As others have suggested, take the guide camera out of the guider (but first use a parafocal ring on it so you don't loose focus)? and insert an eyepiece. Place a bright obvious star in the center of the main scope and then adjust the guider so that the same star is centered in the guide scope. Put the guider back in and re-check focus.
There is another reason (esp. if you don't have a permanent setup) to get this as good as you reasonably can: Field Rotation, which if present causes the images of stars to appear as little arcs center on (and proportional to the distance from) the guide star. Field rotation depends on four things:
1. The precision of the polar alignment.
2. The any difference in the angular alignment of the imaging and guiding scopes ( how well they are co-boresighted, to use the artillery term) - what your question was about.
3. The duration of the exposure. The longer the exposure the worse (if present) field rotation will be.
4. The magnitude and the orientation of the polar alignment error relative to the RA and DEC of the imaging target.
You can not do anything about the 4th unless you just give up and move to another object in a different part of the sky and pray you are lucky. The third you can compensate for by taking more shorter exposures and then using a stacking tool that does a two point stacking alignment.
The first two (polar alignment and co-boresight accuracy) you can do something about. Having the two scopes co-boresighted has two benefits: First, it makes it easier to diagnose (if necessary) polar alignment problems because the arcs of the stars will all be orientated around the center of the image (where the guider was tracking), making it obvious what is going on. Second, a large amount of boresight error can magnify even a tiny amount of polar alignment error.
With regard to 2, the problem (if present) is not less for shorter focal length imaging instruments. This is because the size of the arcs in angular measure (degrees, minutes of arc, whatever)? for a given exposure duration is linear in the angular distance between the star images and the guide star. So while cutting the focal length of the imaging scope in half will reduce the plate scale (Image size in inches per degree, for example) the width of the field, in degrees will proportionally increase (for the same chip, obviously)? so the two effects cancel each other out. Likewise cropping won't do anything in general, nor will going to a shorter focal length (wider field) instrument.
For a general discussion see:
For a detailed discussion, see Michael Covington's book "Astrophotography for the Amateur", pages 276 to 282 where he goes through all the mathematics of polar alignment error, DEC drift, and field rotation. On page 281 he gives some nice illustrations and a summary (B.3.3 How Much Field Rotation is Tolerable). The reading is not required, just a general understanding.
The Calgary (Canada) section of the Royal Astronomical Society has a detailed qualitative discussion of Field Rotation, including the extreme case of ALT/AX mountings:
https://calgary.rasc.ca/field_rotation.htm
The bottom line is that the better the polar alignment the less all this matters. And for off-axis guiders improving polar alignment or shortening exposures (if feasible) is the only cure.
And of course what really matters is how satisfied you are with your final images.
Clear skies and Happy Holidays.
Mark Christensen
PS: Can you do the alignment during daytime? Of course, just make sure the alignment target is far enough away. How far? Far enough so the parallax between the two scopes (guider and imaging) doesn't matter much. If your 60mm guider is the type I am familiar with its focal length is between 240 and 300mm.? Let's call it 300mm. Most guide cameras have small chips in them so let us suppose you want to get within 1mm of the center. That means the parallax error should be reduced to less than 1/300 or so (about 0.2 degrees). If the spacing between the center lines of the guider and the imaging scope is something 6 inches then the alignment target should be at least 300 * 6 inches or 150 feet and 300 feet would be better. Further is you wish to achieve more accurate co-boresighting. The same discussion applies to finder scopes.
