M42 first shot

multiweb (Marc) · #1 26-12-2007, 10:12 AM

Hi Guys, I took that photo last month on a mate's small refractor, but on a very good mount. Can't remember what the scope was though.
10 x 1min exposures at ISO800 on a Nikon Coolpix 995. Stacked and further processed in photoshop [with Jase amazing HighPass filter trick

].

My camera attracts loads of noise especially in the red but I can deal with most of it. Couple of questions though.

1_ Why am I getting purple halos around the brightest stars? Is it a focus issue?

2_ Some stars on the edge of the field have really weird shapes too. Why is that?

Thanks in advance for your help and feedback. All comments most welcome. Got heaps to learn.

jase (Jason) · #2 26-12-2007, 10:52 AM

Marc,

Quote:

Originally Posted by multiweb

1_ Why am I getting purple halos around the brightest stars? Is it a focus issue?

You mention you're friends refractor - what was it? Was it a achromat by any chance? At a quick glance it looks like a chromatic aberration issue. Potentially compounded by soft focus.

Quote:

Originally Posted by multiweb

2_ Some stars on the edge of the field have really weird shapes too. Why is that?

The weird shaped stars around the edge look like a field rotation problem due to inaccurate polar alignment. You can see them rotate in a small arc.

You'll need much greater data for the Selective Contrast Masking (highpass filter) to work. Focus on achieving good polar alignment and sharp focus first, then progress from there.

multiweb (Marc) · #3 26-12-2007, 11:07 AM

Quote:

Originally Posted by jase

Marc,

You mention you're friends refractor - what was it? Was it a achromat by any chance? At a quick glance it looks like a chromatic aberration issue. Potentially compounded by soft focus.

Can't honestly remember what it was. I think he uses it for autoguiding only.
Yeah I had a feeling it was focus. I'm focusing eyeballing the back screen of the camera right now so it is a bit dodgy hey

Got to work on a better method.

Quote:

The weird shaped stars around the edge look like a field rotation problem due to inaccurate polar alignment. You can see them rotate in a small arc.

I have a feeling it's my lens adaptor to fit the camera. I was zoomed out to the max and at the limit of vignetting so it could well be me again there. My mate was in the middle of doing 60 or so star alignment to better his polar alignment on his TAK EM-200 and I believe it must have been spot on by the time I jumped in to take a couple of shots.

Quote:

You'll need much greater data for the Selective Contrast Masking (highpass filter) to work. Focus on achieving good polar alignment and sharp focus first, then progress from there.

I'll keep all that in mind next time I try it. Thanks for the feedback Jase.

multiweb (Marc) · #4 09-01-2008, 07:58 AM

Hi Jase, I reprocessed the pic with some help and hints from JohnG and got some more details from the subs. Provided I sort out my focus and tracking problems, I reckon I can get a bit more out of this camera still. It is a 3 megapixel Nikon Coolpix 995, 1min exposure at ISO 800 max. I'm trying to perfect my skills with it before I move on. One thing I was thinking off is to stick it in a couple of minutes in the fridge to cool it down before taking a serie of shots. You reckon it's ok, or is it a no no?