Round 1
The first round of tests used my Canon Rebel XT to offer a known-standard distance from any reducer to the sensor. Three configurations were tested: Prime focus, a William Optics 0.8x Reducer / Flattener v2, and a Baader MPCC. This last one came as a suggestion from a fellow imager, Scott Anttila. Having seen some of his shots with this setup, I knew it was worth a try. In addition, a 0.63x SCT reducer was attempted (placed as close to the sensor as possible) but the results were poor enough to not report here.
The region around Vega was imaged for 20s and the sharpest frame from several was selected. The central area along with crops near the corners is shown below for each configuration. Click on any for a full-sized image.
Prime Focus
William Optics 0.8x Reducer / Flattener v2
Baader MPCC
Of the three, the MPCC is the sharpest at the corners. It appears to introduce some chromatic aberration, however, and the corners still aren't perfect. The WO v2 reducer produces round stars on the edges, but they are clearly bloated. To my eye, this looks like some combination of spherical aberration and either coma or residual field curvature. Using PhotoShop, I attempted to calculate how big an image circle was rendered cleanly in each setup. "Clean" is obviously subjective and something in the eye of the beholder, so these numbers should be taken with a grain of salt.
| Configuration
|
Effective f/l
|
Diam. mm
|
Diam. arcsec |
| Prime
|
390
|
15
|
130
|
| WO v2
|
310
|
19
|
208
|
| MPCC
|
390
|
22
|
190
|
| SCT
|
280
|
14
|
172
|
Round 2
Round 2 took place using indoor testing of a lens resolution target. This round of testing was designed to assess whether the WO reducer was being run at its optimal spacing. Some evidence was found that it performed better at 50 mm rather than 55 mm of spacing. Results from these tests will appear here shortly. Testing in Round 3 of this did not find that 50 mm performed better than 55 mm, however, when under the stars.
Round 3
Round 3 took place in conjunction with tests on a newly-acquired TMB 80SS. This was done on an APS-sized chip (CCD Labs Q8-HR) at prime focus and with two focal reducers / flatteners I had on hand - a William Optics 0.8x v2 and a William Optics 0.8x v3. Both were used at 55 mm from the CCD plane. The images below were simple 20 second shots (best of 3 selected), demosaic'ed and converted into luminance-only in Nebulosity and then stretched linearly. The full-frame was then rotated 33.3 degrees and cropped so that the full diagonal extent of a frame could be easily seen. At prime focus, the image covers 208' x 139' and the diagonal covers 250'. With either reducer (both were measured to produce 0.8x), the image covers 260' x 173' and the diagonal 313'.
It is worth noting at the outset here, that William Optics recommends the v2 reducer for this telescope but does not recommend the v3 reducer.
Prime Focus
William Optics 0.8x Reducer / Flattener v2
William Optics 0.8x Reducer / Flattener v3
Again, we can see that at prime focus, the corners are not very clean. This is to be expected from a doublet and there is nothing "wrong" with the scope. Using a purely subjective measure of "acceptable", I found that at prime focus, about 47% of the diagonal here was good. The diagonal measures 28 mm on this chip and if 47% is usable, that would make for an image circle of 13 mm - a touch less than reported above (again, note how this is subjective and probably open to the whims of how critical I feel when judging the images). As in Round 1, the v2 reducer / flattener produces round, but a bit bloated stars as we move off-center, looking to my eye like spherical aberration. The v3 reducer appears overall sharper, but the stars appear astigmatic near the corners, becoming more like plus-signs than round stars. This does not come through as well in the JPEG shwon here as in the original image. Of these three, it's a bit of a toss-up betwen the v2 and v3. There are aspects of each I like better than the other. In truth, since these images will end up being reduced for screen display (e.g., examine them with your browser resizing them to the screen vs. at full-resolution), either works well to remove the artifacts seen in the prime focus image.