What is "Differential Flexure"? (Guiding Issues)

[AstroRuz]

Hello all,

So I routinely have issues with guiding. Last night, I was getting some solid sub exposures of 5 minutes on the Flaming Star Nebula. When it got to about 15-20minutes of the meridian, I began getting trails on my stars. Then after the flip, I was still getting trails despite PHD reporting all's well.  I moved over to the Horsehead Nebula and tried 5 minutes there, and it behaved itself. Then I tested it on the Jellyfish Nebula and it also behaved (just about - minor trails). But it just did not want to dig it with the Flaming Star Nebula, which was practically near the zenith (about 2:30am 30/11/19).

I tried recalibrating PHD (I use Pulseguide so the scope knew where it was). I then also recalibrated PHD at Alnitak so it was near the celestial equator (as the PHD best practices recommend). I even checked my Polar Alignment (done with a Polemaster), which was fine. I couldn't find anything wrong. 

The guide scope was slightly off alignment with the main OTA, but that wasn't making any differences before the Meridian. 

 

I did find the helical focuser of the Starwave 50mm guide scope I was using to have some movement in it again when I manipulated the guide camera. So that's something I need to look into. But before I start throwing money at this issue, I see the terms "flexure" and "differential flexure" coming up. Can anyone shed light on what this means please?

I tried manipulating the OTA in its tube rings, that didn't yield any movement either. The piggyback scope is secure, the g/scope is secure in its rings as well, there's no movement in the OTA focus rack and the cameras are secure in their tubes. 

I do have my guide rings quite far back on the body of the telescope to assist with fitting the guide scope piggyback. 

Video of g/scope play: 

 

Flaming Star Nebula. 5 minute sub, post Meridian Flip, about 2:50am (30/11/19). Note the plate solve compass in the top left. Drift appears to be in S/W direction.

Position of tube rings (they're now at the focus end).

I think I am leaning towards needing to move my tube rings (once I remember to get the tools from work). 

 

 

Equipment:

(brand new) Skywatcher EQ6-R Pro, SW Evostar Pro 80ED, Altair 0.8x R/F, Canon 600D, Altair Starwave 50mm Guide Scope, Altair AR130 Mono Guide Cam. 

 

 

[knobby]

Differential flexure is where the guide camera / guide scope moves independently from the imaging scope / camera.

The focuser movement is a good place to start. Everything needs to be solid !

Edit

Just watched the video of focuser ... That's shocking ! Need to find a way of stopping that. The weight of the camera as it flips across meridian will surely cause movement .

Edited November 30, 2019 by knobby
Added info

[Tomatobro]

Take a pin, move it 400 metres away and the distance between one side of the pin head and the other is roughly 1 arc second. That's the angle you are dealing with.

So ANY relative movement between the imaging scope and guide scope will cause problems. If the camera can sag slightly in the focuser or the guide scope body can deflect in the rings. PHD is guiding the guide scope and has no knowledge of what the main scope is doing relative to it.

[michael8554]

As Tomatobro says.

Remember that your 600D pixels are only 4.3 microns in size, so flexing as small as microns will cause those noticeable elongation in your images.

And your focuser could slop millimeters, a thousand times more !

Michael