Ideal Exposition Time - More Explanation/Description



After few month utilization SGP Pro, I need understand and more information regarding the calculation of ideal exposure of a photograph.

I understand we need define the parameters in the Equipment Profile Mnager - Camera:

  • Readout Noise in e-.

  • Gain (Value and e-/ADU)

If our Camera have the gain defined by default (defined by the supplier) in the drive we need define Hight or Low and the Gain Value e-/ADU (Defined by the supplier).

But when we have camera, where SGP not suppot the value (Example QHY10 defined the gain in the driver ASCOM),

How SGP calculates the ideal exposure time? or where we need define more parameters to obtain the good “Ideal Exp. Time” ?

Many thanks for the comments or descriptions, will be very well received.




I believe it follows a paper that suggests a certain ratio between read noise and sky noise. If you do an Internet search, you will find a similar plugin for Maxim DL along the same lines:

t minimum = 5 R^2 / (sky flux)

where R is read noise in electrons and sky flux is in electrons/second.
This is the simplified version. Others exist too. I think the hidden loft obsy site has some.


Hi Leandro, good point.

Rephrasing your question:

From where is SGP getting the info to calculate the Ideal exp. time? From the Equipment Profile Manager or from the ASCOM Driver for the QHY10? (The ASCOM driver is the window that appears after you click the camera settings icon on the sequence´s window).

On the Equipment Profile Manager and for the QHY10, I think you should input gain 0,7 (camera specs) or less depending on how you adjusted your camera after following QHY´s instructions but on the ASCOM Driver for this camera you are supposed to input the gain adjustment position, a value between 0 and 62. This is the range expected for the gain on the ASCOM driver.

After reading the Ideal exp time recommended by SGP for my friend’s 12 minutes Light, I am also confused regarding which info is SGP using because on that reading I got an Ideal exp time of 71 min???




Thank you for your email Buzz.

But I need know if SGP obtain any ideal exposure time for camera (example QHY10) where not have define the gain in the profile device, the camera have only definition in the ASCOM GAIN and Offset, SGP obtain the time?




Ideal exposure time in SGPro is based on this paper by John Smith:

It only works on images that come directly off of the camera (it will not work for images opened off the disk). It requires 5 parameters to be of any use:

  • Integration time (automatic input from SGPro for the current camera image)
  • Mean background ADU (automatic input from SGPro for the current camera image)
  • Gain for the binning used in e-/ADU (user input from the camera tab in the control panel)
  • Readout noise in e- (user input from the camera tab in the control panel)
  • Percent contribution of read noise to total noise (assumed no more than 5%)

I have updated the help file to say as much…


Hello Ken,

We have all the parameters in SGP for the QHY10 camera (use ASCOM where define the GAIN and OFFSET), but each time we saw the ideal exposition time, SGP display very hight time of exposition, example 12, 15,… 71 minutes (see email send Renan).

We have:

Gain for the binning used in e-/ADU = 0,7

Readout noise in e- = 9,0

We don’t understand the higt time proposal for the exposition.

It is possible see in the log the parameters SGP use when calcule of the exposition time?

Thank you,



It seems to me that there may be a discrepancy between John C. Smith´s paper and what SGP calculates for the Ideal Exp Time.

Smith´s paper considers, for calculating the Esky (Sky background flux in electrons per minute), the reading from a calibrated light, i.e., the ADU reading from a “dark-subtracted” light while the Ideal exp time calculated in SGP it looks that considers the MEAN value from a non dark-sbstracted frame giving then shorter ideal exp time results.

(See example on page 2 of the paper).

Checking some of my frames, the difference correspond exactly to the effect of the mean background reading of a dark.




In practice - the bigger issue is often blowout on stars. The suggested exposure plan keeps a certain noise level for backgrounds but does not account for highlights. The times I use are typically judged to make the key areas about 80%. Some ultrabright stars cause issue, (like M45) and so I will take another set at a considerably shorter exposure and combine with HDR in Pix.


Just a suggestion, like we input Gain and Read Noise values, maybe there would be a window to input our Bias Mean value? Then it would be subtracted from Light frames and then do calculation for Ideal Exposure. I think that will add more precision to calculation, since full calibration cannot be applied.