What is this?

[multiweb]

Hi guys, I'm posting some data I acquired last new moon week-end. I was starting to collect some 10min subs on M78. It's quite faint so it's a work in progress and I have only a portion of the data I need.

Something got me stumped in those subs though. :shrug: I definitely know it's not guiding or micro jumps while guiding. Only some of the stars got it and not necessarily the bright ones. And it's consistent on every single sub. I have no idea what is is. A little spike going towards the right of the field, like a little diffraction spike.

Have a look and let me know what you think. :question:

[Geoff45]

Tube currents? Maybe a hot area pooling at the top of the tube.

[DavidU]

Which scope Marc?

[multiweb]

In a newt? My dewshield is fairly long but. :question:


5" newt so open tube. That was at the end of the night around 3:30am so I'd say the mirror would have been to ambient as it's small piece of glass.

[DavidU]

Have you checked the tube for insect webs? Maybe near the secondary?

[multiweb]

Yeah. It's squikky clean. That's a stack of 5x10min subs there. The exact same spike appears in all of them. It's not a registration problem either. And it's not like a foreign object that stayed there for 50min. Never seen that one before. That's what got me puzzled. It's not a read out artefact either because the sensor read out goes the other way. Those frames have been rotated 180 degrees.

[DavidU]

It has to be collimation, it appears the airy disc center is 1/3 to the center of the CCD. It is not in the same plane as the diffraction spikes but 10-15 degrees lower than the RH spike.

[multiweb]

Nope... here's the field curvature for this particular shot. :shrug:

[Terry B]

They only seem to be on the right hand side of the image. I would have thought they are from the image plane not being parallel with the focal plane.

[DavidU]

The depth of curvature distortion area is about 15 degrees to the right.
Difficult one.......

[multiweb]

Actually you're right. I've looked at this picture for too long. I didn't even notice it. Now you mention it they get worse from the middle onwards to the right. I did take the QHY8 off the focuser to check if the glass wasn't fogged up and put it back. I might have tilted it slighty but I thought I had it flash with the focuser face. Seems it doesn't take much at all.. bugger!. Thanks for picking this up. :thumbsup:

[Geoff45]

I would have thought that a tilt would give you symmetric eggs rather than the weird shape you have.
re the tube currents--I just assumed you were using your SCT.
Geoff