Janusz So if my exposure time is like 400 seconds am I OK? Or will it (the PE) catch up to me the next session after first
It is OK for visual, but astrophotography requires autoguiding. Punkt. Period.
The error looks like noise riding on a zero offset sinusoidal. If you are well polar aligned, it will track to within the peak-to-peak (p-p) error for days. But the large PE will preclude astrophotography with no autoguiding.
The period of the sinusoid is 430 seconds, and the p-p error is about 70 arc seconds. If you take the time derivative of the sine wave, you will find that the slope of the error (what autogiding needs to countermeasure) is of the order of an arc seconds (angle) per second (time) at two regions of the sine waveform [remember, time derivative, so result is in arc-seconds per second). And it needs MinMo of PHD2 to be practically zero.
As a result if you use 2 second guide sampling time, the worst case error using PHD2 (ASIAIR is always worse than PHD2) is of the order of 1 arc second RMS. with 1 second sampling time, you get between 0.5 to 0.7 arc second RMS, and with indoor testing (no "seeing problem") with artificial sky that moves at sidereal rate, I see somewhere between 0.25" and 0.5" arc-second RMS with a half seconf sampling period.
With short samplin times, you will need some kind of low pass filtering to work when seeing is poor. ASIAIR does not expose the ZFilter nor the Lowpass2 filter, nor multi-star tracking in PHD2, all of which allows the autoguider to work under poor seeing conditions, so you usually need to run 1 second autoguiding exposures. To track the fast slope ASIAIR is also missing the MinMo parameter.
If you are familiar with the Avalon M-zero autoguiding, it is quite similar (Avalon recommends 0.5 to 1 second sampling time, with Lowpass2 and a MinMo of 0).
A number of people uses the Lacerta MGEN-III wth the RainbowAstro for its portability and also multi-star autoguiding.
Janusz Using an auto guide. My present one is the ZWO 120 MM Mini and a 120mm f/4 scope. Will that be
adequate or I'll need something more robust?
The budget combination you gave is bad on ASIAIR (but will work with PHD2) because of lack of MinMo in ASIAIR (and its erratic frame rate). You can compensate a bit for lack of MinMo by using a guide scope with a longer focal length. I use a Borg 55FL as a guide scope with a focal length of 250mm, together with an ASI290-mini for the small pixel size.
If you want good tracking, get a 10 Micron mount or one of the other premium US manufactured mounts. The RainbowAstro and HOBYM mounts are for portability (airline travel with a carbon fiber tripod and no counterweight). And for old people who can no longer handle mounts that weight over 10 kg.