Mr Blobby comes to Star Town

[Sterrenland]

Annoyingly, the last two times I've managed to get out my stars have taken a turn for the worst and seem to have become elongated. The smaller and fainter the star, the worse it seems. The elongation is over the whole image and seems to be horizontal (i.e left/right and not up/down and it is not curved as when back spacing is off).

Here is my first ever attempt at the Jellyfish Nebula last night. This shows the elongated stars. The only change to the image train is that I rotated the camera-spacer-filter-drawer-spacers-field flattener optical 'block' in order to assist in framing the nebula. As the whole block was rotated and the back focus hasn't been changed, I'm not sure this would have contributed to the elongation.

I have recently been struggling with my guiding and am wondering if this is the casue of the long stars. For some reason I 've been running at about 1.6 RMS. Occasionally it dips below 1.0  I have made sure my guide camera is focussed and I usually do PA a couple of times through the Asiair Pro to pin it down a bit tighter. Using the Zwo 120 mini guidescope I have the gain at about 50-60. I know the seeing hasn't been great, but in the past I tended to easily get the guiding down to 0.3/0.4 I know you shouldn't pay tooooo much attention to the RMS - up till recently my stars were pretty good, but they seem rubbish recently.

The most recent addition was a Zwo filter drawer. I initially had a problem with the back spacing, but fitted one of the Zwo extra spacers and it seemed to cure the problem (I assume it was needed to account for the filter glass thickness).

Any suggestions very welcome...it's annoying when new problems seem to appear from nowhere. Suggestions to improve the guiding especially welcome.

 

Detail of stars from above image...

Image train...

[michael8554]

Total RMS values are irrelevant in this context.

The individual RA and Dec figures matter.

If RA = 1.0 and Dec = 0.5 then stars will be elongated in the RA direction.

I'd guess your stars are elongated due to guide errors.

But a Stack of long-exposure subs is a poor guide to star shape.

A single short exposure Sub, stretched if necessary, will eliminate guide errors and expose any tilt, pinched optics, reducer etc problems.

Michael