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Initial tests
I've just done a test in daylight to measure the reduction factor of the following reducers:
WO Flat III, manufacturer stated 0.8x
Borg DG-L, manufacturer stated 0.85x
WO Flat IV, manufacturer stated 1.1x ~ 1.2x (info recently added to WO web site)
vs native magnification, on the FLT98
I set up the scope in my garage, with a view of a fence about 250m away. This gave me a good reference point for measuring field width in each of the 4 configurations.
For the Flat IV, as it is adjustable, I measured with it in the central position. As you move towards the outer and inner stops, it increases/decreases the reduction/magnifiction factor by a few percent.
On each of the sample images, you will see two small x marks on the fence, which indicate my measurement point. The distances were measured on the original images, using Photoshop's ruler tool. Those distance were then compared with the reference distance to arrive at the following results:
WO Flat III: 80.4%
Borg DG-L: 89.2%
WO Flat IV: 120%
Borg states that the reduction factor of their flattener will vary depending on the thickness of the spacer ring that you have installed, so I knew that it wouldn't be exactly 85%. 89% was no surprise in this case.
The real surprise was the Flat IV, which had a magnification rather than reduction. This would turn my 618mm f/6.3 FLT98 into a 740mm, f/7.56 configuration. WO has recently modified their description of this item to include the magnification factor. At the time that it was shipped, the factor was undisclosed.
I have not yet measured flatness, which will require steadier seeing than I have gotten during daylight hours, but will hopefully get a chance to do that tonight if my autoguiding plays nice.
Regards,
Eric
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