Polar Scope and Cone Error?

[Criffa]

Hi all! I'm a first time poster to SGL and relatively new to astronomy so please forgive me if I mess up with the nomenclature! I've tried to search for the problems I've been having, but it could be because of the terminology that I've failed, so further apologies if this has been covered elsewhere.

The ultimate question is that even if a polarscope is perfectly calibrated and then used appropriately to polar align the mount, will cone error (i.e. the scope tube having a different optical angle from that of the polar scope or the RA rotational axis?) generate star trails in astrophotography?

The reason I ask is that I have spent a lot of time correctly calibrating the polar scope, using it to polar align and have now discovered drift alignment using PHD and the graph output. I drift aligned using this method and for the first time ever I managed to get a 2 min unguided exposure without star tails. Beforehand, 30s was absolute max. Out of interest, I rechecked the position of polaris in the polar scope and expecting it to be on the circle that depicts polaris's orbit around the NCP, but it was way off. This got me thinking that the rotational axis of the polar scope must be different from that of the telescope.

Does this all sound rational or have I missed something? It could just be that I've overlooked this aspect in my previous research, but when it comes to astrophotography, there seems to be great importance about polar alignment and ensuring a well calibrated polar scope, but next to nothing about cone error...

Chris

[Merlin66]

Unless you're using the main scope for polar alignment, cone error won't affect the polar alignment of the mount.

Where it does affect things is GOTO accuracy...

[Freddie]

If you do a 3 star align, the 3rd star is used to take out any cone error. With Skywatcher kit that is.

[UnderTheRain]

You can get rid of the cone error it's not too difficult if you check out the astronyshed videos on youtube Dion does a complete scope setup and in part 5 he explains how to get rid of the cone error. But as previously stated a 3 star allign will allow for this on Skywatcher mounts. And in answer to your question you are correct if you have not corrected for cone error then the scope and polarscope axis are out of alignment and even when corrected they will be out by a tiny tiny amount.

Brian

[Criffa]

This is all starting to make sense now!

I've recently upgraded to the EQ5 with my 150pl (I know it's at the limit, but the HEQ5 was just out of my range - I'll be saving up for the EQ6 or higher next, along with the CCD camera, 10" scope and a house with dark skies!). The upgrade was from a EQ3-2. No matter how much I played and fiddled with the polar scope on both mounts, I was unable to get 30+ subs (before discovering this drift alignment with a webcam). I've discovered that I do indeed have a cone error that I think I've now remedied, so essentially my optical axis *should* be more or less the same as my polarscope axis.

So in a nutshell, using a polarscope is pretty much useless if there is a significant cone error?

I've also been playing around with the 3 star alignment function too. Even if this is done correctly, there'll still be drift if I'm not polar aligned?

Chris

[Freddie]

Yes, you need to get the MOUNT aligned otherwise you will have drift.

[hemihaggis]

30 secs is fine unguided