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"US Government Surplus" round, optical flats on Zerodur blanks to
1/20th wave accuracy are listed by Nova-Optical in the US. A direct link appears at the bottom of this
page:
Please note: All claims as to the quality and accuracy of these optical flats are entirely Nova Optical's responsibility. I am merely a paying customer and have no commercial interest in the supplier. I was personally assured that when a sample of ten flats were tested they showed between 1/20th and 1/26th accuracy and came originally from highly respected US sources. I do not have the necessarily expensive equipment to test the validity of the claimed precision of these polished optical surfaces. Few people do. You buy these optical flats entirely at your own risk. Not mine. I mention this only in case somebody becomes unhappy with their purchase and decides that it must be my fault because they were disappointed.
I received two flat, round mirrors, within a few days of ordering, well wrapped and the coatings in pristine condition to my own, unaided eye. The orange colour of the blanks suggest they really are made from Zerodur. A stable, very low expansion, glass-ceramic material much used for astronomical mirrors and other optical research. An image search will show you the expected colour. I have borrowed these interesting images from Nova's own website since they are relevant to my own purchase for educational purposes.
My enquiry emails to Nova were answered succinctly and
in good time. The correct customs forms were attached to help the
package through Danish customs. The parcel was sent Priority
International Airmail @ $55US for a 6lb package. I have paid more
postage for less weight in Europe and not enjoyed remotely the same,
very quick delivery. When the package arrived i Denmark I was contacted by email and asked to pay the customs clearance charges, VAT and import taxes by the Post Office/Customs service. [Which I did very conveniently on their website from the supplied link.] The package was then released and delivered the very next day. Danish Post Office Parcel Tracking was provided within Denmark once payment of the charges had been made. I must say I was delighted with Nova's and the international postal/customs service I received. It was all completely painless even if it made a large, but expected dent, in my bank account. It was a case of do something, or waste the expense of the objective and time spent building the refractor.
When the package arrived at my home, I saw the box had been opened for inspection and re-sealed with official, Red "Post Office-Customs" tape here in Denmark. Fortunately they had not fingered the Al coating and I am not sure they even opened the neatly taped bubble wrap and considerable layers of protective tissue paper. The customs form described the optical flats as 4" & 5" telescope mirrors.
Those importing such items from outside the US can probably find an online, import charges calculator provided by your own government. My own total [Danish] charges amounted to almost exactly 0.3 of the purchase price. VAT [Moms] is set at 25% in Denmark. There was a £16 [equivalent] charge by the Danish post office for customs clearance. Import duty was quoted as 2%. PayPal charged me about £20 to convert my currency and transfer US dollars to the supplier's email account. I have no idea if my bank would have charged that much but I imagined PayPal was offering me some protection if my funds had simply vanished. It was also extremely convenient to be able to do everything online.
Be aware that VAT is usually charged on the entire transaction. i.e. Purchase price + freight charges + all customs and delivery charges. This all adds to the purchase price but must be accepted as part of "the deal" in buying almost anything from the US. Don't be fooled into thinking you will somehow avoid these fixed charges by some lucky fluke. You cannot "get away" with the supplier under-pricing the value nor claiming the item is a gift. So don't even try. Gift allowance is far too low to affect any likely, optical purchase. Do you suppose that career customs officers are going to turn a blind eye in your special case? The items are listed as merchandise on the customs form with the full purchase price and postage in US Dollars.
I hope all this information will help you to make a useful decision if you are in the market for such items. Even at the full price including all charges these [round] optical flats were considerably cheaper than any competitor's retail prices which I could find by searching online. I don't have easy access to the many small ads services in the USA where secondhand optical flats sometimes crop up. The Nova flats are unused and I just hope the remarkable surface accuracy is as claimed. Only after extensive testing, in the finest seeing conditions, at very high magnifications, will the quality of planetary or lunar images be likely to be discernible from using the objective directly in a straight tube. With reasonable luck and careful collimation, there will never be any visible difference at all when using 1/20th wave flats.
It is a risk I personally consider worth taking for the increased convenience of a far more compact OTA.The present long, straight tube is certainly impressive in size but is simply far too heavy and too unwieldy for me to handle safely. In icy conditions I seriously doubt I would be able to manage it at all. I have nearly dropped the OTA a couple of times in mid-carry out to the mounting. I cannot afford to replace a broken objective even if one was available.
It is highly recommended that, regardless of source, you always oversize optical folding mirrors to avoid using the outer edges or losing light due to the flats being too small. Tilting will always demand more mirror breadth than you might have hoped for. Draw yourself an accurate, full sized light cone on a long roll of paper with a useful circle of full illumination. Then cut it out carefully to your perfectly straight lines. Now fold it where desired and measure the width of the fold directly from the drawing to find the minimum mirror size at that location. If you are restricted to a fixed OTA/tube size then be even more generous with your mirror size allowances. Measure twice. Order once.
All enquiries to:
Optical Flats @ Nova Optical Coatings
PLEASE NOTE: A fellow ATM has contacted me to say that Nova is running out of optical flats. The useful 4" size is no longer available. Contacting Steve Dodds at Nova-Optical yourself will confirm which sizes he still has in stock.
Second update: 4" flats are still available but not coated. You would be responsible for getting your own flats coated.
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