Sligthly used, with fault: Geoptik 30B306 - Flatfield adapter, D=372 mm, for flat field imaging

The Geoptik flat field adapter solves the problem of unevenly illuminated deep-sky images. This effect is often caused by vignetting (inherent to all optical systems, more or less) or prisms of off-axis-guiders inside the optical path.

Problem solution is capturing an evenly lit background. With this "flat field", image processing software is able to remove the vignetting effect from the main picture. Here, we recommend taking several flat fields and combining them to one "master flat". Nearly all astromical image processing softwares (Astro Art, MaxIm DL ...) can handle this.

Advantage of the Geoptik Flatfield Adapter:

At the sky, an evenly lit background is nearly never present. Even if clouds, late dusk or fog appear as absolut evenly illuminated, in fact they aren´t. These errors will then be calculated into the latter picture. The Geoptik flat field adapter provides a true flat field - a first class reference. We recommend taking such flat fields in darkness to prevent stray light from entering the optical system.

Using the Flatfield Adapter:

This is very simple: Direct the telescope or lens roughly to the zenith and put the flatfield adaptor on the top of it. Switch the adaptor on. Now you have an even lighting on the chip, only changed by irregularities in the telescope or on the chip. Make flatfield exposures, we recommend 4. Make them in the darkness to avoid stray light adulterating the results. Now you have a base which can be combined to a "master flat".





The image above shows a typical flat field with obvious and slightly uneven vignetting. This is normal with most telescopes.

Technical details:

Suitable for lenses / telescoeps up to 372 mm outer OTA diameter

27 mm thick. Locking with three nylon thumbscrews

Powered by an external 12 V DC power source

Weight: >1000 Gramms

How do I get a good flat field image?

A good flat compensates for edge shading (vignetting), but also for darkening caused by dust on the filter, sensor or corrector. The background becomes even, the contrast can be raised further to make finer details visible in faint nebulae. The following basic settings should be noted:

The ISO or gain setting must be the same as for the lights, i.e. the actual astrophotography.

Telescope, camera and accessories, such as filters, off-axis guider etc., must be the same as for the actual exposure, also the position of the focuser should be unchanged.

The exposure time should be so short that nothing is "burned out", i.e. overexposed. Only in this way can the flat unfold its full effect. We recommend an exposure of 30-50%. The flat should not be burned out, but it should also not have any unexposed areas.

How do I find the right exposure time with the shooting software, for example Maxim DL?

The ADU value (Analog Digital Units) helps here. Every capture program shows this value when you move the mouse over the image. In the center of the image, where the illumination is highest, the ADU value is highest. The longer you expose, the higher this value becomes. If the ADU value is higher than the maximum value of the camera, the image is overexposed. A camera with 16 bits has a max. ADU value of 65536, one with 14 bits has 16384, one with 12 bits only 4096.

The exposure time of the flat should be so short that in the middle of the flat the ADU value is at most 50% of the maximum value of the camera. Then the flat looks well exposed. We recommend to take at least 10 flats per exposure and to process them to a "master flat".

If the flatfield box is too bright, that means it always produces overexposed images, then a white paper, which is fixed in the flatfield box, helps. This will dim the light and allow for slightly longer exposure times.