Finding a guide star

[Ian]

I was out last night playing with guiding again.

Has anyone any tips for finding a guide star? Is it just a case of wiggling the guidescope around until an appropriate one turns up the CCD or is there a more rigorous approach?

[JohnP]

Ian - That's the way I do it.. I normally just hope that there will be one there but if not I just start adjusting my guide rings until I get one. On a few ocassions I have had to take the guide cam out & put an eyepiece back in & centre a star that way...

John

[Mike]

Yep that's pretty much it. Luckily with a cooled CCD for a guide cam you can find a star 99 times out of 100, but as long as you pick any star along the same declination it will work, even if it's nowhere near where you are imaging.

[Rocket Pooch]

I do the wiggle scope bit works for me.

[Fay]

I usually use a focal reducer, as It gives me more choice

[Mike]

However with a wider field of view the star moves less and you therefore get less accuracy with the guiding.

[Ian]

thanks for the replies, I wasnt sure that I had the best approach.

I'm now going to ponder Mikes comment about the dec. I wonder if guiding might be better if you try and get the a guidestar with as low a dec as possible...

[RobertM]

Hi Ian,

The dec would have to be relatively close otherwise you may run into diffraction effects depending on how much atmospheric the light from your guide star has to come through.  It'll get worse the further from the meridian you are guiding.

You could always increase the guider exposure time.  I use 2-5 seconds with my cmos guider at approx 500mm fl and haven't failed to find something to guide on, even if it is a hot pixel ...

[MarkS]

There's another good reason for keeping the guide star close by - field rotation.   This is not easy to explain without diagrams but I'll do my best.

Even for an equatorial mount, if your polar axis is not aligned dead accurately then you will suffer drift and also mild field rotation - up to a few tens of arcseconds per minute depending on where in the sky you are pointing (it is worst near the pole).  But when guiding, the drift component is removed leaving you with a residual field rotation.  Your guide star effectively becomes the centre of this residual field rotation since it is the object that is kept stationary.  If your imaging scope is pointed well away from this centre of rotation, then the stars being imaged will form slight trails during a long exposure.

An alt-azimuth scope is simply a very extreme case of the above effect.

[RobertM]

That's very good point well explained.

[Mike]

If your guide star and imaging area are in the same declination then field rotation will be irrelevant as they will both be rotating at the same speed.

[Ian]

hmm. Not sure field rotation isn't a red herring. need to do some sums, and I'm currently wearing socks... Getting my head round it empirically isn't working as you don't get field rotation with an Alt-Az at either the pole or the equator, but does that mean it's worse at 45deg, I wonder?

But, looking at this thread I wonder whether a guidestar at a similar altitude is best.

Of course, I could always just STFU and get on with it, but it's cloudy 

[Mike]

No because the earths axis is tilted to the celestial axis and therefore objects at different altitudes will rotate at different speeds. Whereas objects at the same declination all rotate at the same speed and your mount is tilted to the same axis.

[MarkS]

Mike,

It is not sufficient to choose a guide star with the same declination. 
Here's a diagram that illustrates what is happening.

                X
A    P     X                         G
                X

P is the celestial pole
A is where the axis of our misaligned scope is pointing
G is the guide star
The X-cluster is what we want to image.

The relationship of the guidescope, telescope and axis is always rigidly fixed.  In this example the telescope points halfway between the guidestar and the axis and so it starts off pointing directly at the X-cluster.

6 hours later the stars have rotated around the celestial pole (see diagram below).

A    P     
                 
      X
    X  X



      G

But the telescope is still pointing halfway between A & G.  So, from the telescope's point of view, the X-cluster has moved off to the right and rotated - hence the long exposure ends up trailing/rotating.  The same argument holds whereever you choose your guidestar (just place it in your chosen place in the digrams above)

[Mike]

I disagree. If they are on the same declination, not altitude, then they are rotating around the zenith at exactly the same speed. If your guide star is on the same altitude then it will naturally rotate as we are at latitude 53 degrees and as such different areas along the same altitude cross different declinations.