ASIAir - AutoFlat Frame Exposures

 

V1.5.3

This version of ASIAir introduces a new feature I call AutoFlat. Before i go into how to use this let me explain a little about flats and how we used to calculate the proper flat exposure. 

Flats are used to map out vignette, gradients, dust, and other filter variants.  This data is then applied to your target light frames to balance the gradients and remove dust and other optical imperfections so that the resulting image is flat and evenly exposed without the defects of the optical system.

Typically folks use layers of tshirts against the sky, led flat panels, EL or electroluminescent panels, or light boxes.  Some have even used their ipads.  The idea is to use a known even illumination source so gradients and dust can be recorded.  The trick is obtaining the proper exposure.  Previously it was common knowledge to use what is called 1/2 full well.  Some have suggested to use 1/3 full well but like anything we do there are plenty of folks to challenge every step.  The process of calculating this exposure is to slowly ramp up your exposure until your average ADU value shown on the preview screen just reaches 65534 or the maximum value for your camera.  Basically its the exposure where this value doesnt increase anymore.  For 1/2 full well we would take exactly one half of this exposure time.  The 65534 value represents the point where the pixels on average have been fully saturated and will record no more photons; a full well.  Half of the exposure time will record one half full well.  If you are in the 1/3 full well family then you would take 1/3 of the exposure time.  This would then be your ideal flat exposure time.  It is not 1/2 ADU or the middle of the histogram.

So in reality, this is really only important for the purists.  ASIAir makes this legacy model easier since we can immediately see the ADU value on the preview screen.

With 1.5.3, ZWO introduced Autoflat.  When you set up the autorun tile for flat exposure you will find there is now an Auto option for the exposure.  There are some limits however.  ZWO has introduced a 10 second exposure limit. If your exposure time hits 10s them you need to increase the brightness of your light source.  There can be a lower limit with some cameras.  There have been reports that some cameras show banding with exposures shorter than 2s.  This is something to watch out for.  So for this to work reliably, your exposure should be between 2 and 10 seconds when using auto.  

I requested a higher limit like 30s but was told 10s was enough.  Lets hope they bump this up in the future as its difficult to run flats for an entire wheel of filters if you have lum, rgb, and narrowband filters in the same wheel.  For now you just have to run them separately.

Now, folks have also abandoned bias frames with asi cameras in favor of using flat darks.  This is due to the banding issue mentioned earlier.  Flat darks are the same exposure as the flats but with the scope covered.  Fortunately ZWO added a way to do this. After running the flats, go into autorun and reset the autorun, go into the flat tile and click the exposure edit icon.  The previous auto exposure will populate the

exposure field.  Now you can select bias or dark as your image type.  Your choice.  Cover the scope and run the sequence again.  You now have your flat darks.  If you are using a DSLR please use true bias frames, not flat darks.

One will notice that the histogram does not show the hump at the manually calculated 1/2 full well.  I’ve been told that this is due to the exposure being calculated before white balance has been applied and before debayer for an OSC camera.  Honestly I have never seen an issue between a variance here in the fractional full well used except in cases where there are extreme gradients.  The important thing is that for all filters used that the fractional full well value is the same for all filters, somewhere between 1/3 and 1/2 full well is fine, to help with color balance when all are combined to produce a color image.

Previously getting the correct flat exposure was a painstaking effort that needed to be performed whenever you changed gain and filters.  With the introduction of Autoflat this has been made quite a bit easier to accomplish.  Currently this is limited to exposures between 2 and 10 seconds.  You can vary the light intensity to get you in that range.  One option is to use frosted lexan or plexiglass or for very bright led tracing panels you can use cling on window tint from the auto parts store.  Find some that have 20% transmission and are neutral grey in color.  It comes in rolls 2ft x 12ft for only 12.00US.  Just cut with scissors and tape to your led panel.  Stack layers if you need even more light reduction.

One last tip.  If you are using a LED flat panel or light box and normally orient the scope vertically to keep them in place, be sure to park the scope to the home position.  This will set the scope to a known position you can reset to after the flats are taken, disable tracking, and prevent an automatic meridian flip if that is enabled in autorun.  May be a good idea to just turn that off when shooting flats this way.  You dont want any movement to cause your equipment to slide off and get damaged.

I hope this has been helpful and simplifies your calibration workflow.