I have been looking for a long focal length achromatic refractor with an objective lens of around 80 to 100mm mark for a while to do some visual work on planets and because I wanted a long telescope. Ideally, I wanted to get one that met the Sidgwick standard with a focal ratio (F) greater than 3 times diameter (inches). That way, it would ensure that it would be fairly well corrected for chromatic aberration and achieve diffraction limit performance. A good standard to go for without being too unwieldy to use. Yes, I could have settled for a higher ratio and headed towards the Conrady standard but not without making things a little too unwieldy. For example, a 100mm objective that meets the Conrady standard would have an F ratio of F20 to be >5.0 on the Conrady standard. That's a 2m telescope! Hence, I opted to try and find one that would fall within the Sidgwick standard, which would be tolerable for
CA and manageable in terms of use.
After much searching without much luck, deliberating and procrastinating on whether I should opt for a small objective diameter, say, 60mm and or semi-
apo capability but, pay more money, I decided to stick by my guns. And as luck would have it, I saw a 90mm x 1000m Vixen achromatic refractor being advertised for a lowly 15000 yen, which is 135 USD or 98 GBP at today's conversion rate. A positive snatch! This also met my criteria having a Sidgwick ratio of >3.0. So, I placed my bid and sat back, hoping that no one else would instigate a bidding war on it. The day before yesterday, my bid went through without challenge, and today the new edition to my collection arrived.
As we all know, buying from auction sites can be risky, especially when trying to obtain quality glass. So as soon as it arrived, I gave it a thorough going over. Of course, It's not perfect, and there are one or two little scratches here and there on the barrel, but the objective lens, finder scope and focuser are all in excellent condition, few!!!
As mentioned, the telescope is a Vixen achromatic refractor with a 90mm fully coated (not multi-coated) objective at 1000mm, giving it an F ratio of F11.11 and a Sidgwick ratio of 3.135, meeting the criteria of >3.0. Furthermore, this telescope was manufactured in Japan and uses Japanese glass. So, I'm expecting it to be a good performer.
Hopefully, the rainy season will finish soon so that I can get out and use it.