Shiraz
16-04-2017, 09:46 AM
Hi
The ASI1600 allows the user to choose almost any combination of settings -
question is, which is best.
To answer the basic question of how to choose a sub-length, an astrograph model was used to generate tables of suggested sub-lengths for the ZWO1600 under a wide variety of conditions – broadband and narrowband. These could be helpful in navigating between different combinations of camera gain, fNo and sky brightness. Should be good enough to find a starting point if new to the camera, or to see what might happen if you try something different – attempted to keep assumptions reasonable, but your system optics/sky/filter etc may differ, so don't expect extreme accuracy.
BROADBAND GREEN AND YELLOW - 21.5 and 21 sky
http://astrob.in/291972/0/
BROADBAND ORANGE AND RED - 20 and 19 sky
http://astrob.in/291973/0/
NARROWBAND - 21 and 18 sky
http://astrob.in/291974/0/
To use the tables, first determine your sky brightness. If possible, measure with an SQM, but otherwise, find your colour zone from a sky brightness atlas – maybe try this as a starting point http://www.skyandtelescope.com/astronomy-resources/rate-your-skyglow/
When you know your sky brightness/zone, go to the appropriate attached table and identify the column headed by your scope fNo. Then choose a gain from the first column – lower gain for dynamic range, higher gain for low noise short subs or narrowband. If you are new to the camera, suggest that you start out at gain 50 or 75 and that you use the original ZWO intermediate offset default of 21. Now look directly across the row for your chosen gain setting and find the sub-length that is also in the fNo column for your scope. If you cannot find the exact fNo for your scope, or want to use a gain value that is not listed, look at the surrounding data and guess – that will be good enough.
The value in the table is the shortest sub length that you can use (for that scope, sky and gain) and still be shot noise limited (5%RN criterion) - this is the most efficient way to image. The suggestion is definitely not a hard and fast rule though - you can use longer subs with no loss of signal to noise ratio, but more stars will be saturated. You can use shorter subs, but the total exposure time to get to a given image quality will be longer.
To refine the sub length, first take a sub and use the cursor of your acquisition software to look at the ADU values in a background sky region of the scene (no stars and no hot pixels, preferably away from vignetting). Dither the cursor about slightly to estimate the average ADU background value in the clear region. From this background ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average ADU signal in a bias frame taken at the same gain and offset as the light sub). Then compare your sub-minus-bias value with the “expected ADU value above bias” in the final column of the table. If your sub-minus-bias is lower than expected, increase the sub length to compensate – if you get much a higher value, consider reducing the sub length.
The Narrowband tables should be used in the same way as the Broadband ones, although there are more unknown variables to upset the accuracy – so use as a guide only. Because of the wide possible variability in parameters, I chose to generate tables only for the extremes of dark sky and full moonlit conditions.
You could use the same gain setting as for broadband, but you may get slightly better results using higher gain – 200 would be a good value to try at some stage. If you do change the gain, you will need new dark and bias calibration and could also set the offset higher (maybe use 50).
I assumed 7nm filters, so if you have narrower filters, you could use longer subs – (or vice versa).
The estimates should be reasonably applicable to commonly used Ha, O3 and S2 filters.
Some comments:
Don't go by the visual appearance of a sub – with the 1600, they can look very thin, but when stacked, the final result will be fine – so ignore the fact that subs from the 1600 can look weak.
The modelling assumes an average optical efficiency. If you use a high quality refractor, you could possibly get away with shorter subs and if you use a highly obstructed scope, or one with standard Al mirrors, you will need longer subs.
The tables may possibly also be applicable to the QHY163. I don't have access to one, but found a couple of posts on other forums/threads suggesting that 139 gain on the 1600 corresponds to gain 12 on the 163. If so, and if the mapping is the same, maybe divide the values in the 1600 gain columns by 12 and go from there – eg, assume that gain 75 on the 1600 corresponds roughly to gain 6 on the 163.
If you change any of temperature, gain or offset, you will need new bias and dark calibration data.
your bias value should be at least a couple of hundred ADU - if you measure less, then increase the offset value.
If you decide to use very short subs, be aware of the processing load – it can take hours to process a few hundred 16mpix subs.
Thanks for looking, hope the above is useful. Regards Ray
The ASI1600 allows the user to choose almost any combination of settings -
question is, which is best.
To answer the basic question of how to choose a sub-length, an astrograph model was used to generate tables of suggested sub-lengths for the ZWO1600 under a wide variety of conditions – broadband and narrowband. These could be helpful in navigating between different combinations of camera gain, fNo and sky brightness. Should be good enough to find a starting point if new to the camera, or to see what might happen if you try something different – attempted to keep assumptions reasonable, but your system optics/sky/filter etc may differ, so don't expect extreme accuracy.
BROADBAND GREEN AND YELLOW - 21.5 and 21 sky
http://astrob.in/291972/0/
BROADBAND ORANGE AND RED - 20 and 19 sky
http://astrob.in/291973/0/
NARROWBAND - 21 and 18 sky
http://astrob.in/291974/0/
To use the tables, first determine your sky brightness. If possible, measure with an SQM, but otherwise, find your colour zone from a sky brightness atlas – maybe try this as a starting point http://www.skyandtelescope.com/astronomy-resources/rate-your-skyglow/
When you know your sky brightness/zone, go to the appropriate attached table and identify the column headed by your scope fNo. Then choose a gain from the first column – lower gain for dynamic range, higher gain for low noise short subs or narrowband. If you are new to the camera, suggest that you start out at gain 50 or 75 and that you use the original ZWO intermediate offset default of 21. Now look directly across the row for your chosen gain setting and find the sub-length that is also in the fNo column for your scope. If you cannot find the exact fNo for your scope, or want to use a gain value that is not listed, look at the surrounding data and guess – that will be good enough.
The value in the table is the shortest sub length that you can use (for that scope, sky and gain) and still be shot noise limited (5%RN criterion) - this is the most efficient way to image. The suggestion is definitely not a hard and fast rule though - you can use longer subs with no loss of signal to noise ratio, but more stars will be saturated. You can use shorter subs, but the total exposure time to get to a given image quality will be longer.
To refine the sub length, first take a sub and use the cursor of your acquisition software to look at the ADU values in a background sky region of the scene (no stars and no hot pixels, preferably away from vignetting). Dither the cursor about slightly to estimate the average ADU background value in the clear region. From this background ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average ADU signal in a bias frame taken at the same gain and offset as the light sub). Then compare your sub-minus-bias value with the “expected ADU value above bias” in the final column of the table. If your sub-minus-bias is lower than expected, increase the sub length to compensate – if you get much a higher value, consider reducing the sub length.
The Narrowband tables should be used in the same way as the Broadband ones, although there are more unknown variables to upset the accuracy – so use as a guide only. Because of the wide possible variability in parameters, I chose to generate tables only for the extremes of dark sky and full moonlit conditions.
You could use the same gain setting as for broadband, but you may get slightly better results using higher gain – 200 would be a good value to try at some stage. If you do change the gain, you will need new dark and bias calibration and could also set the offset higher (maybe use 50).
I assumed 7nm filters, so if you have narrower filters, you could use longer subs – (or vice versa).
The estimates should be reasonably applicable to commonly used Ha, O3 and S2 filters.
Some comments:
Don't go by the visual appearance of a sub – with the 1600, they can look very thin, but when stacked, the final result will be fine – so ignore the fact that subs from the 1600 can look weak.
The modelling assumes an average optical efficiency. If you use a high quality refractor, you could possibly get away with shorter subs and if you use a highly obstructed scope, or one with standard Al mirrors, you will need longer subs.
The tables may possibly also be applicable to the QHY163. I don't have access to one, but found a couple of posts on other forums/threads suggesting that 139 gain on the 1600 corresponds to gain 12 on the 163. If so, and if the mapping is the same, maybe divide the values in the 1600 gain columns by 12 and go from there – eg, assume that gain 75 on the 1600 corresponds roughly to gain 6 on the 163.
If you change any of temperature, gain or offset, you will need new bias and dark calibration data.
your bias value should be at least a couple of hundred ADU - if you measure less, then increase the offset value.
If you decide to use very short subs, be aware of the processing load – it can take hours to process a few hundred 16mpix subs.
Thanks for looking, hope the above is useful. Regards Ray