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Walking on the Moon

whats the cause

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I do my guiding in maxim and everytime no matter what my guiding looks good to me for the first hour or so but suddenly the Y axis (usually) starts going off from something like 0.07 to 1.5 2. something then carries on creeping up, yet for the first two hours its spot on, any ideas whats doing this? if it was polar alignment wouldnt it show right at the beginning? took a screen shot everything to me looks good and it will stay like that for an hour or so then it starts going off


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Not so easy to answer without seeing the guiding graphs etc. If you manage to set a really good PA then you can try turning off DEC corrections and you should be able to go to ten minutes with no dec trailing.

While trying to identify guiding problems always leave the camera perfectly aligned, X to RA axis and Y to DEC axis so that you can be sure there is no interaction between them.

It might just be backlash in the DEC axis becoming more noticeable as the evening progresses and the mount is pointing higher, the Skywatcher EQ5 and EQ6 mounts do suffer a little from backlash in the dec axis.

Also make sure that the mount is always set east side heavy and camera end heavy when balancing so that guiding corrections are always pushing against the same side of the ring gears and not bouncing back and forth between the teeth, you don't need to be much out of balance, around 0.5Kg equivalent hanging on the telescope is enough.

When the mount pointing creeps up towards the zenith the effect of the unbalanced weight becomes less for the DEC axis and I used to find for an accompanied mount (rather than remote operation) a length of thin bungee cord tied around the camera end of the scope and down to a heavy weight on the ground (or tripod leg etc) provided a more constant pull on the dec axis than off setting the balance point but you do need to remember it is there before slewing off to a new target else it can get knotted and tied around the mount!

The guys at Diffraction Ltd have finally decided that the guiding module in the current version of Maxim DL6 v6.13 is not working that well and are currently working on a rewrite of the guide module routines, last I read they hope this will be out in the next revision due shortly (v6.14).

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its just started again now after an hour, checked cables all ok, strange thing is if i switched guiding off then i will stay on target all night whereas when guiding it will go off. heres another snapshot the Y axis is on 9 now yet X is stil 0.33


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Can you clarify, are you saying that after a certain time the target drifts off the camera in the DEC direction while the Y error increases to ~ -9 and that if guiding is not stopped then the target will drift off the camera completely?

If so, this could be a warm pixel appearing on the guide camera and the guiding module is locking on to the warm pixel after a guiding move. Once it locks on in this way then the guiding movements will forever be in a single direction as it tries to recenter the unmovable warm pixel. Warm pixels often appear after the guide camera has been in use for a lengthy period. It might even be that you selected a guide star that is too close to an existing warm or hot pixel.

To help you further diagnose the problem, in the options menu of the guide tab, turn on "Record Tracking Images. This will copy the guide camera images into the My Documents\Maxim DL 6\GuiderTrackingImages so that as you carry out a guiding run once the problem appears stop the run and look at the last few guider images to determine if the system has locked on to a hot pixel.

Also, consider activating the Multi-Star Guiding option, this selects several stars together for guiding and Maxim will decide how to guide by averaging all those stars it automatically selects. This helps avoid the system locking onto a single warm pixel after a move. Define a sub frame size in the guiding setup tab that has plenty of guide stars for Maxim to use otherwise Maxim will use the whole guide camera sensor size for Multi-Star Guiding and that slows the guide frame acquisition rate..

In the last image you posted the guide star is overexposed, the 3D profile of the guide star FWHM should look like Mnt Everest, not Table Mountain, you need to shorten the exposure time until the peak becomes pointy similar to that shown for the simulator image below.

When you have finished a guide session remember to delete all the guide camera images from the My Documents\Maxim DL 6\GuiderTrackingImages folder and once the problem is resolved you can turn off the option to save the images. To save disk space the option to save guider images is turned off automatically each time you startup MaximDL.


Edited by Oddsocks
Clarified Multi-Star setup and destination folder
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1 hour ago, brrttpaul said:

Just woke up and checking my data now will get back to you but already learned something (for some reason I always thought that peak was focus not exposure )

Can't speak for Maxim, but guide star profiles are both (in PHD2). A clipped star profile is usually undesirable since the software can't reliably calculate the centroid. Still, you wan't to aim for a high S/N (signal to noise ratio - does Maxim give you that?) but not saturated. The focus is actually less important, within reason. I believe a slightly defocussed star can give better guiding, presumably because of the wider diameter (greater fwhm) which allows more precise centroid calculation. But a noisy sky (which I suffer from - a lot!) isn't so good...


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Am I reading your screen shot right that you are guiding at DEC 89 deg? That can be difficult as RA and DEC are not necessarily orthogonal. I switch off RA guiding in that situation

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Um hang on im getting lost here a little, the dec89 i can see it dosnt it do that auto? im not sure. I will try and get my head around it ,quite a lot to take in, strangly enough when i woke this morning it had stayed on course but I did reduce the auto save to 120 secs not 300/600 so im wondering would that have anything to do with it

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5 hours ago, brrttpaul said:

for some reason I always thought that peak was focus not exposure

It is both focus and exposure time. The 3D profile of the star will change from wide and low to narrow and pointy as you sharpen focus but as you do this more of the stars photons are focussed into a smaller number of pixels which become saturated and then the profile becomes flattened on top. You have to adjust the focus and as the 3D profile becomes steeper but flat on top you also reduce the exposure time until you reach best focus and shortest exposure time without a flat top.

As posted above you can then defocus the guide camera a little and the 3D profile will drop in height and become wider requiring a small increase in expoure time again to bring it back up. The reason to do this is to spread the star over several pixels as a sort of averaging function, it helps to reduce the guider from reacting to the seeing conditions as a sharply focused star occupying a single repeatedly jumps between adjacent pixels as the star "twinkles" and "shimmies" in response to an unstable sky.

The same result is achieved by binning the guide camera, for the focal length you are working at binning the guide camera 2x2 will smooth out seeing variations, help increase guide camera sensitivity and reduce the chances of single nearby warm or hot pixels becoming mistaken for the guide star after a guiding move which often happens when binning 1x1.

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