SBIG autoguiding

[Andy82 (Andrew)]

Hi everyone,

Just wondering if anyone can help me with calibrating a Losmandy G11 with a SBIG STL 11000. Regardless of different settings, focal lengths etc, it refuses to calibrate, with either the messages " invalid motion in x or y axis" or "star to close to edge after move". Changes to times, aggressiveness etc don't make a difference, although the mount will guide with a Orion guider.
Any and all help greatly appreciated,
Andy

[gregbradley]

You can get that message with any camera.

Uusually it means that the framing box is too small and the star moved out of the framing box or the software is picking up a hot pixel in the image and confusing it for the guide star.

Make sure your camera is at high cooling.

You can use the main imaging sensor instead of the autoguider and CCDsoft will automatically adjust for that. I have done that successfully with an STL11.

Just use the focus subrame checkbox and drag a box around a bright guide star (don't use a double star, don't use a star which has another brighter star nearby that will come into the frame when the mount moves).

What callibration exposure times are you using?

As I recall you want the image to move 50 pixels which isn't very much so depending on your focal length that may only be 10 second callibration times. If you have an FSQ it may be 30 seconds.

I also once used a plug in to remove hot pixels for CCDsoft when it was hot pixels causing the callibration to fail. Google it, its a free plug in.
It worked but for some reason it disabled the auto select button on the autoguiding tab which is handy to use as it centres the guide star.

Greg.

[bert (Brett)]

Make sure you are not using the same guide cable as the orion. The sbig autoguide cable is different.

Brett

[cventer]

I would also check you have the right guide cable for the SBIG to losmandy.

Its different than the Orion Startshoot to Losmandy cable.

[Andy82 (Andrew)]

Thanks Greg,
The exposure times I've tried are between 5 & 25 seconds, at either 930mm or 2000mm focal lengths. How bright should the guide star be to calibrate on and do I need to recalibrate on each different object during a session?
Cheers,
Andy

[gregbradley]

The software calculates the centre of the guide star to a fraction of a pixel. I think the normal guideline in the manuals is a bright single star but not fully saturating the sensor. Not a double star as that could confuse the calculation of the centre of the star.

I found with autoguiding that not all guide stars give the same results.
The first thing I would do if I were getting poorer than expected guide errors was to try another guide star. Often a medium bright and sharp star worked better for me. Depending on your setup sometimes edge stars are not as round. They still work but I would pick a round one if I could. Sometimes you don't have much of a choice.

Check your cable works by hitting the NSEW buttons on the autoguiding tab and make sure you can slew your mount from those buttons. If not your autoguider is not connected to the mount.

With my STL I would chose the main imaging chip, have it as cold as possible, do 2x2 binning, make sure it was focused, download say a 4 second image with luminance filter, check the subframe box in the focus tab, drag a reasonably sized box around a medium bright sharp star (with no hot pixels and also no brighter star just outside the box). Then do the callibration with 15 seconds exposure time. Most of the time it worked. You should also use autodarks as the 237 chip is pretty noisy.
There may be other settings with a G11 that may need to be sorted like guide speed.

I tend to callibrate again if the object is on the other side of the meridian or far away from where I callibrated initially. Not every object.

Often times I only callibrate once. If guide errors become higher on another object then I may recallibrate in an attempt to reduce them. I am not sure it helps that much. I have read others who think it does. It can only help.

Callibration is much faster and easier on a Paramount mount and the whole thing only takes about 15 seconds and it rarely fail as above.

If its really buggy then you can copy down the callibration results from a previous time it worked and you can enter them manually if you have the same setup. I did that once when I was stumped and it worked.

Greg.

[Andy82 (Andrew)]

Thanks,
As soon as the weather clears up I'll give thses another try.
Cheers,
Andy

[bmitchell82 (Brendan)]

Just one thing about round stars greg i guide on cigar shaped stars very successfully here is a photo of the type of stars i guide on and a result of a 15 minute capture do remember this isn't a G11 ro PME/X only a EQ6. I have always noticed as long as you have a clearly defined Centroid or a bright patch that is regular in shape then it doesn't matter what shape your guiding on. This is my experience and my images speak for themself as getting a EQ6 to track a for 15-20 minutes accuately isn't easy.

One thing that i have also found that really effects guide quality is your actual calibration, in maxim it moves the star left right up down if it doesn't produce a nigh on perfect " L " ill keep trying till it does. Things that affect this L is wind, polar alignment and balance.

[gregbradley]

Yes I 've seen that too Brendan where odd shaped stars still worked. In fact I did that last weekend and I was surprised at how odd the star shapes were but I still got good guiding. That algorithim for calculating the centroids obviously is very good.

Greg.