Finderscope flexure?

[Mark_C]

Hi all,

I have recently started out imaging DSO's using a Canon 60D and am a bit new to this. I am using a QHY5L-II mono connected to my Skywatcher 9 x 50 finder/guider scope on a 120ED with a 0.85x focal reducer and am using PHD2 to guide. I've checked the PHD2 graph and the RA/Dec graphs hovers a very small margin above and below the central line but nothing significant.

I am finding that even with guiding, I am getting star trails in some instances and has left me baffled as my polar alignment is next to perfect (using the D.A.R.V align method). It seems to be worse in certain parts of the sky than others. I tried to image M13 last night and while it looks ok, it is obvious the stars are trailing ever so slightly and is becoming an annoyance. :-( I slewed then over to the M57 and took a 3 minute sub - again there is evidence of star trailing. I then had a go at imaging the M101 but there seems to be hardly any evidence of star trailing... very confusing. I've checked the balance of my scope and that seems to be fine, I've also checked to make sure there's no cable snag - which there isn't, so am starting to become at a loss as to what is causing this, a fault with something? Or just me doing something stupid? (good possibility   ) I've wasted a few decent nights trying to resolve this issue now and am close to giving up. :-(

I am starting to wonder whether the Skywatcher finder is suffering from flexure? I am using the original white bracket that came with the 120ED that has a sprung loaded screw and the 2 black screws? they're nylon I think? Can't be 100% though.

Anybody have any ideas before I go insane?!

[ChrisLX200]

If the guiding trace is good then flexure (of some description) is strongly indicated. Could the actual scope moving, the camera where it attaches to the finder, cables pulling etc. It takes only a thou (0.001") or two movement to give you eggy stars or worse. If you use a separate guidescope it has to be rock solid.

ChrisH

[sloz1664]

I use the Skywatcher finder. I tighten the two screws  hard against the sprung loaded screw to stop any flexure and I can guide 20 mins.

Steve

[Mark_C]

If the guiding trace is good then flexure (of some description) is strongly indicated. Could the actual scope moving, the camera where it attaches to the finder, cables pulling etc. It takes only a thou (0.001") or two movement to give you eggy stars or worse. If you use a separate guidescope it has to be rock solid.

ChrisH

I fear this is what is may be happening. I will have to see if I can get a normal screw to replace the sprung loaded one as that introduces movement quite easily when pressure is applied against it, I didn't realise minuscule movements would have such a drastic effect on the image. Replacing the nylon screws with metal ones should sturdy it even further.

I use the Skywatcher finder. I tighten the two screws  hard against the sprung loaded screw to stop any flexure and I can guide 20 mins.

Steve

I had thought of doing this but my only concern is that the finderscope will no longer be in alignment with the axis of the main OTA if I tighten it up completely, will that not put guiding out?

I use plate solving through the finder to centre the object on the DSLR sensor, so I relied on both scopes pointing at the same object... unless there's a better way of doing it. I really don't know why SW didn't just use 3 solid metallic screws to align to finder.. the sprung loaded screw seems a bit daft, unless I'm missing something!

Thanks,

Mark