Interesting (very expert and logical) argument but from someone who is ignoring one of the most common "environmental variables", that is, the depth of our wallets. That is a real-world variable that we have only some control over. How about this for a variation on the question - "At what price point does a refractor always beat an SCT"? Will a $1000 refractor always best a $1000 SCT (OTA's for common ground comparison). For that variable, using OPT as a benchmark sorted by price, it buys a pedestrian 80mm Chinese-made triplet APO or a better ED doublet with FPL-53. Compared to a $900 8 inch SCT Celestron or Meade. Which would win in categories - planets, moon, splitting double-doubles, large globs, nebula and other faint fuzzies (and throw in color fidelity, contrast, observable distortion)??? Then, to change the environmental variable that we have control over, what about $3000, then $10000. Would the conclusions be different? Sounds like the higher in price you go, the refractor wins. BUT, at the lower price point, the SCT is at least the equal and probably better at most categories. (I bookmarked the a-mart discussion, it is very in-depth and uses lots of big words, so will have to dig into it over time).
Or am I wrong here??
Just a warning...you will probably not like my answer...sorry
When one tries to make a decision based on a brand or on a design type (to a degree), IMO the outcome is no different than picking blindly. Lots of folks IMO place too much stock in reputations and branding. I've had many experiences where cheap wares beat top tier premium wares. It's just not uncommon. So basing expected performance on cost just won't be reliable. And of course you will hear that it will increase your odds of getting good wares by going with cost or branding. That is fine if you are one that likes to play odds to meet goals. My experience is that it is easy to get excellent ware, all you have to do is inspect the one you intend to purchase or inspect and return after purchase (insuring the vendor has the proper no fault return policy).
I think there is way too much misguided information out there also. With refractive optics it's about the combination of glasses used and not about any single element in the mix. So hunting after FPL-53 ensures you little other than increased cost. And even then, I've looked through some not so great FPL-53 triplets, that should have been great. Why they weren't was of course the execution was wrong because of a QC issue or was simply executed wrong due to ignorance of all the issues (unsaavy builder). So it's more than optics too...incorrect attention to baffling in the tube or blackening of components in the tube and focuser can easily allow a less rpecise optic beat a more precise optic. Lesson is that the view a scope puts up is controlled by way more than simply the precision of the main optic.
The bottom line IMO is that one's theoretical analysis should be basic (aperture class needed relative to intended targets, max TFOV capability desired, max illuminated field, size of diffraction limited field). After that, one should then start focusing on real world issues and not the theoretical -- thermal management, CO impacts, collimation ease, size, weight, usage ergonomics, cost, upkeep, longevity. These latter real-world issues will come more from user reports of handling characteristics in the field (i.e., experiences), and less from technical paper exercises.
Given all this, now back to your question: "At what price point does a refractor always beat an SCT"? Answer - given all the variables in the real-world, at all price points it can beat it, and at all price points it cannot beat it. It's just too complex of a system to boil it down to just one variable (cost) being so sensitive that it becomes deterministic. In reality, it's the exact mixing of all the variables matched against the exact needs which will determine the answer.
As example why questions like this have no answer...here's a real-world experience from me:
- My 80/100/152mm APOs beat my 8" SCT when I want to view large TFOVs
- My 8" SCT beats my 80/100/152mm APOs when I want to go as deep as possible into DSO
- My 80/100/152mm APOs beat my 8" SCT more often on planetary when the SCT is not thermally acclimated sufficiently
- My 8" SCT beats my y 80/100/152mm APOs sometimes on planetary when they are all thermally balanced and the seeing is better than 3/4 arcsec (so rarely).
- My 80/100/152mm APOs beat my 8" SCT when I am viewing star points in the off-axis as the SCT has coma
- My 8 SCT beats my 80/100/152mm APOs on any target when I want more image scale for a given brightness of the view
- Many times, my 8" SCT and 152mm APO will show equivalent planetary views because the local seeing is restricting resolution and at 3/4 arcsec or greater
So there is no clean answer. And no single aspect about a scope is deterministic as to whether it will beat another scope overall. Even aperture is not deterministic entirely (e.g., the 8" SCT goes slightly deeper on Globs than the 152 APO, but the 152 APO shows a more precise view of the Globs as the star points are more point-like the vast majority of the time as the SCTs thermals are rarely completely tamed like they are in the refractor and of course the CO will always throw more light into diffraction rings around the star). So what does "beat" mean and for who? It is a soft term and slides around depending on exact circumstance. So the real answer is that my 8" SCT in some ways beats and in other ways does not beat my 152 APO on Globs.
So the resolution to it all is, like most all things, not something easy to achieve. It requires work. First and foremost one has to really know well what are all the most vital and important factors for them, to include observing situations (think of the Glob example above). Then once they know all that about themselves, it becomes an easier task to match up the type of instruments that will best satisfy *most* of the factors. The reality of the situation will probably always be that the solution to all the needed factors will be multiple scopes of multiple designs. That's life!
Edited by BillP, 17 October 2014 - 10:46 AM.