Telescope Service TSQ65 backfocus distance - triangular stars

[Geordie mc]

Hi all. Does anyone know the backfocus limits on this scope please. The 4th lens acting as a field flattener is in the tube. As the focus tube has to move to reach focus I presume that their is a range for the backfocus rather than a set distance. When I image with my ASI1600mm - cool, with good focus and guiding showing a total RMS error under 0.7 using the ASI air pro, when I zoom in on the image the stars look slightly triangular. I think this means I’m too far past the ideal backfocus distance, but I’m not sure. I’m confused as to what I can do. The sensor has to be at that distance to be in focus. Is the 4th lens in the draw tube of the focuser? If that’s the case I could remove a spacer and get the ASI1600 about 3mm closer to the focus tube. Image is one Ha sub of 300 secs at minus 20 degrees. Any advice very much appreciated. Thank you.

[david_taurus83]

If the correcting lens is fixed then it makes no matter if you remove a spacer as the focal point won't change. Something doesn't look right though.

[Geordie mc]

Thanks David. It’s a msystery. I think I’ll just have to experiment. I have a QSI660 I could try with the scope and see what happens. As you say, if the flattener is in a fixed position then trying different spacers should make no difference. Maybe it’s the ZWO filters? But then you’d think others would be reporting issues. Hoping I can do something in Pixinsight to reduce the issue. 

[newbie alert]

The ts65 quad is well known to have pinched optics, thought they'd rectified it..