What is this?

multiweb (Marc) · #1 23-11-2009, 06:24 PM

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.

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.

Geoff45 (Geoff) · #2 23-11-2009, 08:50 PM

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

DavidU (Dave) · #3 23-11-2009, 08:58 PM

Which scope Marc?

multiweb (Marc) · #4 23-11-2009, 09:10 PM

Quote:

Originally Posted by ghsmith45

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

In a newt? My dewshield is fairly long but.

Quote:

Originally Posted by DavidU

Which scope Marc?

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 (Dave) · #5 23-11-2009, 09:12 PM

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

multiweb (Marc) · #6 23-11-2009, 09:20 PM

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 (Dave) · #7 23-11-2009, 09:38 PM

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 (Marc) · #8 23-11-2009, 09:50 PM

Quote:

Originally Posted by 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.

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

Terry B · #9 23-11-2009, 09:55 PM

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 (Dave) · #10 23-11-2009, 09:57 PM

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

multiweb (Marc) · #11 23-11-2009, 10:01 PM

Quote:

Originally Posted by 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.

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.

Geoff45 (Geoff) · #12 24-11-2009, 12:22 PM

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