How to build a flat field box

Tired of all the blemishes on your deep-sky images, and a night sky that looks brighter in the middle than at the edges? The answer is to make a simple accessory.

Written by Martin Mobberly.

Skill level: advanced

Image Credit: 
Martin Mobberly

Astronomical imagers strive for perfection, but when you start off in the hobby and go to process that first set of deep-sky pictures on your computer, you can often end up with something murky and far removed from the crisp field of stars you had in mind. Fortunately, we’ll show you how by building a flat field box, you can beat two of the top culprits behind murky images: vignetting and dust doughnuts.

What you need do get started

Box material - The basic box structure can be made from wood, plywood, hardboard or thick, rigid cardboard. Go for materials that are lightweight yet strong. The inner surfaces need to be white.

Bulbs - Low voltage, low wattage bulbs are best. Bulbs over 10W will saturate the CCD detector, while 240V mains can be dangerous outside in the dark, potentially lethal. Use torch bulbs for the illumination and a 12V power source. A battery is ideal.

Hose clips - To fix your box to your circular telescope tube, make a circular sleeve out of flexible H-section UPVC and tighten it to a friction fit with a metal hose clip. Hardware suppliers stock the H-section and hose clips – you will need the clips to be 3.5x the diameter of your tube in length.

Power tools - Elaborate tools are not needed but some power tools, such as a jigsaw and electric drill, are very useful. A hacksaw, superglue and even strong tape can be employed if the parts are light.

Translucent material - A slab of frosted/translucent acrylic sheet or Perspex will diffuse the light from the bulbs entering the telescope. Hardware stores such as B&Q stock suitable materials.

Easy-to-spot problems

These problems are easy to recognise. Vignetting is where the centre of your image looks much brighter than the edges. It’s because the light cone from the primary mirror is pinched as it travels through the optical system. This can occur for a variety of reasons. For example, your Newtonian’s secondary mirror may be too small and may not be able to reflect the whole diameter of the light cone from the primary mirror. Your focuser drawtube may be so long and thin that the light cone cannot get through it without being chopped off at the edges. Accessories such as focal reducers, filter wheels, motorised focusers and camera adaptors can all bite a chunk out of that light cone too.

Dust doughnuts look like fuzzy rings or Polo mints and they are a sure sign that there are specks of dust on the glass surfaces above the CCD detector. The size and intensity of the doughnuts will depend on how big the dust grains are and the f-ratio of your system, but it’s also a sign of how far they are from the CCD chip. The further a dust speck is from the CCD, the bigger and fuzzier its effect will be: specks on the CCD glass can cause small, sharp doughnuts, while specks on filters, Barlows and other accessories can cause huge, fuzzy doughnuts.

Fortunately, the flat field box can solve these imaging horrors. But what is a flat field? Well, the best way to think of it is an image that contains a record of the optical deficiencies in your telescope and nothing else – no stars, no galaxies – just the dust and vignetting effects. In a perfect telescope, the flat field image would just be a neutral shade of grey, with no details visible. You then apply the flat field to your astro image with imaging software to subtract any vignetting effects or dust doughnuts. Don’t confuse the flat field with the dark frame: a dark frame is a record of the thermal noise in the CCD detector and nothing to do with your scope’s optics.

Light matters

For a flat field image, you need a neutral shade of diffuse light, and there are really only two practical sources for this: a clear twilight sky with no clouds in the way, or a home-made flat field box. Twilight flat fields can work well, provided the sky is uniform just after sunset. But you need to wait until the Sun is about 5˚ below the horizon before taking the image. If the Sun’s too high, the light sky will overexpose your CCD; if it’s too low, bright stars will appear out of the twilight background. Being prepared for this narrow twilight window can be a hassle.

And that’s not the only problem: suitable crystal-clear twilight skies are never around when you want them. This is typically immediately before or after an imaging session, especially if your dust specks have moved about a bit since the last flat field was taken. The solution, then, is to make your own flat field box to simulate twilight illumination.

This box has to fulfil three main functions. Firstly, it has to fit over the end of your telescope easily – it will not work on a different aperture unless it is modified. Secondly, the illumination seen by the telescope must be very even. And thirdly, the illumination must be pretty low, as it is mimicking the dim conditions of twilight.

Reflective surface

The key here is to illuminate a translucent material – like white Perspex – from behind, but not by direct illumination. The bulbs that generate the light should be very low wattage and they should not point towards the telescope. There needs to be a white reflective surface that spreads the light around to make the Perspex front appear totally flat in illumination. We cover these considerations in more detail in the six steps below.

Once you have a flat field image, it merely remains for you to apply it to your astronomical image with imaging software. This is a simple step because all astro software packages have flat field routines and commands. The software then divides the blotchy pixel values from your image with the blotchy flat field values. And like magic, your image will become perfectly smooth and clean.
















Step 1 - The box must fit snugly over the telescope end so a circular sleeve of the same diameter as the tube works well, especially with SCTs. Making the sleeve a friction fit can be achieved by tightening it with a hose clip.

















Step 2 - A translucent or frosted screen needs to be fitted inside the box so that the telescope stares at a diffuse and even illumination and cannot ‘see’ individual bulbs. If the illumination looks uneven, you may need more light diffusion.
















Step 3 - The bulbs need to be fitted inside the top of the main box to illuminate the back and sides of the box, which should be painted white. Fitting a translucent 35mm film container over the light bulbs can diffuse and attenuate the harsh light from the bulb.
















Step 4 - The bulbs will not need to glow brightly, so extra resistance will probably be required to avoid CCD saturation. Fit a potentiometer, which is like a dimmer switch, onto the outside of the box to adjust the bulb brightness during the initial trials.















Step 5 - After construction, attach the flat field box to the scope and carry out very short exposures with your CCD camera to see if the illumination level produces a similar result to a flat field image taken at twilight. Adjust the brightness or bulb positions accordingly.











Step 6 - The acid test! In your imaging software, apply the flat field image to an astronomical image of a star field in a slightly bright night sky (moonlight or dark twilight) and, fingers crossed, those background brightness variations and dust doughnuts will magically vanish.



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