Goto mounts and autoguiders.

[Devil20]

I noticed whilst sniffing around some of the telescope suppliers that they sell autoguiders. If you have a goto mount do you also need to buy an autoguider. I'd have thought with a goto mount it would lock onto the star and then continue to track it once it had found it. Will a goto track accurately enough for AP purposes. Stand by because I'm going to ask another daft question in a minute.

Ian

[knobby]

Goto / tracking will keep the object in the eyepiece fairly well but not accurately enough for long exposure photography. Auto guiders will make constant minute corrections to the mount to keep tracking far more accurate

[Devil20]

Goto / tracking will keep the object in the eyepiece fairly well but not accurately enough for long exposure photography. Auto guiders will make constant minute corrections to the mount to keep tracking far more accurate

This is what I suspected. Does the autoguider take over from the mounts motors or do they continue to run and the autoguider just superimposes corrections on top. Is the mount tracking open loop control and does the autoguider provide closed loop feedback? How much more accurately will your average guider track compared with say an EQ5 Synscan?

[knobby]

From my limited knowledge of guiding the mount continues tracking and the guider will just interupt to make corrections as required...

[ronin]

A goto mount will go to where it believes the object is that you want to view.

There is no guiding once it has gone to that position, it will simply track (move) based on the location of the object.

If the mount is incorrect, likley you have moved it manually , then the scope will not point at the correct position.

E.g. You asked the mount to goto M42, then manually move the scope to Betelgeuese (can be via the handset on most) then the scope is looking at Betelgeuese but it believes it is pointing at M42. Subsequent movement is based on the position being M42 not Betelgeuese so will all be wrong.

[dmahon]

A GOTO mount will go to where the object should be - accuracy depends on your mounts/software's model. It will then track at sidereal rate. Expensive mounts at short focal lengths, if properly polar aligned, will track accurately for a few minutes upwards (usually more expensive ones do better). Guiding is superimposed on it to correct any inaccuracies.

If your mount cost less than £5k, you probably want guiding.

[RikM]

On an NEQ6 with a 750mm FL scope, unguided I can get 2min subs before star trailing becomes unbearable. With an autoguider I can get 30 min at least (never tried for longer).

[Devil20]

So it would seem that the mounts are open loop control based on the Earth's rotational speed but in error due to innacurate polar alignment of the mount whereas guiders are closed loop and possibly make use of the luminosity in the FOV to centre the scope to the brightest object in view via a superimposed correction. Makes sense I suppose.

[brantuk]

"I'd have thought with a goto mount it would lock onto the star and then continue to track it once it had found it."

Fraid not - the mount doesn't "lock on" to anything at all. The goto mounts just goto any chosen object using positional information in it's database, and based on mount set up parameters and initial alignment. You first polar align the mount so it's right ascension axis is parallel to the Earth's axis at your observing position (using the pole star in the northern hemisphere).

Then you give it the time and date and daylight saving, followed by one, two, or three alignment stars. So it knows where it is on the Earth's surface and how the sky is set out above it. Then it can track at the same speed as the Earth's rotation, but in the opposite direction - in a single plane (RA).

Now the important bit. How accurate this tracking is, will depend entirely on the accuracy of your polar alignment and your  pointing accuracy of the 1, 2, or 3 alignment objects. If all your alignments are perfect, then the tracking should follow the exact path followed by the stars as they rise, traverse the sky, and set. Mostly the tracking is, at best, very good for observing by eye.

But for astro photography we are talking about keeping streams of photons from a single object millions of light years away, in exactly the same position on a ccd, dslr, or webcam chip, with pixel perfect accuracy. Much finer tollerances are involved. Exactly the same principle as trying to follow a race car with a camcorder - very difficult to do with perfection, avoiding blurring, and avoiding corrective movements.

To make the tiny tiny mount corrections required at these levels of accuracy, we use a second camera to follow a nearby star, in order to guide our imaging camera by passing movement corrections to the mount. This can be in the form of a separate guide scope and guide camera, connected to a computer using guiding software like Phd. Or it can come in the shape of an autoguider that feeds the corrections direct to the mount using "on board" software (typically via an ST4 port).

The software can detect when the guide star moves on the guide cam or autoguider chip, by how much, and in which direction. It then tells the mount how to correct both RA and Dec axes of the mount in order to maintain image position of the object on the imaging camera chip. Hth

[brantuk]

It's a one way open loop from guide scope to guide cam to computer and back to mount. Or from guide scope to autoguider and then direct to mount. There's no feedback the other way.

[cantab]

I think unguided tracking would be the open loop control - the controller sends drive signals to the mount but gets no information back. Guiding would be closed loop - the controller monitors the orientation of the mount using the guide cam and guidescope/OAG, and sends drive signals to control the mount orientation.

[Devil20]

Thanks for the comprehensive answers chaps. Cut and pasted into my astronomy useful stuff file.

[bomberbaz]

Handy thread this, does this work for webcam type view ad erll?

[dph1nm]

So it would seem that the mounts are open loop control based on the Earth's rotational speed but in error due to innacurate polar alignment of the mount

Don't forget  inaccuracies in the worm gear etc of the mount (periodic error). Except at very short focal lengths, these are likely to get you well before polar alignment does.

NigelM