SharpCap Forums

This article describes a methodology for establishing the minimum exposure to avoid photon starvation (a search for 'photon starvation poisson noise' returns some interesting reads).

Thanks to Brian (@oopfan) for encouraging me to investigate this technique and for his help/encouragement along the way.

We came to the conclusion that spending all night collecting data and finding out the next day that the data mediocre was not a good use of our time.

Summary: the calculations below lead to minimum exposures to avoid photon starvation for the equipment involved. This could lead to less-than-interesting images. The recommendation is a 50% uplift to get "on top of the curve". Consider increasing the exposures by 2x or 3x but check for star saturation by 'scouting' (capturing a limited set of exposures using an L filter). Doing it for Narrow Band may be out of the question if exposures are already long but at least do it for LRGB.

These were the steps used to calculate the exposures.

• Run the SharpCap Sensor Analysis Tool to obtain the sensor read noise.

• Use the Quantum Efficiency graph for the ICX285AL (Atik 314L+) to estimate the sensor QE for each of the colours.

• Run the Sky Calculator which can be found at https://tools.sharpcap.co.uk/ .

• Use the Read Noise and calculated Sky Electron Rate to calculate the minimum exposure in the formula
  Minimum Exposure = (Read Noise)*(Read Noise)*(Factor)/SER

STEP 1: Run the Sensor Analysis within SharpCap, the Read Noise returned was 4.14e- (manufacturer figure was 4e-).

STEP 2: Use the Quantum Efficiency graph to estimate the QE for each of L (400-700nm), R (600-700nm), G (500-600nm), B (400-500nm). The lines for Ha, OIII and SII filters were drawn in.

STEP 3: Use the Sky Electron Calculator to calculate the SKY Electron Rates.
Data to be supplied: Bortle Number, Telescope F ratio, Camera Pixel Size, Quantum Efficiency, Monochrome, Selected Filter (Red/Green/Blue or None (Luminance)). The outputs are the Sky Electron rates (SER) for each of the 7 filters.

STEP 4: Use the Read Noise and SER to calculate the minimum exposures using the formula (Read Noise)^2*(Factor)/SER. A Factor of 10 was used (recommended by Brian). These were built into a spreadsheet. The calculated exposures were rounded up to sensible, easy to use figures. This is the resultant spreadsheet for an Atik 314L+ in Bortle 6 skies using a Vixen 81s refractor with 0.67x reducer.

Calculating the minimum exposures will avoid photon starvation but, depending on the object, rates of 1.5x, 2x and 3x might be applied. To test this out, it was decided to 'scout' an object. The Double Cluster is next on my list of objects. Star clusters are not easy objects as they are a severe test of good exposure and delicate post processing. The decision was made to use the L filter and take 15m sets of exposures of 10s, 20s and 30s (1x, 2x, 3x minimum exposure). Data sets captured with the L filter [90x10s, 45x20s & 30x30s] were processed in PixInsight.

Using the Statistics process the analysis for the 10s exposures was:

The analysis shows that star saturation was on the edge. Therefore increased exposure was liable to lead to blown out stars. The decision is, in this instance, to proceed with the minimum calculated exposures.

The above process, which cost about 1h in time at the end of a session, will hopefully help avoid wasted nights.

As with all good projects a plan is involved. This is my imaging plan for September to December 2025.

Transit times are included to help with planning imaging. The transit times will move earlier by ~30m per week. Of course weather and the moon will add to the fun.



Dave

Hi Dave,

Everything you say is true except the requirement not to saturate stars by looking at the maximum pixel value. If you are doing photometry, fine, but if you're looking to create an attractive image, star bloat can add visual interest.

Your test subject of the Double Cluster is very bright by most standards. However, there are lots of clusters that are faint that would benefit from longer exposures than the minimum. I suggest experimenting with short equal size stacks at different exposures like you did but don't stop at the first hint of saturation. Let your eye be the judge. Just avoid going below the minimum but feel free to expose longer if it's more visually appealing.

Brian

Thanks Brian. I will experiment with the 50% exposure uplift next time I am imaging.

Dave

I have something positive to say about the 50% uplift:

Establishing exposure to the east of the meridian by sampling the sky background with SharpCap can be challenging. I've noticed at my Bortle 5 location that the sky brightness differs by 0.3 Bortle from 30 degrees altitude to the zenith, so if you establish exposure at 30 degrees altitude the sky will darken as your telescope approaches the meridian. Darker skies require longer exposures, so your stack will have more Poisson noise. The 50% uplift in exposure guarantees that you are safe. Basically I'm saying, respect the minimum exposure but you should not be fearful about going over it.

Brian

Hi folks,

I think this is basically a different approach but arrives at essentially the same result as the calculations done by the SharpCap 'Smart Histogram Brain' calculation. Those are detailed in the thread on 'Picking the correct exposure for deep sky', and in this post - viewtopic.php?p=2054#p2054 - we arrive at the same equation - that the exposure is some factor times the read noise squared divided by the electron rate from the sky background. The factor I would tend to recommend is between 5 and 10, which lead to the total stacked image noise being about 10% and 5% (respectively) above the absolute minimum that could be achieved from a single extremely long exposure of the entire integration time.

cheers,

Robin