Hi all
Interesting thing I have noticed recently....I have a BORG 76ED to which I have the dedicated DSLR 0.85x focal reducer and which is supposed to yield a flat field. I have an ST10XME attached. I have noticed that on one side of an image, stars are elongated significantly. So obviously I checked the usual things i.e is the filter carousel okay, are the filters screwed in properly, is the effect apparent through different filters etc and does the effect appear in short unguided exposures. The answers are all yes, the elongated stars remain.

The optical train are all screw fittings, so flexure could possibly be ruled out, however, I have also moved the focus mechanism to the front the scope, so at the rear end, the optical train is 20mm spacer, focal reducer, ST10XME.
If the elongation was common in all 4 corners, then I could easily say it was off-axis aberrations, but its limited to one side only. Plus, the focal reducer is designed to flat-field out for large DSLR chips, and the ST10 chip is much smaller.
I would appreciate any other tips as to where I might look to track this down.

tks
Martin

Hi Martin,

"Flatness" is a much abused/misused term that variously refers to different aspects:

Illumination flatness: this is usually "vignetting" and is corrected via flat-field processing. This usually has virtually nothing to do with optical aberrations or field curvature or anything like that.

Field flatness: “simple” optics (few elements) produce a curved focal “plane”. The focal “plane” is actually a part of al sphere. If the detector (film, CCD, CMOS, eyepiece FP, etc.) is flat then detector will intersect the focal-sphere in a way that prevents best focus for all parts - i.e. if you focus at the center of the detector then the edges will be out of focus.

Field aberrations: this usually has nothing whatsoever to do with flatness but I’ve repeatedly seen this referred to as field flatness (perhaps due to CCDI terminology?). Coma is a classic case of field aberration.

Of course any scope/camera will likely have all 3 “flatness” issues and it can be useful to address each issue separately because they each have different causes (for the most part). I’ll defer to Paul here (if he is listening) but CCDI cannot separate field flatness from aberrations because there is insufficient information in any single image to differentiate these effects: it would be necessary to create a true 3 dimensional map by carefully changing the focus in small steps while taking numerous exposures. So what is often called “flatness” is not necessarily actual flatness but simply a measure of the variation of resolution over the field. But a single image cannot really say why the resolution varies – is it due to being out-of-focus from actual curvature or is it off-axis aberrations like coma or astigmatism or some complicate convolution? (This is made worse because off-axis out-of-focus stars usually exhibit exaggerated aberrations that are minimal when in-focus).

The situation you describe sounds to me like optical misalignment in all 3 axis. The corrector is probably not at the correct distance from the focal plane (that is usually a VERY critical constraint). And the scope’s optical axis is not exactly aligned with the corrector axis, which results in non-symmetrical errors. Furthermore, “correctors” can only do so much and are often specified with a generous “spot size”, which means that they can produce OK results with grainy film but using small pixel CCDs with good guiding will reveal inherent limitations.

Stan