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This is beyond weird.

ollypenrice

By ollypenrice
in Imaging - Discussion

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ollypenrice

ollypenrice

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Shooting Jones Emberson One. TEC 140/Atik 460 capturing in Artemis capture. Guiding via ST80 and Lodestar in PHD2. Mount is Mesu 200.

I start off whith JE1 in the middle of the chip. Take a sub. Fine. It's a windy night, though, with poor seeing. My next sub is free from trailing but JE1 is well to the lower left of its original position. Errrm... Odd. I suspect that the wind has nudged the mount and PHD has picked up on the wrong guide star and carried on. I put the mount back in position and start again. Guiding seems happy, the trace is perfectly normal. But this sub comes out as two subs in one. The target appears in two places as do all the stars, but both are sharp. The direction of the second image is towards the lower left again. The guide trace is fine but I'm getting the bongs and it has stopped guiding. I restart PHD afresh, I recentre JE1 and try again, this time using a brighter star which I know to be saturated but I want to be sure I'm not losing the guide star. The first sub is fine. The second sub has moved JE1 to the lower left again but the sub is sharp. The third sub s a clone of the second and sharp. The trace is fine.

It seems that the whole system has decided that JE1 should be in the lower left of the image and it is damned well determined to put it there! It's as if PHD is doing a big dither but I haven't asked it to and it doesn't communicate with Artemis and once it's dithered JE1 to the same lower left position it doesn't want to dither it again.

Now if we imagine a clevery contrived, consistent and instantaneous differential flexure we could explain this behaviour. Something flopping quickly between two positions. Usually just between exposures. Not very convincing, is it?!

Or it's a bug in the guiding. A curiously consistent bug that likes to strike between exposures (two out of three times.) And in a system in which there is no communication between guiding and capture.

I think I'll switch to guiding in AstroArt and see what happens then.

All thoughts welcome!

Olly

 

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JamesF

JamesF

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Sounds like everything is broken and needs to be replaced.  To save yourself a whole pile of bother why don't you just pack everything up and ship it to me so you're shot of it.  I won't even charge you a penny to take it off your hands.  I can't say fairer than that, can I?

James

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michael8554

michael8554

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The PHD2 Guide Log should show you whether there was a star lost situation, whether the move started before corrections (suggesting stiction or dragging cable or wind) or was caused by spurious corrections by PHD2.

Did you not notice anything in the guiding graph at the time?

Michael

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ollypenrice

ollypenrice

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23 minutes ago, JamesF said:

Sounds like everything is broken and needs to be replaced.  To save yourself a whole pile of bother why don't you just pack everything up and ship it to me so you're shot of it.  I won't even charge you a penny to take it off your hands.  I can't say fairer than that, can I?

James

I can live with JE1 in the lower left so long as it's sharp!!!

Olly

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ollypenrice

ollypenrice

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1 minute ago, michael8554 said:

The PHD2 Guide Log should show you whether there was a star lost situation, whether the move started before corrections, or was caused by spurious corrections by PHD2.

Did you not notice anything in the guiding graph at the time?

Michael

Nothing at all.

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steppenwolf

steppenwolf

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If the guiding log and graph show nothing happened then this must surely indicate that the telescope or imaging camera have moved. If the movement is linear (i.e. no rotation) and you still have a well focused image then it must be the telescope that has shifted? As you say though, not a convincing argument. The AA test will remove PHD from the equation so that has to be done after a quick spanner check to remove any doubt there.

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ollypenrice

ollypenrice

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1 hour ago, steppenwolf said:

If the guiding log and graph show nothing happened then this must surely indicate that the telescope or imaging camera have moved. If the movement is linear (i.e. no rotation) and you still have a well focused image then it must be the telescope that has shifted? As you say though, not a convincing argument. The AA test will remove PHD from the equation so that has to be done after a quick spanner check to remove any doubt there.

 

7 hours ago, michael8554 said:

Then PHD2 was happily guiding on the same star and your imaging camera had moved.

Michael

The problem is that in trying to decide between sudden flexure and a PHD issue we still have to account for how it managed to do it in the five second gap between subs, a trick it pulled off twice. Only on the third example did it jump during a sub. Then again, whichever explanation you prefer the same 'coincidence' applies. Hmmm....

Thanks for the input. I'll get the spanners onto the flexure hunt in the daylight and try AA for guiding tonight.

Michael, what I'm wondering is whether, perhaps, PHD is changing its mind about which star it's guiding on, slewing the mount to a different one and carrying on regardless.

3 hours ago, alan potts said:

I would go for Dark forces at work, if you find out it is something in PHD, let us know. I am battling away with it at the moment and find it don't work at all if you leave the cover on the guide scope.

To solve this problem I lost the guide scope lens cap years ago but keep the lens scrupulously immaculate by cleaning it once every three years... :icon_mrgreen:

Olly

 

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carastro

carastro

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Just found this dated 30th October 2017:

https://www.express.co.uk/news/science/873096/nibiru-planet-x-earthquake-alps-big-one-nemesis-end-of-the-world-brown-dwarf

FRANCE has been hit by a huge number of earthquakes over the last month, leading to fears that the BIG ONE is on its way.

In the past 40 days, the French Alps have been rocked by 140 quakes which has led officials to warn locals that a more powerful tremor may be on the way.

Earthquake observation organisation Sismalp says that the strongest tremor was a 3.8 magnitude quake, but such a large swarm can be a precursor for a massive earthquake.

 

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ollypenrice

ollypenrice

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52 minutes ago, carastro said:

Just found this dated 30th October 2017:

https://www.express.co.uk/news/science/873096/nibiru-planet-x-earthquake-alps-big-one-nemesis-end-of-the-world-brown-dwarf

FRANCE has been hit by a huge number of earthquakes over the last month, leading to fears that the BIG ONE is on its way.

In the past 40 days, the French Alps have been rocked by 140 quakes which has led officials to warn locals that a more powerful tremor may be on the way.

Earthquake observation organisation Sismalp says that the strongest tremor was a 3.8 magnitude quake, but such a large swarm can be a precursor for a massive earthquake.

 

Eek! I haven't heard a word about this.  I think I'll plug my ears to be sure that it carries on that way...

There was no other imaging rig going but the 14 inch visual SCT remained stunningly accurate on GoTo throughout the night, though it was nudged by gusts at times without losing its bearings.

Here's a theory:

A sudden gust of wind (there were sporadic gusts through the night) nudges the scope. PHD loses the chosen guide star but picks up on another and latches onto that, holding the scope in the new position. The gusts come from the same direction each time so it is the same new guide star which is picked up each time. This explains the sudden nature of the jump, the fact that it was always in the same direction (which was not aligned with either axis) and the fact that PHD is satisfied that it has held the mount on course. It doesn't know of the change in star. There is no trailing because there was a jump rather than a drift and, apart from the duration of the jump, the guiding was working fine. 

I think this might well be it.

Olly

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The Admiral

The Admiral

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It does sound weird, even for a complete noob when it comes to guiding. Besides which, I hesitate to offer any thoughts to "The Master". However, trying to apply a bit of logic to this, surely a gust of wind would just shift the 'scope temporarily, after which the mechanics of the mount would return to the original position, more or less. In other words, behaving a bit like a spring. So if there was a change of guide star, the mount would have to motor to its new alignment once it had 'sprung back'. Would any of your traces show whether this occurred? If there wasn't that movement, then I don't see how your explanation can hold true.

Ian

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ollypenrice

ollypenrice

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2 hours ago, RayD said:

The PhD log would also show that a new star had been selected, so worth starting there.

Would it know it was a new star?

I think that, just maybe, the gusts might have been enough to cause slippage in the drive. PHD would then pick up on the nearest star, perhaps 'thinking' it was the original. To complicate matters all stars, and the target, were very faint indeed. They might not record during a short period of searching.

The sensor is small and the FL quite long so the slew to the second imaging position is only the equivalent of a couple of seconds input at slow guide speed.

I take on board the objections to my 'explanations' though. Whatever I can think of meets with an objection, that's the problem!

Tom will be joining us this evening so we'll have more (and better!) grey matter on the job...

Olly

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ollypenrice

ollypenrice

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20 minutes ago, gorann said:

Maybe this could only happen to a Mesu mount with friction drive. Other mounts with worm wheels would bounce back (like Ian suggests), while a Mesu can slip.

Yes, this has to go on the list of possibilities.

To be honest the wind had so messed up the seeing that it was hardly worth collecting data at this resolution but I was keen to try. And, in fact, I lost only one sub. Since the Ha I was doing is only there for the main nebula it doesn't matter where on the chip it is as long as its there!

What remains mysterious is how it moved and then delivered a perfect sub in a new place. 20 minute subs need guiding, especially at this resolution.

Olly

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swag72

swag72

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We all battle our gremlins that's for sure and we always find something new to slip us up.... while I think that your wind gust theory is sound, I can't help thinking that it is perhaps a massive coincidence that both times they occurred at the exact time of a download of the previous exposure..... If this is the case, can you please give me the lottery numbers for the Spanish El Gordo lottery on the lead up to Christmas and I promise I'll buy you a present with the winnings  :) 

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pete_l

pete_l

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18 hours ago, ollypenrice said:

The first sub is fine. The second sub has moved JE1 to the lower left again but the sub is sharp. The third sub s a clone of the second and sharp.

My first question would be: where is "lower left" in terms of RA and Dec?

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michael8554

michael8554

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Hi Olly

I would post on the PHD2 forum, which is regularly replied to by the gurus.

https://groups.google.com/forum/m/#!forum/open-phd-guiding

They'll need the guide log and a list of your kit and connections.

IIR the guide star position is continuously logged (after all this is the essence of guiding) so they will surely find the problem.

Michael

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