SDM Size Does Matter.
The biggest refractor I have ever looked through was a 6",cant remember
What make it was,(probably a Tak?) but it didn't hold a candle to my 16' Newtonian.
Reading his article and the comments, to me it came across as a lot of money for a small increase in sharpness.
Cheers
Call me a cynic, but I'm not sure how much of a revelation that is. I always understood the function of an apo was to bring the three primary colours to focus at more or less the same point (more or less depending on the quality!) Maybe that was my misunderstanding
In a kind-of astro facebook way, his personal journey of revelation is profusely documented on CN. I remember a while back he used to advocate and even defend the use of a large SCT with sound reasoning, and I don't just think it was because he owned one at the time. His talk about contrast transfer with a central obstruction was quite enlightening.
Maybe the pills/wine were particularly good that night when he felt the need to espouse that an apo should be able to focus all the visible light more cleanly than an achro...or have I missed something?
The one area that refractors have no pier is the low level of light scatter in the optics... The image (what there is of it) looks cleaner and arguably more aesthetically pleasing.
By putting an image intensifier in the loop, he is throwing that one saving grace in the bin...
It's like putting a tablespoon of salt on an oyster to enhance the flavour.
I just don't get it?
APO's are hard to beat but cost makes them impractical above about 7 inch aperture. I had a TEC180 for quite a while and it was as large as I would want in an APO.
Also the term APO is a pretty loose term these days. You have AstroPhysics APOs, Takahashi APO's and TEC APO's. AP and Tak being the highest grade.
Chromatic aberration is still the weak area of an "APO" although a good Tak and AP scope does not really have any to comment on. TEC has a small amount weak in the red as I recall.
If you want to get longer focal length then you need something else with larger aperture to offset the longer focal length.
APOs don't make great galaxy imaging machines really. There are some good images from APOs of the larger brighter galaxies but pretty much any large aperture long focal length scope will surpass it.
APOs then are then, somewhat limited to widefield and medium widefield where they do best.
I can appreciate the numerous advantages of both reflectors AND refractors and that these differ with focal length and that, as has already been stated it is a matter of Horses for Courses, .......
BUT, Please. Please don't castigate me for this..... As beautiful as I genuinely find many of the images from reflector telescopes, due to their large aperture and the low noise capabilities they open up, I find the diffraction spikes, from those with spiders/etc, highly distracting. Once my eyes picks one up, I just look for more. To me it makes the image much less natural, dare I say, fake. I would take a little Chromatic Abberation over diffraction spikes (in fact a sharp spike can sometimes contains contains some by way of a spectrum, depending on the sharpness of focus/target).
As an aside-
To the expert image processors out there in the community - Is there a Diffraction Spike Reduction/Elimination tool out there ? ( Similar to the Red-Eye Tool in portrait photography). If not, Software Engineers - there is your next $1,000,000 money-making project - In the software, hover an adjustable diameter target circle or target 4 spike tool over the 4*-spiked diffraction spiked star and select "DESPIKE", which could have a programmable strength/size parameter.
(*- or more, or less depending on spider)
Having said all that I look forward to purchasing/building a reflector scope one day. I think it would use a 90 degree 2 leg spider to minimize spikes. A 1 legged spider looks even more unnatural - at least on the various simulations I've seen.
The context for the article on CN was about finding a wide field telescope so I assume that's why the focus is on the shorter focal length refractors. I liked the 'jounrey' through the testing process but I am still relatively new to it all so it doesn't sound too blantantly obvious, as it might to others :-)
I think his post came out of another discussion in the CN-EAA forum that I visit. It was all about wide field EAA and whether an 80mm apo or a small reflector was better. But the comparison was to a pretty poor reflector.
Just about the only reason to buy a refractor is because you want and like refractors. My 12"dob easily outperforms my Televue 101 and is less than 20% of the price. But I wouldn't be without the Televue.
Geoff
SDM Size Does Matter.
The biggest refractor I have ever looked through was a 6",cant remember
What make it was,(probably a Tak?) but it didn't hold a candle to my 16' Newtonian.
Reading his article and the comments, to me it came across as a lot of money for a small increase in sharpness.
Cheers
Well, in another life I was a professional astronomer in South Africa and the observatory had a 26.5" refractor. Great instrument. One night with superb seeing I was looking at Mars with 1000 power. Absolutely amazing clarity and detail. It's something that I'll never forget.
Geoff
Well, in another life I was a professional astronomer in South Africa and the observatory had a 26.5" refractor. Great instrument. One night with superb seeing I was looking at Mars with 1000 power. Absolutely amazing clarity and detail. It's something that I'll never forget.
Geoff
A rare and costly instrument, again one would expect that sort of crispness for a scope without obstructions.
It would have been a great experience.
Remember it was a scope just a bit bigger than that,that Lowell saw his canals.
Cheers
Worms mate ,,, can's of worms ....
Or a well collimated and cooled C9.25 .
