Triplet Semi-Apo?

I am curious if anybody is familiar with the type of scope that is in auctions right now. It is large refractor listed as a "semi-apo" and every time I hear that term it makes me cringe. Does anybody know if these designs had an ED element. If not, how does the third element work exactly?

Clear skies,
Kevin

Kevin Brennan said:

I am curious if anybody is familiar with the type of scope that is in auctions right now. It is large refractor listed as a "semi-apo" and every time I hear that term it makes me cringe. Does anybody know if these designs had an ED element. If not, how does the third element work exactly?

Clear skies,
Kevin

Kevin: Since the scope in question was made by Roger Ceragioli, I think he is probably the man to ask... Fortunately he is an member of Astromart and an infrequent poster here... He is highly respected, he knows his stuff and is a straight shooter... Look up his user profile and PM him or respond to one of is classified ads.

I suspect the scope being sold is this one:

http://www.astromart.com/classifieds/details.asp?classified_id=362150

This what Roger had to say in that ad:

I have a 200mm f/9 semi-apo OTA for sale. See pics. It is an oil-spaced triplet with a very good figure in a machined aluminum cell. RMS wavefront error is 0.03wv at 532nm; Strehl ratio is 96%. Focal length is about 71". Definition and contrast are excellent. Color error is equivalent to a 6" f/15 standard achromat in a more compact format with larger aperture. Comes with machined aluminum end rings and an AP 2.7" focuser. More information and interferogram on request.

Jon Isaacs

Kevin Brennan said:

I am curious if anybody is familiar with the type of scope that is in auctions right now. It is large refractor listed as a "semi-apo" and every time I hear that term it makes me cringe. Does anybody know if these designs had an ED element. If not, how does the third element work exactly?

Clear skies,
Kevin

For a modern apochromat of reasonable focal length and superb color correction the obvious candidates are fluorite or a fluor-crown like FPL-53. Either is an expensive choice, and availability is a problem is you are an individual making a single lens. An alternative is to use a slightly abnormal glass to produce a lens with better color correction than a traditional achromat, but still has some secondary color. The first "super-planetary" lens Roland Christen showed at Stellafane, a 5" f/12 triplet, used BaFN-10 and KzFSN-4 to synthesize a flint element with the desired optical properties, and combined it with a BaK-1 crown to produce a well corrected lens.

Back before the days of modern apochromats, lunar and planetary observers would have been quite happy with an 8"f/15 achromat. Of course, such an instrument is long and difficult to mount. A modern 8" f/9 with superior color correction and a superb figure sounds very attractive. It certainly tempts me, but I have enough telescopes.

Clear skies, Alan

I got curious about prices in this aperture realm...

D&G 8" f/15 achromat $3695
TEC 180mm f/7 fluorite oiled triplet $19,000
APM 203mm f/9 triplet $26,695

Prices are for OTAs, and may include a few accessories.

Clear skies, Alan

Hello fellow refractor lovers.

I have posted some additional info in the classified ad and the auction listing which should prove helpful to serious lookers who know what they are looking for. I have more information as well, but for some reason I am not receiving any private messages through Astromart.

https://www.astromart.com/auctions/details.asp?auction_id=5518

My direct email address is [email protected].

This is a superb lens for visual work.

Best Regards,
Gary Osoba

Probably it would be best described as ED?

Everybody seems to be getting all hung up over the color correction. No suprise here.

To me, the seller has been very careful and clear that this is a specialized instrument for visual use. Looking at the spot diagram, the blue does not seem that far out to me at all for such a fast scope. How people react to that visually is the stuff arguments are made of.

However, based upon the 6" F10 oiled triplet I got from Roger, I can speculate the figure of the lens for visual use is exceptional and will take your breath away upon first view.

Jeff