Guiding log review please

[iapa]

Not sure this is in 'Beginners' other than we are all beginners in thi shabby, there ar eno 'professional' amateur imagers

I have couple of issues with my guiding, log file attached, where the DEC goes running way off after a while. I'd watched it for about 30mins and all was OK.

Last night I manage to get some 240s exposures of M51, Backyard EOS was used, ISO 1600 to try to get rough some of the haze. Of the some 23 where usable. Ive attached an example of an unprocessed unusable image..

The result is below, not compensation applied as yet. Debayers/demosaiced, intensity normalised, aligned and cropped (to remove uneven edges from stacking and drift of image) using Nebulosity 4. As was background colour offset, auto colour balance and some stretching. A bit of leaves in Photoshop SCC2015.

The OTA was balanced in DEC after cameras etc. added. Balance is RA was with OTA east, with a over balance to that it would tend to drop downwards if released from about 45deg about horizontal - 'East heavy'

M51 did not go past the meridian until c 04:50 this am at which time capture was halted.

Average outside temp c -5C.

 

 

PHD2_DebugLog_2017-01-26_212935.txt

[wimvb]

From your description, I would guess that the behaviour in dec is caused by a combination of polar misalignment and dec backlash. The suggested remedy is to try guiding in one dec direction only. But you will have to fugure out which one. As with ra, the dec gears should always work in only one direction.

Good luck

[alacant]

Hi. Could you post the guide log? The debug log is too geeky for me! Saludos.

[iapa]

On ‎31‎/‎01‎/‎2017 at 06:30, wimvb said:

From your description, I would guess that the behaviour in dec is caused by a combination of polar misalignment and dec backlash. The suggested remedy is to try guiding in one dec direction only. But you will have to fugure out which one. As with ra, the dec gears should always work in only one direction.

Good luck

PA was done with PoleMaster

Dec guiding was Sth only, which worked fine for the 1st half hour.

Probablynever get to the bottom of it as I'll not be able to replicate it again .

Ho hum  

[iapa]

On ‎31‎/‎01‎/‎2017 at 07:22, alacant said:

Hi. Could you post the guide log? The debug log is too geeky for me! Saludos.

Thanks

Will do that tonight.

[iapa]

Guide Log file attached.

There were a few shots at setup in there

 

TIA for assist

PHD2_GuideLog_2017-01-26_212935.txt

[kens]

Looking at the first of the two four hour sessions I can offer the following guess at what is going on. It's just a guess so don't take it as gospel. First up you started guiding at -5h30m which is down near the eastern horizon. Your were guiding north only and all was fine for 30 minutes as you said. From there is looks like you had a PA error that caused you to drift north so the drift was not corrected. 

Down near the horizon the PA drift is mostly due to altitude error. At -5h30m you are some 7 degrees above the horizon. After half an hour you are 15 degrees above the horizon. And once you are ore than about 20 degrees above the horizon the azimuth error starts to come in. By the time you get near the meridian the azimuth error is the main contributor. Near the meridian you had drifted some 4' after 4 hours so your PA error is in the vicinity of 4' or 5' which is quite reasonable.

Looking at the graph in Pempro Log Viewer and removing the Dec corrections I'd guess that your altitude error is causing a drift south which is corrected but as the azimuth error kicks in it is in the opposite direction. In fact, the early drift suggests the altitude error is worse than the azimuth error. Down near the horizon, refraction and general murk may also be having an effect.

Best suggestion I can make is to check your PA and for long imaging sessions, with guiding in one direction, try to get the altitude error to drift the same direction as the azimuth error. Unfortunately, this will only work on one side of the meridian. But after the meridian, it will be ok till the altitude error kicks in on the west which will be late in the session.

[iapa]

Thanks for the analysis - the first 4 hr session is my main concern, from 00;15hr to 04:30hrs approx. The rest was 'what the hell' stuff. I slept through most of that until a late breakfast ?

itll take me some time to digest tho' - slow thinker. ?

[iapa]

1 hour ago, kens said:

Looking at the first of the two four hour sessions I can offer the following guess at what is going on. It's just a guess so don't take it as gospel. First up you started guiding at -5h30m which is down near the eastern horizon. Your were guiding north only and all was fine for 30 minutes as you said. From there is looks like you had a PA error that caused you to drift north so the drift was not corrected. 

Down near the horizon the PA drift is mostly due to altitude error. At -5h30m you are some 7 degrees above the horizon. After half an hour you are 15 degrees above the horizon. And once you are ore than about 20 degrees above the horizon the azimuth error starts to come in. By the time you get near the meridian the azimuth error is the main contributor. Near the meridian you had drifted some 4' after 4 hours so your PA error is in the vicinity of 4' or 5' which is quite reasonable.

Looking at the graph in Pempro Log Viewer and removing the Dec corrections I'd guess that your altitude error is causing a drift south which is corrected but as the azimuth error kicks in it is in the opposite direction. In fact, the early drift suggests the altitude error is worse than the azimuth error. Down near the horizon, refraction and general murk may also be having an effect.

Best suggestion I can make is to check your PA and for long imaging sessions, with guiding in one direction, try to get the altitude error to drift the same direction as the azimuth error. Unfortunately, this will only work on one side of the meridian. But after the meridian, it will be ok till the altitude error kicks in on the west which will be late in the session.

First stab then is that the PA done using the polemaster may be off?

given its good reputation, unsure that's a valid assumption for me to make 

I would have thought that If I've got my altitude set off the I'd not have Polaris in view? Ditto for azimuth?

i did the PA from start as if removed the polemaster last tear down.

could mechanical issues be a cause ? Gearing or something f?

[kens]

5' is not that far off, in fact for AP in general it is quite ok as long as you guide out the drift and would not take you out of the FOV

On mass market mounts its generally pretty hard to get much closer than that except by fluke as the adjustments are just not that fine.

Overall your polar alignment is fine but with single direction guiding from horizon to meridian its beneficial to get the polar alignment error in the right direction. Either that or switch the guiding direction when appropriate.

The other possible problem is flexure which is mechanical and often hard to track down. In fact my normal reaction would be flexure but looking at the guide log the drift curve looks too smooth

My recommendation would be to follow up your polemaster alignment with an unguided drift at the eastern horizon and east of the meridian and ensure the drift is the same direction in both cases. In my experience, its easier to adjust the azimuth so do that till it drifts the same way as the altitude drift. Check again with your polemaster and it should still be aligned within the possible precision

There's no way it can be gearing as you were drifting in the absence of guiding pulses

 

[iapa]

Again, thanks for the response.

Reassured that it's less likely to be mechanical. Flexure - piggybacked refractor with rings to camps to dovetail on top of OTA. I've a build (like the back garden cludgie) that allows for good torque to be applied manually to the bolts holding clamps etc.

Focuser was tight - glad you don't need to have sharp focus for guiding - it moved a bit when I tightened it up, improved the guide star capture nicely  

It's going to be a while before I get another shot by the looks of current forecasts

Will bear your thoughts in mind for that time.

[ollypenrice]

On 27/01/2017 at 12:07, iapa said:

Not sure this is in 'Beginners' other than we are all beginners in thi shabby, there ar eno 'professional' amateur imagers

 

 

In best pantomime voice, 'Oh yes there are!'  

lly