I looked and it’s only on the L filter images and after I shot M42 but before
M36. Between those two sequences, I had swung over to M45 to see how it looked.
That’s when I first noticed the “dust” showing up. Must have shaken some dust
loose when I slew over there.
But flat fields won’t cure all that can go wrong. That dust might have landed
during a sequence if I had been really unlucky. Then some images would have
dust sports and other s wouldn’t. Or they could move around. You almost need to
take an image, take a flat, take an image, take another flat, …. To be really
paranoid, that is. :-)
And if I’m changing exposure times and gain settings for each subject, then I
need darks for every sequence too. (Do I need separate darks for each filter?
Shouldn’t. It’s dark after all!) There’s potentially a large number of
combinations of exposure time, gain, and sensor temp.
I guess I was naively hoping to spend more time taking actual images than on
all of the enabling stuff.
Did you roll your camera thru the dirt before before mounting it on your scope?
:-) Your dust “donuts” look all the same size whereas I have at least two
different size dust particles on mine. Not that I think that has any
significance, just interesting in a weird sort of way.
Hmm! Come to think of it, I did bump the tripod with my foot about the time the
dust shows up. I was removing the dust cap from taking my darks for M42. Tripod
didn’t move (PHD2 showed a nice spike! Alignment was still good afterwards.) as
I have a bucket of bolts hanging down from the accessories tray to keep it
steady during the wind we had a couple of days ago. That might have jarred
loose some dust in the scope to fall on the L filter.
Mike McDonald
mikemac@xxxxxxxxxxx
On Jan 25, 2019, at 2:33 PM, Tom Polakis <tpolakis@xxxxxxx> wrote:
Mike McDonald wrote:
But why did the dust show up when it did? There weren’t any dust spots in the
earlier images that night. I hadn’t disconnected the camera and filters from
the
scope, so no new dust should have been able to get into the system when it
did.
(Unless the backside fo the ZWO camera is open to the fan or something.)
I bet it's on all of your images. Adjust the levels such that the background
is light gray, and you will see the same dust motes on all of the images.
And how do I get rid of the dust? I know, nearly impossible in Arizona!
Guess I
need to check the RGB images to see if it’s on the camera or one of the
filters.
As Paul mentioned, the answer is flat fields. You should take at least a
dozen flats for each filter, and median-combine them. Since new dust
inevitably gets to the sensor and filters, you'll need to refresh the flats
periodically. I showed this comparison in my recent photometry presentation
to demonstrate the magic of calibrating images.
https://i.imgur.com/k0p275p.gif
Tom
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