[jase (Jason)]

Originally Posted by pvelez

I think there is a difference in nomenclature here. The full well capacity of the ST 8300 chip is about 25,500 e- which I understand converts to about 65,000 ADU. The Sky Flat Assistant sets by reference to ADU and I had thought that Maxim's graph function did the same. Feel free to correct me if I'm wrong on this.

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.

Originally Posted by pvelez

Last night I tested flats with target ADU counts from 8,000 (co-incidentally) up to 30,000. It was late when I twigged to the error factor and dropped from 10% down to 2% and then 1%. Flats at 8,000 ADU yielded a very nasty vertical dark stripe down the image. The sweet spot (though still not that sweet) was at around 19,000 to 20,000 with a 1% error setting.

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.

Originally Posted by pvelez

Its still not perfect - I am using 5 minute subs. I suspect that the uneven light will be less an issue at shorter subs. If so, applying the same flats will be worse rather than better I guess. Thats logical so its probably wrong.

5min flat subs! Crazy. 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.

Originally Posted by pvelez

This bit is more of a challenge for me. I've set Calibration Wizard to the correct file and its identified in Set Calibration the relevant dark, bias and flat files. I've then created a master flat from the flats in that folder using median combine. The settings are as per your earlier post (albeit under a tab marked Advanced). I then open the light and calibrate it from the Process tab. I'm not sure how to level the data as you suggest. Is this an alternative to median combine?

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.

Hope that clarifies.