Effect of Misalignment Between Optical Axis and Motion Axis

[mjkzz]

I do recall meeting some frustrations with this approach, such that I didn't think my method mature enough to post out. But as it's been a few years, I don't recall what those frustrations were. I do think the approach has merit.

I think when calibrating alignment, the stacking distance is irrelevant, so one can use the full travel distance to calibrate it. In this sense, the merit of your method is obvious -- if the white paper is far away, say a meter or more, the shift of laser dot (or line or cross) will be significant enough to show the difference.

In other words, if there is misalignment, the shift will be heavily biased toward the misalignment, so even if you have rail shifts, the end result will show the bias, statistically speaking.

[mjkzz]

My newer cameras don't have hot pixels to the same extent and misalignment is harder to identify.

Have a cylinder of say, 5mm in length and 1mm in diameter. At beginning, make sure you only see the top section and make it a circle. Move your rail to 5mm forward, check your image.

If it is perfectly align, what you see is a perfect circle, too, with same side as the first one. If you do not see it, you have a misalignment. This requires a telecentric lens (or very close to).

Just a thought.

[enricosavazzi]

I find this very interesting since I have almost finished my manual horizontal rig. At the moment I will only be using a MP-E and a few reversed enlarger lenses. I was wondering how 'critical' it was if my lens was perpendicular to my specimen? If so, I was wondering how would I verify that.

perpendicular to the specimen: this is not a requirement at all. In fact, many of us often shoot stacks of oblique subjects.

I guess you mean the requirement that the optical axis of the lens must be parallel to the direction of movement of the camera (or stage if the latter is the moving part). If so, yes, then this is the requirement that this thread is talking about.

[mawyatt]

With my old Pentax *ist DS camera I was plagued with hot pixels. As annoying as they were, the streaks they created in a stack did a marvelous job of pointing out the axis of the actuator. When the streaks pointed at the center of the image the axis were aligned. If they wobble, the actuator wobbles.

My newer cameras don't have hot pixels to the same extent and misalignment is harder to identify.

Keith

Keith,

My D800E has a hot pixel I use exactly for this, tells me in the stack if the rail has wobble or serious misalignment.

I think most, if not all sensors that we get in our cameras have a few bad pixels, which may be hot (read out as full) or cold (read out as empty). Possibly these are "hidden" at the factory by the replacement or average neighbor in the camera readout firmware. If a pixel goes bad later then it shows up in the images like my D800E.

Best,

Mike

[mawyatt]

My newer cameras don't have hot pixels to the same extent and misalignment is harder to identify.

Have you considered batch editing your test stacks to add some markers in fixed pixel positions? A Photoshop action would be pretty simple.

--Rik

Rik,

That's an interesting idea, introduce a "built-in" alignment indicator marker prior to stacking. My D800E has a hot pixel which I use for this purpose, but not my D800 nor D500. I think the D800E hot pixel appeared recently, as I don't recall seeing it in stacks from the distant past.

I'm not particularly good with PS nor LR, but agree it seems a simple means should be available to introduce the indicator marker. One could use this for actual alignment as well, by simply moving the rail to extremes and capturing a few images there and in between, then looking at the marker movement in the stacked output.

Best,

Mike

[ray_parkhurst]

My T2i has a hot pixel (Red) in lower left corner that I used to assess telecentricity in my early lens tests. I don't find it useful for alignment since it's far from center, so can't help when aligning non-telecentric lenses. I suppose things like lasers and markers and such might help, but it is actually really easy just to pixel peep two images from a detailed subject.

I also just realized my application is a special case. I move the subject, not the camera. So my process may not make sense to other folks who move the camera and keep subject fixed.

[Beatsy]

This is all starting to sound quite similar to drift alignment in astrophotography. Perhaps some useful ideas hidden there? That process is slow but pretty much automated now, isn't it?

[mjkzz]

finally got time to finish it. I have not found any way to correct misalignment, but thinking backwards, I guess you can use the width (or height if streaking shows up vertically) of streakies (have to borrow this word), to estimate amount of correction to be applied. And once setup is corrected, if you do not make significant changes, you only calibrate it once.

Detailed Blog Here

[rjlittlefield]

Another way is using a laser pointer shooting through view finder, this was done by Chris (forgot which Chris). That method might not work for electronic view finder type camera, but could be very useful for other type, as you can make the laser to come out at center of the lens. With this, you do not need to use an objective, just a regular lens.

It was I, in the post Laser aiming and focus in photomacrography.

I have indeed used this laser to align my rig. With the tube lens assembly on the camera, but with no objective attached, the laser projects nicely onto a white piece of foam core mounted on the subject stand. Make a dot where the laser hits, then slew the rig through its full range of focusing movement. If the laser doesn't move, I consider the rig to be aligned. If the laser does move, it's obvious which way to adjust the rig to correct it.

I do recall meeting some frustrations with this approach, such that I didn't think my method mature enough to post out. But as it's been a few years, I don't recall what those frustrations were. I do think the approach has merit.

--Chris S.

I agree the approach has merit.

The biggest issue I see is that the laser will come straight out the front only to the extent that it is pointed straight into the eyepiece. If it enters the eyepiece at an angle, then it will emerge from the lens at some angle also. Assuming a 200 mm tube lens, then the off-axis angle leaving the lens will be only about 1/4 as large as the off-axis angle entering the eyepiece, but if you're going for precision it matters. In Chris's rig, careful assembly of the bracket and a tight fit on the eyepiece may take care of this aspect.

The potential errors could be greatly reduced by having two small apertures, both centered, and as far as possible apart along the optical axis. Given the stuff in my own kit, one easy approach would be to mount a 200 mm telephoto, stop that down to minimum, and add an iris at the end of some extension tubes in front of the lens.

--Rik