Quote:
Originally Posted by Merlin66
For more than 300 years after the invention of the telescope, the choice for refractors was basically limited to Achromats.
All the discoveries made during this time were made with Achromats.
It's only in the past 60 years that multi element, exotic glass has been available and used to produce the "APO" type telescopes of today.
The long and illustrious tradition of the Achromat should not be forgotten.
Bang per buck they can still do very useful work.
My 2c
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The achromat is perfectly fine if the focal ratio is f15 or greater and that is what it was for most of those 300 years. Short refractors (f5) have become popular in recent years but to truely appreciate what they can offer one needs such a short refractor to be an APO. There are many middle ground instruments between f5 and f15 and some are excellent around f9, e.g. TV-102 or similar quality instrument as these use exotic glasses to help reduce the chromatic aberration.
I have an ED80 (f7.5 with ED glass) but don't use it as much as my little Tele Vue 60 (f6) which provides me far more enjoyment. I bought my son an ST80 (f5) and for wide field low power it's great but for planets it has far too much chromatic aberration to be enjoyable and he prefers the view though the ED80 and TV-60 or Newtonian or SCT that I have.
The very expensive APOs with quadruple elements such as the NP-101 offer the best of both worlds but at a high price. You get a flat widefield for visual and photographic as well as high power views of planets that show no signs of chromatic aberration. Previous generations of the 101 (TV-101, Genesis) can be found on the used market and they can represent great value. There are other brands such as Takahashi which seem to pop up on the used market often.
It all comes down to what you enjoy viewing and if low power wide field then a short achro is great. If you prefer planetary viewing than a longer achro is preferred. If a long achro is too long and heavy for your mount then a shorter achro with exotic glass will be fine. I also have a TV-101 and use it more than my C9.25 but I use my TV-60 more than my TV-101 simply because it is more convenient and provides stunning images of planets in the evening twilight which is when I usually observe them (when they're up of course).
Note though aperture still rules and whilst the smaller refractors provide stunning high contrast images at the drop of a hat, my C9.25 and more so my 10.1" f6.4 Newtonuan with premium mirror trumps them all for planetary detail and on a superb night this is my preferred instrument. However superb nights are few and far between and on most nights (except those with very poor seeing) the refractor usually provides the more enjoyable view and the high contrast they offer due to the lack of central obstruction should not be underestimated.
If you can get to a star party and look through a few different refractors then you will see for yourself and if you find enough enjoyment from a lower cost instrument then that's more you can spend on eyepieces.
Whatever you choose, enjoy!