcam840k, 5fps, @ F/22. 
--------------------
|
Vic Menard
Post Laureate
Reged: 07/21/04
Posts: 3066
Loc: Bradenton, FL
|
|
Is there any possibility that the elongated moon and shadow are the result of motion? In 80 seconds the planet rotates about 1/2 of 1-percent of the diameter, moon speed could be more. It's certainly possible that the elongation is a collimation artifact, but looking at the rest of the surface detail, I don't know... Are you convinced?
--------------------
|
auriga
scholastic sledgehammer
Reged: 03/02/06
Posts: 794
|
|
Thanks Vic, this is a very clear and useful summary (except for the autocollimator section of course). I have marked this as a "favorite thread" so I can refer to it in the future.
(But if it reaches 300 posts with 100 diagrams I am out of here. :-) )
Regards,
Bill Meyers
|
Jason D
Post Laureate
Reged: 10/21/06
Posts: 3293
Loc: California
|
|
Quote:
But if it reaches 300 posts with 100 diagrams I am out of here. :-)
    
--------------------
XT10 classic with premium optics
Tri-knob CR2 with compression rings
Round Table Platform
4.5" StarBlast
6" StarBlast6
TV EPs
|
RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1175
Loc: Grand Rapids, Michigan
|
|
Vic, I am pretty convinced that it is (was) a collimation problem. The raw frames show the elongated moon shadow.
Have not had a night with a shadow transit to verify now that I have circle shadows though. Hope to have the scope out tonight.
Vic, another thing that is bugging me, I know that the secondary is centered in the tube via measuring the distance from the center screw of the secondary mount to the tubes edge. Anyway, when you shine a lightsource down the tube, or are pointed at the moon, and you look down the tube and see the shadow of the secondary, the circular shadow is noticibly off center, like by 3/4" if not more. I have tried with a high intensity flashlight to see if the shadow changes location when the flashlight is held at different locations around the circumference of the tube, but the shadow doesn't move. Its always biased toward the focuser.
Is this normal?
|
sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
|
|
Yes. There are enough threads about diagonal offset elsewhere not to pollute this master thread with a more thorough explanation .
--------------------
400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
|
RedIrocZ-28
Pooh-Bah
Reged: 09/18/05
Posts: 1175
Loc: Grand Rapids, Michigan
|
|
sixela, you're right. Mods, remove my post if you feel necessary.
--------------------
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 