PHD guiding with wonky shaped stars

[SteveA]

Santa brought me a new guide camera at Christmas and last night was the first time I had a chance to set it up and test. Up until now I've been using a QHY5-II and although its been okay I have struggled with finding guide stars on occasions and despite my best efforts have been dogged with noise and horizontal banding, guiding became a real pain in the proverbial! I opted for a QHY5-III 178 mono and having rigged it up last night the sensitivity is amazing and I have high hopes for it.  I did however end up having to rearrange my imaging train in order to achieve focus. With the old QHY5-II, I managed to shove the OAG prism deep inside the throat of the camera having removed the screw in filter that sits in front of the chip. I was less inclined to do this with the new camera in fear of damaging the chip, but having got the prism as close as possible to the front of the camera I was still not achieving focus. What I have had to do is put the OAG in front of the coma corrector, this allows me to focus my main imaging camera (just) whilst allowing me plenty of flexibility on the guide camera. Not surprisingly however, star shapes are now fairly distorted in the guide camera clearly as a result of coma. I only managed a quick guiding session last night and sadly guiding wasn't great and I suspect PHD is having issues not seeing a nice centroid. I'm hoping tonight to try and position the prism further inside the light path in the hope that I can reduce some of the effects of the coma. 

My question is...and maybe I already answered this ....but how tolerant is PHD of a less than perfectly shaped star. My star shapes were either  "U" shaped or small arcs. I'm hoping I can at least reduce some of this distortion by better positioning the prism in the light path but I can't imagine that without the corrective effect of the coma corrector that I will get anywhere near perfect shaped stars. Has anyone else guided with wonky shaped stars?

Steve

[ChrisEll]

A pic of your optical train would be helpful. If you're getting banana stars you're pretty close to being in focus. I use the QHY5-II Mono with an OAG and it seems to work quite well.

[SteveA]

27 minutes ago, ChrisEll said:

A pic of your optical train would be helpful. If you're getting banana stars you're pretty close to being in focus. I use the QHY5-II Mono with an OAG and it seems to work quite well.

I'll pop a pic up here after work....You're right it may well be a focus issue, I had limited adjustment on the focus with the Baader focusing eyepiece holder that I use with the guide camera, so it was a bit hit and miss. I've re positioned the prism now and moved the OAG turret to a better position so focusing should be easier.

Steve

[spillage]

although I use the older version of this camera I have issue when perfectly focused and find being just out of focus helps.

[SteveA]

Moving the prism further into the light path has pretty much sorted the distorted stars. However..and as is normally the case,  solving one issue has allowed me to move on to the next problem which is a significant spacing issue between the coma corrector and the main imaging camera.  I had conveniently ignored whilst setting up the new guide camera... kind of back to the drawing board really

Steve

[ChrisEll]

Pushing the prism into the scope will make the image brighter but shouldn't actually alter the focus of image.

[SteveA]

24 minutes ago, ChrisEll said:

Pushing the prism into the scope will make the image brighter but shouldn't actually alter the focus of image.

Agreed...but the shapes of the stars are significantly less distorted.....a fact of life with a newt!