# Using Ideal Exposure time statistic

#1

I’ve got this statistic running for 1x1 binning or no binning. What I’ve noticed is this statistic fluctuates with the exposure time that I use to test it. If I use a 20 minute image exposure it is different than if I use a 5 minute exposure. Is there no real ideal exposure time? How valuable is this statistic. Is anyone using it, and what are your comments?

Rob

#2

Hi,
yes, I’m too find that this calculation is too much rounded and could be much precise.
Authors say that the calculations based on following study http://www.hiddenloft.com/notes/SubExposure.pdf

where:

Ideal Exposure Time = (9.76 x ReadNoise^2) / SkyFlux

SkyFlux = (BackgroundADU x Gain) / TestExposure(Min)

We can see now that SGPro takes all of the parameters from what we have provided him and from test exposure.
The problem is I believe, that the BackgroundADU should be taken from nebula/star free region in the test frame and more important dark-substracted, which doesn’t happen inside SGPro. Seems like it just takes Median value of the test frame and applies calculations.

I can only suggest to implement an option to enter our Bias frame median value, (profile specific) as we do with gain and readnoise and use this formula:

Ideal Exposure ADU = 9.76 x (r^2 / Gain) + Bias Frame Median ADU

With this you don’t even have to take test exposure frame, after inserting all the details, SGPro can show you your desired median ADU value that you need to reach, so you’ll adjust time accordingly by your self.
Giving it a second thought, it can also be used with taking test frame.
Took 3 minutes frame, noted median ADU and from that suggested corrected time needed to reach desired median ADU.

Sergio

#3

Sergio,
Thanks for the reply. How does your formula work with different filters?

Rob

#4

That is the beauty of it, once you calculate your desired background ADU you must reach with your camera, it won’t be matter if you switch the telescope or filters, you still have to reach your ADU adjusting the length of exposure.

#5

Well, I wanted to revive this thread to inquire about adding a dark or bias substraction option… according to Starizona, the test frame should be dark subtracted…in which case you get a completely different Ideal Exposure Time. I’m imaging NGC4490 at the moment and according SGP, the IET is 3.7 minutes. This is confirmed by the Starizona calculator too… however, once I enter the dark calibrated frame’s BG ADU into the Starizona calculator, the IET jumps to 8 minutes. This means, I have been underexposing all my 200+ frames for this target Starizona’s calculator can be found here

www.mainsequencesoftware.com