Thanks for this spreadsheet! Great work!
I bought a 533mc a few weeks ago and have been testing it out on clear nights. I am pretty new to AP having completed my scope rig in janurary so i could start imaging with a dslr. In late April, i upgraded to a 533mc pro. I am still learning but my biggest issue is determining the best gain/offset/sub length for my target.
Can you give a overview on how best to use this spreadsheet to get the suggested gain and exposure length?
I am afraid to give an overview, as it might completely derail the thread, which is really just to point to the location of the spreadsheet.
But I will anyway.
The original goal of the spreadsheet was twofold:
1. To determine a gain and subexposure that results in swamping the read noise by 10X
2. To be able to use actual image data to determine your sky background
Part 2 is much more difficult, as it requires actual sky survey data, in various wavelengths, and then many assumptions regarding how much of the sky photons makes it into a digital signal. So quantum efficiency, optical transmission efficiency, etc, all have an effect. But I did the best I could (so far).
Part 1 is much easier, if you already know your sky background. (via an SQM, or using dark sky maps, etc).
Why 10X? That is just a good rule of thumb, and there are multiple CloudyNights and other threads on this. Put simply, 10X means you are at 95% of the best Signal-to-Noise (SNR) you can get for that particular sky.
So once you have agreed to the 10X rule of thumb, it's easy:
Sky Background = 10 x (Read Noise)^2
So if you pick a region in your image of blank sky, the ADU values (in whatever system you are in, typically 16 bit), should be 10X greater than the square of the read noise in that same system.
Note that SharpCap Pro does this too, using actual image data while in the filed. I think the Pro version is about $10 a year. So if you don't like my spreadsheet, which is perfectly fine, there are alternatives.
Also note, the 10X rule might not be practicable for narrowband and/or true dark sky sites.
Regarding the spreadsheet:
Only enter values in the blue boxes (or bluish-green boxes, if you know those values)
If One Shot Color (OSC), or RGB filters, enter 100nm for filter bandwidth (Cell D15) wavelength, and for filter wavelength (Cell D14) enter 550 for OSC, or 650, 550 and 450 for Red, Green and Blue, respectively.
For straight luminance / monochrome, enter 300 nm for filter bandwidth (or whatever the bandwidth is for you luminance filter). This has the effect of reducing the subexposure by 3X (and hence why people like to shoot in luminance, another long topic on CN...)
Transmission efficiency is still uncertain, but I am finding 0.3 to 0.4 is now working for me (I have data from a Bortle 3-4 area, from last night, I want to try).
If you know your sky background and dark current you don't need to use the middle portion of the spreadsheet.
The right portion is the recommendations to achieve Swamp 10X for that scope and camera, in that sky.
I hope that helps a little.
Edited by StevenBellavia, 13 May 2020 - 09:16 AM.