interpretation of PHD graphs

[Sammyb]

Hello,

I was using the graph output the other night for the first time and I noticed I could get a nice flat trace but the Dec line started drifting and then coming back into line with the RA trace. This cycle would keep on happening.

Has anyone seen this before? What is happening here?

I also got these traces where the dec trace would suddenly leap what is the cause of this?

I've attached some screen grabs to illustrate what I was seeing.

I was using an SPC 900NC on a 9x50 finderscope on a SW explorer 200P and EQ5 synscan with EQMOD etc.

Any ideas folks?

Sam

[Mav359]

I'm no expert and i have a Celestron mount so there might be difference's but if its a cycle that repeats itself wouldn't that suggesst Periodic Errors in the gears. Its my understanding that what ever the mount and no matter how well it was put together there will always be slight deviation in a full cycle.

In my mount its suppossed to take about 8mins for a full cycle which would highlight the error which you can then train out using Period Error Correction (PEC)

Possibly!!!

[ollypenrice]

No, it's not Periodic Error because it's in Dec and all being well Dec isn't being driven at all, only nudged by the guider. It should stay put in a perfect world.

In the Brain section are you guiding in Dec in both directions? It is best to guide only in one since all Dec corrections will be in the same direction. I'm not sure what's happening here. I'd begin by disabling the Dec corrections on either N or S (trial and error) to see if that helped. I've only just started using PHD so I'm not used to it. I normally guide in AstroArt.

One other possibility would be a lazy connection on one one of the ST4 pins, meaning the mount is slow to respond to Dec drift (normally due to polar misalignment.)

Finally I'd try altering the dec balance making one end a bit heavy. It may just be that the dec axis is oscillating either side of the backlash and then the guider is taking a long time to pull it back. If it is out of balance it won't oscillate.

Olly

[RobH]

Looks to me like somethings moving. Is everything tightened down solidly? I give all my guide rings etc a small amount extra with molegrips.

Are any cables snagging?

These are the things you need to look for first.

Olly is right re. PHD.

Your mount is unlikely to be aligned perfectly in RA, so your DEC corrections will be either (mostly) N or S either side of the meridian. Set PHD to this once you've figured out which it is.

This keeps the gears correctly meshed all of the time and means that there's no backlash to take up. If you are correcting in both DEC directions, you will always be dealing with backlash, meaning that it will take a while to catch up with the drift, and then often overcorrect, a cycle which repeats.

Rob

[Sammyb]

Thanks everyone. That's plently for me to think about.

Sam

[FrankieValley]

Good suggestion already, but the last graph makes me think something is moving and that it moved pretty quickly on the last graph.

You have Max Dec pulse set to 150 as is the default and the graph tells me that this isn't enough to keep the Dec on the straight and narrow. PHD tells you what it is moving and by how much along the bottom bar of the main screen. You will be seeing lots of N 150 and S 150 as the Dec pulse is initiated to its full extent. You can increase the Mx Dec setting to whatever is needed to keep it on the level until such a time as you resolve the cause.