Please find attached tables of suggested sub-exposures (broadband and narrowband) for the ASI183MM camera from ZWO. With adjustment, the tables are also applicable to the QHY183M and also to colour cameras using the 183, as explained in later text. The tables should be helpful in navigating between different combinations of camera gain, fNo and sky brightness. Should be good enough for designing a new system, or to see what might happen if you try something different.
BROADBAND - 21.6, 21, 20, 19 sky brightness and
NARROWBAND - 21 and 18 sky
To use the tables for the ASI183MM, first determine your sky brightness. If possible, measure with an SQM, but otherwise, use the Bortle scale (http://www.skyandtel....ark-sky-scale/) or find your zone from a sky brightness atlas – maybe try this as a starting point http://www.skyandtel....-your-skyglow/
When you have selected the table that best describes your sky brightness/zone, choose a camera gain from the first column – use lower gain for more dynamic range, higher gain for low noise short subs or for narrowband. Suggest starting out at gain 50 for broadband. Now look directly across the row for the chosen gain setting and find the sub-length that is 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% effect criterion) - this is the most time-efficient way to image. The sub length suggestion is definitely not a hard and fast rule though - you can still get images if you use longer subs, but more stars will be saturated and your mount needs to be better. You can use shorter subs, but the total exposure time to get to a given image quality will be longer than need be.
If you wish to refine the sub length, first use the camera to take a sub and, with the cursor of the acquisition software, look at the DN/ADU values in a background sky region of the scene (no stars and no hot pixels - and away from vignetting). Dither the cursor about slightly to estimate the average DN/ADU background value in the clear region. From this background DN/ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average DN/ADU signal in a bias frame taken at the same gain, offset, temp as the light sub). Then compare your sub-minus-bias value with the “expected sky DN above bias” in the final column of the table. If your measured 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 reduce the accuracy – so use as a guide only. Because of the wide possible variability in parameters, tables have only been generated for the conditions of rural dark sky and full moonlit conditions. With narrowband, you could use the same gain setting as for broadband, but may get slightly better results using higher gain – 150 would be a good value to try. 7nm filters were assumed and, if you have narrower filters, you could use longer subs – (or vice versa). The transmission estimates should be reasonably applicable to commonly used Ha, O3 and S2 filters.
The tables can also be used with the QHY183M - just divide the ASI gain figure by 10 - eg gain 10 on the QHY should give results close to those from gain 100 on the ASI. The broadband tables could also be used with a 183 colour camera or with RGB filters - work out the sub exposures for the mono version of your camera and then multiply by 3x to get the sub exposures for the colour version (not exact, but should be close enough). The expected DN value will be the same for both colour and mono cameras.
The tables optimise signal-to-noise/time and do not take into account dynamic range limitations (eg due to skyglow or amp glow) - if dynamic range is an issue, reduce the gain and/or sub length.
Don't go by the visual appearance of a sub – short broadband subs with the 183 may look very thin, but when stacked, the final result will be fine
The modelling assumes an average optical efficiency. If you use a high quality refractor, slightly shorter subs may suffice and if you use a highly obstructed scope, or one with standard Al mirrors, you may need longer subs.
Any change of temperature, gain or offset will require new bias and dark calibration data.
The offset should be chosen to give at least a couple of hundred DN/ADU in the bias frame - if you measure less, then increase the offset value.
Thanks for looking, hope the above is useful. If you find anything that looks obviously wrong, please point it out.
Edited by Shiraz, 30 April 2018 - 06:37 PM.