What is auto guiding ?

[Vega]

Hi

I'm kind of new to this fine tuned polar aligning lark. Having done my first successful drift alignment I noticed auto guiding being mentioned in these parts...

Can anyone explain to me what this involves ?

Can it be done with an EQ3-2 mount ?

Cheers

A newly aligned Matt

[themos]

I am a complete novice so I will take a guess and make the others laugh. Auto-guiding seems to involve a hideously expensive CCD (charge-coupled device), a computer for processing the digital image that comes out of it and determining which way stars are drifting and motors that can be controlled from a computer. What I am not sure is whether this happens at the same time as the CCD is collecting your "real" picture or not, whether you use another piggyback scope for this guiding and many other things besides. We will soon be told...

[GazOC]

Thats pretty much it Themos. To guide you can either use a seperate scope, a camera with two chips (a guide chip and a imaging chip) or one camera and scope alternating inbetween guiding and imaging. Although the last option sounds by far the easiest/ cheapest it has its own set of problems - mainly the increase in time needed to image an object.

No, I'm afraid the EQ3/2 can't autoguide without some major mods.

[Kaptain Klevtsov]

Auto guiding is where, as Themos suggested, a camera is used to monitor a star. This guide star, as its called, starts off in a certain position on the guide camera's chip. Software monitors the position on the chip and detects movement away from the start position. If a movement is detected the computer issues commands to the mount in an attempt to move the image of the guide star back into the correct place on the camera chip.

The commands to the mount can be done via USB, a guideing input on the mount head (on goto mounts usually) or a splitter box between the handbox and mount for non-goto mounts. The output from the computer is either via the parallel port or a USB port.

Most often the guide camera sits in a smaller piggy-back mounted 'scope arranged so that the guidescope can be aimed at a bright star independantly of the main 'scope.

Another common way of fitting the guide camera is to use an off-axis guider. Basically this is like a diagonal with most of the mirror smashed away and a cameraa fitting on the back. The guide camera sits on top of the diagonal while the imaging camera goes on the back. Most of the light goes straight through to the imaging camera but some goes to the guide camera because of the little bit of mirror that's left. Or a prism can be used instead. This makes finding the guide star more important than framing the image so it is much more difficult to set up. It does perform better as there is no chance of the guide scope moving relative to the imaging 'scope as they are one and the same.

Guide cameras tend to be webcams or modified webcams. These have small pixels therefore making the camera more sensitive to drifting of the guide star.

HTH

Captain Chaos

[Vega]

I see... very clever/expensive stuff. Kind of like a computer doing on-the-fly drift alignment on steroids. I like it

Looks like trusty ole time consuming manual drift alignment for me still

Cheers for the responses all

Matt

[Kaptain Klevtsov]

Its more than drift aligning, as it compensates for any lumpiness of the drive. This can be caused by the drive worm not being centred within a thousandth of a millimetre, slack in the bearings, tiny imperfections in the polar alignment etc. It keeps the target exactly where it should be measuring and adjusting constantly. It's an absolute must for exposures of more than a couple of minutes as the only other option is to manually guide using a reticle eyepiece and guidescope. That has it's own set of problems such as you have to look down the guidescope for hours without falling asleep, getting bored or touching the 'scope. You also have to quickly move the guide star back on the cross hairs in the correct direction whenever it slips off. An hour of this and you'd prefer Eastenders I'm sure, even though I haven't tried either. Imagine the guide star staying where it should be for ten minutes while you watch it like a hawk, that must be against the Geneva Convention or something.

Captain Chaos

[Jeff RV]

Think you just about covered that one CC, well put. Eastenders is on now in the background,He he he!!

Jeff.

[themos]

CaptainChaos that's a good first stab at a FAQ entry, thank you. Surely there's a similar gizmo for auto-focusing...

And while I'm making a fool of myself, I just had a hare-brained idea: Isn't it true that one can improve an imperfect mirror by blanking certain areas of it? And couldn't that be done by some LCD screen under computer control (if not the blanking at least the determination of the position and size of the bad regions)?

[Kaptain Klevtsov]

CaptainChaos that's a good first stab at a FAQ entry, thank you. Surely there's a similar gizmo for auto-focusing...

And while I'm making a fool of myself, I just had a hare-brained idea: Isn't it true that one can improve an imperfect mirror by blanking certain areas of it? And couldn't that be done by some LCD screen under computer control (if not the blanking at least the determination of the position and size of the bad regions)?

That's what's known as using flats. Take a picture of something totally flat white with no details on and use that to calibrate the CCD / 'scope system. Dead awkward to actually do though.

Captain Chaos

[MartinB]

It's wrong to think that you need expensive gear to autoguide. You do need a guiding capable mount so an HEQ 5 or GP clone will suffice. You could probably get a guiding capable handset from AWR technology for the EQ3 but the cost might not be justified. You could then use an SC1 modded web cam - Caz has had hers up for sale for £60, a cheap achromat guide scope, and a set of guide rings. You use your existing camera be it DSLR modded web cam or full blown CCD. There is free software that will do the job for you but you will need a laptop.

The fact is that autoguiding takes a lot of effort to set up. It isn't an easy option and is often best avoided. The people who are auto guiding tend to be extremely committed (obsessive?) and this is reflected in what they have been willing to fork out for their kit.