Posts Made By: Roland Christen

Lens coatings don't come off easily. If they are hard multi-coatings, they will need to be ground off. They will never etch off with any kind of acid. That, of course means repolishing and refiguring, which is very costly.

If it's a single layer mag-fluoride, then some form of agressive acid eching can be used to remove the coating, but it will also damage the glass underneath. It will open up all the fine sleeks that may be present and turn them into nasty deep score marks. That means polishing and re-figuring again.

Lens coatings are not the same as mirror coatings. In a mirror, the aluminum coating sits on top of the glass in a relatively thick layer, and has a very tenuous adhesion to the glass. Aluminum etches very quickly with ferric chloride solution, which is quite harmless to the glass underneath. A lens does not have a reflective coating, rather it is an anti-reflective coating. This coating is ultra-thin. When it is applied correctly, the glass is heated to 600 degrees F, which increases the mobility of the glass surface molecules. The glass essentially "opens up" and allows the coating material to fuse well into the glass layer. In effect, the coating and the glass bond very strongly to a certain depth, and in order to remove it, you have to rip the surface molecules of the glass apart with the acid etchant. Weak etchants like ferric chloride won't touch this coating. You need to use hot sulphuric acid, and let it bathe for a time.

If the lens is any good at all, I would leave it alone. The other option would be to send it to D&G Optical and ask Barry Greiner to repolish it for you. He has the expertise to do this right.

Roland Christen

How many people would be interested and what size scope are you contemplating?

Roland Christen

You either need a guide scope or a very accurate mounting so you can do unguided imaging. Another requirement would be a fast telescope - F4 to F6 if possible. Otherwise you get almost no signal.

Roland Christen

Thank you all for your kind replies.

I hope to do some deep sky imaging again as soon as the Moon goes away.

Roland Christen

>When manufactures mention color correction like 8X color corrected on refractor lens what exactly are they comparing the color correction to? >>

An achromat defocuses the red and blue wavelengths by 1 part in 1800. That means that an 1800mm focal length lens would focus green about 1mm closer to the lens than red and blue. If a lens of 1800mm FL has 8 times better color correction, then the green focus would lie only .125mm from the blue and red.

All achromat lenses have approximately the same percentage of color error. The perceived color error depends on the focal ratio of the lens. A fast lens with normal achromat color will look like it has more color error than a slow achromat. The reason for that is simple geometry.

In order to make a fast lens color free, one needs to use special glass called ED of fluorite and combine it with the right matching glass in a doublet or triplet configuration. Doing so can reduce the color error, or difference in focus point to be anywhere from 4 times to 20 times better than an achromat.

Whereas an F9 lens might look quite color-free with 8 times better than achromat error, an F5 lens with the same design will show color error on bright objects. The only way to match the performance of that F9 lens would be to have an F5 lens with 14 times better color error.

Correcting color error is only one of many defects that will be present in a lens, and it is not the main component to having a sharp view. One can have achromat color error, but have a superbly polished and figured lens where all the other Sidel aberrations are well corrected, and as a result that lens will easily beat a poorly made apo lens on the planets and even deep sky objects.

My personal feeling is that a large number of people are being unduly influenced by one or two aspects of a refractor, and thinking that if this was perfected, then the scope will perform to the utmost limits. If the cost is low, then so much the better. When low cost is the goal, and people want something like low color error, manufacturers have no choice but to concentrate on that one aspect while reducing the performance of the other fundamentals.

Many years ago Zeiss made a line of semi-apo lenses. These had only 2x better correction than a normal achromat, but they were made to very high performance standards, and will easily outperform any of the better color corrected inexpensive offerings coming from China today. The views through a Zeiss AS objective are outstanding. I sold a number of 6" and 8" lenses when they were still available, and still remember the magnificant planetary views. Even the little 4" AS lenses were exceptional. And yet, their color correction was only 2x better.

Roland Christen

Do a Google search.

Roland Christen

Excellent! Wish I had been down there. We will be next year.

Roland Christen

Nice, but what's the best way to get color out of this camera?

Roland

Cool!

RC

I think the original image is most natural.

Roland Christen