I'm hoping that someone can ID what looks like a possible issue with field rotation.
Nice clear night, Dark sky site, M16 found & centred, scope polar aligned using PHD drift method as is my usual procedure.
It was cold & dewy, had a dew heater running on the 'Scope - but not the guidescope. Some lens fogging issues noted, hairdryer on standby to clear as required.
DY red line was running smooth & flat on Alt & Az on the phd graph as normal. Completed Polar alignment, calibrated and started guiding as usual.
This is a single 10 min sub, dark applied.
Using EQ6, ED120, Orion Miniguidescope, SBIG 8300c OSC
I shot a 5 min sub which looks better, but still not great, also a 7 min sub and it appeared that the longer the exposure was the worse it looked.
I haven't had this happen before, I didn't know you could even get field rotation on a GEM and it's a bit frustrating travelling out to the country and not getting a good return from a long night.
Definitely field rotation. Given the position of the eagle to get that much field rotation in that part of the sky in a 10min sub your polar alignment must have been miles out. Something went wrong in your drift alignment. Maybe you've mistaken RA for DEC in the graph?
Yep, looks like field rotation due to poor polar alignment. If it was that severely out, your PHD graph should have started shooting off the graph within seconds?
Alternatively, if you were set up on soft ground (e.g. wet grass) the scope may have settled under its own weight and thrown out the polar alignment (though usually not by this much).
It might also be worth trying one of the software-assisted approaches (AlignMaster, PoleAlignMax, etc) to see how you like it?
Thanks guys,
Well diagnosed, both of you. I was on soft wet grass and the graph went crazy at first. Tool a while to settle it on Alt, went to az and it was pretty good. Then went back to Alt and it was spot on. It must be as you said Naskies
Trying again tonight on concrete at home. Looks perfect and back to business as usual first time around, go figure!
Another variable to add to the long list - Thanks fellas
Cheers
Andy
I find that placing three heavy Besser bricks / thick pavers on the grass (or even better - half burying them into the dirt) helps to stabilise the mount.
Thanks guys,
Well diagnosed, both of you. I was on soft wet grass and the graph went crazy at first. Tool a while to settle it on Alt, went to az and it was pretty good. Then went back to Alt and it was spot on. It must be as you said Naskies
"Spot on" worries me ... In your OP your said the red line was flat. You are always going to get some movement. Is it possible that you forgot to turn Dec guiding back on? If so, and you went back from dX/dY to RA/Dec in the PhD graph, the red line would be dead flat, but that's because you are not guiding in DEc, not because you're perfectly aligned.
That could explain how the scope could sink into soft ground but nothing showed up in PhD.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,435
Yes it looks like the bright star above the nebula is your autoguide star. You can get field rotation whenever the Earth's rotational axis and the telescope polar axis don't point in the same direction. It's just that an alt az mount is an extreme example of this effect but it can happen with a GEM. That's why they have to be precisely polar aligned even when you have an autoguider.
Many, many thanks to the IIS community for your input and replies.
I did a test shot of M16 last night from my light polluted suburban backyard setup on a concrete surface with much improved results.
(It's only an 1hr of data, with minimal darks - but there's no field rotation).
Amazing, after doing all the usual unpack, setup, initialize, 1 star align, Phd drift align Az, Phd drift align Alt, recheck Phd align Az, 2 star align etc. calibrate, frame, focus, finefocus, in record time, (about 90 mins) - M16 was smack in the middle of the crosshairs on the goto, first time, just as it should be!
(I should have mentioned earlier it wouldn't work at all on sat night, probably as the alignment was so far off.)
Jon, I did notice the DX graph on Sat night, there was some, marginal movement, deftinately not a flat line as such.
Happy days, shame about wasting a night's imaging under a beautiful dark sky, but it's all a learning curve eh?
Guess we will set you up in the driveway instead of the stables yard next time if there has been any rain. The drive is inches deep in crushed rock that has been down for years.
And a good excuse for me to carve off a bit of the dam bank for a pair of piers (one for you, one for me)
Bluester, it's all good. as of 2 years ago I didn't even own a telescope (until you kindly donated me your old newt, "show your kids the moon you said"). Now I'm totally hooked and attempting shots of these incredible objects that are turning out semi- half decent and continually improving.
Even on amazing clear dark sky nights such as last Saturday, when the skies were perfect but my technique and a new variable let me down, nothing was wasted, I learned some more from a failure and there's plenty of clear nights a comin' to practice... and practice makes perfect they say!
Damn this AP thing is addictive
Special thanks to all the support from the IIS members too.
Bluester, it's all good. as of 2 years ago I didn't even own a telescope (until you kindly donated me your old newt, "show your kids the moon you said")
Oh, dear, that probably makes it all my fault. Don't tell R!
Still, even with a learning night rather than the result that we were looking for it was a fun night and a good chance to spend hours out looking at the stars. How many times did we (I) find the wild duck while hunting for the right target by eye as the goto was not right!
Amazing, after doing all the usual unpack, setup, initialize, 1 star align, Phd drift align Az, Phd drift align Alt, recheck Phd align Az, 2 star align etc. calibrate, frame, focus, finefocus, in record time, (about 90 mins) - M16 was smack in the middle of the crosshairs on the goto, first time, just as it should be!
Andy
Hi Andy,
I have found one flaw in your procedure. You should not do a star align before you start drift aligning. Even with a one star align, your scope will start making corrections that can affect your drift align procedure.
You can slew to a star before you start to get focus, but don't tell it to sync.
Hang on a minute, Tigerdes. After initializing the controller, date/time/location etc.. doesn't one have to do at least a 1-star alignment to set the EQ6 motors into tracking mode?
Cool shot. Makes a hell of a difference hey? 
