Differential Flexure

Guiding Troubleshooting
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Tony Gondola avatar
Tony Gondola avatar

There have been more than a few threads here and elsewhere taking about differential flexure, usually in reference to why OAG is the better way to go. One thing I’ve never seen is someone actually measuring it. On a very basic level, it would seem that a good general test would be to point your system at the lowest altitude you would normally image and take a long exposure, say 10 min. If guiding remained good through that exposure yet the stars in the sub are not round, that would certainly indicate that something is moving. No way to tell if it’s the image train, guide scope or anything else but it would demonstrate that there is a problem. If the result of the test is round stars then all is good and if you benefited from an OAG, it wasn’t because of DF but something else.

I could also imagine running two instances of NINA so that you can plate solve the main camera and guide camera. Start off pointing at the zenith, plate solve, step down to a lower elevation and repeat, down to your personal elevation limit. You could then take that data see if the pointing between the main scope and guide scope diverges with elevation.

Thoughts?

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andrea tasselli avatar
andrea tasselli avatar

Mirror flop (for cats and reflectors in general) can give the same results as “differential flexure” in that context. Unless the guide scope is mounted on foam or some other crappy material I can’t see a physical justification for actual differential flexure in the classical sense nowadays (let’s say for typical exposures not exceeding 600s). Incidentally, that would manifest itself as a shift in the direction of the gravity vector whilst the former (DF) would manifest in a shift not strictly in the direction of the gravity pull, generally speaking.

Tony Gondola avatar
Tony Gondola avatar

Good point and I would expect that it could represent that way if you took the data. Measuring the difference in pointing would only tell you that something is shifting, not what is shifting. That said, I would expect that the shifts caused by a mirror mount with to much freedom of motion would show as a sudden change in pointing where that due to flexing due to gravity would tend to show something more like part of a sine curve as you go through different elevations.

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andrea tasselli avatar
andrea tasselli avatar

There wouldn’t be just a sharp difference in position but, once reached a critical inclination, there would an initial faster drift followed by a slowing down as the mirror adjusts to an equilibrium position (and I know for direct experience). Differential flexure harks back to the time when we used to take hour-long integrations or even before when people used big slow refractors to take emulsion-plate exposures of several hours. Allan Sandage used to ride the cage of the Hale great reflector for nights at a time, guiding on off axis reticule to avoid such a thing (he was particularly proud of not having to take a leak the whole night long). But I digress; the point of differential flexure is that, if a thing at all, would need to manifest over some length of time and our typical integrations are shorter than the time it take for the gravity vector to change enough to manifest a different tracking path between main and guide scope.

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Tony Gondola avatar
Tony Gondola avatar

That was my initial take on the subject also, that with typical integration times used today that DF is much less of an issue. Still, I think it would be interesting to actually measure it as a sanity check if nothing else. This is what happens when you don’t have any clear nights for several weeks at a time. The rabbit holes become tempting!

One thing I didn’t realize when I posted on this is that NINA already has a plug-in to measure and and correct DF. It might be interesting to add this to a session, just to see what the system looks like.

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Alex Nicholas avatar
Alex Nicholas avatar

To what end?

I think that with regard to differential flexture in an imaging system, knowing ‘how much’ is not valuable… Knowing it exists is 100% important, but knowing that its say, 5”/hr or 1.23” per degree of scope tilt…

The complication will be (as I’ve found in systems in the past) that differential flexture may affect you worse at certain declinations, depending on where and why the flexture is occuring…

I think, as long as you can identify that your guiding is reporting no star drift, and your images are showing star trails, then you know… and you fix it. Spending time measuring ‘how much image degrading differential flexture exists’ seems like a completely unnecessary set of mental gymnastics… Identify that there is flexture - identify and rectify flexture…

Its kind of the same as if I indentify that I have a stone in my shoe… I’m not going to spend hours trying to think of all the places from which the stone may have originated, or trying to discern the size of the stone by how much it’s impeding my ability to walk… I’m just going to remove the stone and get on with my day.