Flatfields for H-alpha solar imaging

I have been experimenting with materials suitable as diffusers for use with a H-alpha solar scope, to place in front of the objective and capture flatfield images. I had success using material I purchased from Amazon UK at a very reasonable price: It is 0.1mm thick, translucent thermoplastic polyurethane, frosted Waterproof Material.

Initially I experimented with holding a single thickness of the material in place with rubber bands at the front of a Coronado, Solarmax II 60, BF 15 H-alpha scope.

Click on an image to get a closer view

First experiments with rubber bands

Initial experiments were successful, so I mounted some of the material on the front of a short piece of acrylic tube so that it could be placed over the front of the scope very quickly and with no chance of accidentally moving the scope.

TPU material mounted on a quick fitting tube

Inside view showing three small, plastic standoffs to prevent the front of the scope from touching the material

Outside view

AstroDMx Capture was used for imaging and for the capture of flat fields via a wizard that does this. We had it set to capture 20 images and to save the average as a masterflat.

AstroDMx Capture capturing flat fields

When capturing flatfields it is essential to use the same gain (the equivalent of ISO) as is used for capturing. This gain is established before capturing data begins. Then the exposure is set so that the histogram is close to the centre of its horizontal axis as shown in the screenshot above.

Whilst an exposure of 16ms was required for capturing light data frames at a gain of 140: with the diffuser material in front of the scope and the scope still pointing at, and tracking the Sun an exposure of 3s was required at gain 140 to capture the flat fields.

Master Flatfield

It can be seen that in our optical system which comprised the Solarmax II 60, a x3 Barlow and an SC432M CMOS camera with a global shutter, there was lots of dust and some vignetting that would adversely affect uncorrected data.

With AstroDMx Capture, it is possible to activate real-time calibration so that the data are captured flat field corrected (calibrated)

A point worth noting is that Flats will not correct for Newton's rings should they occur. This should be done by a tilting mechanism which can remove the interference fringes. The SC432M CMOS camera showed no signs of Newton's rings even though a Barlow was being used. This could be due to the fact that this camera has a global shutter which is generally accepted as avoiding Newton's rings, although it is a poorly understood solution.

AstroDMx Capture capturing a flat-calibrated 1000 frame SER file

Two, overlapping panes were captured as SER files. 90% of the data were stacked in Autostakkert!. The panes were stitched with Microsoft ICE and the resulting image was wavelet processed in waveSharp and post processed in the Gimp 2.10. 

The Sun in H-alpha on May 19th

Steve Wainwright and Nicola Mackin