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Upgrade from 80mm to 100mm refractor- is it worthwhile?


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Depending on seeing conditions, how dark your site is, and your targets, it could well be a worthwhile upgrade. A dark site helps to bring the best out of the scope so that will yield the most improvement from the extra 20mm. The improvement will be most noticeable on the large planets, particularly if seeing is very good. Deep sky will show some improvement, especially if the target was only just visible in the 80mm. Clusters will resolve more clear stars, but globular clusters are generally still going to remain only partially resolved.

Hope this helps,

Ant

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Agree with Ant, it will be a worthwhile and noticeable difference if your skies are not bad. The ED80s and 100s are both very good scopes. Arguably a good 80mm ED will be better than a so so 100 achromat.

As it happens, I have a superb 80mm Pentax achromat which gave me more pleasing views of double stars than my ED100, which is why I sold it..but I think that is the exception rather than the rule.

HTH

Dave

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I think I have reasonable skies- I'm able to see both Leo galaxy triples as well as many of the Virgo galaxies around this time of year. Globulars hint at resolution, and most memorable view is probably the Veil nebula (with OIII filter) last summer.

I tend to struggle with planets, have seen some detail on Mars at last opposition, and a few Galilean transits in the last Jupiter opposition.

I used to have a 10inch dob, but have found the ZS80 to be so much more convenient and easier to use that I sold it last year. My thinking is that a 100mm refractor is about the limit for me in terms of convenience, aesthetic views and maintains the ability to see all of M31 or the Veil in a single view.

I'm considering a Revelation (F6) or Ikharos (F7) to keep the wide field options open.

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I like 4 inch apos. I sold my nice old Genesis and do miss it as a visual widefield. I'd want to keep the FL short enough, though, to get those classic 'whole Veil' or 'whole Rosette' views, though this might slightly compromise the planetary performance.

Hard to say what the extra light/resolution really amounts to. I can still see the whole Veil or Rosette in the 70mm TV Pronto I bought to ease the pain of losing the Genesis. You do get half as much light again in the 100 over the 80.

Olly

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This is my experience going from a ST80 to a ST102. The biggest jump in performance I found was when viewing DSO's at 80 - 100x magnification. Using a 5mm plossel the view with the ST80 at 80x mag was to dim for my liking and looked like an unremarkble smudge. While using the same eyepiece with the ST102 at 100x mag. the view was brighter and more pleasing to the eye. More detail was resolvable and the DSO's shape could be seen.

Lower power DSO's and higher power lunar and planetary view were marginly better. Bear in mind I was using ST achro's so I do not know how this would translate to 80ED Vs 100ED

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Hard to say what the extra light/resolution really amounts to.

Olly

Surely arithmetic can easily nail what the extra light amounts to, in terms of detecting objects. Using a nominal 7mm as the diameter of the dilated pupil, we can express the light grasp of the 80mm as 131x the grasp of the unaided eye, and 204x as the light grasp of the 100mm. The difference in light between objects of one apparent magnitude is 2.512 (Pogson's constant, the fifth root of 100). So our 80mm scope will enable us to see objects 5.3 magnitudes dimmer than the naked eye, and the 100mm 5.8 magnitudes dimmer. That's the difference between Rhea and Tethys (and Iapetus and Enceladus, but they'd be tough targets in any four inch!). So the four inch can buy us satellites and stars half a mag. dimmer, and a bucketful of similarly dimmer faint fuzzies.

Simples, as we meerkats say.:)

Edited by neilmack
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Surely arithmetic can easily nail what the extra light amounts to, in terms of detecting objects. Using a nominal 7mm as the diameter of the dilated pupil, we can express the light grasp of the 80mm as 131x the grasp of the unaided eye, and 204x as the light grasp of the 100mm. The difference in light between objects of one apparent magnitude is 2.512 (Pogson's constant, the fifth root of 100). So our 80mm scope will enable us to see objects 5.3 magnitudes dimmer than the naked eye, and the 100mm 5.8 magnitudes dimmer. That's the difference between Rhea and Tethys (and Iapetus and Enceladus, but they'd be tough targets in any four inch!). So the four inch can buy us satellites and stars half a mag. dimmer, and a bucketful of similarly dimmer faint fuzzies.

Simples, as we meerkats say.:)

Indeed, but that's not what I meant. What I was (not very coherently) saying, is what does an X percent increase in light grasp really look like, subjectively? The 70mm Pronto shows the whole Veil so that you can see it. The Genesis did likewise, a bit more brightly. But how much is 'a bit.' I simply don't know. I know that light grasp goes as the square of the aperture and that an exit pupil greater than 5mm is no good to me but it is hard to know how the subjective view is affected by all this.

Olly

Edited by ollypenrice
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Life, as they say, is a compromise!

I have a 4" refractor (achromat) and an ED80. I can "see a lot more" with the 4", despite its slower f ratio, on very clear nights (there aren't many in S. Oxon), but it is a lot more cumbersome to set up and carry around - so my ED80 gets a lot more use, as I can get it out and set it up quickly and on 9/10 nights, the smaller aperture is not apparent.

I have a professional mount for a 16" fork 'scope in my garage, with 12" high precision 360 teeth worm drives, but it is unlikely to ever acquire an OTA worthy of it, simply because there are too few nights with really good viewing here, and every time I blink, another light or house appears nearby to ruin my view even further! I could of course move back to someone darker....

So, even though we all suffer from aperture fever, reality (at least in the S of England) might indicate that smaller, lighter scopes might often be a better compromise for most of us??

Chris

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Thanks for all of the above opinion. I guess I aspire to a Genesis (it's a hangover from reading Sky and Telescope ads in the 1980's) but will look for a compromise in another 4 inch refractor. I wonder whether I have the skies or observing skill to justify it.

I'm trying to balance the convenience of quick setup and refractor clarity with a degree of aperture fever that can only be sated by a large dob!

I doubt anyone can really help me decide if it's worth it, but the fact that I've asked the question makes me think, I'll keep looking for a bit longer, and see if something turns up.

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