96 replies to this topic

[cheapersleeper]

The other day after working on my spider for a while, I had to figure out which way to move my primary to get the barlowed laser circle OFF MY CEILING and back onto the secondary. Had to do the piece of paper in front of the tube to find that blob.

[Starman1]

After using the Catseye tools for a while (all are 2"), I find the sight tube is most useful for centering the secondary under the focuser and assessing gross misalignment of secondary tilt. A laser can do the latter, but not the former.

The cheshire is the best at primary alignment. I can see extremely minute misalignments of the primary using this tool. Since there are not multiple misregistrations possible with this tool, as there are with barlows, blugs and tublugs, a cheshire is an accurate tool for primary alignment.

But the XLK autocollimator is simply amazing. With it, I can align secondary tilt to such tiny amounts a slight pressure on a secondary collimation screw changes things noticeably, and align the primary to a better level of collimation than the errors I can see in the cheshire.

I can switch back and forth between cheshire and sight tube and see no obvious errors at all, yet the AC will reveal there are still errors.

And even after I get a perfect stack of 4 center markers in the central hole in the AC and the cheshire shows no visible errors in primary alignment, the lateral pupil in the XLK AC still shows I can improve on the secondary alignment. It is SO sensitive that making the alignments in the lateral pupil usually shows me no change at all in the central pupil in the tool except, perhaps, to make the field around the stacked markers even blacker.

Alternating between this tool and the cheshire sometimes shows that extremely small changes may be necessary in both mirror alignments.

When the Cheshire and AC (both pupils) all agree, there is no doubt you are collimated. But will the scope hold that collimation?

Well, if you have some time to tinker, and you have an autocollimator, you can improve the collimational stability of your scope significantly, and that's not a bad thing.

Properly collimated lasers (and the OP's laser probably was not) can do the job. But I find the "passive" tools more repeatably accurate, and especially find a cheshire ten times easier than a barlowed laser. And the accuracy achievable with an autocollimator isn't achievable with a laser. A laser (with barlow) can be close, but won't beat an AC.

So, lasers are nice to check collimation with, but i wouldn't want to rely on them for my initial collimation when setting up my scope.

IMO, of course.

[cheapersleeper]

How is secondary tilt assessed with the passive tools? Is it strictly an autocollimator function? I am using a sight tube, then laser and then cheshire and can't see a way to set secondary tilt with those tools.

[smee]

Actually, that's not what I suggested. Use the autocollimator to assess the focuser axial alignment with a carefully decollimated primary mirror axis (using the signature alignment of the two center reflections to align the focuser axis), and then use the Cheshire to align the primary mirror axis (which "closes" the system). Using a "darkened" autocollimator by itself is not a conclusive alignment--the carefully decollimated primary mirror signature is a conclusive focuser axial alignment (with 2X magnification), as is the Cheshire signature (which magnifies the primary mirror axial error 2X).

Can I jump in here and ask?

You mention in the quote and in other replies "signatures".

Could you elaborate on that term, I'm unsure what it could mean. Is it to do with the "appearance" or "presentation" of the parts wrt their alignment?

[Vic Menard]

Sight tube: get the secondary centered under the focuser
Laser: Get the cetnered secondary aimed at the center spot of the primary
Cheshire: get the circle, or triangle, or radioative sign into the glowing circle

There it is. And I did not say "axial" one time.

I agree with your "short, short version"!
FWIW, I tend to focus on the assessment and correction of the alignment of two axes, the focuser (eyepiece) and primary mirror (optical) axes--in the short, short version, your second and third steps.

There are measureable tolerances for both of these axes.

The first step in your short, short version is secondary mirror placement. Since the secondary mirror has no axis, we deal instead with the circles, or the edges of the components that are being aligned. There's no specific tolerance here, a residual error in the placement of the secondary mirror only affects the illumination at the edge of the fov, optically. Secondary mirror placement also affects the mechanical alignment of the orthogonality of the optical axis relative to the mechanical (altitude/azimuth, RA/Dec) axes.

That's a lot of axes!

[Vic Menard]

How is secondary tilt assessed with the passive tools? Is it strictly an autocollimator function? I am using a sight tube, then laser and then cheshire and can't see a way to set secondary tilt with those tools.

I'll answer your question with my next post regarding alignment signatures.

To add to the signatures post, the secondary "tilt" adjustment is used to correct focuser axis errors. Secondary "placement" is usually brought about by adjusting the secondary's rotation and the secondary's positioning, closer to or farther from, the primary mirror. That's a bit over simplified, but it should answer your question.

[Vic Menard]

Can I jump in here and ask?
You mention in the quote and in other replies "signatures".

Could you elaborate on that term, I'm unsure what it could mean. Is it to do with the "appearance" or "presentation" of the parts wrt their alignment?

Yes. There are various signatures that indicate a specific alignment is correct (to the read accuracy of the tool being used).

Sticking with cheepersleeper's short, short version, let's start with secondary mirror placement.

The secondary mirror is optically aligned to the pupil of the sight tube when certain circles are aligned. These circles are:
The bottom edge of the sight tube
The actual edge of the secondary mirror
The reflected edge of the primary mirror
The reflected edge of the bottom edge of the sight tube, and in the center
The pupil of the sight tube

The only edge not centered is the reflected edge of the secondary mirror, usually seen in silhouette surrounding the reflected edge of the bottom edge of the sight tube. This edge reflection will appear offset toward the primary mirror. You don't have to make any special mechanical accommodations for this offset to happen--when you center the secondary mirror under the focuser, you've already generated the required offset.

In the real world, it's more complicated than that. There are a variety of errors that confound most Newtonian owners when assessing, and correcting, the secondary mirror placement. But for now, let's leave this signature alignment as it is--the concentricity of certain edges.

The next two alignments take place in the center of the fov (not the edge).



First up is the focuser axis signature. With a sight tube, this is represented by the alignment of the primary mirror center spot to the intersection of the actual sight tube cross hairs. If you did a good job with the secondary mirror placement, this should already be close. Some people have difficulty getting the cross hairs and the center spot in focus at the same time, so the read accuracy is not always very good. Don Pensack suggests backing your eye away from the sight tube pupil until the focus differential improves. Others just use a laser.

Which brings us to the second focuser axis alignment signature. Using a simple thin beam laser, when the outgoing beam intersects the primary mirror center spot, the focuser axis is aligned. The read accuracy is usually better than the accuracy achieved with a sight tube, and depending on the application, may improve image performance.

The third focuser alignment signature is achieved using an autocollimator. With the focuser axis already carefully aligned with a sight tube and/or laser, and the primary mirror axis carefully decollimated to separate the reflections seen in the autocollimator, when the two reflections in the middle are properly aligned--the focuser axis is collimated. This signature alignment magnifies any residual focuser axis error 2X (the read is twice the actual error), so the precision is very good.

Moving the primary mirror alignment screws has no affect on each of the focuser axis alignment signatures. This is why the primary mirror alignment is the last step.



There are several primary mirror alignment signatures.

With a sight tube, the primary mirror alignment is correct when the reflection of the sight tube cross hairs is aligned to the actual sight tube cross hairs. Read accuracy varies, especially when the focuser axis is not already precisely aligned.

With a Cheshire, the primary mirror alignment signature is the perfected alignment of the primary mirror center spot relative to the bright Cheshire ring (or the darkened pupil seen inside the bright Cheshire ring). Read accuracy is excellent even when the focuser axis is not precisely aligned, and the read error is 2X the actual primary mirror error.

With a simple thin beam laser, the return beam is often used to align the primary mirror by aligning the return laser dot to the laser emitter in the focuser. There are inherent problems with this signature. The focuser axis must be precisely aligned, and the laser dot tends to drop in the emitter "hole" before "precise" alignment is achieved. Pattern generators help to alleviate the second problem, but the alignment is still dependent on the focuser axis alignment and there's no error magnification.

The Barlowed laser works similarly to the Cheshire eyepiece. The signature alignment is the silhouette pattern of the primary mirror center spot precisely overlapped on the Barlowed laser target aperture. Read accuracy is excellent even when the focuser axis is not precisely aligned, and the read error is 2X the actual primary mirror error.



Interestingly, there is no specific primary mirror alignment signature using an autocollimator. A "perfect stack" (no reflections except the center spot on a darkened background) indicates the two axes intersect at the center of curvature (2X the focal length). Combined with a precise Cheshire alignment, the perfect stack indicates the two axes are closely aligned to each other. The read is meaningful in that a "perfect stack" should be present when both axes have been precisely corrected.

The read accuracy of a perfect stack can be further improved with a second, off axis, pupil. This idea utilizes another signature, and is the result of Jason Khadder's out of the box thinking after analyzing the behavior of the autocollimator reflections. The off axis pupil sees the reflections separated on two parallel axes (the pupil axis and the primary/optical axis), the resulting signature is two individual stacks. With the Infinity XLK autocollimator, the user can assess the focuser axis with a carefully decollimated primary and parallelism of the two axes. Combined with a BlackCat Cheshire (and the matched CatsEye triangular or HotSpot center spots), these two tools provide very precise axial alignment.

[cheapersleeper]

It makes a bit more sense each time...almost there.

Regards,
Brad

[Vic Menard]

I added a bit to my response to your post to clarify how secondary mirror tilt is usually assessed and corrected. (But you still have to be careful not to induce combined rotation/tilt errors... )

[uniondrone]

OK, I'm still not sure I understand what you're saying. When you say "registration of the drawtube" do you mean the registration of the drawtube relative to the focuser body? (Modern Crayford focusers have all but eliminated that problem.)

That's exactly what I am saying. And this is exactly why I dumped the rack & pinion focuser and upgraded to a MoonLite Crayford.

That's certainly the way they're sold, but simple thin beam lasers are not as accurate as Cheshire eyepieces and their derivatives when assessing and correcting primary mirror axial errors. If you think about it, the laser's return beam is simply the reflection of the focuser axis. It doesn't represent the primary mirror axis unless the focuser axis is perfectly corrected. If there's a residual focuser axis, one half of the residual error will be propagated back to the laser emitter via the return beam, which means a perfect return beam alignment is actually imperfect! And there's no magnification factor either.

So, while you can expect excellent alignment accuracy with a Cheshire eyepiece or collimation cap, it's quite possible to end up with a significant error (the kind that actually impacts image performance) when you use the unBarlowed return beam from a simple thin beam laser to align the primary mirror.

That doesn't mean there aren't some simple workarounds to get the job done with a good simple thin beam laser. The 2-inch Glatter is machined to sufficient precision to all but eliminate pesky registration errors, and with the standard 1mm aperture stop, the diffraction pattern around the laser dot can be used to bring the silhouette reflection of the primary mirror center spot back to the emitter/target. With this tool, I can get a precise read of the focuser axial error and the primary mirror axial error.

Thank you for the insights on this!

[David Pavlich]

In keeping with the theme of top flite lasers vs so-so lasers, I finally bought a Glatter 2" laser and TuBlug. For anyone that's been around mechanical stuff and precise machining, it's easy to see why Howie's stuff is so well received. The tolerances are very tight between the laser and the TuBlug. I would venture a guess that if I heated the TuBlug with a heat gun, it might be too tight to put the laser in, it's that good. Pricey? All things are realtive and if results are critical, then it's worth the price of admission. Just so you know, I am not affiliated with Howie Glatter other than a few emails asking for advice. He is a good guy as well.

David

[Starman1]

How is secondary tilt assessed with the passive tools? Is it strictly an autocollimator function? I am using a sight tube, then laser and then cheshire and can't see a way to set secondary tilt with those tools.

Vic answered this very well.
The sight tube parts important in secondary alignment are the peephole (to center the eye), the inside lip of the front end of the sight tube (to gauge concentricity with the outer edge of the secondary mirror's reflective surface) and the crosshairs (to center the primary mirror's center marker behind by adjusting the secondary tilts).

There is also a distant reflection of the bottom of the crosshairs that will appear to align itself with the near-to-the-eye actual crosshairs when the primary mirror is aligned.
Hence, a grossly miscollimated telescope COULD have both mirrors very roughly aligned with just a sight tube if, after the primary's center marker is aligned behind the crosshairs near to the eye by adjusting the secondary, the distant reflected image of the crosshairs are aligned behind the near crosshairs by adjusting the primary.
This isn't accurate enough for primary alignment, but if the scope is grossly out of collimation this step allows one to use a cheshire or barlowed laser and at least have the primary's center marker be visible in the field of view.

In the same way, though the return bounce of the simple beam laser is not accurate enough to align a primary, it does allow a grossly miscollimated instrument to be adjusted to where the appropriate primary mirror collimation methods will at least find the images in the field of view.

How often is an instrument so grossly out of collimation? When a new scope is assembled for the first time, or when a new mirror cell or secondary is installed in the scope. Removing mirrors for cleaning and reinstalling them doesn't usually put things that far out of whack, but if the secondary is removed and cleaned, it could.

[cheapersleeper]

Man, Don, you have just proven yourself not to be an ATM. I tear stuff down regularly to the point where when I try to collimate in the living room, I end up with the laser missing the secondary and hitting the cat. Betting my fellow ATMs have similar problems.

Regards,
Brad

[Jason D]

For anyone that's been around mechanical stuff and precise machining, it's easy to see why Howie's stuff is so well received. The tolerances are very tight between the laser and the TuBlug.

When I slide my Glatter inside the Tublug, I can hear the sound of the air whoosing out. The fit is impeccable.

[smee]

thanks for that considerable detail.

[Mark Harry]

I always judge the centration of the laser by rotating it in the focuser. (I also rack the focuser in and out to see what slop is there too)
If the laser spot wanders, just keep rotating and watching, and the dot will circumscribe a circle of sorts. The center of the circle is where the laser dot ought to be pointed. Now, you can index the thing for reference, and the laser can be collimated. Simple.
If after getting the laser properly aligned, then you can collimate the 2 mirrors with a double bounce. Place the first reflection off the secondary into the center spot on the primary.
Peer at the reflection of the secondary off the primary, which will show you the image of the laser aperture. There will be a row of multiple reflection dots. Touch the side of the focuser, or spider vane, and apply just a bit of small pressure. You will notice a stray dot somewhere on the face of the laser move slightly. Tweak the primary so that this dot is aligned precisely with the aperture of the laser and disappears.
You're done. Enjoy the view. Most times, it takes me about a minute if I know where the allen wrench is at to tweak the secondary. If I don't, I can fuss around for 20 minutes to locate the thing! I'd say the secondary is the culprit around 80% of the time if things are a little off. Generally, one of the 3 screws needs less than 1/8th turn to get things straightened out. (barring any severe banging around with the OTA.)
Thought I'd add these few tidbits just in case you want to still play with the laser. It's how I do it; and collimation has been relegated to pretty nuch a non-issue. I never had anywhere near the precision I wanted by using the passive tools- Always had to follow up with fiddling while viewing to get it dialed in, etc.
M.

[Jon Isaacs]

I always use the barlowed laser rather than the double bounce because it is essentially only sensitive to the tilt of the primary. It does not even require a well collimated laser and the fit in the focuser is not critical.

A good experiment is to put the laser in the focus and as Mark describes, push on it. The return beam moves a considerable amount. If you try that with the barlowed laser, it does not move because you are only looking at the reflection of the shadow of the ring. You can move the laser a lot and the position of the shadow does not move.

When Nils figured out the Barlowed laser technique about 10 years ago, it was the final step in making the laser an effective collimation tool.

Jon

[Jason D]

I always use the barlowed laser rather than the double bounce because it is essentially only sensitive to the tilt of the primary. It does not even require a well collimated laser and the fit in the focuser is not critical.

Statements like these give the impression that barlowing any low-cost low-quality laser collimator will give the same results as higher cost quality laser collimator. That is untrue. First, what about the secondary alignment? Barlowed laser can’t be used for that.

As far as “only sensitive to the tilt of the primary” is also somewhat untrue. If there is too much slop and you apply pressure then the target (where the center spot shadow is projected) will move and the “virtual laser point” will also move.

Jason

[Vic Menard]

I always judge the centration of the laser by rotating it in the focuser. (I also rack the focuser in and out to see what slop is there too)
If the laser spot wanders, just keep rotating and watching, and the dot will circumscribe a circle of sorts. The center of the circle is where the laser dot ought to be pointed. Now, you can index the thing for reference, and the laser can be collimated. Simple.

This assumes laser/focuser drawtube registration is consistent (or worse, laser/2- to 1.25-inch adapter/focuser drawtube registration). It also assumes that the registering cylinders are actually round (when they're not, the pattern may appear ovoid and may cycle in a "spirograph" pattern around a common center, again, assuming registration is consistent). I'm not saying it can't be done, nor that the error can't be visualized. But it isn't always "simple".

...If after getting the laser properly aligned, then you can collimate the 2 mirrors with a double bounce. Place the first reflection off the secondary into the center spot on the primary.

This clearly represents focuser axial alignment. It's easily visualized on a high humidity night, where the beam is clearly visible all the way to the primary mirror center spot.

Peer at the reflection of the secondary off the primary, which will show you the image of the laser aperture. There will be a row of multiple reflection dots. Touch the side of the focuser, or spider vane, and apply just a bit of small pressure. You will notice a stray dot somewhere on the face of the laser move slightly. Tweak the primary so that this dot is aligned precisely with the aperture of the laser and disappears.
You're done.

For longer focal ratio optics, I would agree. Below f/5, I think most Newtonian users will be able to do better by Barlowing the laser or using a Cheshire eyepiece for the primary mirror axial alignment. Again, I'm not saying it can't be done (you're obviously getting an alignment that meets your performance expectation), I'm just saying that other users may not have the same degree of success as you have with this alignment procedure--at least, that's my experience.

...I'd say the secondary is the culprit around 80% of the time if things are a little off. Generally, one of the 3 screws needs less than 1/8th turn to get things straightened out. (barring any severe banging around with the OTA.)

I'm not quite sure what you mean here. If you mean the secondary mirror normally needs adjustment to "fine tune" the focuser axis alignment, then I agree. In fact, with many users moving to plastic washers and no-tool knobs to facilitate easy adjustment of tilt and rotation, I find it very common that both (tilt and rotation) need adjustment.

Thought I'd add these few tidbits just in case you want to still play with the laser. It's how I do it; and collimation has been relegated to pretty much a non-issue.

Considering the focuser axis alignment error tolerance is about +/- 3-percent of the primary mirror diameter (0.3-inch radius from the center of a 10-inch mirror without a coma corrector), the internal laser alignment doesn't need to be perfect, just capable of maintaining tolerance. Of course, using the (unBarlowed) return beam to correct the primary mirror axis error is a different story. The acceptable (high performance) tolerance is only 0.005 times the focal ratio cubed, +/- (about 0.03-inch radius from the center of the laser emitter for an f/5 mirror--assuming the focuser axis alignment is perfect).

I never had anywhere near the precision I wanted by using the passive tools- Always had to follow up with fiddling while viewing to get it dialed in, etc.

Interesting. If you don't mind me asking, what passive tools were you using, and how was the precision lacking with respect to using a simple thin beam laser?

[Vic Menard]

...A good experiment is to put the laser in the focus and as Mark describes, push on it. The return beam moves a considerable amount. If you try that with the barlowed laser, it does not move because you are only looking at the reflection of the shadow of the ring. You can move the laser a lot and the position of the shadow does not move.

To which Jason responded, "As far as “only sensitive to the tilt of the primary” is also somewhat untrue. If there is too much slop and you apply pressure then the target (where the center spot shadow is projected) will move and the “virtual laser point” will also move."

To clarify, this is what Nils Olof described as a balanced Barlowed laser procedure. His original Barlowed laser experiment placed the "virtual laser point" above the fulcrum of the focuser drawtube and the target below the fulcrum. Thus, if the top of the focuser drawtube is torqued away from the primary mirror end of the OTA, the bottom of the focuser drawtube will move toward the primary mirror end of the OTA, effectively tracking the silhouette (shadow) of the primary mirror center spot by moving the target along with the shadow.

[howard929]

I've found it to be quite easy to NOT torque, lean on or otherwise cause the focuser draw tube or the laser so that they are out of the position they're normally in while collimating with a barlowed laser. Which so far has worked well for me.

Howard

[Jon Isaacs]

I always use the barlowed laser rather than the double bounce because it is essentially only sensitive to the tilt of the primary. It does not even require a well collimated laser and the fit in the focuser is not critical.

Statements like these give the impression that barlowing any low-cost low-quality laser collimator will give the same results as higher cost quality laser collimator. That is untrue. First, what about the secondary alignment? Barlowed laser can’t be used for that.

As far as “only sensitive to the tilt of the primary” is also somewhat untrue. If there is too much slop and you apply pressure then the target (where the center spot shadow is projected) will move and the “virtual laser point” will also move.

Jason

Jason:

I was trying to be brief because I was writing it on my cell phone. The double-bounce is used to set the primary tilt, the Barlowed Laser is to set the primary tilt, another technique is necessary to set the secondary tilt. If a laser is used, a ggod fit and alignment of the laser are critical.

I bought my first Howie Glatter Laser 10 or 11 years ago so I am coming at this from the other side, that is, one can actually achieve reasonable collimation with a lesser instrument if one is careful and aware.

As far as "essentially only sensitive to the tilt of the primary", this rests on the interpretation of "essentially." Essentially I use "essentially" to mean to "a first order."

In any event, performing the experiment is instructive, when using the double-bounce, it only takes a small pressure to move the dot on the face of the laser, when using the Barlowed laser, even comparatively large pressures and displacements result in no visible motion.

Jon

[Mark Harry]

Yes, Vic, I was pretty brief. But I can add, my expectations are pretty high, imo. I just covered the major points. I was also assuming that reasonable accurate components, and pre-collimating assembly is carried out correctly. (I didn't write a whole chapter). My methods work WELL from F/3.3 and up, fyi. In fact, I've never had to fiddle with tweaking after this double bounce method was used. (If there was a defect, I yanked the optics and tested them/ checked for pinching, etc.)
Cheers,
M.

[Vic Menard]

...You can move the laser a lot and the position of the shadow does not move.

Apparently, while everyone was being brief, so was I!

I commented that the procedure needed to be balanced to remove the sensitivity to angular error contributed by the focuser drawtube and/or registration. The reason I felt it was important to note this particular sensitivity is because many users simply insert their economy windowed laser in a Barlow (seated in their 2- to 1.25-inch adapter which is then seated in their 2-inch rack and pinion focuser). While this does allow the shadow of the center spot to be observed on the laser target in the window, the procedure is not balanced and is more sensitive to angular errors than the balanced procedure described by Nils Olof.

I know that Jon uses a Glatter laser, and I'm sure his focuser is also top notch. My only concern was for the readers of this thread who might be less well equipped.

[Vic Menard]

...My methods work WELL from F/3.3 and up, fyi. In fact, I've never had to fiddle with tweaking after this double bounce method was used.

I can't argue with your expertise. In fact, that's why I stated, "Again, I'm not saying it can't be done (you're obviously getting an alignment that meets your performance expectation), I'm just saying that other users may not have the same degree of success as you have with this alignment procedure--at least, that's my experience."

I'm on record (numerous times in these forums) stating that after dark, I turn to my Glatter laser with 1mm aperture stop to assess and correct the axial alignments of my 22-inch f/4 Dobsonian, because "...it gets the job done." Of course, I'm also comfortable using "passive" tools and regularly advocate the use of CatsEye tools for various alignment signatures.

I'm not trying to bait you in this discussion. I truly respect your contributions in these forums. But I don't usually assume, "that reasonable accurate components, and pre-collimating assembly is carried out correctly" in a thread entitled, "Collimation is about to make me dump both my Dobs". So while the content of my post may have seemed provocative, my concern was for less expert readers.