Last night as often before I do some imaging, "not that I got any in last night" I sometimes take some flats before the event.
This is all OK, and usually they turn out quite well, however last nights lot, after combining 20 or so, ended up with one fairly distinct small square, smudge, dot, what ever you want to call it, and two not so distinct square, of the same.
Now that i have the master flat of this event with these marks in them, will this have effect on the usefulness of the Flat, or will it not matter.
I am very perticular about dust, and air blow the sensor very regular, or could this have been caused from some dust on the Optical train, and/or the actual white cover i used for the flats.
My question really is, not about what it is, but will it matter.
If you have dust in your imaging train then that is what the flat is for, removing those shadows so, yes, your flat should have dust donuts on them.
A question for you though, do you take your flats before you have focused your setup or do you focus, take flats then take images without adjusting focus?
If we ignore other optical system effects such as vignetting, a 100% clean sensor would not require any Flats to calibrate the image. Let’s say that some dust particles have over time, settled on your sensor, in various locations.
Simplistically, each dust particle location will have a specific x-y coordinate, such as x=259 pixels across and y= 748 pixels down for a particular dust particle. Your Flat will capture the shadow of each dust particle for each x-y location.
If you clean off the dust from your sensor, then all x-y locations on the sensor are now clean, but the “old” x-y locations on the Master Flat still show the dust shadows. If you now use the “old” Master Flat to calibrate the newly cleaned sensor, you will be removing “imaginary” dust shadows and therefore introducing unwanted artefacts into the light frame.
So, whenever you clean the sensor, ideally you should generate a new Master Flat to map to the current “dust spot” status of your sensor.
Hi Paul, as I usually use the Tak for imaging, the focus is mostly there all the time, by this I mean, other than a temp change while it is idle, the focus is not moved from the last imaging session, so therefore I expect that it would be very close to focus for the next session.
However on this occasion, although i did not alter the focus, i did have the camera off, and re attached it last night, so therefore it would not be in the same place as last time, and i also move its orientation.
So i realise that i would have to take new flats for the next session of images.
Now that the camera is back on, and the sensor is air dusted I will do another batch of flats tonight, and see how that goes.
Dennis I understand what you have described, and thank you for your very well described information.
Ah yes, those lovely dusty motes. It's a never ending battle we fight Leon.
I know you like to keep your chip nice and clean, but if you take good flats then they will take care of the problem usually. Just something to consider versing the chance of causing some damage while cleaning your chip cover.
Yes Paul, I do know what you mean, I probably get a bit carried away with dust, I hate it, but having said that my cleaning procedure is probably more preventative rather than physical effort.
I don't touch the surfaces at all, but take extreme care to prevent the dust from getting there in the first place.
I sometimes take some flats before the event.
![[1ponders]'s Avatar](../vbiis/customavatars/avatar45_9.gif){kind=avatar}
It's a never ending battle we fight Leon. 
