Posts Made By: Roland Christen

If the ring of fuzz seems to disappear or lessen on one side of focus and get worse on the other side of focus, then you have a classic case of turned edge on the mirror. Quite common on a machine polished optic.

Roland Christen

The difference is huge. Planetary viewing is not affected by local light pollution. Seeing is the only criteria. The seeing in front of my house, in the middle of Rockford, is superb for planets, even though I can only see a few bright stars due to the car dealerships, Walmart/Kmart/Target/Mega malls, etc. right down the street.

Roland Christen

One thing you might check is the polish of the secondary. If the secondary mirror is not fully polished out, and there are lots of pits left over, this would indeed cause a glow or fuzz to be seen around bright objects. This assumes you don't have a turned edge. You would need to remove the corrector to examine the secondary suface.

Turned edge can be easily tested for by masking 1/4" to 1/2" all the way around the outer diameter of the aperture. If the fuzz is still the same, then the edge is not at fault. If it goes away, then you have a turned edge.

RC

I realize that the public can be fooled into believeing any sort of marketing hype. However, people on this newsgroup have the ability to reason for themselves. Here are some facts for you to consider:

The term ED means Extra-Low dispersion. A lot of glass has extra-low dispersion (generally any glass having a Vd value > 70). Not all glasses having this property will produce measurably better color correction than a standard crown-flint combination achromat. Measurably better would be better than 1 part in 2000 color error over the C-F spectrum (red to blue-green). Therefore, you can easily claim to have a "Semi-ED" lens if you use the common design FK5 crown and SF1 flint, yet have exactly the same color error as any normal achromat.

It is not the ED or "semi-ED" that produces better color correction. It is the fact that some glasses, notably the ones using fluorite elements in their construction, have a property known as abnormal dispersion. This property allows two dissimilar materials (crown and flint) to have opposite and equal color errors which cancel when they are combined. The result is an apochromat. Glasses with normal dispersion properties (the vast majority of all glasses) do not have equal and opposite color error, so when any two of these are combined in a doublet, there will always be a small error in the way the colors line up and come to focus.

To summarize, it is NOT the "ED" (extra-low dispersion) which allows for better color correction. Rather, it is the Abnormal Dispersion properties of a glass which allows for better correction. Most (not all) of these glasses with abnormal dispersion also are Extra-Low dispersion glasses, but the two properties should not be confused.

Roland Christen

That is really excellent resolution. There is actually more to be seen in this image if you re-align the color channels. I did a quick re-alignment in MaximDL (hope you don't mind).

Roland Christen

BK7 in high quality level costs approximately $22 per lb. Have him weigh the glass and you can decide what it's worth. I would not pay for the grinding time, since it probably was not done right anyway.

Roland Christen

That's a very nice image. I hope you don't mind my punching it up a bit in photoshop.

Roland Christen

Closeup of the right side

Roland Christen

Close up of the left side

Roland Christen

Excellent processing. Very sharp.

Roland Christen