Thanks again for your time Jase
Quote:
Originally Posted by jase
For 16bit cameras the maximum pixel value is indeed 65,535...however you're not calculating the flat saturation value from this figure!!! In doing so it would mean that every 16bit camera would have the same saturation value regardless of the CCD chips characteristics = incorrect flats!!! You need to calculate the value from the CCD chip's full well depth - 25,500 for the ST8300. Forget about the 16bit ADU for the time being.
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OK, so I aim for about 8,500 as the target ADU in Sky Flat Assistant. I'll give that a go again.
Quote:
Originally Posted by jase
The vertical stripe you note is probably characteristics of the camera. Short exposures can exhibit this. Do your bias frames show a similar feature in the image? It is normally corrected during the bias/dark frame application to the flat subs prior to being combined.
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Until recently I didn't take bias frames. Now I do so that Maxim can autoscale the darks before subtracting them from the flats. To be honest I haven't looked too closely at them. I'll check this out too.
Quote:
Originally Posted by jase
5min flat subs! Crazy.
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It would be! I was only referring to 5 minute lights. My thinking was that the uneveness of illumination on the sensor becomes more pronounced the longer my light subs go. If my flats are too bright - the contrast with a shorter light sub will be more pronounced than if I applied the same flat to a long sub.
Quote:
Originally Posted by jase
Long subs will work if your flats are controlled, i.e. you're using a lightbox or similar, but reaching the target ADU faster (shorter subs) is preferred especially if you're doing real sky flats as the longer the exposures the more changes you'll pick up stars in the dawn/dust sky and your time window to acquire the flats are short given the sky luminosity changes quickly at dawn and dusk. As a reference, the longest I'll go for sky flats is 120secs. I do reach this value when acquiring flats through the 3nm bandpass SII and OIII filters as the quantity of energy being passed is small compared to LRGB broadband filters. Long or short, you need to ensure you're scaling your dark frames to match the exposure times. Flats work best when the exposure time is altered to reach the target ADU - unless you have a way of controlling the luminosity of the FOV. Subsequently, I don't take flats shorter than 10 seconds. This is due to the Apogee camera's Melles Griot shutter leafs causing a star fish shape at the edge of the flat field. SBIG camera's use a different shutter mechanism, hence you wont have problems going shorter than 10 seconds for your flats.
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Now this may be an interesting avenue too. I've been using a light box - luckily it has a potentiometer so I can step down the intensity. Using a target ADU count of 20,000, my flat subs are less than a second - in fact for the green filter its about 0.18 seconds - quite close to the spec minimum of 0.1s seconds. I can turn the intensity down a bit more and with the reduced ADU count I can probably increase the exposure time a bit. I wonder if the flat is affected by the shutter sweeping across the sensor so that I don't have even illumination across the field.
So I'll turn down the pot and take the subs as long as possible - the 8,000 target will make this easier.
Quote:
Originally Posted by jase
Yes, we're using different versions of MaximDL. I'm running 4.6, not version 5 hence the screen capture differences however the fundamentals are the same. You don't level the data manually, the algorithm used for data rejection is doing this for you. Its is determining which pixels should be considered outlier and rejecting them. Median combine is more than ample to produce calibration frame masters. Try getting around 8 flat subs per filter, but 3 will work. You want to ensure that you're not adding noise to the light frame subs as a consequence of bad flats.
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OK - got that (I think). I've been taking 10 flats and median combining them. I've relied on Maxim autoscaling the darks and subtracting them so I might try doing that manually.
Looks like I need to get the kids to bed early tonight so I have a chance to sort this out - all part of the learning curve.
Thanks again
Pete