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Flat frames or ‘flats’ are a crucial step in image processing. By taking flats and applying them to images the effects of vignetting and any foreign objects such as dust on the optics or filters can be removed.

Initially, I experimented with taking flats of the sky, the observatory wall and also using a white cotton T-shirt stretched over the telescope. The latter worked reasonably well but I really wanted a portable, reproducible and easy to use option so I could quickly take a batch of flats at the end of the nights imaging. The exposure using the T shirt method varied enormously with the sky brightness and indeed changed over a few minutes.

I purchased an A4 size EL Panel from Earlsmann including a converter and battery holder for 8xAA batteries to power it.

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Initial tests showed that the exposure was very short and the brightness of the panel needed to be attenuated somehow. I added a sheet of the best quality white paper, then another and ended up with four sheets between the panel and optics to get a reasonable exposure of a few seconds. Inspection of the resultant frames showed that the fibres in the paper were visible.

Therefore I needed a different approach.

Following some research I sourced two Opal plastic panels from eBay – I think they are normally used for illuminated advertising panels.

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Placing these in front of the EL Panel did indeed attenuate the brightness nicely and also gave a uniform brightness. We were in business!

Being an amateur woodworker I decided to make a wooden frame just under A4 in size and used a router to cut three grooves equally spaced in the inner edge of the frame. This would allow me to support the EL Panel on some thin plywood close to the bottom edge, then a small gap around 8mm to the second groove for the first Opal plastic sheet and finally a 8mm gap to the third groove for the second Opal plastic panel.

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I am really pleased with the performance of this unit and it has been used with LRGB, Ha, OIII and SII filters on a Takahashi FSQ-106EDXII at both f/3.6 and f/5 and also on a Hasselblad Medium Format lens at f/4.

The exposures in the table on the next page give around 30,000adu in the centre of the field . . .

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Table of exposures
with QSI583WSG cooled to -20°C

The technique I used is to point the telescope vertically and place the light box on top of the dew shield, switch it on and run a series of exposures in Maxim.