[gregbradley]

Interesting results Ray and as usual you have done a thought provoking post.

Did you know PDH2 has a guide assistant that actually measures the seeing and PE and recommends the settings from that? Its quite good.

I have used it on both PME and AP1600. Generally it recommends the same exposure length each night of between 2 and 4 seconds.

I had a Tak NJP mount and it typically gave better results with short guide exposures like 1 second if I could get it.

PME seemed to get best results around 4-6 seconds and AP1600 around 4 seconds.

I think its highly mount dependent with the smoother better mounts with larger gears etc doing better with longer guide exposures and lower aggression and those with more PE shorter exposures as there is more moving around from the PE. I have often got best results with aggressiveness at 3 on mounts like PMX, PME but usually I use about 5.

Roland also recommends to increase aggressiveness on nights of good seeing so the number of secs before the mount is corrected back is lower.

I tend to tighten up max move as sometimes you see a bit of seeing induced larger corrections that seem to be outliers and worsen the guiding for a few secs. So now the mount is correcting the corrections.

A night of good seeing also may be skewing your results. Also mounts like PME/AP use a different system to EQ8 and its direct guide or pulse guide (AP version) rather than relays and this further improves guiding.
Short guide exposures on a night of worse seeing may in fact chase the seeing much more but in good seeing short exposures would work better.
FWHM may also not be the best measure as seeing fluctuations could account for some of the results although there is no arguing there is a definite trend there in your results. PI has a measure of eccentricity of stars you could use if you have PI. If not you can download a trial of PI.
At the end of the day round stars is what you are after so it would be the better measure. Still affected by seeing though.

There is a fabulous video at the PHD site where one guy talks about guiding and all its aspects. I found it very educational and the best resource around.

Another possibility is that using a guide scope differential flexure may also favour shorter exposures as that occurs usually over longer time periods. It may not and its just a thought.

Autoguiding is not often talked about yet it is critical to good results and a continual source of problems. So it would be good for a long thread on it and see what others use and what results they get.

It also may come down to software too. I have found CCDsoft my main autoguiding program over the years mainly because it was the first software I used from my 1st camera which was an SBIG.

The Sky X also seems to be the guts of CCDsoft transferred over to Sky X with a few extra features. I use that now sometimes as my AP mount needs pulse guide and CCDsoft does not offer that as an option but Sky X does.

PHD2 is very sophisticated. I am not sure if it gives better results than CCDsoft though. It will do Pulse Guiding or Direct Drive via an ASCOM plug in which is free.

A lot use Maxim. It has more bells and whistles and I am not sure if they ever mastered multiple star guiding or not but it sounded interesting. I have never used it.

Using a guide scope usually means flexure to some degree although I have managed to get 10 minute subs with round stars on a PME with 4 second guide exposures and 2 metres focal length. But it usually means elongated stars beyond 3-5 minute exposure times plus a guide scope tends to unbalance your scope at different angles due to raising the centre of gravity.

Getting round stars at 10minute exposures has always been the starting point for any astrophotography. Its quite a hard target no matter how good your gear is!

Not really on topic but probably interesting to a lot is I measured the PE of both my AP1600 and PME recently using the same setup. The AP1600 was considerably better at around 1.5 arc secs error often less. The best I saw was .6 arc secs and .8 is common. The PME was more like 3.5. Both give round stars when properly setup though. The PME has been a reliable performer for me so I was surprised the AP guide graph was so much smoother and less peaky than the PME's (about a 10 year old PME which may have better PE than modern versions which state higher PE in their specs). This is with PEC turned on. Home position on the PME is a hell of a useful feature though.

Another point I would make is the guide star selected. I find it can change guide errors a lot. If I get increased errors the first thing I do is select another star perhaps one less saturated and nice and round. Also tighten up guide star focus which improves results a bit. Rounder stars should mean more accurate centroid calculations than an odd shaped star. Sometimes when using OAG depending on the scope, guide stars near the outer edge of the FOV are distorted and aren't the best choice for a guide star. This can be worse with some scopes compared to others. Refractors don't usually have a problem but some scopes do give distorted guide stars in the corners and sides.

Greg.