Odd PHD graph

[lukebl]

Hi,

Just wondered if there is anything significant about this PHD guiding graph from last night. The guiding seemed fine, but I noticed that the RA and Dec graphs were mirror images of each other. Does this mean anything particular?

Also the Osc-index was only 0.01, when it's normally around 0.30. No idea what the Osc-index means! Should it be high or low?

[SteveL]

OSC-Index is the chance of the guide direction swapping to the other direction, where 0 is never, and 1 is always. If your polar alignment is out a little bit, it can happen that your guiding "nudges" are always in the same direction like your graph above.

[lukebl]

OSC-Index is the chance of the guide direction swapping to the other direction, where 0 is never, and 1 is always. If your polar alignment is out a little bit, it can happen that your guiding "nudges" are always in the same direction like your graph above.

Thanks, Steve. So is a lower OSC-index figure better than a high one? and what about the RMS figure? Sorry for the questions, but I just can't find the answers in plain English!

[SteveL]

A lower OSC is better... within reason! A OSC thats too low probably means your polar aligmment is out, and so longer expsures will start to show field rotation.

RMS is the Root Mean Square value of the diatance of guiding nudges required to keep it on track during the graphing period. Think of it as the average correction its had to make, much easier

[themos]

stupid question: how do you get this graph window up?

[SteveL]

PHD -> Tools menu -> Enable Graph

[themos]

much obliged sir!

PS: sure looks better than the EQMOD pulseguiding graph...

[chrisshillito]

PS: sure looks better than the EQMOD pulseguiding graph...

oh but look at the shear size of the thing! It's easy to look good when taking up half the screen

The PHD graph and EQMOD graph actually show different things. The PHD graph is I believe plotting the guide star pixel movement whereas the EQMOD graph shows the direction and duration of the correction pulses being applied to the mount.

Chris.

[themos]

As soon as I wrote that I thought "uh-oh, Chris will think I'm having a go!"

[swag72]

I thought that it was best to get an osc-index between 0.3 and 0.5? I read that in another thread somewhere, that 0.3 was the holy grail of the osc index! Don't tell me I've got it wrong again!!!!

[chrisshillito]

I noticed that the RA and Dec graphs were mirror images of each other. Does this mean anything particular?

It wonder if it could indicate your guide cam had rotated since calibration. When analysing other folk's PHD graphs in PECPrep I've sometimes noticed worm and gear periods associated with the RA axis present in the DEC data (albeit at a much lower level) and had always assumed this was as a result of either camera shift or poor/inaccurate calibration.

Chris.