Saturday, March 21, 2015

Flats - TAKE THEM AT THE BEGINNING OF THE SESSION

Well, I think I learned the hard way that you should take flats at the beginning of each session.   I setup tonight to continue to take more subexposures of M101 and for some reason the field is turned about 30 degrees!    I can't figure out how it happened.  

Rotating the focuser presents a problem:  the defects that the flat will correct (spots on the lens and the sensor) are partially unique to the orientation of the camera.   So, if I take flats now, some of the flat data will incorrectly correct for defects that are about 30 degrees off.  This will be interesting as I will need a week before I get to the data.


Tuesday, March 17, 2015

NGC 3628. Two steps forward and two steps back.

So I finally got out the big scope for a general shakeout and possible first frames of a longer project on NGC 3628, the Hamburger.


This was shot through the 8 inch f/6 reflector.   10 minutes, 1600 ASA.   Single frame with some curves, levels, and color balance adjustments in Photoshop.   It is the full-frame from the modded Canon T3i.   I actually took 6 exposures just to see if the guiding was up to snuff.   And it was.

The stars are warped around the edges of the frame due to coma inherent to reflectors.

To guide this setup, I have to use an off-axis guider.   Originally, I also had a Baader coma corrector (the MPCC) attached to the off-axis guider.  But I was getting warped stars which was really puzzling.  I was worried that it was guiding, but in fact, I finally deduced that it was the single screw on the focusing tube which was pinching the optics of the coma corrector.

I have to either switch coma correctors or get a new focusing drawtube.   It's always something.

And here's a crop of an image taken earlier with the coma corrector in place:


Those slightly warped stars come up in short 10 second exposures as well.  So that meant it wasn't guiding/tracking errors.   When I removed the coma corrector, that elongation disappeared.   Another possibility was that it was the scope optics -- luckily that wasn't the case.

[Update:  Will be getting a new compression ring drawtube for the focuser.   I plan to send the focuser to Moonlite Focusers for the replacement after I finish with the M101 exposures.]