On-Axis Guider - How Often Do You Lack a Guide Star?

Does anyone use an on-axis guider?

Mike

Hi Mike,

How often you will find a useful guide star depends on many things:  size of the guider chip/pickoff, ability to offset field, telescope aperture, focal length/field of view, etc.  The advantage of an on-axis guider unit is that they are typically ahead of the filter wheel, and so you have more light available.  The disadvantages are that they are mounted pretty far off-center and so may have poor image quality, and they take up back focus space (which may not be available on a Newtonian system).  The old SBIG STxx cameras had the nice feature of an embedded guide sensor, but with the disadvantage of having that guide sensor look through the photometric filter and thereby not being able to go as faint as an OAG.

There are 41253 square degrees in the sky.  A typical good catalog for choosing guide stars is the Tycho2 catalog, which has 2.54 million stars.  This averages out to about 62 stars per square degree. There are fewer out of the galactic plane, and more in the galactic plane, but most variable-star observers will find this to be a lower bound.  If your guide camera has 10arcmin FOV, then you will have 1-2 guide stars in almost every field of view at the 11-12mag limit of Tycho2, and if you can off-center your target star, then the available field for choosing a guide star increases.

A separate guide scope increases your available field of view since you can use a bigger sensor, but probably doesn't go as faint as your main scope.  You do have to worry about flexure between the guide scope and the main scope, though in most cases of <10min exposures, this probably is not a major issue.  It certainly is easier to use a guide scope than to have to purchase an OAG, so it might be useful to try that approach first.

Many planetarium programs will display both the imaging field of view as well as an OAG  guide camera field of view, so that you can manually move the centering around so that a good guide star is on the guide sensor.

Arne

I have gone from an SBIG ST-8 with guide chip to the QHY.  Having used it now for a few months I wish I had switched earlier. 

1.  It's in front of the filters, so more sensitive

2.  Duty cycle, ie proportion of time collecting photons, is higher because guiding continues through filter changes and image downloads

I have not had a problem getting guide stars.  But there is some work to be done to be able to get to that point.  You have to locate where the guide field is in relation to the main field and set up those FOV in your planetarium.  When I plan observations I will sometimes offset the target to get a good guide star.

Another setup step is to get the guide parfocal with the main camera.  Because my CMOS main camera can do video I just point the assembled camera + OAG + guide camera to a distant object then adjust both to be in rough focus.  Fine focus can then be done on a star.

TG