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Stackshot, maximum magnification?

 
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ChrisR
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Joined: 14 Mar 2009
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Location: Near London, UK

PostPosted: Sat Apr 23, 2016 11:29 am    Post subject: Stackshot, maximum magnification? Reply with quote

What's the highest magnification you've had success with, on a Stackshot?

I'm assuming high precision mode, standard motor.
As reported and discussed before, to some extent, the minimum step and its accuracy/repeatability puts a limit on what can reliably be used.

On a longer Stackshot I find the per-revolution screw eccentricity a nuisance.
(In other words the guide rails don't guide very well)

Has anyone tried swapping to a 400 step per revolution motor?
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Chris R
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jsp



Joined: 28 Mar 2015
Posts: 215
Location: United Kingdom

PostPosted: Sat Apr 23, 2016 12:19 pm    Post subject: Reply with quote

Thanks for asking this. I'm listening in. :-)
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rjlittlefield
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Location: Richland, Washington State, USA

PostPosted: Sat Apr 23, 2016 6:53 pm    Post subject: Reply with quote

I assume this thread is motivated by related discussion in jsp's thread at http://www.photomacrography.net/forum/viewtopic.php?p=189719#189719 and above.

Most of what I know about this issue has already been written up and posted out in years past. The most relevant thread is What does StackShot "High Precision" mean? That thread's summary statement is that
Quote:
As you can see, the StackShot with High Precision OFF is prone to periodic big jumps every 16 microsteps (7.936 microns), with an especially large one every 32 microsteps (15.872 microns).

With High Precision ON, that pattern is not completely suppressed, but it's cut way down. In this run, the largest single microstep is 1.38 microns versus 0.496 microns average, and the largest double microstep is 2.06 microns versus 0.992 microns average.

So, for high magnification work such as 40X or 50X, what I do is to set a nominal step size of 1 micron (2 microsteps), knowing this means that most of the time I'll get a little less than 1 micron and once in a while I'll get a hair over 2 microns. This is sufficient to avoid focus banding while still getting good rendition as illustrated at http://www.photomacrography.net/forum/viewtopic.php?p=103997#103997.

Based on those measurements, and backed up by further experience, I would confidently expect good results in using that particular StackShot unit at 20X NA 0.40 or even 40X NA 0.50 (as illustrated at http://www.photomacrography.net/forum/viewtopic.php?t=11482).

That said, the variable step size makes using the StackShot less efficient than using a repurposed microscope focus block, which can reliably and repeatably make much smaller movements, for example as documented at http://www.photomacrography.net/forum/viewtopic.php?t=11519.

Worse, the results that I've reported are for a single StackShot rail, operated by me. There is no good reason to think that somebody else with a different unit and a different working environment would get similarly good results. In fact there is some reason to expect that they sometimes won't, given reports like the screw eccentricity on ChrisR's long unit, and other reports I've heard regarding units that have suffered some sort of mishap and never worked quite as well again.

As it happens, I have all the necessary parts on hand to try the experiment of swapping in a 400 step motor or even a geared motor. But I've never taken the time to do those tests, and the current discussion makes me even less enthusiastic about doing them now than I was before. The reason is simple: positive results would not be reliable for other users (for the reasons discussed above), and negative results would be of little use to anybody. From my standpoint, that's kind of a no-win situation.

--Rik
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jsp



Joined: 28 Mar 2015
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PostPosted: Sat Apr 23, 2016 9:58 pm    Post subject: Reply with quote

Thanks, that explain a lot that I had not understood before.
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