William Optics FLT98 – flattener test – part 1.

With the switch to a DSLR with a full size sensor (Canon EOS 5DII, 24x36mm) I wanted to check the degree of vignetting when used with different types of flatteners. First part of this test is shooting a flatfield image with a lightbox (Aurora flatfield from Gerd Neumann).

The following flatteners were tested:
1. William Optics flattener 68.
This is the dedicated FLT TMB flattener designed by the late Thomas Back. It is supposed to be the best flattener for the FLT range of telescopes. My FLT98 (the first generation carbon OTA) is upgraded with a FeatherTouch 3035 focuser. A special end cap is available from Starlight Instruments for attaching M92/1 William Optics male threaded accessories. This provides a very sturdy, flexfree connection to the focuser. Highly recommended!
Two adapters are included with the flattener 68:
a) T adapter. This is the ‘old’ cone shaped version.
At the flattener┬┤side it has a clear opening of 68mm, the side that attaches to the T-adapter is approximately 37mm.
b) CCD adapter. This is a short adapter with SCT thread.
Although it’s supposed to be used with CDD camera’s, I wanted to see if I could figure out a way to use it. Enter the Borg… In the past I’ve used a mini Borg 45 in different configurations. These configurations come with a lot of adapters. Going through this pile of adapters I found a M57/60AD adapter. This connects SCT thread to the Borg thread. Good! Borg also has it’s own camera mount system with Canon EOS adapter with a 48mm. Looking good so far. Probably need a bit more distance, so I added a 16mm extension tube. Time to measure the total length of this combination …. drumroll …. Wow, exactly the recommended distance.
2. Televue TRF-2008.
I’ve been successfully using this flattener/reducer(0.8x) for several years with different refractors.
3. William Optics flattener 4.
Bought this flattener/reducers(0.8x) recently after reading a lot of positive reviews.

With every flattener connected to the telescope, I focused on a distant building before taking a flatfield image. Exposure time for each image was adjusted so the histogram would peak roughly halfway down the dynamic range.
In PaintShopPro the histogram was stretched to reveal the degree of vignetting. Comparing the results, fhe flattener 68 in CCD-Borg configuration looks really promising. Next test will take place ‘under the stars’.