Posts Made By: Roland Christen

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Roland Christen

A lot of the old glass types contained lead and arsenic among other bad substances. This exposed the workers at the glass plants to these toxic substances. It also fouled the air around glass plants to some degree because the pouring of hot glass released fumes into the air and contaminated neighborhoods in the vicinity of the plant.

The new glass types have identical optical characteristics to the old. Some glasses have been dropped altogther, but these were not generally used in amateur optics anyway.

The main concern for us lens makers has always been the internal quality of the glass, which can vary from excellent to really poor, depending on how the glass is poured, cut, annealed, etc. This has always been the case, old glass or new eco-friendly glass. Glass companies do not guarantee their quality in the price range that amateurs wish to pay, so it is sometimes a total gamble for the lens manufacturer. It is possible to get an entire batch of excellent glass, or conversely it is possible to get hundreds of blanks that are so bad, they are unuseable. It is up to the lens maker to throw them away, eat the loss and go on, or make lenses out of them and hope nobody notices the astigmatism, coma and other defects that arise from that poor glass.

There is unfortunately no way around at the present cost structure of amateur optics. To get guaranteed quality in each blank will cost more than the entire telescope is worth in some cases.

Rolando

Beautiful work!

Rolando

Full image reduced to 1/4 size.

Rolando

I added 3 hours of S2 to my Ha-O3 data to get something similar to the Hubble palette (red = S2, green = Ha, blue = H3). I am not entirely happy with the results at this point, but with no clear skies to add more data, this is the result.

Rolando

>>I've given into the idea of trying some other types of scope and am looking for something that will match or beat a 6" APO, as a photographic instrument.>>

Match or beat? You will not do either with any other kind of instrument. You will not match the field of view or the resolution or the signal/noise that you can get with a good 6" refractor UNLESS you spend about the same amount on another type of instrument.

A 6" refractor (F7 to F9) is pretty much a universal imaging machine. It can be telecompressed for wide field views, at prime focus it has excellent pixel scale that matches pretty much the limitations of the atmosphere, and it can pull in small objects like galaxies at prime focus or with a Barlow. It is an instrument which you will use for years and years and not run out of capability. When you are ready to move on to another size, you can almost always recoup 100% of your investment.

You might also consider some of the lower cost scopes - a bit smaller, say 4.5 to 5.5 inches aperture, some of which can also produce extremely good results. Also, don't forget the used market - you may find a bargain or two that will do what you want.

Rolando

Guiding is good, focus seems to be off. Try bracketing your focus positions before taking an actual image to establish the point of best focus. The stars should be pristine points, not little discs. When you have achieved best focus, you should be able to resolve the small triangle of stars in the middle of M27.

Rolando

Here's how I see optics evaluated by a typical evaluators in a side by side view:

less than 80% - somewhat soft but not bad at all for the money, (maybe the seeing is bad)
80 - 85% - sharp as a tack, (sometimes, when the seeing co-operates)
85-90% - Holy ....! Never saw Jupiter better, got a real good look at the Red Spot (when the seeing settled down)
90-95% - mmm, how come this scope is so sharp even in this crappy seeing?
95-99% - nice view at any power, the seeing seems to be really steady tonight!

Rolando

Your question about Strehl is legitimate, but by itself it does not answer the whole question about an optic. Can you have high Strehl and have a not-so-good optic? The answer is yes. In these next 4 images I have captured the process of hand correcting an optic that starts out with a respectable Strehl of .91, and then over a period of about 4 hours convert it to an optic that I can place into a tube assembly and sell.

Optical glass is never really perfect so that if you place a perfect figure on each surface you would get a final perfect wavefront. The internal quality of each element must be extremely high for that to happen, and this is not always the case. Small broad imperfections exist internal to the glass, so one must apply spot hand corrections to achieve the highest level of performance. In my case I can spend anywhere from 2 to 8 hours (or more in some cases) to slowly bring the optics to perfection.

This lens started out with .91 Strehl, 1/20 RMS, 1/4 wave P-V. The numbers, especially the Strehl, sound ok, and I would guess that this lens would pass some company's specs and be on the way to some lucky amateur. Does it work at this level? Well sorta. It would show detail on Jupiter at most powers, but there would always be a bit of something lacking.

The fringe pattern shows a strong S shape which means a non-rotational aberration caused by tiny differences in density internal to one of the elements - a high spot on the wavefront toward the front and a depressed area toward the rear. Spot figuring must be done to flatten the wavefront. 1:00 pm - time to start the hand figuring (basically working the polisher for 5 minutes or so, and then letting the lens sit for 15 minutes on the interferometer until the glass has reached thermal equilibrium so a new reading can be taken).

RC

Baader filters are also available from Astro-Physics and Anacortes.

Rolando