Frequency of generation of calibration data (bias/flat/dark)

@schakravarthi That is correct, though not entirely. Bias, dark and dark flat in general may be shot about once a year if you have a cooled camera and if you use the same gain, offset and temperature. If you have a non-cooled camera then it becomes more difficult to reuse bias, dark and dark flat since the temperature of the sensor varies and with that the dark noise. In winter the ambient temperature generally is lower than in summer and with that the sensor temperature.

Flat frames you should shoot with every imaging session. However, if you do not remove the camera and filter wheel (if you use one) from the telescope and if you do not transport the telescope (so dust specs do not move on the lenses, filters and sensor cover window) then you can reuse them during various nights.

Thanks for the response @wvreeven. I am shooting with a DSLR and not using any cooling.

I am wondering what if any calibration files I can generate now after the imaging session. From the above, it seems I can not generate flats. Would it make sense to try to set the camera out on a night that is similar to the one I previously imaged and then run an imaging session and then generate bias, dark, and dark flat so that these are similar to what I would have to first order? 

Also I am not sure if these are useful without the flats.

The issue is that unfortunately all I have are light frames taken at a dark site and could not do another imaging due to continued bad weather. It may be a long time before I get to a dark site again so I am trying to salvage what I have.

@schakravarthi I didn't mean to give the impression that you cannot shoot flats. On the contrary! Flats are essential so you should shoot them with every imaging session. Most people do it immediately after al lights have been shot since then the telescope is focused which is necessary for good flats.

Regarding the bias, dark and darkflat frames: for bias and dark the temperature needs to be the same or close to the temperature of the lights. Many people put their dslr in the fridge to shoot flats. Darkflats need to be the same temperature as the flats so if you shoot the flats much later (again without removing the camera from the telescope and without touching the focuser) then you can immediately shoot the darkflats to have an as close as possible temperature to the flats.

thanks for the quick responses @wvreeven. Do you have a suggestion to my problem  (copied below) or is all those images garbage without the calibration files?

Problem: The issue is that unfortunately all I have are light frames taken at a dark site and could not do another imaging due to continued bad weather. It may be a long time before I get to a dark site again so I am trying to salvage what I have.

@schakravarthi Did you try to process the images? If yes, what was the result? You can always try to shoot bias and dark and see if those improve the result. As for flats, if you don't see any "dust devils" (i.e. dark circles where incoming light has been blocked by dust specs) then you can try to create an artificial flat and apply those to the lights to see if those help. In order to do that you'll need to search the internet since APP doesn't support this. Depending on the object of your images this may be doable or impossible.

Perhaps you can post an integrated image here so we can see what the "damage" is?

Here is the result of the integration

Also the result after

post processing. there is so much red. Is this real? Most of the stars are looking red and there are so many. I wonder if I am looking at hot pixels @wvreeven. 

@schakravarthi As far as I can see there are no obvious dust devils in your image so it may be possible to create an artificial flat to correct for the vignetting in the corners.

Regarding the colors: yes they are real. Hot pixels would be very small (literally the size of a pixel) so that's not what you are seeing. Unfortunately your image is unsharp which is a pity because it looks very beautiful otherwise with the Orion Nebula and Horse Head Nebula!

@wvreeven thank you for the feedback. 1) regarding the image being unsharp, would using the sharpening in the APP help while saving the image. 2) can you explain a little more on what I am looking for in doing the artificial flat. You mentioned APP does not support artificial flat. So it is confusing. 
I did find this link in the forums for creating artificial flats. Looks like I need to create (i suppose artificial) dark, bias, and then the artificial flat. Would that be correct or do I just only do artificial flats

https://www.astropixelprocessor.com/community/tutorials-workflows/how-to/#post-449

@schakravarthi

1) No, you will need to make sure that you focus your camera better next time. A very good way to do this is by using a Bahtinov mask. There are a lot of good online resources about those masks and how to use them for focusing.

2) Sorry, I forgot about that option so you can give it a try. Yes, you will need to take dark and bias frames. You could put your camera in a fridge to cool it. You may not get rid of all the dark noise that way but at least you will get rid of some noise.

Perhaps this way of creating an artificial flat may work for you:

http://www.ianmorison.com/producing-a-flat-frame-from-a-light-frame/

Note that you will need to take special care of all the nebulae in your image (even the faint ones) to make sure that they don't become part of the flat. But I'd try using APP first to see if that works for you.