a question about other than light images !!

[cal1985]

Hi all, im quite new to astrophotography and loving the learning curve!! however im a little baffled regarding flats and bias??

i understand that a dark sub effictively cancels out the hot pixels and noise and that you should generally use as many darks as lights at the same exposure and time. however i have no clue as to what a flat does or what it is.. or what a bias sub is or what it does.. please can someone explain what they are and the purpose thay have on a final image ?

many thanks

Cal

[Merlin66]

A flat is an exposure through the telescope of a bright even background...the sky at twilight etc. This shows any dust on the various optics ("doughnuts") as well as any vignetting (where the field of view is unevenly illuminated). By dividing the image of the target by the flat, these 'defects' are removed, giving a cleaner more even image.

The Bias frame is a very, very short exposure with the telescope covered. This records the amount of noise generated every time the camera transfers data from the CCD chip. Bias is contained in every image. It's needed to compensate for the use of darks taken at different exposure times to the target images (Lights)

Hope this helps.

[ollypenrice]

Whether you need the same number of darks as lights is debatable. Most CCD imagers ignore this completely and make a master dark for each temperature, bin mode and exposure length they expect to use for their lights. Opinion varies as to how many darks should go into a stack, but between 15 and 40 would be the middle ground if you took a sounding, I think.

The killer problem with darks for DSLRs is knowing what the chip temperature was at the time of exposure because if the darks and lights don't match the darks can do more harm than good.

On one camera I use I have ditched darks entirely in favour of a bad pixel map but that's (yet) another story!!

Olly

[cal1985]

thats great, has really helped me to understand what is going on... at the moment im taking a dark straight after the light, so there is no temp difference.. but this is extremely long and turns out at the end of the night, i may have only a few light subs!

if i could create somekind of master that would be such a help. as then i could just work on the lights.. but i move my scope every time i use it as it is not on a perm fixture. so the camera is being rotated and moved... wouldn't this stop me using darks made from another session?

[Merlin66]

You can build up your "library" of darks at anytime....

The darks are related to the camera performance not the telescope or optics.

The exposure and temperature justs needs to match the lights...

[ollypenrice]

Indeed, it is just the flats that have to photograph, in effect, the unaltered light path of the system. Darks don't even have to be in the scope.

Losing half your lights in order to shoot darks cannot be worth it.

Olly

[cal1985]

brilliant this has helped me so much. thank u all for this