Master Flats and Masterdarks

@russ-telfer Hi Russell,

In order to be able to properly calibrate flats so a master flat can be created, either dark flats of the same exposure time, temperature, gain and offset are needed or darks of the same temperature, gain and offset (but a different exposure time) and bias are needed. If the dark flats have the same exposure time as the flats then bias is not needed.

However, for proper calibration of the lights, both a master flat and a master dark are needed. The master dark can be created from darks of the same exposure time, temperature, gain and offset as the lights or of darks of the same temperature, gain and offset as the lights (but different exposure time) and bias frames.

In short, bias frames are needed if for either creating the master flat or master dark or both the exposure time is different than that of the frames that get calibrated with them (which are the flats for the master flat and lights for the master dark).

The flats are used for calibrating the lights as well but once the master flat has been created, no dark flats or master dark and master bias are needed anymore. The master dark always is needed for calibrating the lights and if the exposure time of the darks is different than that of the lights then a master bias also is needed.

HTH, Wouter

@wvreeven 

So if I have dark frames at -5 degree C for Gain 26, offset 50 but different exposure times, can I create one master dark for all exposure times, or do I need to create master darks separately for each exposure time?

@tsk1979 You need darks for each exposure time. I setup my system to the desired focal length and then once the backspacing is correct I use Stellarium to pick targets for that month or so. I find a rotation that will work good for the targets I picked and then I lock down the cameras and will not change rotation until I have finished the selected sessions. I then use my light panel and white tshirts and a knitting hoop with some white cloth and start taking flats with each filter. I have ZWO ASI294 cameras and bias frames will not work with them as the sensor has problems with exposures under 3 seconds so I make sure my flats are between 3-9 seconds and that my average ADU is 26000. I then take dark flats at the same settings and make sure they are at the same exact exposure time. They say that darks should be taken at the same exact temperature, but I have found that since I have a cooled camera I can get by with taking darks seasonally. On a cloudy night I will setup my rig to capture 30sec,60sec,120,sec,180sec,300sec,600sec and 1200sec subs. I have my rig in a garage that is dark with the scope covered so it will not hurt that the session will run into daylight. I would not take them during mid day though as the garage can get a good bit warmer that what the night time air will be for that season. I take 30 subs each of the flats, dark flats and darks and then load them into APP and build the master files. I am then set for that season with master darks and will not need to make master flats or master darks until I rotate the camera or change the focal length by adding/removing the reducer.

I did not notice about the calibration warning in your original post. I think APP has a bug right now as I get the calibration warning every time anymore(been meaning to make a post asking about it myself). I get the calibration warning with most filters, but not all. I use NINA flat wizard which calculates the exposure time for my desired ADU and I just need to adjust the led brightness or white cloth layers to make sure it will fall in my 3-9 second window. After it takes 30 frames it has me turn off the light panel and it takes 30 dark flats at the same settings so all frames definitely have the same settings. It still creates the masters and they work fine so not sure what is wrong with the warning popping up.

Make sure they are all at the same bin setting as well.