M51 and Mirror Flop - Doh!

[wolfman_55]

Finally got the C11 and EQ6 to work together. Tracking seemed good, even better than the HEQ5 I think, so gave M51 a go:

Not bad, but a little disappointing for 25 x 10 mins I thought. The stars seemed a little trailed so I stacked the images without using an alignment star:

Clearly even though the tracking is good, something is moving. The guidescope is mounted to a dovetail on top of the C11, so I can only put this down to mirror flop.

Does this seem reasonable?

Dave

[glider]

Poor polar alignment?

[philjay]

As an ex c11 imager thats not mirror flop. Never had any problems when imaging with my c11. Yes the mirror moves and can flop when slewing fast but when tracking at sidereal rate even going over the meridian wont have an effect until its way over balance. Theres alot of myth about mirror flop Imo, it does exist but not to the extent some folks would have you believe.

as noel suggests thats more likely pa

just remember your imaging at f10 if not using a reducer so guiding needs to be good. I guided at f12 with mine using a mak127 as guide scope and got stonking guiding

Edited April 11, 2014 by philjay

[Guest Kheldar]

As above, I call PA on that one

[wolfman_55]

Hi guys, thank you for the feedback. I'll give the PA another look. Hope you're right, that should be easier to fix.

 

I guess this was what made me think about mirror flop:

 

http://www.marsastro.org/presentations/MARS_Mirror-Lock.pdf

 

Dave

[Guest peter shah]

beautiful shot with super colour.

Its not Polar you would get field rotation and not an arc, The movement is likely to be flex, the gentle arc suggest to me micro movement in the optical train as the scope tracks, not very noticeable in each sub but over several hours will show. Flex most likely to be in the in the focuser (if you have a crayford fitted) or if you are using a guide scope the differential flex between the two scopes. If you are using  an off axis guider then it will help but you can still see movement between each frame. On the SCT it more likely to be movement in the primary.

 

To be honest it does look manageable your results prove this.

[wolfman_55]

Cheers Peter. Just ordered an OAG and will check my polar alignment over the coming days. 

 

I'm still thinking differential flex or mirror flop to be honest.

 

As it happens, its the first image I've taken for some time without a light pollution filter. Might explain the nice colours.....

 

Dave

Edited April 14, 2014 by wolfman_55