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what size refractor comparable to 8" SCT?


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I'm curious to know what size refractor would be considered comparable to say an 8" SCT - at least in terms of the potential viewing capability and assuming one were using similar EP's in each.

I know there is a whole separate debate about the viewing quality of each type and there are obvious distinctions such as a much wider FOV with refractors. However, ignoring these issues, I'm just curious whether the refractor that is closest to an 8" SCT would be so large as to render it ridiculously heavy & expensive. If my thoughts are correct, presumably this is where Newtonians come into their own, in terms of managability & cost at this size.

Some informed views welcomed.

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For deepsky you would need at least an 6" refractor to match an 8" SCT.

For the planets a good 4" refractor would challenge an 8" SCT and a good 5" refractor would beat it.

An 8" Newtonian would beat an 8" SCT as it has better cooling properties and a smaller secondary obstruction.

If you want a good all rounder then the Newtonian design cannot be beaten - cheap too.

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I don't think it is as simple as saying 'matching,' really. I feel sure that I see more planetary detail in our TEC140 than in the Meade 10 inch SCT. But it may just be that the 'look' is sharper and more contrasty. Very hard to say but I would always haul out the TEC for the planets.

On the deep sky it is all about pleasure, for me. In the TEC you have near perfect optics and it shows. The stars are tiny, nebulae are fainter than in the SCT but more contrasty. The view in the TEC is exquisitely beautiful and that's about it, really. I don't think you see more, I just think the view is more gorgeous. Not very scientific but does that matter?

The TEC is far from ridiculously large, it is quite compact at F7 and only weighs about 7Kg. Compared with a 6 inch Takahashi it is positively mucky cheap, but at 6000 dollars all things are relative!!

In DS imaging I very much doubt that an 8 inch SCT would match a premium 5 or 6 inch apo but a well collimated MN190 is another matter. That does put the cat among the apo pigeons as I freely admit. It is more diffucult to use but optically it is astounding.

Olly

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I don't think it is as simple as saying 'matching,' really. I feel sure that I see more planetary detail in our TEC140 than in the Meade 10 inch SCT. But it may just be that the 'look' is sharper and more contrasty. Very hard to say but I would always haul out the TEC for the planets.

On the deep sky it is all about pleasure, for me. In the TEC you have near perfect optics and it shows. The stars are tiny, nebulae are fainter than in the SCT but more contrasty. The view in the TEC is exquisitely beautiful and that's about it, really. I don't think you see more, I just think the view is more gorgeous. Not very scientific but does that matter?.....

I agree - the views though a quality medium to large aperture refractor are immensely satisfying :)

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I owned four different C8s over a dozen years or so. For planetary observation the Televue Genesis and later a really good four inch fluorite doublet (Takahashi) I also owned provided much more attractive images. However the best of the C8s, properly collimated, and used on nights when the seeing allowed the use of a the bigger scope, actually showed a little more detail, albeit not easy to detect. The Takahashi fluorite scope in particular had great clarity and contrast; the Celestrons painted the universe in middling tones of dinge and murk.

The deepsky also showed the four inch refractors outperforming expectation, while the C8s with their signature grey backgrounds and oddly imprecise focus almost always disappointed. Yet for planetary imagers the best of SCTs can produce marvellous results : as witness the masterpieces (is that too strong a word? I don't think so) of Damien Peach and the very fine work of Christopher Go in the Phillippines.

It's also worth saying that a good refractor destroys any other design when it comes to double star observation, another interesting field that gets too little discussion these days. But all this optical lusciousness (note that copy writers always claim 'refractor-like' images as their highest accolade) comes at a huge financial premium.

Fifty or sixty years ago J B Sidgwick (Observational Astronomy) advised enthusiastic but financially prudent amateur astronomers to invest in a sensibly proportioned and well-mounted Newtonian. That really remains the best advice; but there will always be a few hardcore refractor fiends for whom no sacrifice is to great. A nice example of Roland Christen's superlative 160mm refractor has just changed hands for $15,000!

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Hi Tony,

As some have mentioned here, a good refractor can give much better images than an SCT of more than double its size. I've never seen a 10 or even 12-inch SCT that will beat my 133mm f/12 Apomax for lunar and planetary detail.

That being said, it is a 7-foot monster that tips the scales at over 22 kg and it takes a very large and expensive mount to handle it at all. Still, once the big fellow is properly mounted, there are very few scopes to compare with it. A truely wonderful scope, but it was hand made (there are only 23 of them) and expensive. I literally couldn't replace it today even if I wanted to. I had it appraised recently for insurance - they said the value was about $12K - but that was for a comparable replacement from Tak or Zeiss. I actually don't know how 'comparable' they would be - and I'll never sell my Apomax.

None of my 3 sons (or so far my grandkids) are that interested in astronomy, so I've left instructions to have it donated to my Alma Mater when I'm gone (hopefully in 30-40 years from now!!) If I can afford to give the College a dome to put it in, they may even name the observatory after me! :)

Dan

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Thank you to everyone for your insightful comments. There is a specific reason for my question. Namely, I am in the very fortunate position of having been left some money - £2,500 to be precise. It came with a request that it not be squandered on financial investments, but used wisely on fast women/loose cars etc :). In fact, the benefactor suggested I indulge myself in my love of astronomy and I cannot think of a more fitting tribute to them but to invest in a decent bit of astro kit.

Until recently, I had a Nexstar 8SE, though I had issues with the vibrations and had therefore been saving my pennies until I could stretch to a CPC800. The priority for me was (relative) ease of set up (the alt/az mount) with some portability (though mainly used in back garden) and with my viewing concentrated on planets, though I know this will mature over time to include more DSO's. My 10 x 50 bins keep me busy for the moment, but I would like to return to being a scope owner.

This unexpected gift has given me a rather nice dilemma in what to go for. Having alway discounted refractors because of the cost per aperture issue, my knowledge of refractors is rather limited, hence my question about their capabilities in comparison to say an 8" SCT. I will probably end up (in time) with 2 different scopes (refractor & SCT/Dob) taking advantage of their different strengths, but I need to try & roughly allocate out funds accordingly. AP is not an issue at the moment and may never be. I've looked at the Skywatcher Evostar 150 and the 120ED DS Pro. Both look accomplished but wonder if they are difficult to set up/balance etc. If you were to be given £2,500 to spend on a refractor & EQ mount, what models might you consider?

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Ooooohhh, the vicarious pleasure of spending someone else's money!

Takahshi or TeleVue 4 inch or somethng a little less outrageously high quality but larger. A matter of taste. I would try to get to a star party and see what really appeals to you.

Olly

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I'd agree with David (Dweller), that would be a superb rig. I'd also suggest a look at the Meade 127 Triplet..a friend of mine has one and loves it to bits...and at F7.5 like the Equinox it could be used for AP to good effect if you ever want to. The Meade is heavier though, but does yield about 12% more light than the equinox, so will do better on deep sky.

Decisions, decisions...what a lovely dilemma to have. Good luck with your shopping.

By the way, just out of left field, you could consider a good quality Maksutov. A good Mak will kill a normal SCT and give a refractor a good run for its' money too. My Lomo 6" beat a Skywatcher 8" F5 Newtonian I had (itself a good scope) hands down. Image quality through a Mak is very good too, and contrast is nearer to a long focus Newt than an ordinary SCT.

HTH

Dave

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I think 127 vs 120 is a non-issue, but a doublet achromat vs. a triplet Apo is a definate JUMP in image quality (and photographic quality as well.) Especially in the shorter focal lengths.

I have a student who just took delivery on a Meade 127 triplet - and it is absolutely fantastic. Highly recommended.

That said, if the SkyWatcher 150 refractor is available in your area for less, I would consider it carefully.

What someone else said here about getting out to the local club to see some examples of nice refractors is important. It is very difficult to describe the experience at the eyepiece with different scopes; it's rather like writing about fine wine, I guess. Much better to be appreciated in person than described. :)

Dan

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  • 4 years later...

The Chinese 127 f7.5 triplets are excellent, a Meade etx125 good but behind. For visual use - especially just for occasional pleasure - a 4" triplet refractor remains the best choice giving high image quality, transportability and robustness. On the other hand if you want a light bucket for spectroscopy the story would be very different ....

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"An 8" Newtonian would beat an 8" SCT as it has better cooling properties and a smaller secondary obstruction."

 

This I have to disagree with based on my own experience.  A lot of people I observe with have 8" reflectors and the views through them are no better on DSOs and certainly not as clear as I have seen on planetary on my C8.  Granted these are not premium mirrors, which I suspect would give the C8 a run for its money if not beat it, but most people do not buy the premium mirrors but get Skywatcher standards.

 

Now I cannot confirm any detail on collimation of the reflectors, but I know my C8 is well collimated and cooled down properly before use.

 

The bigger secondary of the C8 would/should degrade viewing but it certainly has made no difference to me when comparing SCTs and reflectors visually.
 

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I've looked through a 120mm refractor and a 140 Mak with 1/8th wave optics. While both had much better contrast than my C9.25, neither came close to matching it for detail.

The Mak would easily give better views on double stars (except for resolution) due to it's 1/8th wave optics - yes, it does make a difference ;)

If I had a choice for planets, and I had the space, a 10" f8 Newtonian with a undersized secondary and optics of 1/8th wave or better, would spank the lot :)

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"An 8" Newtonian would beat an 8" SCT as it has better cooling properties and a smaller secondary obstruction."

 

This I have to disagree with based on my own experience.  A lot of people I observe with have 8" reflectors and the views through them are no better on DSOs and certainly not as clear as I have seen on planetary on my C8.  Granted these are not premium mirrors, which I suspect would give the C8 a run for its money if not beat it, but most people do not buy the premium mirrors but get Skywatcher standards.

 

Now I cannot confirm any detail on collimation of the reflectors, but I know my C8 is well collimated and cooled down properly before use.

 

The bigger secondary of the C8 would/should degrade viewing but it certainly has made no difference to me when comparing SCTs and reflectors visually.

I had did a side by side comparision bwtween my Orion Optics 8" F/6 Newtonian with 1/10th wave optics and a 22% central obstruction and my Faster based C8 with XLT coatings. The 8" Newt and the C8 were the same price. On deepsky there was nothing in it. The Newtonian gave slightly sharper planetary views and revealed slightly more low contrast details. The Newtonian cooled down long before the SCT, did not dew up and and matched the ambient temperature drop as the night went on. There wasn't much in it but the Newt had the edge. I really wanted the SCT to win as I find using Newts uncomfortable but the best views are what it's all about.

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I had did a side by side comparision bwtween my Orion Optics 8" F/6 Newtonian with 1/10th wave optics and a 22% central obstruction and my Faster based C8 with XLT coatings. The 8" Newt and the C8 were the same price. On deepsky there was nothing in it. The Newtonian gave slightly sharper planetary views and revealed slightly more low contrast details. The Newtonian cooled down long before the SCT, did not dew up and and matched the ambient temperature drop as the night went on. There wasn't much in it but the Newt had the edge. I really wanted the SCT to win as I find using Newts uncomfortable but the best views are what it's all about.

I can believe all of that with 1/10th wave optics - but you pay more for that than the average person who has purchased an off the shelf OO or Skywatcher, and I also accept that an SCT is more expensive than both of these standard reflectors.  If you are going to upgrade the optics then yes, it should be better.  If you are not then I would go for the SCT every time.  Plus as you say it is amore comfortable viewing experience, and a much smaller package to store between sessions.

When I upgrade my OTA it will almost certainly be to a C11 rather than any other design.

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Nice problem to have :)

I would broadly agree with the comments made above. I have a Tak FC-100C (a fluorite doublet), a Vixen 150ED and an 8" EdgeHD SCT.

For doubles I would choose the refractors every time for all the reasons already given. The SCT is just not as precise in its focus, star shapes aren't as well defined and I have been able to split sub arc second doubles with the 6" but not the SCT (so far)

I've not had the SCT long enough to have much time on planets, but the few views I have had show more detail and contrast in the 6" than the SCT. The Tak is pretty close too, especially under less good seeing conditions.

For DSOs the SCT wins out generally through aperture. On dimmer stars the difference in star shapes is less pronounced and fainter nebulae definitely benefit from the extra light grasp.

The balancing issue for me is portability. I live near Heathrow......... I can take my Edge away on holiday with me to darker skies in the UK and use it on a manual alt az mount side by side with the Tak and have great views. The Vixen is f9, 1350mm focal length and weighs somewhere around 20kg fully loaded. There is no way I can fit it in the car with everything else required for a holiday (without leaving a child or two out!)

I used to have a 106mm triplet which was a lovely scope. I've since moved to using ED or Fluorite Doublets as I am purely visual. The cool down time is less, they are lighter and have slightly better transmission and contrast for visual.

As a generalization, with a 6" Apo scope I would expect you to see closer doubles, comparable if not better planetary detail and the views you get will be beautifully sharp and contrasty. The SCT will go deeper for DSOs and be lighter and move convenient to mount, but is fussy on collimating, cooling, needs a dew shield and likely a dew strap too. I fitted a second focuser on the visual back as I do not like mirror shift when focusing.

I would suggest something like a 5 or 6" ED Apo refractor although even these are very different animals from a mounting point of view. I have a 120ED which is a lovely scope too, at 4.7" f7.5 it is dramatically smaller and lighter than the 6" f9.

A Tak FS128 would be pretty special I reckon but I've no idea what they go for. Pretty rare too but a 130mm ish ED doublet would be nice.

Last point on mounts. Is there a particular reason to move to an EQ mount? For visual I find AltAz much more convenient, so something like an AZEQ6 (which I use in AltAz mode) gives you goto and tracking with the benefit of being able to switch to EQ if you end up imaging. An AZEQ5 would probably suffice for a 5" doublet.

Normal advice also applies, if you can try to see and look through some of these scopes first to get an idea of views and size/weight for yourself before committing.

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Location is also an important factor. I've had better lunar and planetary views with my Tenerife based C8 than with any refractor in the UK. :smiley:

Excellent point Peter, that's half the problem in the UK, seeing conditions often suit smaller scopes at high power so we rarely get that combination of higher resolution views with steady seeing :(

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The UK can have good seeing conditions.  The key is being out there to take advantage of them.  Just look at the fantastic results posted on this forum from the last Jupiter apparition - the vast majority taken with well collimated SCTs. 

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I've found that the Chinese 8" newts can rival the views that an 8" SCT gives. There is some variation in quality but Chinese mirrors are at least as good as Celestron / Meade SCT optics, on average, I reckon. Based on the examples of both types that I've owned.

It's a bit off topic from the original question but that was posted over 4 years ago now !

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The UK can have good seeing conditions. The key is being out there to take advantage of them. Just look at the fantastic results posted on this forum from the last Jupiter apparition - the vast majority taken with well collimated SCTs.

SCTs are definitely the scope of choice for planetary imaging where the best frames can be selected from thousands, and contrast boosted with post processing, but I think given the variable conditions for visual things are far less clear cut.

Of course, when everything comes together ie cooling, collimating, seeing etc an 8" SCT will deliver great results, but those things aren't always easy to bring together. Refractors are more robust in terms of the collimating side. Easier on the cooling and with smaller aperture are generally less affected by poorer seeing whilst having great visual contrast.

To John's point, I also think Newts are easier to cool and probably a little less fussy, or perhaps just easier to collimating so will tend to give better views unless in expert hands. I'm sure cooling issues are responsible for many of the 'characteristics' discussed around SCTs such as slightly fuzzy stars and less distinct diffraction patterns.

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Some good points and I would not overly dispute many of them, but many SCTs owners criticise their scopes without ever really understanding their requirements in terms of cool down and collimation. Personally I've used most kinds of scope - but always happy to get back to my couple of Cats.   Given a choice between a five inch refractor and say a C8 Edge I'd take the latter every single time as an excellent all rounder.  Visually when it does come together the planets can be stunning.

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