The PHD graph from hell

[Ford Prefect]

Hi

Managed to spend a good few hours in the Obsy last night trying to resolve my guiding issues. I had previously got PAM to work so I knew my polar alignment is good and the scope is also balanced but I still get this awful PHD graph!!!!

Problem is, this is a new set up for me (C11 edge, QHY8, OAG, Meade DSI) so I have no idea if this is the best i can hope for, however I think not!!

I have tried changing the RA agr between 60 to 100 and the exposure between 1 and 3 seconds and still no luck.

Anyone got any other ideas,

cheers Jon

[JCJC]

Not sure but how did the stars look on the images Jon?

Jordan

[Merlin66]

The pixel size of the DSI guider on an OAG at f10 (I assume?) may well show lots of movement...is this transferred to the image??

How did the calibration process go? many steps?

Does the DSI allow binning? If so, you could try a 2x2 bin to quieten things down.

( I use a C9.25 at f10 with a beamsplitter - similar to an OAG - with a QHY5 guide camera....doesn't show as much movement as your graph and holds a star on a 25micron slit for ever.)

[Ford Prefect]

For a 10 min exposure the stars we slighty elongated west to east.

[Ford Prefect]

The pixel size of the DSI guider on an OAG at f10 (I assume?) may well show lots of movement...is this transferred to the image??

How did the calibration process go? many steps?

Does the DSI allow binning? If so, you could try a 2x2 bin to quieten things down.

Indeed it is at F10, calibration did around 20 steps in each direction, I belive that the DSI does support binning, i try that next time im out.

Cheers Jon

[Merlin66]

You seem to be heavily correcting both in RA and Dec...

Was the guide camera reasonably orientated East-West - should make a lot of difference but sometimes.....

[Glider]

Try increasing your Minimum Movement so avoid chasing things that aren't really there?

Increase the Max Dec to 260?

[RobH]

Having DEC guiding set to auto means that you're dealing with backlash every time it changes from north to south guide commands.

Unless your PA is perfect, on one side of the meridian, your DEC guide commands will be N,or S, but not both. This will reverse after a meridian flip.

You need to figure out which way it is for your setup, and then set the DEC guiding to this. I have my mount very slightly offest in azimuth so that I can do this. When I've had it perfectly aligned, I get oscillation in DEC guiding.

Regarding RA, did you try increasing your minimum movement before guiding commands are issued? At F10 with a DSI, any slight disturbance in seeing will be causing guide commands to be sent out.

Do you have the mount slightly east heavy, again so that you're not running into backlash issues?

What was the seeing like?

Cheers

Rob

[swag72]

I have just found this link to an Oz forum where there's quite abit of discussion about the PHD graph.

[Ford Prefect]

Many thanks for all your replies, they have been most helpful

Rob, I may be being a bit thick here but can you explain more around how to work out whether I should use North or South Dec guiding?

Cheers Jon

[RobH]

Hello Jon.

Just calibrate etc as per noprmal, then start guding on something a fair way either East or West of the meridian.

Once the guiding settles down, after a few minutes, watch the guide commands given for DEC.

The great majority will tend to be either North or South.

Once you've figured out which, switch the DEC guide mode to this only.

If you've got it wrong, it'll become obvious fairly quickly as your star will drift in Dec and no guide commands will be given to correct it. If that happens, just change the DEC guiding to the opposite of what you've got it set at.

HTH

Rob

[Ford Prefect]

cheers Rob

[SteveL]

This is the same north only / south only option that Ant accidentally had issues with, so you`ll know what the issue is if you see it happen.

[Ford Prefect]

Good point, I had forgotton about that