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Greenfields

Joined: 25 Nov 2010
Posts: 90
Location: Nottinghamshire, England

rjlittlefield

Joined: 01 Aug 2006
Posts: 17987
Location: Richland, Washington State, USA

Posted: Mon Dec 16, 2013 9:37 pm    Post subject:

Henry, this is a very interesting thread.

To cross-check the numbers, I ran them through the MTF diffraction limit formulas copied HERE. Those formulas produce a cutoff wavelength of 16.9 microns per cycle on sensor = 0.42 microns per cycle on subject, assuming lambda = 550nm. This is reassuring to me because the words used at http://www.microscopyu.com/articles/formulas/formulasresolution.html have never made clear to me what they mean by "smallest resolvable distance between two objects".

 Quote: When scrolling through images there is consistent evidence of nutation [nodding] from frame to frame. It's very consistent: one frame slightly to the left and the other back again. So consistent that away from the centre and perpendicular to the direction of nutation the trails from "stuck pixels” in the stack form two parallel tracks like this:

This part puzzles me. I can easily understand that the rail could be consistently left/right/left/right, for example due to alternate stick/slip on each of the rails. What I don't understand is why this should result in parallel tracks that are horizontal as shown in the crop. If the frames are alternating left and right, then a single stuck pixel should result in two clusters that are positioned left and right of each other, with a clean gap between them. Combined with any motion at all on the other axis, this would result in two vertical streaks positioned side by side, not two horizontal streaks positioned one above the other. What I see in this crop looks like a consistent up/down nodding (actually more like NNE/SSW), combined with a more or less continuous drift on the horizontal axis.

What have I missed?

--Rik
soldevilla

Joined: 16 Dec 2010
Posts: 458
Location: Barcelona, more or less

 Posted: Tue Dec 17, 2013 4:28 am    Post subject: We can not forget: we're talking about 0,001 mm displacement. That's about 100 times the thickness of a paper. Get manufacture a mechanism that is repeated systematically below 0.01 is not easy task. Here are small defects accumulate in the parallelism of the guide rods, in the eccentricity between the shaft, which is coupled to the flexible coupling, and the thread. The shift may also be due to a small angle, not a theoretical 0º, in the parallelism between the guide rails and the platform, and even between the optical axis of the camera and tripod engagement platform. This produces small movements but added in the image on the chip. Y dead pixels or dust spots on the chip is not displaced ... All this can easily be avoided by applying a flat in the images automatically before Stack ... _________________http://www.digitalphotomicro.com
Greenfields

Joined: 25 Nov 2010
Posts: 90
Location: Nottinghamshire, England

 Posted: Tue Dec 17, 2013 7:10 am    Post subject: Thank you Rik, I can't explain the observation, but soldevilla makes some good points. In case it's relevant, I am aware that my Stackshot has a larger than average backlash. [Cognisys, as always very helpful, have confimed that the value they use is an average value]. To work around the backlash when stacking with very small slice spacing, this is my process: 1] Decide what the start points and end points of a sequence look like on the LCD screen. 2] Draw the carriage right back until I am certain that I have travelled further than the backlash. 3] Move forward without allowing any reversing to actually set the start and end points. 4] Draw the carriage well back again then move it forward to approach the start point [but not cross it] 5] Start stacking - but check that the first shot look right. Stackshot usually starts at the right place, but often does not in which case I go through the process again. One thing I have not tried is adjusting the Stackshot motor's speed. It's currently set at 600. Maybe a lower speed would have an advantage with a small slice spacing. soldevilla: I understand what a "flat" means in astrophotography, but how. in practice would you make one for photomacrography ? I presume that you would then subtract it from each image before stacking ? Can Photoshop do this or would you need an astro image editor ? Henry_________________Feel free to edit my images.
rjlittlefield

Joined: 01 Aug 2006
Posts: 17987
Location: Richland, Washington State, USA

Posted: Tue Dec 17, 2013 10:48 am    Post subject:

 Quote: I can't explain the observation,

If you have any reason to investigate more deeply, one source of information is the XOffset and YOffset values computed by Zerene Stacker's alignment process and recorded in its console log and project files (*.zsj). That's the way I got the data on what High Precision really means. See HERE for detailed discussion of what the values mean.

 Greenfields wrote: Maybe a lower speed would have an advantage with a small slice spacing.

At nominal 1 micron focus step, the StackShot's motor will only be advancing by 2 microsteps. I'd be pretty surprised if any of the "speed" parameters have any detectable effect in that regime.

 Quote: To work around the backlash when stacking with very small slice spacing, this is my process:

That's the same process I use. In Zerene Stacker there's support for automating it, via the "Prerun distance" parameter in the StackShot controller configuration panel.

 Quote: In case it's relevant, I am aware that my Stackshot has a larger than average backlash.

I would not expect that to have any effect on the one-way operation that you've described. As far as I know, the variations in backlash are just a matter of tolerances in the thrust bearings and mating of the lead screw and nut.

There are a couple of procedures for calibrating backlash, described HERE. But even when properly calibrated the correction is not exact because the rail does interesting things like twitch significantly from side to side when the direction is reversed. My usual recommendation is to not worry about calibrating backlash, but just know that it exists and work around it with prerun. I also recommend shooting a few extra frames at front and back, not because the equipment does anything wrong but just because it's hard for me the human to tell exactly where the start/stop points should be.

--Rik
soldevilla

Joined: 16 Dec 2010
Posts: 458
Location: Barcelona, more or less

 Posted: Tue Dec 17, 2013 4:24 pm    Post subject: I did some tests strongly reducing the speed of my StackShot and have found improvements when I photograph at x20 (the maximum that I can use, I have no more powerful lenses). A flat can efficiently remove all the dead pixels and dust spots on the chip, it allow to do disappear those "worms" of points, clearly visible on blur backgrounds. The operation to apply a flat is the division, and Photoshop does not this way of applying a layer. But Gimp can, I did tests with Gimp to automatically recover a correct color balance (the flat was a Median of the original image). Maybe we can find an automatic process to apply a flat before stacking, I'll try ..._________________http://www.digitalphotomicro.com
seta666

Joined: 19 Mar 2010
Posts: 860
Location: Castellon, Spain

 Posted: Sun Dec 22, 2013 2:10 am    Post subject: Hello, This is one of my favorite lenses indeed, and while others may think it is difficult to use I always get good results with it; even with not so flat subjects like this bee tongue 40x with 5D mkII Full size http://farm8.staticflickr.com/7162/6695022043_f067a61495_o.jpg stereo http://farm8.staticflickr.com/7145/6695384687_c336c4f97b_o.jpg In my opinion is the best high magnification lens you can get at low cost Regards Javier_________________http://www.flickriver.com/photos/seta666/ www.macrosmuymacros.com
enricosavazzi

Joined: 21 Nov 2009
Posts: 799
Location: Stockholm, Sweden

 Posted: Sun Dec 22, 2013 11:14 am    Post subject: I suspect - but don't know for sure - that the holes are for a centering tool. The tool has pins that press sideways through the holes against an internal element or group of elements until it is centered according to a collimating instrument. The groups/elements are then blocked in place by tightening a ring that presses on the element mounts in the axial direction._________________--ES
Greenfields

Joined: 25 Nov 2010
Posts: 90
Location: Nottinghamshire, England

 Posted: Thu Jan 02, 2014 6:51 am    Post subject: That fits. I received different Nikon objective with similar holes. It was not the item ilustrated and had a broken front element but the seller refunded the payment at once. He did not want it back. I tested it in case it was an example of how well a damaged lens could perform but the image was not usable so I disassembled the objective out of interest and even managed to match is with a Nikon patent. Build quality was impressive. Shame about the damage. The elements were stacked together in cells and the whole column retained by a screw-on cap, all inside the outer barrel just as Enrico suggested. Henry_________________Feel free to edit my images.
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