Autoguiding...

Ah, now I understand! That's interesting Jed. I didn't know that. So basically, PEC is to help you get started in astrophotography until you get the autoguider or if one only wished to take relatively short exposures.

Michael - as far as I'm aware, if you use PEC, it will result in less corrections for the autoguider to issue back to the mount. It least that's the case with the AP I think. Not sure about Meade mounts though.

The key thing you need is polar alignment, once you have that as good as you can get, then a PEC recording will improve the mounts tracking ability - though the question of whether you can then get better polar alignment with PEC on, arises. The AP seems to know the difference between errors in the mechanics of the mount (worms, gears, motors - i.e. a periodic error) and an alignment error - and corrects/allows for it.

When autoguiding, it seems that the autoguider will correct for as many errors as it can - periodic, alignment, orthogonality, flexure etc. If you can minimise the periodic and alignment errors, you'll have better guiding because the guider is correcting for fewer errors.

Bear in mind too, that for wide-field imaging, guiding is not as crucial as for narrow-field imaging. Also, as you say, when you get to longer exposures (greater than say 5 or 10 minutes), then autoguiding starts to become a necessity.

I'd be wary though of assuming that with an autoguider you can disregard periodic error and PEC...

€0.02

Dave

Jed - just out of curiosity...

The AP1200 says that PEC should be used at all times to increase accuracy (though the mount is accurate to 5 arcsec). I think the PEC signals the mount to correct every 100ms. One tip I did see was that autoguiding corrections need only be set to every six seconds or so.

Do you think that autoguiding should be used without PEC in this case?

Cheers

Dave

I think we can treat your mount as a special case Dave

Most mounts PEC is not as accurate and thus hinders the autoguiding, but some mounts have significant periodic error and the only way you can guide is to use PEC and a large integration window to avoid occilation.

Later,

Jed.

Ah, now I understand! That's interesting Jed. I didn't know that. So basically, PEC is to help you get started in astrophotography until you get the autoguider or if one only wished to take relatively short exposures.

No PEC is there to reduce large errors so that it is possible to autoguide. As I said it is most often not desirable to use PEC (unless you have a very good mount like the AP) but some of the less accurate mounts require PEC to make autoguiding possible.

As I mentioned before, you should check on the Meade groups to see if PEC is required to autoguide or not on your particular mount. If it is required then just make sure you use larger intergrations than the PEC correction frequency.

Later,

Jed

I can imagine a situation where the mount might go quickly out and come back to center before the autoguider can compensate and the autoguider sees this and then tries to compensate thus actually inducing a second error and getting itself in a real knot.
IF you carelessly program your PEC, its easy to induce the above situation.

Look at it in these steps,
1,The mount is unprogrammed (PEC wise) and starts to "wonder" due to error.
2, In you PEC programming, you see this occuring and correct quickly but with a delay

If you mount has alot of erratic error, then the autoguider will see rapid ins and outs and will completly make a dogs dinner of tracking.
Rapid error is what kills the autoguider, be it caused by inherently bad gears or a messed up PEC programming.

As for checking it on meade yahoo sites, they will come back and tell you its pot luck, just like with image shift, your scope might have it good or bad. The only way is to look through the eyepiece for the 8 minutes and see what happens to the star position.
Its also recommended to use polar mode, as now (in theory) one drive is shut down, less chance of error.
It s also critical to get the scope balanced in declination if using altaz mode, otherwise you'll get different PEC behavour depending on whether the scope is moving north or south. Imagine the "fun" in that situation

Some people recommend putting the guidscope slightly out of focus to reduce the effects on seeing and make the guiding smoother.

I have been ingesting all of this information you have posted Dave G as I am on the same boat and am just getting the guiding setup and functioning at present. I know that the 300d mod is on the cards so I would be most grateful for any help/advice when the time comes.

Clear Skies


Markdj