I kindly ask that no other DPAC images be added to this thread so as not to confuse the reader. Unless it’s of the same make/model scope that I’ve tested (SVX180). Hopefully a robust exchange of ideas and opinions will follow especially how it relates to claims and verification as well as thoughts on what is truly important in a high performing Apochromat.
Look, you know it, I know it - it’s never been more confusing to buy a high-end telescope… We’ve got test reports, supply chain crunches, and lotteries to navigate. And, for those lucky enough to purchase an APO, questions remain. I mean, how many threads have come/gone with the OP asking how to interpret the test report? or how can I to tell if my scope is good or bad?
I’ve been a member here on CloudyNights for nearly 19 years; I’ve been an amateur astronomer more than twice that long. And, in all this time, I don’t ever recall seeing such a keen interest in testing refractors and the metrics used (Strehl, PtV, RMS) to quantify, describe and communicate those tests to the amateur astronomer. Why all the interest? Might it be: (1) Fancy looking certs being distributed with the OTA’s; (2) Manufactures making claims of unverifiable quality; (3) Manufacturers making no claims of quality other then it “meets our standard”. The reasons run the gamut, but all have to do with that slippery term “quality” and how its defined.
A few weeks back I was asked to check out and evaluate a StellarVue SVX180 F/7 APO. The owner dropped it off in late February and I had possession of it for 6 days/evenings spending about 20 hours in total crunching the data, processing the photos and writing a report. This was the first time I’ve had a critical look at any StellarVue offering. Sure, as I’ve written previously, I’ve seen a couple of SVX152’s at Cherry Springs but have never critically viewed through one. Please understand that at low power, or if a camera is in the focuser hooked up to a laptop, every scope looks like it’s a Strehl of 1.0.
I’ve always been a straight-shooter, often with an abrupt but to-the-point delivery. My tone can be terse, but it’s not personal, rather, like a lot of you, I grow frustrated with all the handwringing and debate that occurs over, what amounts to, hearsay, rumor, and anecdotes labeled as truth. Nothing beats a good set of facts and that’s what I deliver as quickly, concisely, and directly to as many as possible. In other words, you may not like the messenger, but the message is usually on-point.
I am reminded of an exchange I recently had with another CN member. In a calmer moment, he asks why I place such an (over)emphasis on bench testing? A fair question and easy one to answer… Testing, in this manner, is the only neutral arbiter of a telescope’s quality.
At the end of the day, I care deeply about the collective joy this hobby brings to us all and am empathetic to the trials and tribulations we as observers go through when trying to determine if a telescope is good/bad/mediocre/great. It’s rarely easy and is compounded by an unprecedented level of marketing and misinformation bantered about. Vendors and manufacturers don’t participate often on CN, so it’s up to us to keep cognitive dissonance, hearsay, and innuendo from becoming the memes of CloudyNights.
From a fit and finish standpoint, StellarVue got this right. For decades I’ve admired the look of the StellarVue OTA’s even when I disagreed with the company’s message. These SVX optical tubes look like telescopes and that’s important me. The SVX180 – as all large APO’s are, is nose-heavy. My sense is the OTA weighs in at about 40lbs (I’ve not verified this). Even so, StellarVue did a terrific job of removing as much excess weight and bulk from the overall package.
Over the course of several days, I performed multiple tests utilizing several different methodologies but will focus on Ronchi Autocollimation (aka “DPAC”) because it’s the current craze here on CN; is visual; is easy to understand; and has few places in which error can creep in to skew the results. In other words, its highly reliable, dependable, repeatable, and defendable. It’s perfect for determining if an optic is good/bad/mediocre/great.
I tested in three wavelengths R-G-B plus white. My commentary on the actual test will be brief as the bench photos tell the story. Thank you for reading this far!
Edited by peleuba, 13 March 2023 - 09:52 AM.