EQ6 Stripdown and re-grease and Image creep!!

[steppenwolf]

I am proposing stripping down and re-greasing my 'old style' EQ6 which I have modified with the SkyScan upgrade. Although I am generally very pleased with the mount, I have noticed 'image creep' in my images and some non-periodic quick random movements in RA which are making my stars 'egg shaped'.

I have done some research and this seems not to be a trivial exercise! Has anyone on this forum done this to their EQ6 - I am starting to panic at what I have read but the poor stars are also starting to get to me!!

On the subject of image creep does anyone else get this? In my most recent Horsehead image (24 subs of 300 seconds each), I have checked the position of a central star using the 'information crosshair' in Maxim DL and found to my horror that despite a drift align and autoguiding, the 1st and 24th images have the following movement!!

Sub 1

x = 794.833

y = 517.841

Sub 24

x = 764.182

y = 502.489

This means that the image has crept 30.6 pixels in RA and 15.3 pixels in DEC over 2 hours and 4 minutes. OMG that seems terrible!

Any assistance would be greatly appreciated as I may be missing something simple!!

This is the finished image in question, reduced in size for the web but although post processed, nothing has been done to correct the shape of the stars and the egg shape (but NOT 30 pixels X 15 pixels worth!) can be seen in the image!

[steppenwolf]

Some additional information that may help with diagnosing the problem:-

My sampling rate for this image was 1.108674 arcseconds/pixel (10" 1200mm FL Reflector and SXVF-H9C CCD)

I have rotated the image above 90 degrees CCW for aesthetic reasons. However, If I stop the RA drive whilst taking a series of test images, the stars move to the LEFT. This means that the stars would have moved DOWN in the image above.

The autoguider error plot in MaximDL showed peak errors of 0.72 pixels in RA and 0.35 pixels in DEC

[Kaptain Klevtsov]

Could be differential flexure then where the mounting of the guide 'scope bends slightly as the 'scope tips over during tracking.

Is the movement of the central star all in one jump, or does it move gradually between frames? A jump would indicate some kind of slippage rather than a gradual bending. 40 ish arcseconds isn't a lot of bend in anything, so it could be your mounting setup.

Kaptain Klevtsov

[George]

Stripdown & regrease are pretty straightforward and just basic tools are needed, set aside a couple of hours per axis.

Here's a couple of links describing the job:

http://www.beevo.com/rework.htm

http://www.jamesrobertcook.co.uk/EQ6_dismantle.htm

http://avex.org.free.fr/dossiers-pratiques/eq6/index-eng.html

http://www.eisystems.be/astronomy/menu_EQ6.html

And of course there is the yahoo EQ6 group for info and help http://tech.groups.yahoo.com/group/EQ6/?yguid=286900793

Hope this helps

George

[MartinB]

Is that drift in RA or the X axis Steve? The only reason I can think of for RA drift is an out of balance scope overloading the motor, flexion as KK suggests or slipping of the clamp. If your gears are in need of a good service you may be getting some big errors but they should all reverse themselves over the course of the worm cycle therefore not cause drift. I calculate your sampling rate as 0.98 arc secs/pixel with the H9. Lets call it 1 so thats around 30 arc secs of drift over over 124 mins - slightly more if you take the delays between subs. So roughly 1.2 arc seconds or just over 1 pixel per 5min sub. Just enough to show up on your stars if the seeing is magnificent. The seeing is very unlikely to match your sampling rate though, 2 arc secs per pixel would be good.

My guess is that the drift is down to polar alignment and the fact that, very sensibly you aren't working at 100% aggression with the guiding. I think your mis-shapen stars are down to the RA errors which are probably much bigger and occuring in short rapid bursts therefore difficult to guide out. I wouldn't worry about the drift and, like you, I think it's time to service the mount.

BTW - from everything I've read super lube grease is the stuff to use rather than white lithium grease.

[jcm]

I have noticed some "creep" along the RA axis during a recent 2 hour continuous imaging session. ( it was the M33 in the deep sky section)

The scope was operating at F12. I think the movement was about 16 pixels. ( will have to check this). Two of the images had to be discarded , these occurred about the time the mount went over the meridian.

The aligned/stacked image came out OK

The slow creep - poor polar alignment?

The 2 discarded images - change in weight distribution going over the meridian? or flexing of guide scope relative to main scope?

John

[Kaptain Klevtsov]

When the 'scope crosses the meridian the backlash will allow the mount to move as the loading on the worm shifts from one side to the other. If the 'scope is heavy it will fall forward when it reaches the meridian, and if the 'scope is light the mount will pause until the slack inthe gears gets taken up.

Kaptain Klevtsov

[MartinB]

This would occur over 2 frames maximum. You can run an animation utility in maxim. Open all the sub frames then run the animation tool ( towards the bottom of the view menu) to see whether the drift is regular or confined to 2 or 3 frames

[steppenwolf]

Thank you for your input folks.

At this moment in time I am looking at the differential flexure aspect as I have identified a problem which may very well resolve the issue. I am confident that the mount is capable of guiding out the RA drift and the autoguider plots confirm that this is happening. I am also confident that I have a good Polar Alignment. What I am not confident about is the fact that I have been relying on the electric focus unit to 'lock' the focus tube in position once I have achieved focus but having checked the camera for lateral movement, I was astonished at how much it would move without tightening the focus tube lock screw!! I need a clear bit of sky to confirm but the difference to the physical stability of the camera with the lock tightened is amazing so I have reasonable hope of this being the problem - I still have the new grease if I am wrong!

[daz]

SW

The strip down is not as bad as you might think actually, and once you've done it once....

I still want to replace the worm bearings on mine, and there is a guy called Gerlinger who machines a replacement worm set that is supposed to *top* notch. About £200 though!!