Thursday, October 25, 2012

This is gonna be technical...

... but I promise there will be a pretty picture at the end. And a pony.

We continue to test the Mosaic filter adaptors to use narrow band  (and other) filters, where in the center we place the filter of interest, and above the outside detectors there are cut outs to allow guiding the telescope. The cutouts are covered with clear glass to ensure they are parfocal with the science target area.

The dilemma is now that the clear glass lets so much more light pass than a narrow band filter, and scattered light can make its way to the central detector array. The image below shows a flat field simultaneously using the small H-alpha and the large ODI z'-band filter. Why two filters? By combining a z' and an H alpha filter, the central area under the narrow band filter should not receive any direct light. The outer detectors with clear glass in the filter module should see the equivalent of a flat field taken in the z' band.

Flat field simultaneously using a large ODI z' band filter and a small Mosaic H alpha filter.
One can clearly see the three well illuminated outer devices (a level of about 40000 electrons).  Zooming into the central 3x3 area below, we can see that some scattered light is making its way there, while under ideal circumstances we should only read noise. The level in the central array is about 0.5% to 1% of the level in the outer detectors. This scattered light will ultimately constrain the flat field quality for narrow band imaging with ODI, and we might need to opt for neutral density filters, or roughly color-matched glass inserts to cover the outer detectors instead of using simple clear glass. We expected to see such stray light, but for a first feasibility test, clear glass was cheap enough and works well for now.

Zoom into the central area of the flat field.
The next image shows a false color image of a genuine, H alpha-only flat field of the central 3x3 detector array. The scaling is such that red is near 100% illumination, green is around 50% illumination, and dark blue indicates near zero throughput. Thus, a substantial fraction of the central imaging array will be useful with narrow band filters.

Color coded H-alpha filter flat field.

Which finally brings us to the pretty picture that I have promised earlier: A 300 second exposure in H alpha of the horse head nebula, bias, dark, and flat field corrected. See? A pony!


Horse head nebula, 300s in H alpha

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