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.
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???
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?
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%)
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?
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.
How would one measure flux?
Does someone have and example somewhere showing exactly how to set up the ideal exposure and where to take the numbers from?
Your book is excellent. I have read many sections several times. Is there a section I might to specially review again.
Often it is hard to read something and then to be able to connect all the dots and apply it. Thank you!
Thank you very much. You are one of the experts i hope to meet one day. I will have a look.
If you were me and have a color zwo 1600, APO 714 mm, good equtorial mount and focuser, live in a light polluted area, have Orion Sky Glow filter infront of my flattener, Saturday night is going to be clear and you want to get many more subframes of M51(gosh the moon is out), how would you take a stab at exposure and gain and the number of subs frames you would take if I can grasp a tremendous amount of the theory people are posting about here.
Yes - I found the reference, it is on page 15 of Gendler’s book, - about sky-limited exposures. Still trying to find any files on my Mac which I may have downloaded.
It is interesting and all that. Ultimately though, I use judgement on my exposures - if I’m clipping bright details, I either switch to a two-tiered exposure plan, or scale back.
skyrate =(background-pedestal) * gain/exposure time
I will carefully re read through this. But does SGP have a way to measure or see a histogram only on the area of interest eg a nebula?
What I am getting is using a higher gain of more then 100 isn’t a bad thing? I think of gain like ISO I would like to go as low as I can go - but that is with a single image.
