Odd Problem With AO-X

Hi,

I image with an STX-16803, STX-guider, and AO-X, mounted on a 14.5" RCOS, sitting on a Paramount ME.

All of a sudden a few weeks ago, the stars started being oblong. All stars, and the same amount of oblong whether a very short exposure (five seconds) or very long exposures (30 minutes).

A LOT of troubleshooting has shown the following:

1. The fainter stars show double lobes, with a streak between the lobes; the brighter stars are oblong. The semi-major axis on all stars is the same, and the semi-minor axis on all stars is the proper width of the star. The misshapen stars are identically distorted across the entire chip.

2. As mentioned, duration of the exposure does not affect the amount/size of the streaking.

3. The direction of the streaking moves with the rotator adjustments.

4. Turning off the mount (and not guiding, of course) shows the oscillation continuing; when the streaking is perpendicular to RA, the tracing of the star when the mount is turned off still clearly shows the oscillation of the image, and some irregularity to it. I have attached a photo of that happening; that photo is a two-minute exposure, with the mount turned off after ten seconds. It shows clearly (i) the double-lobed nature of the star while tracking, and (ii) the streak thereafter has some perturbations, not being the perfect, thin line it should be if the problem were related to the mount.

5. Guiding errors are always tiny; typically less than 0.2 pixels, with both AO-X and guiding with the mount (not using the AO-X, but with the AO-X still attached. The exact same problem exists whether guiding with the AO-X, guiding with the mount, or not guiding at all.

6. Replacing the camera with an STL11000/AO-L/CFW results in perfect stars. This is, of course, another indication that the problem is neither the mount nor something with the telescope.

7. Putting the STX-16803 back on, without the AO-X results in round stars.

So, clearly, something's up with the AO-X. It's as if it has a fairly high-frequency (3-4Hz, at least) constant oscillation, but then does its guiding also. Very strange.

Any ideas?

Thanks.

Mark de Regt

 

This evening, I'm imaging with the AO-X back in the stack (but not powered), and the stars do look a bit elongated (though not nearly as elongated as they did when the AO was operating).

How do we check the functioning of the camera shutter? Have you seen this behavior when a shutter is malfunctioning?

 

Ok; I'll pass that on to the guys at SRO.

But I am confused. The problem is elongated stars, bilobed. The photo I attached to the initial post was to show that the mount is not the culprit--the trail was because the mount was turned off (not tracking), so the stars left a trail (as would be expected). The trail still showed the bi-lobed nature of the problem.

When tracking for an entire image, there is no trailing, just bi-lobed stars. See attached.

Thanks.

 

Here's a .fts file.

 

We did that last night. The results certainly were better than they had been, but not perfect. Maybe one-third of the images have round stars; one-third of the images have noticeably (but not horribly) eccentric stars (meaning that bright stars look reasonably round, and fainter stars are clearly a little oblong), and one-third of the images have badly oblong/streaked stars.

Edit: Out of 25 images taken last night, 13 are good enough to use; 7 aren't horrible, but probably not good enough to use, and 5 are horrible. This is worse than I would expect from guided images, but immensely better than it had been with the AO powered.

 

I am inclined to agree that the AO likely is the culprit. We'll have to get some shims so the system can come to critical focus with the AO removed.

I'll update this as necessary.

Mark

 

Hi Mark.

You’re using a Paramount I think?

The Paramount, when tracking is stopped, doesn’t turn off the power to the motors, they are dynamically braked with a medium frequency AC (in the higher audio range) applied constantly to the motors to hold them still.

If the worm belts are worn or loose, or the worm spring plungers loose, or the worm itself worn, the position feedback from the motor encoders can start up a harmonic oscillation of the stationary axis, most likely the DEC axis when tracking and RA and/or DEC when parked or tracking stopped.

I’m ~70% deaf and can’t hear higher frequencies at all and when I had my Paramount a neighbour once called round to say an alarm had been sounding in the observatory for a couple of days. On investigation it wasn’t an alarm, the DEC worm belt had become loose and while parked the DEC motor had begun oscillating, creating a large self-reinforcing harmonic oscillation of the entire Paramount and steel pier!

If you want to try your shutter open, mount stopped test then you would need to shut off the power to the Paramount, just turning off tracking doesn’t stop the motors “singing” while stationary.

William.

 

Thanks, William; I always appreciate your help!

I did it both ways--stopped tracking, and with the power turned off to the mount. Same result, so I don't believe that the mount is the issue.

 

Hi Mark.

Only similar image artefacts with bi-lobed star shapes I’ve seen (only twice, both lightweight ZWO cameras) were from camera’s with failed cooling fan bearings, shaking the camera.

As you can’t switch the camera cooling fan off you can never remotely diagnose vibration from a failed fan causing the AO mirror or optical element to oscillate in harmony.

Will be interesting to see what the cause does turn out to be….

William.

 

I imaged last night with a replacement AO-X in the imaging train, but not powered on. The problem is not at all evident now (that is, I'm not getting any bi-lobed stars, regardless of exposure time).

Yay!

But in examining the guiding seriously for hours, I noticed another issue.

Randomly, for a period of a few minutes to many minutes, the guide errors will become large (1 to 2 pixels; normally they're well below .25 pixel), and stay that way (no sign of actual corrections being made). Eventually, it'll correct and be fine, but that image is ruined (teardrop stars).

I've tried changing all sorts of settings on the guide tab, but it doesn't seem to matter. It happens on a minority of images, but it's annoying (obviously).

Any ideas?

Also, clearly my AO-X is malfunctioning. How can I get it fixed?

Thanks.

Mark

 

I have checked, rechecked, and toyed with the guider settings, which now are back to the default settings. I tried several different delay settings, min and max move settings. Nothing helped.

The problem occurs most blatantly on the X-axis, but there is nothing much happening on the Y-axis either, when this is occurring.

It's a Paramount ME. There should be no backlash. And, again, this is an intermittent, fairly rare thing; it comes and goes.

On the off chance that it's a rogue dither, I have turned of dithering from ACP.

I have attached a screen shot of the Maxim guiding, showing that, guide interval after guide interval, nothing much is happening.

 

I'm assuming that doesn't happen automatically. So I'll figure out how to do that tonight.