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### Peculiar flat fields for research

Posted: 08 Jun 2022, 13:53
Hello, for a peculiar UBVRI research, the following specification were needed for flat fields:

1) Taken every night, on the sky, at sunset or dawn.
2) Taken for all filters (UBVRI) using an optimal exposure of 2/3 of dynamic range.

The main problem was to find the correct exposure. It was solved taking a test exposure then measuring the average value of the image. Knowing the bias level of the camera, the correct exposure time could be calculated as:

Here is a prototype script were the bias is set at 100 ADU and the target ADU is 40000.
Flats are taken near the zenit switching off the telescope tracking.

Code: Select all

``````seqDir = "D:\Temp\"
sunAltitude = -5.0
nExpo = 3
initExposure = 5.0

WaitSun
GotoZenith
Sequence("u", nExpo)
Sequence("b", nExpo)
Sequence("v", nExpo)
Sequence("r", nExpo)
Sequence("i", nExpo)
Message("Script ended")

end

Sub WaitSun
while true
a = SunAlt
cls
print "Sun altitude = " + Format(a, "0.000") + "°"
if a < sunAltitude then break
Pause(2)
end while
End Sub

function SunAlt
lon = Observatory.Longitude
lat = Observatory.Latitude
az,al = EquatToAltaz(ra,de,lon,lat)
return al
end function

Sub GotoZenith
lon = Observatory.Longitude
lat = Observatory.Latitude
ra,de = AltazToEquat(175, 89, lon, lat)
print "Moving telescope to zenith ..."
Telescope.Goto(ra, de)
Telescope.Wait
print "Telescope arrived"
Telescope.StopTracking
End Sub

Sub Sequence(filter,images)
print "Moving filter wheel to " + filter + " ..."
Wheel.Goto(filter)
Wheel.Wait
print "Test exposure ..."
exposure = CalcOptimalExposure
print "Optimal exposure for " + filter + " is " + Str(exposure) + " s"
for i = 1 to images
Camera.Start(exposure, true)
Camera.Wait
fileName = BuildFileName(filter,i)
print fileName
Image.Save(seqDir + fileName)
if i > 1 then Image.ClosePrevious
Next i
Image.Close
End Sub

Function CalcOptimalExposure
Camera.Start(initExposure, true)
Camera.Wait
Image.Close
return Round(initExposure * mult)
End Function

Function BuildFileName(filter,idx)
return "Flat " + filter + " " + Date() + " " + Format(idx,"00") + ".fit"
End Function

``````

### Re: Peculiar flat fields for research

Posted: 09 Jun 2022, 12:39
I'll give it a try.
Thank you.

Alessandro