PHD2 not giving sufficient corrections causing severe star trails.

I assume that you have had good success with this guiding setup previously.

I agree with apennine104.
Any time that PhD2 has acted irrationally for me in the past (not often at all), I start from scratch with a new profile and default settings. Create a new dark library, calibrate PHD2 near the meridian/celestial equator, and definitely run the Guiding Assist.

Note: I have had PHd2 act wonky at the start of a session and a pc restart corrected the issue.

Good luck!

Jim

What mount are you using ? This may be an issue. I had frequent problems with my iOptron mount with encoders.
Phd2 is a good software but it is what I call an "engineer's software", with so many settings that the simple amateur wastes entire nights fighting with it.
With such a short focale, you should ba able to image without guiding.  Anyway, I solved years of guiding issues when I started using ASIAIR. This cheap micro computer does all the job, and it starts guiding in a few seconds, for hours, meridian flip included !

Philippe

Hi all,

For all it is worth, here is my opinion on it:

Guiding accuracy comes down to math. 

If we look at a short focal length scope, the dither does impacts the guiding quite a bit. Here is an example:
I have a small imaging refractor with 480mm focal length. Imaging camera is a ZWO ASI1600mm with 3.8 micron pixel size. If I calculate the resolution for this setup, I get 1.63 arcsecond per pixel for the imaging train (go to Atronomytools.net for the guidescope calculator).

So, to generate a dither of 5 pixel, then my displacement is 5 pixels  x 1.63 "/px = 8.15 arcseconds. 

If my aim is to autoguide at less than 1 arcsecond, then the guide camera just got a big jolt in one direction or another of over 8 arcseconds. PHD will then will try to automatically compensate for it. That is why we are seeing huge spikes in tracking. PHD is simply doing what it is supposed to be doing.

For a longer focal length imaging scope, it is not impacted as much by the dither. For example, I have a Ritchey-Chretien at 1645 mm focal length. With the same camera as above, I get 0.48 "/px for this combination.

A 5 pixel dither, in this case, will generate a deviation of  5 pixels x 0.48 "/px = 2.4 Arcsecond. When guiding at less than 2 arcsecond, the 2.4 arcsecond displacement is not major impact.

I image under a Bortle 9 sky. My seeing is normally not all that great. Therefore, a 2 arcsecond guiding is normally good for my setup.
I have however improved my guiding to under 1 arcsecond by doing the following:

1) do a good polar alignment of the mount. It is surprising how much impact the polar alignment has on good guiding.
2) limit the dither to 1-2 pixels at shorter focal length of the imaging scope. Limit the dither to 3-5 pixels for longer focal length.
3) ensure that your setup is well balanced and the backlash is properly adjusted.
4) properly adjust the guiding pulses in PDH. I typically use 1 to 1.5 seconds at shorter focal length and 2-3 seconds pulses at longer focal length imaging scopes. The longer pulses smooth out the guiding. There is a chance, however that you will get drifting if you use too long of a guide pulse. PHD instructions recommend pulses from 1 to 4 seconds maximum.
5) make sure that the guide scope is properly matched to the imaging scope. I try to keep mine under 1:5 guiding ratio. Again, go to Astronomytools.net. They have a guidescope calculator that will give you the guidescope ratio, imaging resolution (arcsec / pixel), based on the focal length of your 2 scopes and the pixel size of both the imaging camera and the guide camera.

Don't overthink it and have fun !

François

Ensure you are not trying to use a hot pixel for guding.

Paul

I have had this issue till I did a hy[ertune on my EQ6-R Pro. Problem is the settle time. I dither 5 to 10px, before the hypertune I was also getting star trails. After the tune my mount can settle within 6 to 10 seonds.

How do you manually dither in dec?  How do you tell phd2 that you’ve manually dithered in dec?

Well i auto dither in RA every single frame. After 3-4 frames i pause the program in APT and then stop the guiding, i move the dec manually and start guiding again. This takes about 10 seconds. So far this has always worked perfectly and didn’t give me any strange guiding behaviour. The downside is that i have to be constantly monitoring the sequence.