FAQ
How important is flat fielding?
That depends on what you are imaging. For small bright targets like globular clusters where you aren't doing a lot of stretching of the image and will be keeping the background dark, not so important. For dim nebulosity that requires lots of stretching and digital development, critical.
For photometry, flat fielding is important for accuracy. Since it is impossible to keep the target on exactly the same set of pixels over a photometry run even with the best autoguiding and mount performance, any inconsistencies in pixel response have to be characterized and removed. That's what the flat does. I have used the Flip-Flat on my NP-101 to get millimagnitude accuracy for exoplanet transits. Try that without a flat field!
How much better is a flat frame from your products than a dome flat or twilight flat?
What Alnitak Astrosystems products offer are stable, repeatable, and extremely uniform flat field images, with uniformity which deviates less than 0.5% from a mean value. The 0.5% is a conservative estimate. In testing we routinely get less than 0.1% variation from uniformity.
Your alternatives are twilight flats or dome flats. Twilight flats can give good results when the telescope is pointed to a null point in the sky, but just where that null point is depends on lots of variables, some of which are changing from moment to moment.
In addition, since you are using the light of the sun below the horizon to illuminate the sky, the brightness is always changing, so your flat durations have to constantly change. While there are automated solutions out there, CCD Autopilot being one of the best, you still are racing with the clock to get enough flats (say five to ten in each filter) for good noise statistics.
As your camera/telescope field of view gets wider, it becomes harder and harder to avoid some gradient across the image when using twilight flats. Any gradient in the flat field images will ADD a gradient to your flatted images, just what you are trying to avoid!
Dome flats can be a good alternative to twilight flats, but they have their own set of problems, chief amongst them being uneven scatter off the white surface that has been illuminated by an off-axis light source. Open tube telescopes will suffer most from this scattering. We have seen good dome flats, but none with less than 0.5% deviation from uniformity, and usually with some sort of radial gradient.
What about spectral flatness? Don't I need to use a uniform light source that has the same spectral signature as the night sky?
Ideally, yes, but I challenge you to find such a light source. Twilight flats come close, but you loose the spatial uniformity. The rule of thumb is that as long as you use the same filter for flatting as you do for imaging, the spectral response won't be an issue.
For critical scientific photometry, professional observatories often combine dome flats (rarely less than 1% deviation from uniformity) with high quality sky flats to combine the best of spatial and spectral flatness.
A sky flat is a median combine of hundreds (or thousands) of images of a star free section of the night sky, free of light pollution gradients. It is virtually impossible for amateurs to make a master sky flat since it would take many evenings of imaging to do so, and who of us has gradient free skies?
What about narrowband filters?
Our products are perfect for narrowband filters. The broad spectral output of the electroluminescent panels means that there is plenty of light at each of the three major narrowbands (Ha,OIII, S2) to reach ADU counts of 20,000 to 30,000 in 5-20 seconds, depending on the quantum efficiency of your detector.
Will your products remove the dust donuts on my images?
Yes, but only if your filter wheel positions the filters in EXACTLY the same place as they were when you imaged the target through them. We recommend taking flats between filter changes. This is easy to do when you have on-demand uniform light sources as provided by our products. Try that with twilight flats!