Help with some basic questions about scope setup please!

[coenie777]

I hope someone can assist me with the following two questions:

Question 1:

When I mount my scope to a dovetail plate and the dovetail plate to the mount saddle, would a deviation left/right or up/down in the telescope's axis relative to the mount's north/south axis (say you moved the scopes axis 0.1mm to the left in mounting it to the skew dovetail you picked up), cause unguided tracking errors? In other words should your telescope's axis (front to back as a line), be exactly in line with your telescope's north/south axis line for optimal results?

Question 2:

When guiding with a second scope, should that scope's axis (front to back as a line) be exactly parallel to that off the tracking scope? If I extended a line from each of the telescope's into infinity, should they remain a constant distance from each other for optimal results?

What I am trying to get at with this is the following:

- Will misaligned axis of the telescope relative to the mount cause tracking errors or do you cancel these out in any case when drift aligning with the said missaligned setup?

Hope you can help.

Regards

[lensman57]

The answer is yes to point 1. Your guide scope should also be pointing more or less to same spot in the sky as your telescope since if it points to another part of the sky there will be a rotation error in the final stack. Different part of sky rotate differently to each other so best to keep guider as close to the target area as possible.

A slight misalignment of the scope to the mount axis can be corrected using the 3 star align routine.

A.G

[coenie777]

Thanks Lensman57

If I understand you correctly, the optimal position would be to have the two aligned exactly then? No need to compensate for anything by the mount.

This would suggest that an OAG is then a more accurate method of guiding than setting up a second scope, as you would need to make sure of alignment every time?

[Marky1973]

Now, I'm no expert so I may be getting come of this wrong but, if you are using good quality, solid dovetails and tube rings to mount your scope and guidescope, then they should be pretty well aligned from the off. If you are using collimating rings on the guidescope, you may not be so closely aligned between scopes, but the recommendation is probably not to use them (depending on your set up of course).

My scope/guidescope are mounted in rings and connected via a dovetail bar, all solidly screwed together, and I tend not to take them apart, so they are set in one position all the time - I guess if you were taking them apart after every session you may get problems with slightly different alignment each time?

From my reading I am not sure if the OAG is more or less accurate than a guidescope, but they are suitable for different jobs and set-ups so you need to take that into account - on a short focal length I get the impression that a guidescope is easier to work with, whereas an OAG comes into its own on longer focal length imaging.

Probably.... 

[steppenwolf]

Question 1:  Left or right is not an issue as this is corrected when you do your first star align. Up and down is potentially more significant as it results in 'cone error'.

Question 2:  The guide scope does not have to be perfectly aligned with the imaging 'scope for autoguiding to work - a small offset left or right is not a problem although again, cone error (up/down) is best  avoided.

Having said the above, I have never had an issue with cone error and have never checked for it either so I really wouldn't lose to much sleep over this.

An off axis guider *can* give more accurate results than a guide 'scope but it has nothing to do with pointing, more to do with limiting differential flexure which is the issue of the guide telescope and imaging telescope moving fractionally with regard to one another.

[coenie777]

Thanks for the feedback.

[kalasinman]

Being a novice, I erred on the side of caution. I found I had cone error, which I corrected with washers between the ring and the dovetail. Goto is pretty good with a one star align.