Just ONE shot for the FLAT image is all that is needed. Same with the Dark and Bias images. Just one shot is all that is needed
What you have to understand here is the difference between a 'standard' ex[posure and an image that has been created from several exposures. When people talk about just using one Flat (or Dark or Bias) what they mean is to use what is more correctly referred to as a MASTER frame (i.e. a MasterFlat, a MasterDark, or a MasterBias).
These 'Master' frames have to be created from a number of otherwise 'standard' individual frames - and the number that you choose to acquire has to be 'statistically significant', and has to be within your available frame captiring session.
Now, by 'statistically significant', what I mean is that you need to capture enough of these 'raw' frames to be able to eliminate some of the noise that you will always acquire - simply by actually capturing the image in the first place. Typically, a good 'rule of thumb' would be to capture as many of these raw 'calibration' frames as you hope to acquire by way of raw Light frames (i.e. those that contain the image data that you are hoping to acquire in the first place).
So - sounds like a whole load of effort - making life almost impossible for an average imager? Not really. First, you don't need to reacquire all of these calibration frames every night (for example 30-each for Biases, Flats and Darks - plus a further 30 for your Lights or, worse, 30-each for L+R+G+B+Ha !!!)
Biases are very short exposures - in fact the shortest exposures you can possibly set out acquire, and so don't really take up too much time. But, careful analysis of your Bias frames may leave you realising that you might not actually need these at all. Depending on your circumstances (camera used, local environment, etc.) you can get away with either a 'Pseudo-MasterBias' (where you use PI to create a MasterBias comprising pixel values all set to be the same value as the average value you usually encounter when acquiring raw Bias data), ir you can simpy ignore the bothersome issue of Bias frames altogether (they are only really ever used if you did not have, or did not acquire, Dark Frames at the same exposure length as your Light Frames - but the penalty is that you really need the CCD temperature to be 'the same' for the acquisition of both the Darks and the Lights - and that usually means some form of TEC cooling system on your imager).
A quick word on TEC cooling might also help here - if you can run your imager night-after-night at a fixed, repeatable, temperature, the you definitely do not need Bias Frames. All you need is a good collection of MasterDark frames, one each for the various exposure times that you feel you are most likely to encounter. Best yet - the raw Darks needed to create these MasterDarks can be collected 'anywhere' - you don't need to be connected to your telescope, you only need to be connected to your TEC-cooled CCD imager. And, I lied, even better - you can gather copius numbers of DarkFrames whilst you are asleep, at work, out, etc., etc. - just make sure that no stray light can get anywhere near the imager. (And, you may be ahead of me already here, if you 'really' wanted to collect Bias frames, these can also be collected when you are not actually imaging - I do this, occasionally, just to double-check that the Bias levels on my imager have not changed over time - and, since buying the QHY10 many, many, years ago, they haven't changed enough for me to be able to detect the change using any of the tools available in the PI arsenal).
In summary then, depending on your imager - you don't need Biases, and you can create MasterDarks when not actually in the middle of an imaging session.
That just leaves your Flats and your Lights. I work from an observatory - nothing changes in my optical train for months at a time. Sometimes I don't even bother with focus because environmental conditions remain so stable. So, I take Flats 'every so often', using a large piece (A3-size) of white foam-board pinned to the observatory wall about 2m (6 feet) from the aperture of the OTA, and I 'point' the OTA (very roughly) 'at' the board. This is illuminated by a 60W (eq.) CFL lamp about 4m (12 feet) away (and slightly to one side). Iis the illuminated field 'even'? Again, I haven't found any means within PI to tell that it isn't. Is the illumination 'perfectly white' - no, it isn't, it's anything but white - but I have a very simple routine in PI that sorts all that out before the MasterFlat actually gets used to calibrate the raw Lights.
Great! You can now acquire your Flats before your imaging session even starts (using the overhead sky, perhaps through a white, cotton tee-shirt; perhaps, like me, through another sheet of white foam-board, with the OTA pointed 'straight up').
And that then leaves your entire night-time session free for imaging. Or, like me, watching for clouds, tripping over things in the dark, falling asleep at the computer (or, worse, on the floor - waking up to find spider's webs strung from my nose to my ears), drinking hot chcolate by the pint or just waiting for the gentle 'bong-chime' of PHD to tell me that the sun is now well above the horizon!
Hope this helps.
