Image drift across capture screen?

[kingboya]

Hi all, I am having problems.

 

I am guiding succesfully on NGC6888 using the ST80 and QHY5 and PHD. Calibration takes about 10 steps in all directions.

Scope is the AA 115 ED triplet with 0.79 reducer, but I am getting drifting. Long exposures (10min) reveal elongated stars, and shorter exposures (5min) just about eliminate the effect. Polar alignment is spot on with the EQMOD routine and balance is such that I have a bias to north (the weights end) to try to eliminate backlash.

 

I am stumped because this is a recurring problem for some time now. Thoughts please?

 

[nmoushon]

One suggestion would to increase your callibration step upmore. like 1200-1500. Thats what I use with my setup.

[Freddie]

Differential flexure between the imaging and guide scopes. Make sure everything on the guide scope attachment and rings is as tight as possible.

[johnrt]

Yes as Freddie says something is flexing somewhere. Nail everything down good and tight.

[Freddie]

Increasing the calibration step duration wouldn't be a good idea as that would reduce the number of calibration steps and you are already at the low end.

[kingboya]

Thanks all for the pointers. I think maybe my confidence in polar alignment maybe misplaced, so on others advice, I am the proud owner of alignmaster, to really get alignment bang on. Failing that, we are looking at flexure I am sure.

I use a dual bar adm Losmandy setup, but maybe focus rotator etc might have a small amount of play in it? I shall find out soon hopefully

[kookoo_gr]

As others said the most likely culprit is flexure at one or more elements at your setup, after making sure you have almost perfect polar alignmnet. I would check the following

1. your guidescope mounting. if you use rings with adjustmnet screws you need to remove them and use fixed rings. Thankfully the guide scope gives you a wide FOV and you'll always have a guide star

2. The focuse of the guidescope or the main scope has flexure. Make sure that the tension screws are tight so that they don't allow slipage. Also if your imaging scope's focuser drawtube is extended outwards at the focus potition this will introduce bending of the drawtube thus having flexure. Use extension rings at the focuser so that the focuser drawtube is more inside and have better leverage.

3.The mounting rings of the imaging scope. If you use the classic rings that most scopes have with a small holding area for the dovetail it will introduce flexure. The best solutuon is to change them with e.g. parallax (it's what popped in my mind but you get the picture) that have a larger mounting area for the dovetail.

4. The saddle of the mount can introduce flex if it has small area. At my scope i have an adm saddle and it is rock solid.

5. It is possible that the difference of f ratios at your scopes gives you this drift. Although the best solution is to go for an OAG you can change the Min. motion at phd so the guide scope can giude correctly. Small numbers can cause the setup to chase seeing and big numbers can make the system to respond slowly. The calibration step affects only the size of the calibration steps (small numbers for small FOV and large numbers for large FOV) and does not affect guiding at all.

6. Balance of the mount. After servicing my mount and meshing the gears i have found that perfect balance is the way to go. Even a fraction imbalance can make my setup to have Dec drift. Check the balance of the scope at horizontal postion, vertical position and when the scope points at Polaris. Also the cables at various positions can cause imbalance at a large degree.

7. At one point i discovered that the screws that hold the dec shaft were loose, i found it after dissmantling the mount 5 times in search for a  major dec drift issue i had last year.

8. Lately i found that the holding screws for the focuser can get loose. Tighten them with a pair of pliers. Also at my ed80 i changed it to a baader clicklock and the camera is held rock solid despites its 2kg of weight and i am going to do the same for my rc scope.

[ollypenrice]

Do the frame edges from the first and last image of the run line up once the stars are aligned? If they do you can believe your software derived PA. If they don't your PA is the culprit. I'd risk a fiver on this. 

It could be flexure but this has a strong whiff of PA misalignment to my nose.

Olly

[Komet]

Could be flexure but more likely to be field rotation. Remember you are guiding your guidescope and not your imaging scope. That is purely along for the ride. If your guide star is a way off the point you are actually imaging then there will be a rotation of the field of view. This is common in twin scope setups, its visibility depending on the difference between FoVs. The only real way to eliminate totally is to use an OAG. You could do worse than spend some time checking that the centre of the FoV of your guidescope is looking at the same thing your main OTA is looking at.

[kingboya]

Wow thank you all for such a great response. Lots to consider.

Olly, the images definitely do not align when stacked, so I guess that points to either polar alignment or field rotation. If the problem still persists, I will look more into the flexure side of things, but will ensure things are tight to start with.

I will keep you posted of progress.

[ollypenrice]

Wow thank you all for such a great response. Lots to consider.

Olly, the images definitely do not align when stacked, so I guess that points to either polar alignment or field rotation. If the problem still persists, I will look more into the flexure side of things, but will ensure things are tight to start with.

I will keep you posted of progress.

I really thought this would be so and I see that Mick is in agreement. It isn't 'either/or' PA/rotation: rotation arises from polar misalignment.

Software is software and reality is reality. You can't argue with a drift alignment... It's a primary source of information. Mick has a point about the position of your guidescope but personally I've never paid much attention to this. He's dead right about twin scope setups, though. Very sensitive.

Olly

[kingboya]

Well, I grabbed the opportunity last night to get a couple of hours under the stars to test alignmaster which was recommended to me for polar alignment. I was using the EQMOD polar alignment routine before and frankly have no idea what I was doing wrong. 

Alignmaster for me is superb, and alignment was complete within about 10mins of first use. Now for the tests.

I set artemis to run a series of Ha images ranging from 300 sec to 1200 sec subs, and testing for any star drift etc.

Olly Penrice, ALL images stayed exactly where they were on the capture screen! I feel like I have won the lottery!

This is a single 20min sub taken from Woking with hight wispy cloud at time, no darks flats or bias frames, only the data, and processed with PI.

To say I am chuffed is an understatment

Thankyou all for your help and Ideas sorting this out. I owe you a beer!

Tony

[ollypenrice]

Good news. I might even look at Alignmaster myself! Shifting of the stacked frame edges is a key PA indicator.

Olly