PHD 2 GUIDING PROBLEMS

[red dwalf]

hi all, i`ve tried everything i know, which isn`t much to get rid of my elongated stars, the image shoes a 10 minute sub with egg shaped stars which i believe are caused by the DEC motion, i noticed it more this evening when i was trying to focus on stars and the image was moving side to side causing very egg shaped stars and sending my focus readings all over the place, i`ve managed to calm the phd 2 graph down a little which is the reason for my current settings, i`m fairly sure i`m very well polar aligned, using the excellent shapcap 2.9.

any other tips i could try please.

 

[MarsG76]

The graph is telling me that your star is guided quite well, and it doesn't look like there is a stretch or bias in one direction over another, so, in theory, your stars should be round.

Initially I thought that your problem is mirror flop, but looking at your scope, that theory went out the door... The next theory I have is that you're using a guide scope and guiding on a star that is too far from the object being imaged and that you might not be as accurately polar aligned as you assume. If this is the case than that would explain your star drift since guiding on a star too far away from your object will cause the effect like your imaged object acts as if it's rotating around you guide star.

I think that if you guided on a star that is either slightly east or west of the imaged object, but not north or south of the object than the guided arc will be closer and that should improve on the eggy stars... if guiding too far off the object, especially north/south than unless perfectly polar aligned, you will be guiding on a slightly different arc. One of the reasons why i'm predominantly guiding using a OAG.

 

[red dwalf]

many thanks,

my guide scope and camera are piggybacked onto the william optics scope and they two scopes are pointing at the same object, i will check my polar alignment again. 

[sloz1664]

Ayup Rob,

I assume you have set up PHD2 correctly and used the guide assistant. Have you balanced "east heavy" I can see you are guiding at 1.5 sec which I think is too short. I guide at 5 sec which negates chasing the seeing. I chucked my guide scope in favour of OAG. Less weight and removes the chances of deflection which can cause oval stars. I can now guide in excess  of 30 mins.

Steve

[Owmuchonomy]

Is your chip aligned with your Dec and RA axis?  If not, set it so that it is then identify definitely whether the 'egginess' is associated with Dec or RA.  That will enable us to suggest some causes/remedies.  Also, check the 'Trendlines' and 'Corrections' boxes on the PHD screen and rerun the guiding so that we can spot drift in your capture.

[red dwalf]

hi Steve, i`m not sure on my balancing, i`ll check that out, i usually go for 3 seconds but tonight i seemed to be jumping all over the place, starting to wonder if i have something loose on the setup, this warm and cold weather can cause problems, i`ve had them before in the field,

here`s a picture of my phd target, it`s not very nice.

[sloz1664]

What are you imaging Rob?

 

[red dwalf]

10 minute subs in OIII on the horse head nebula, i have 7 hours of HA, 4 hours of Sii so just need the OIII to finish

 

[kens]

Can you post the guide log from that session? The screen shot shows only 75s of guiding.

[red dwalf]

where would i find the logs, i`m using windows 10

 

[red dwalf]

just chedcked my alignment again and it was slightly off, now redone it.

before

after

[michael8554]

Tick the boxes mentioned by owmuch... and also select arc-secs instead of pixels.

The guidelogs are in the phd folder.

Michael

[kens]

1 hour ago, red dwalf said:

where would i find the logs, i`m using windows 10

 

By default they are under Documents\PHD2 - otherwise the location is on teh Global tab of the brain

[red dwalf]

I couldn't find any logs in the PhD documents folder, will check the brain later.

 

[red dwalf]

managed to find the logs, i tried a few different settings during the night but nothing seemed to help much.

guide log

PHD2_GuideLog_2018-01-20_194506.txt

de bug log

PHD2_DebugLog_2018-01-20_194506.txt

[kens]

That log looks pretty good. The calibration is good - only the steps are a bit large.

There is a bit of backlash takeup which can be avoided by making sure the last move you make when navigating to the guide star is north and west.

Did you run the guiding assistant? If not do so - let it run for a good 20-30 minutes and tick the option to calculate backlash. When done accept is recommendations.

Despite the backlash during calibration it does not appear to be adversely affecting your guiding - possibly/probably due to the belt mod. So it seems safe to continue guiding in both directions and to dither on both axes.Also, whilst I'm not a fan of backlash compensation it seems to be working for you.

So why the eggy stars? If the guiding is good (and it is) then the most likely cause is differential flexure between the guide scope and imaging scope.

Its rather hard to track down so start by making sure all your mountings are rock solid, all bolts tight, cables not dragging (even infinitesimally). One trick I've used is to point at the pole and take an exposure while slewing the mount in RA. Have the guide camera looping at the same time and compare the main and guiding images after. The star trails should follow the same trajectory. However its not foolproof as any optical aberration will give a false reading. But gross issues sometimes jump out. If you use a "CCD" imaging camera a better way is to run two parallel PHD sessions  and compare the plots.

[red dwalf]

Thanks for the advice, how can I decrease the step size ?

[kens]

6 hours ago, red dwalf said:

Thanks for the advice, how can I decrease the step size ?

Use the calculator in the guiding tab of the brain. 

[newbie alert]

On 20/01/2018 at 21:40, red dwalf said:

many thanks,

my guide scope and camera are piggybacked onto the william optics scope and they two scopes are pointing at the same object, i will check my polar alignment again. 

Your rms figures say that your PA is pretty good to me..

Are you definately guiding on a star as if the guide scope isn't focused that well it can give you false  readings..amazing rms values but egg or tear drop stars

[red dwalf]

The focus issue is a good point, I've often be told that you don't need to have the best focus for guiding, I do try to get them as good as I can but it can be difficult. 

[red dwalf]

a little update, 

the other night i started imaging and as usual the cloud started to roll in, so i thought  i`d re-visit the guiding problem and try to re focus the guide cam, for the life of me i couldn`t get it quite right and thought that something else was definitely wrong, so after about half an hour of tinkering i though i`d check the lens of the guide cam and would you believe it , somehow the guide camera had come loose it the guide scope, how it had not rotated in the guide scope was a mystery, so i re- seated it properly and did a 3 minute and a ten minute sub and the stars looked miles better, can`t wait to try it before the light nights take hold.