Image jitter in long exposure astrophotography
Charles Bradshaw
bw_msgboard
Hi Charles. The short answer is that you aren’t going to “fix” this with guiding, it’s a mechanical problem that occurs too abruptly for guiding to handle. But I wouldn’t start by tearing down the mount. The Dec axis is idle for most of the time so it’s very hard for the Dec drive system to produce these kinds of problems. I think the problem probably lies outside the mount drive system and it may take some patience and experimentation to track it down. To start with, you’re re-using a bad calibration. The Dec guide rate that came out of the last calibration (5/13/2021) is too low by 35% which is why you got an alert message about the rates not looking right. That’s bad – it means that the Dec guiding will be prone to over-correction. So you should start by redoing the calibration and getting one with no alert messages. With your mount, you *must* clear the large backlash manually before starting the calibration. Start PHD2 looping on a star field, then use the ‘North’/’Up’ button on the hand-controller until you can clearly see the stars moving consistently in the display. Then start the PHD2 calibration. You should do this every time you need to do a calibration.
That said, this isn’t the cause of your problems. Here are some of the potential sources that come to mind:
- Cable snags on a stationary part of the mount. If you have cable ties or non-smooth cable wrap, those can produce these rhythmic events.
- Something slightly loose in the guide assembly that allows tiny amounts of shift from gravity as the mount tracks west. There are lots of places for that – the guide scope shifting inside the rings, the focuser tube moving, the guide camera attachment moving inside the focus tube, etc. etc.
- Substantial weight imbalance on the Dec axis that causes periodic (tiny) rotation of the OTA in Dec. This can be compounded by a clutch that isn’t fully tightened. If you are running with a heavy load on the mount, these things become more likely.
Before you dismiss these as possibilities, consider that these “big” Dec jumps correspond to unwanted movements of the guide camera by 3-4 pixels. And with your setup, those correspond to movements of 11-15 microns – a human hair is typically 50 microns thick. So you probably won’t have anything in your toolkit that will see movements that small.
Beyond that, I can see some evidence that the Dec axis isn’t moving as smoothly and freely as it should. The reversal delay you see may not be pure backlash, it may include static resistance on the axis.
Hope you can track it down,
Bruce
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Charles Bradshaw
Charles Bradshaw
bw_msgboard
Yes, this looks a lot better, well done. You should have gotten nice round stars all night. It’s worth keeping an eye on things, especially regarding the cable routing and the balance. Those things can come back to bite you again, sometimes depending on the specifics of the scope pointing position – I speak from bitter experience. L
Bruce
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Charles Bradshaw
Sent: Sunday, June 06, 2021 3:46
PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding]
Image jitter in long exposure astrophotography
Last night's log is at: https://openphdguiding.org/logs/dl/PHD2_logs_vvvc.zip. This one is after incorporating the ideas I received from the forum.
1. I re-adjusted my DEC backlash
2. I carefully re-balanced the mount
3. I re-routed the cable leaving the mount to drop straight down from the rear focuser of the guide telescope.
4. I re-calibrated PHD2 after first removing the DEC backlash by pushing the Up/North button until the star moved before calibrating.
PHD2 thought I had change equipment because the calibration changed so much.
Thanks all for making the night a success.
Regards,
Charlie
On Saturday, June 5, 2021 at 5:21:09 PM UTC-4 Charles Bradshaw wrote:
Thanks for the ideas. It gives me things to check and I will redo the calibration. I only have a single wrapped cable that has power and USB3 data, but it is wrapped with spiral plastic sleeve, which, if catching on something, could cause regular jumps. I have tried hard to eliminate cable drags, but it still could be a problem. The north/up clearing of backlash, I have not been doing and I knew better. Sometimes I get in a hurry.
Thanks,
Charlie
On Saturday, June 5, 2021 at 4:21:48 PM UTC-4 bw_m...@earthlink.net wrote:
Hi Charles. The short answer is that you aren’t going to “fix” this with guiding, it’s a mechanical problem that occurs too abruptly for guiding to handle. But I wouldn’t start by tearing down the mount. The Dec axis is idle for most of the time so it’s very hard for the Dec drive system to produce these kinds of problems. I think the problem probably lies outside the mount drive system and it may take some patience and experimentation to track it down. To start with, you’re re-using a bad calibration. The Dec guide rate that came out of the last calibration (5/13/2021) is too low by 35% which is why you got an alert message about the rates not looking right. That’s bad – it means that the Dec guiding will be prone to over-correction. So you should start by redoing the calibration and getting one with no alert messages. With your mount, you *must* clear the large backlash manually before starting the calibration. Start PHD2 looping on a star field, then use the ‘North’/’Up’ button on the hand-controller until you can clearly see the stars moving consistently in the display. Then start the PHD2 calibration. You should do this every time you need to do a calibration.
That said, this isn’t the cause of your problems. Here are some of the potential sources that come to mind:
1. Cable snags on a stationary part of the mount. If you have cable ties or non-smooth cable wrap, those can produce these rhythmic events.
2. Something slightly loose in the guide assembly that allows tiny amounts of shift from gravity as the mount tracks west. There are lots of places for that – the guide scope shifting inside the rings, the focuser tube moving, the guide camera attachment moving inside the focus tube, etc. etc.
3. Substantial weight imbalance on the Dec axis that causes periodic (tiny) rotation of the OTA in Dec. This can be compounded by a clutch that isn’t fully tightened. If you are running with a heavy load on the mount, these things become more likely.
Before you dismiss these as possibilities, consider that these “big” Dec jumps correspond to unwanted movements of the guide camera by 3-4 pixels. And with your setup, those correspond to movements of 11-15 microns – a human hair is typically 50 microns thick. So you probably won’t have anything in your toolkit that will see movements that small.
Beyond that, I can see some evidence that the Dec axis isn’t moving as smoothly and freely as it should. The reversal delay you see may not be pure backlash, it may include static resistance on the axis.
Hope you can track it down,
Bruce
From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Charles Bradshaw
Sent: Saturday, June 05, 2021 12:04 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] Image jitter in long exposure astrophotography
Last night I was taking 10 minute subs, guided by PHD2 and I got double star images. My log is posted at: https://openphdguiding.org/logs/dl/PHD2_logs_p9gF.zip. There appears to be regular DEC jumps. I've tried to remove DEC backlash, but so far, not too successful. I have a fairly heavy dual telescope/camera rig on a G11 mount. I'm using a Vixen 200mm F9.5, reduced to F7 for imaging with a QSI-683 and a SV115 with a ZWO ASI224MC as a guide camera. The guide/image arcsec/pixel ratio is close to 1. Any good ideas for fixing this with guiding is appreciated, but I may have to tear down the mount.
Regards,
Charlie
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