|
I get this problem occassionally as my CDK is very fussy with flats and they often only work so well.
I'd say you need to take some fresh darks. Take say 16 and sigma reject combine.
You also need to take flat darks and subtract them from the flats.
The scope needs to be focused when you take flats so in case you took flats and then later focused the scope to image then they will be off to some degree (not sure how much).
As far as a bias goes you could try that. I usually use flat darks on the basis the bias is included in them plus the noise at the time of the flat.
The flat also needs to be taken at the same temp as the lights or at least the dark flats do.
Camera needs to be in exactly the same position.
I usually use dusk flats with a white t-shirt over the end of the scope and take them quite quickly.
Perhaps also take more flats. I was using 3 but have started taking 6 each of the filter flat + the flat dark (I share the flat dark between the filters and binnings so I usually only need 2 flat darks - the 2x2 one for the rgb flats and the 1x1 for the luminance).
Ideally you take a flat for each rgb and Luminance + any Ha or other filters. They need to be fairly fresh as dust can shift and change.
You have to have a flat for each filter if you want to get rid of dust motes which may be specific to each filter.
Flats are not a good substitute for cleanliness as I rediscovered recently. Its far easier to clean your camera and filters often so flats are less needed.
In fact with some recent cleaning and my TEC110 and ML8300 I did not need any flats as the images were so evenly illuminated and there were no dust motes. That is better than needing flats due to dust motes.
But if you stretch your images hard to try to make up for lack of exposure time then dust motes will raise their ugly head.
You can also use the healing or clone tool in photoshop set to luminosity and clone them out later without too much trouble so if the flats are too hard work then cleaning everything and photoshopping can be a plan B.
How are you taking flats?
I have found variable results on a tricky setup (reducers/correctors often create a hot spot in the centre of the image that is hard to flat out). Different brightnesses between late dusk (too late to get a decent flat) and then the next day with the roof closed and a white cover over the end of the scope worked better. I think a certain amount of trial and error and get your procedures worked out.
Richard Crisp has done a paper on flats and that would be worth studying. He also does daytime flats using aluminium foil. Worth trying. It gets rid of the rush to get it done quickly but in my experience and in his recent posts on the subject dusk flats are hard to beat. You have to be fast and efficient as the light fades rapidly and fast camera downloads becomes more important. Some use electro panels and have no problem.
Perhaps Mike Sidonio can chim in here as he seems to be having no trouble with flats and he uses a Newt with a corrector and 16803 camera so flats would be vital. He uses a light panel from Astronomiks I am pretty sure. Spectrum of the light from these can have an effect as well. Too bright, too dim. With some cameras like FLI flats have to be at least a 5 second exposure otherwise you see the shutter causing shadows in the flat which are not of the scope/camera system and are useless.
I wouldn't mind hearing more about flats as they have been a source of problem for me at times with my CDK. I think part of that is I need to flock the scope and its carbon fibre trusses as they are reflective and throw the flats off sometimes.
Greg.
|