Mmm...next question.

TheDecepticon · #1 06-01-2009, 09:07 PM

I don't really know how to put this, but what is the cause of when your imaging and you're locked onto a guide star and all seems to be humming along nicely, then exposure after exposure, the image drifts across the screen. Most notable when going from one filter colour to the next, and then when you line up the frames when processing, each colour is successively shifted by a few mm to the previous. Is this normal, is this field rotation requiring higher polar alignment. ****..how much better would it need to be. I can keep a star on the chip on the screen for over3/4 hour before it has drifted away to the edge-not good enough??

Gama · #2 07-01-2009, 01:02 AM

how are you guiding ?.

Field rotation is easily recognised by the fact the guide star is a nice point, but as you move away from the guide star, the stars begin to show larger and larger arcs (centred around the guide star).
If the trails are straight, then it may be many things, like DEC error, flexing etc.

Theo

TheDecepticon · #3 07-01-2009, 01:04 AM

No trails,in general guiding is good.

Gama · #4 07-01-2009, 01:27 AM

Again, how are you guiding ?, as this may be the clue needed.

Theo

Dennis · #5 07-01-2009, 07:34 AM

A good candidate for what you describe is the "drifting" of the colour sub-frames due to sub-optimal polar alignment for your set up?

Cheers

Dennis

tempestwizz (Brian) · #6 07-01-2009, 08:39 AM

I've thought I've had drifting during guiding issues in the past, but I can basically put it all down to differential flexure in the system. If the guide scope and imaging aren't absolutley rock solid together, this will occur.
In some shots I've taken I've measured aproximately 7arcsecs shift between successive 5 minute exposures. 7arcS is not a lot in real terms, but when stacking it becomes significant.

Try doing a stack without aligning, and check out the path that a particular star makes.
As stated before , if there is rotation, you will see the alignment star as a dot, and the rest forming larger arcs as you mave radially away.
If you are not guiding in Dec, any polar mis-alignment will show up as lines in the Dec plane only.
If PEC or other RA issues then there will be a line in that plane.
If you get a curved path ( showing shift in both Dec and RA), I'd put it down to differential flexure between guidescope and imaging scope.
This caused by gravity effects bending components at different rates as you track across the sky. You may even find tthat while imaging in some parts of the sky, the problem appears to be not so bad, while in others it is terrible.

HTH.

BC

TheDecepticon · #7 07-01-2009, 09:29 AM

Quote:

Originally Posted by Gama

Again, how are you guiding ?, as this may be the clue needed.

Theo

I use a guide scope mounted onto the top of the imaging tube. It is bolted down very well and I have to wrench it around a lot to make it move. It is then connected to PHD and uses pulse guiding through EQMOD to an EQ6. I don't get any trails or arcs or streaks and generally everything works out in the end, I just notice over the course of a few hours and when stacking the LRGB exposures that the image has shifted slightly per colour.

Gama · #8 07-01-2009, 08:45 PM

Are you correcting for both R.A and DEC ? or just R.A .

Theo

TheDecepticon · #9 07-01-2009, 09:05 PM

I'm correcting for both.

g__day (Matthew) · #10 08-01-2009, 10:14 PM

I had alot of this with a DSI Colour and PHD - switched to DSI II Pro and guiding was perfect - maybe your guide stars aren't sharp enough for your guide program to find the centroid!

TheDecepticon · #11 09-01-2009, 09:33 PM

Mmmmm....strange you should say that!! I have just about had enough of PHD. Heaps use it no worries, of course.

I use EQMOD and Starry Night for planetarium software and PHD doesn't like to operate too well in this environment, with cutting of communication to the mount, Starry Night hangups, PHD hangups, spoiled frames etc. All works great until I open PHD and it doesn't matter what order I open or do things, somewhere along the line-kerblam!!

Last night I gave it the flick. Paid for MaximDL 4 + DSLR and never use it, daunted by its plethora of functions and options. Last night, Maxim guided perfectly, with round stars to the edge of field in 8min exposures of NGC3324. Now to go for 20min subs and see what happens.

Gama · #12 09-01-2009, 11:03 PM

You may have the aggresivness too high, or the backlash adjustment may not be right, but looks to be some setting issue if your guiding fine with Maxim.

Theo

TheDecepticon · #13 10-01-2009, 02:02 AM

It's the software problems as well that are ticking me off, not just a setting in PHD alone, and I have had a good play in the brain as well. Ive got a fully capable program, just gotta learn how to use it. I did have to set Maxim up tonight though, it wouldn't do anything well at all. So now I get 15 min subs with round stars, next test is 20 & 30. Then I can move back to the original problem I posted about and see if it re-occurs.

Jeff · #14 10-01-2009, 07:22 AM

Could it be that you're disturbing the mount ever so slightly when changing filter colours?

Zuts · #15 10-01-2009, 11:55 PM

Quote:

Originally Posted by Jeff

Could it be that you're disturbing the mount ever so slightly when changing filter colours?

This happens to me as well. For example I image with a ST2000 XCM which is a 2 megapixel camera. Over the period of say three hours taking 10 minute subs my images 'move' about 10% top to right and need to be cropped out after stacking.

I guess this 'pixel creep' is caused by inaccurate polar alignment. The guiding can still keep the guide star centered but over a three hour period it 'wanders'.

On a large format camera or DSLR this would not be a problem but on my camera you lose nearly 10% of the image after stacking.

Cheers
Paul

TheDecepticon · #16 11-01-2009, 01:13 AM

My filter wheel is electric with a hand box hanging down the pier, just tip-toe over there and press the button.
I am gonna re drift and see where that gets me, but I'm pretty close already.
Yeah I reckon I loose 10% as well.

Ken · #17 11-01-2009, 04:08 PM

If your Polar axis or mechanical axis are not perfect what you discribe is the norm if you are guiding with a seperate guide scope. If you use a off axis guider every image will be the same.
Clear skies Ken C.

Bassnut (Fred) · #18 11-01-2009, 04:34 PM

Ive always had this effect, each image OK themselves, but a few mm difference between them. But not with SBIG internal guide. Dont worry about it if the stars are round in each sub. IMO, its differential flexure between the image and guide scope, theres nothing you can do about it.

edit... umm, thats assuming a mm or 2, if its driftting off the screen, then theres a mounting problem on the guide scope.

Zuts · #19 11-01-2009, 06:15 PM

Quote:

Originally Posted by Bassnut

Ive always had this effect, each image OK themselves, but a few mm difference between them. But not with SBIG internal guide. Dont worry about it if the stars are round in each sub. IMO, its differential flexure between the image and guide scope, theres nothing you can do about it.

edit... umm, thats assuming a mm or 2, if its driftting off the screen, then theres a mounting problem on the guide scope.

This happens to me using my SBIG 2000 XCM using the internal guide chip, so I reckon its innacurate polar alignment in my case.

Cheer
Paul