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Old 28-02-2014, 08:29 AM
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Andy01 (Andy)
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Help please: Dithering "Requesting Current Distance"

Hi there everyone.

I apologise if this topic has come up before.

I'm having trouble dithering. Which is frustrating as I know I'll get better results using it!

This message " requesting current distance" keeps coming up after the first frame is completed and stalling the shooting sequence. I have tried all combinations of dither settings from small to extreme, and <.1 to <.5 both paused and active. The PHD link appears to be connected and working.

Doesn't matter what my settings are / it still stalls.

I'm using a MacBook, nebulosity 3.0 and the latest version of PHD1. I have a William Optics FLT 110 refractor on an EQ-6 mount guiding with an Orion Starshoot Autoguider and shorttube 80 guide scope.

There's a P-flat 4 focal reducer/ .8 Flattener in place and the camera is an SBiG 8300c OSC CCD.

Can anyone help /advise me with this issue please?

Cheers
Andy
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Old 28-02-2014, 08:53 AM
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I have never seen that message before.
I just found out the other day that PHD2 has manual dithering
see here:
http://www.iceinspace.com.au/forum/s...d.php?t=117669

but your version PHD1 doesn't so I can't understand how you are obtaining dithering -
it sounds like it's through Nebulosity.
Therefore it might be a bug in Nebulosity.

Maybe download PHD2 & try to manually dither?
Maybe reload Nebulosity?
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Old 28-02-2014, 09:10 AM
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Shiraz (Ray)
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phd and Nebulosity talk to each other and that message is part of their conversation - Nebulosity will not take any more images unless phd tells it that tracking has resumed after a dither offset and that the error is within the limit you specified (eg <0.3 pixels or whatever you set it at). Have a look at the tracking graphs. On a dither, the curves should jump randomly and then come back to the axis as tracking resumes. Sounds like yours are not.

Possible causes are:
1. poor seeing which is causing the track point to wander so much that you never get the tracking error within the error bound you specify
2. poor polar alignment which can introduce an offset in the dec axis
3. poor balance which can introduce a RA offset
4. dec mode set to only track on one side

try setting the phd dec mode to auto, Nebulosity dither error bound to the maximum (<.5), phd exposure to about 2.5 seconds and the phd minimum motion to say .25 and view the graph to see how its going. If the two error curves do not go back close to the axis and imaging resumes, check 1-3 above.

Last edited by Shiraz; 28-02-2014 at 10:07 AM.
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Old 28-02-2014, 09:11 AM
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mithrandir (Andrew)
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Quote:
Originally Posted by alpal View Post
I have never seen that message before.
I just found out the other day that PHD2 has manual dithering
see here:
http://www.iceinspace.com.au/forum/s...d.php?t=117669

but your version PHD1 doesn't so I can't understand how you are obtaining dithering -
it sounds like it's through Nebulosity.
Therefore it might be a bug in Nebulosity.

Maybe download PHD2 & try to manually dither?
Maybe reload Nebulosity?
Nebulosity tells PHD to move the scope and waits for it to reply the guide star has settled. I've had problems in the past with PHD1 not being happy and never giving Neb the OK message. I haven't tried it with PHD2 yet.
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Old 28-02-2014, 01:18 PM
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Andy01 (Andy)
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Quote:
Originally Posted by Shiraz View Post
phd and Nebulosity talk to each other and that message is part of their conversation - Nebulosity will not take any more images unless phd tells it that tracking has resumed after a dither offset and that the error is within the limit you specified (eg <0.3 pixels or whatever you set it at). Have a look at the tracking graphs. On a dither, the curves should jump randomly and then come back to the axis as tracking resumes. Sounds like yours are not.

Possible causes are:
1. poor seeing which is causing the track point to wander so much that you never get the tracking error within the error bound you specify
2. poor polar alignment which can introduce an offset in the dec axis
3. poor balance which can introduce a RA offset
4. dec mode set to only track on one side

try setting the phd dec mode to auto, Nebulosity dither error bound to the maximum (<.5), phd exposure to about 2.5 seconds and the phd minimum motion to say .25 and view the graph to see how its going. If the two error curves do not go back close to the axis and imaging resumes, check 1-3 above.

Thanks everyone - Ray, I'll give that a go tonight if it's clear again in Melb. Much obliged.

Cheers

Andy
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Old 01-03-2014, 07:42 AM
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Progress report:

I rebalanced the 'scope. It was unbalanced as I had installed a Flattener and the new focus position was enough to unbalance the scope. Good tip thanks!

I tried setting the phd dec mode to auto, medium dither setting, Nebulosity dither error bound to the maximum (<.5), phd exposure to 1 second (2.5 sec didn't work) and the phd minimum motion to .25

It worked ok for 9x5 min exposures then it seems that it lost the guide star and when it resumed imaging all I got were exposures of streaks!

Any ideas?

Cheers

Andy

Last edited by Andy01; 01-03-2014 at 08:00 AM.
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Old 01-03-2014, 08:11 AM
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Shiraz (Ray)
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Progress.

What bit didn't work at 2.5 sec? thats odd, should be fine at that exposure

It possibly locked onto a hot pixel after it lost the star - a dark frame will probably fix this if you have not already done so. Also suggest you use 3x3 median noise reduction if camera noise is a problem.

What are the tracking graphs like? - you can diagnose many problems just by seeing what they are doing

Last edited by Shiraz; 01-03-2014 at 08:28 AM.
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Old 02-03-2014, 12:48 PM
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Quote:
Originally Posted by Shiraz View Post
Progress.

What bit didn't work at 2.5 sec? thats odd, should be fine at that exposure

It possibly locked onto a hot pixel after it lost the star - a dark frame will probably fix this if you have not already done so. Also suggest you use 3x3 median noise reduction if camera noise is a problem.

What are the tracking graphs like? - you can diagnose many problems just by seeing what they are doing
At 2.5 sec it was stalled out. "Requesting current distance" again. I went back to 1 sec and it worked again. Until it lost the guide star. I'll try again tonight using those tips and this time I'll try to remember the graphs!
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Old 03-03-2014, 10:31 AM
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Progress report 2:

Success! Thank you to Ray

Took a dark frame and it appears the issue has been resolved. Managed 18 x 5 min frames in sequence before the clouds rolled in last night.

The graph spiked after each dither, but they eventually returned to the axis and imaging resumed. Curiously, the longer the sequence went on the quicker imaging resumed after each dither.

I'll post some graphs up tonight.

Cheers

AC
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Old 03-03-2014, 11:15 AM
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Shiraz (Ray)
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Quote:
Originally Posted by Andy01 View Post
Progress report 2:

Success! Thank you to Ray

Took a dark frame and it appears the issue has been resolved. Managed 18 x 5 min frames in sequence before the clouds rolled in last night.

The graph spiked after each dither, but they eventually returned to the axis and imaging resumed. Curiously, the longer the sequence went on the quicker imaging resumed after each dither.

I'll post some graphs up tonight.

Cheers

AC
good to hear
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