Author Topic: "Refractors are best for planetary viewing"  (Read 45 times)

Joe Hall

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"Refractors are best for planetary viewing"
« on: December 25, 2017, 02:23:04 PM »
I see statements like that all around the Web, on reputable websites.  The justification usually given is that, because of the fact that refractors do not have obstructions, and because lenses do not shed nearly as much lighting as mirrors, refractors waste much less light and therefore have higher sharpness and contrast.  That means being able to see more detail and also a higher quality image overall.

Though the above is accurate, it's also a fact that a scope's theoretical resolving power is directly tied to aperture.  I have also heard it said that additional scope layouts can make up for the lost light through brute-force, with a bigger aperture.

So this makes me wonder if that widely repeated claim about refractors being better for the planets and Moon is accurate or good advice.  If you had to pick between the Celestron 6" achromat and the Celestron C8 SCT, which is it?  Are refractors just better at the same aperture, or are they the best buy?



boysagiskest

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Re: "Refractors are best for planetary viewing"
« Reply #1 on: December 29, 2017, 05:30:54 PM »
The refractor design does have several advantages that make them very good for planets, assuming you can deal with CA through ED glass, F ratio, chromacor or at least a minus violet filter. Clear aperture for maximum contrast, use of lenses instead of mirrors for less light scatter (especially if you use a prism diagonal ), they hold collimation extremely well, cool down fast, and can make the most of subpar seeing conditions. Light transmission is probably more beneficial on faint DSO than planets, but yeah that is a benefit of refractors. Keep in mind with an achromat their is some light lost since not all the light comes to focus, so refractors may not be as efficient as they seem.

All that being said, it tends to all be relative to aperture. I have two good planetary scopes. An excellent mass produced Chinese 6" Mak, and a premium Japanese 4" Apo. Other than a much more narrow field of view, the Mak has a clear advantage on deep space. But the Mak also performs as well or better on moon and planets. I would guess the Mak is 90-95% strehl and the frac is probably more like 95-97% strehl. So both very good, but the refractor is probably slightly better optical quality. And the refractor is the better design for viewing planets. But the Mak has more aperture, allowing it to provide just as nice, if not a touch nicer, views of the moon and planets. Assuming the Mak is cooled.

Think of it this way. You can win a lot of small battles, but still lose a couple major ones and lose the war. Lenses versus mirrors is beneficial for planetary viewing, but it is a pretty subtle difference. Same with more efficient light transmission. Clear aperture makes a more significant difference, and aperture makes a big difference. Is my 4" F6.5 achro a good planetary scope? Uh, no. Is my 4" premium Apo a good planetary scope? Yes. But a good quality 8" SCT would probably easily best it despite SCTs not really being known as planetary scopes. Assuming good seeing conditions, precise collimation and a cooled scope. So in some ways what makes refractors good planetary scopes isn't so much that they will always give the best views, but simply that you don't have to worry much about seeing, collimation and cooling.

Scott

bamrocorna

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Re: "Refractors are best for planetary viewing"
« Reply #2 on: January 09, 2018, 08:09:13 PM »
Quote
If you had to choose between the Celestron 6" achromat and the Celestron C8 SCT, which would it be?
Both are not particularly superb on the planets, but my choice would be the 6" achromat. I've actually tried both side by side and the achromat was a much more reliable performer. The C8 *could* give better performance, but usually didn't and was very fuzzy most of the time, even when I paid great attention to cooldown and collimation. Others may have different experiences. Now I use a far cheaper 6" f/8 newtonian that soundly trounces them both on the planets. A good 8" f/6 newtonian would be vastly superior to both the 6" f/8 achromat and the C8.Clear skies!
Thomas, Denmark

Manuel Ghumare

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Re: "Refractors are best for planetary viewing"
« Reply #3 on: January 10, 2018, 10:22:16 PM »
I'd say that depends on what "best" means by the OP.

Aesthetically speaking, for lunar and planets visual, I definitelyprefer my 3" ED or 5"APOto6" Newt or 8" CAT underaverage seeing. Yes, even the 3" ED! That'sfor aesthetic reasons because high power of the 8" CAT is out of the game anyway. Thehigher contrast image from a smaller aperture (<5") is still a pleasant one to me.

But, when seeing is excellent, the 8" CATdoes give more resolution and brightness enough to outweigh that aesthetic experience. Also the 8" CAT nearly always beats the others on DSO.

So it all depends on where, when you are using the scope to give you the best experience you like.... but not everyone cares about the aesthetic aspect of a refractor image. YMMY.

IMHO, refractors are very user friendly andmorelikely to providethe most stable performance.

Clear skies,
Samuel

Chaudhari Evans

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Re: "Refractors are best for planetary viewing"
« Reply #4 on: January 13, 2018, 07:19:04 AM »
Refractors are really nice and do wonderfully on planets and the moon. I had my 115mm ed triplet out tonight and had a lot of fun on the moon and some nice clusters. For fun, used exit pupils down to 0.4mm on the bright moon before conditions went south.

But the best planetary I have seen was Jupiter on a great night with a 16" Dob.

Of the two scopes you mention and from your urban location there is no question I would go with the SCT. It is more flexible. It does a very good job on planets - best Uranus I've seen was via another's C8 at an outreach event in town. They're great onthemoon.

But they're also very capable DSO scopes, something that's more difficult to say about a refractor from a bad location.

THE best is the one you have out on a perfect night. Conditions rule.

Darkz Tousa

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Re: "Refractors are best for planetary viewing"
« Reply #5 on: January 15, 2018, 11:31:41 PM »
As you can see from my sig line I have both SCTs and Refractors. I'm a SCT guy at heart but in the last 4 years I've acquired a 4" and 5" APO refractors. I find the view thru the refractors to be very good but it takes short FL eyepieces to get a decent image size in the eyepiece. I like the largest image in the eyepiece that I can get without losing image quality. The refractors do give me darker contrast in my light polluted skies! The best views I've had of the planets and DSOs have been with my Meade 2080 8" SCT and my Meade LS8 ACF 8" SCT. I have a used C9.25 and haven't had a chance to try it out on anything but the Moon and it does and excellent job on it. Now that fall is here we should get some very good skies in between storms!! For me the refractors and SCTs are used for different objects and each has it's own niche. But I do prefer SCTs for the planets and DSOs. To each their own!!!!

Cory Bass

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Re: "Refractors are best for planetary viewing"
« Reply #6 on: January 17, 2018, 12:14:44 AM »
Quote
I see statements like this all over the Web, on reputable websites. The justification usually given is that, due to the fact that refractors do not have obstructions, and because lenses do not lose nearly as much light as mirrors, refractors waste much less light and therefore have higher contrast and sharpness. That means being able to see more detail and a higher quality image overall.

While the above is true, it is also true that a scope's theoretical resolving power is directly tied to aperture. I've also heard it said that other scope designs can make up for the lost light through brute-force, by having a larger aperture.

So this makes me wonder if this widely repeated claim about refractors being better for the planets and Moon is true or good advice. If you had to choose between the Celestron 6" achromat and the Celestron C8 SCT, which would it be? Are refractors only better at the same aperture, or are they the best buy?


As others have said, this is a complex issue. As someone who loves both reflectors and refractors, here'sfew thoughts:

- The central obstruction issue is relevant when comparing telescopes of similar apertures, it does affect the contrast to some extent but refractor advocates tend to ignore the importance of aperture which is the main player(besides optical quality and seeing)in terms of planetary contrast.

- Sharpness is a difficult issue to define. Is it resolution? Certainly a larger aperture scope has better resolution than a smaller aperture scope.

- In getting the good planetary views, the biggest player is the seeing, the stability of the atmosphere. If the seeing in your location is typically on the poor side, a refractor is probably a better choice. This is because of the thermal stability of refractors. If the seeing in your location is typically good to excellent, a larger aperture scope, and that means a reflector,will likely be the better scope for viewing the planets.

-In my situation, I have a very nice 4 inch apo, a very nice 120mm F7.5 ED/apo and several larger aperture Newtonian/Dobsonians.The seeing here tends to be on the good-excellent side of things. Within the limits of their apertures, the refractors provide very good planetary views but they are limited by their aperture. My $240 on Astromart 10 inch F/5 Dobsonian provides more detailed, crisper planetary views than the refractors.

- Reflectors require more tinkering, more care and attention than refractors. My 10 inch Dob requires more than an hour with a sealed back fan cooling the scope to reach some level of thermal equilibrium. To get the really good views, I had many nights this summer where Saturn, Mars and Jupiter were clean and crisp at 410x, it might take 2 hours if the indoor-outdoor temperature difference is significant. The scope must be accurately collimated as well.

- In comparing a 6 inch F/8 achromat with an 8 inch SCT, there are a number of factors to consider. A 6 inch F/8 achromat has a significant amount of chromatic aberration and that does definitely reduce the planetary contrast. Filters can help a certain amount but one is always fighting against the defocused image of a fast achromat.The SCT has issues as well, they take a while to cool and the central obstructions are quite large and with a relatively small aperture difference, it is an important player.

In comparing the two, I will simply say that neither would be my choice, I think my 10 inch Dob, less expensive than either the SCT or the achromat, butit's abetter planetary scope.

Jon

Richard Acosta

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Re: "Refractors are best for planetary viewing"
« Reply #7 on: January 17, 2018, 12:22:46 AM »
my best view of Jupiter was a 6" refractor
my best view of Saturn was a 10" Mac
my best view (and only) of Pluto was a 22" dob
my best view of comets was with binoculars
my best view of meteors and Northern Lights was naked eye

as stated above-it is complicated

edj

cytiwitqua

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Re: "Refractors are best for planetary viewing"
« Reply #8 on: January 18, 2018, 02:41:49 AM »
If all else was equal, Refractors would probably be best. But all else is never equal. I'd get the 8" SCT, I feel like it's going to give similar performance on planets but be better at many other things.

plethenofin

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Re: "Refractors are best for planetary viewing"
« Reply #9 on: January 18, 2018, 06:17:36 AM »
A well-cooled Mak or Newtonian can beat a refractor any day of the week IMO. I've never seen Saturn's shadow on it's rings with an APO but I can do it with my ETX-90.

Kenneth Naim

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Re: "Refractors are best for planetary viewing"
« Reply #10 on: January 20, 2018, 07:55:42 PM »
Quote
my best view of Jupiter was a 6" refractor
my best view of Saturn was a 10" Mac
my best view (and only) of Pluto was a 22" dob
my best view of comets was with binoculars
my best view of meteors and Northern Lights was naked eye

as stated above-it is complicated

edj

I like this answer. Different tools for different jobs.

There isn't one perfect scope that does it all, which is why people wind up with two or three (or 10) telescopes.

I'm a refractor guy. Love my 6" f/8 achro. But it is a big scope on a big mount, and sometimes I'm not up for all that. Which is why I grabbed the MUCH smaller 6" SCT.

Between a C8 and a 6" achro, that's a tough call. In most fights aperture wins. The C8 sure would be more compact and easier to mount, and it has no CA. CA is a personal thing though... some are bothered by it, some aren't.

buckfeedssapfai

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Re: "Refractors are best for planetary viewing"
« Reply #11 on: January 22, 2018, 09:51:32 PM »
The best view of Saturn and the moon were through a well collimated Meade 8" SCT ACF Cat. I compared the moon with a 5" APO and 6" achromat and while both refractors put up good views of the moon there was much more detail in the SCT. Saturn through the SCT looked like a photo from an astronomy book. I do have to add that seeing was excellent on that night.

Anthony Cejudo

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Re: "Refractors are best for planetary viewing"
« Reply #12 on: January 23, 2018, 05:02:51 AM »
While I use a refractor for planets and lunar, its more of a convenience, especially for cool down....but for more serious viewing, the refractor gets dobsmacked....4" just can't compete with 10".

anficonco

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Re: "Refractors are best for planetary viewing"
« Reply #13 on: January 25, 2018, 07:17:20 PM »
Interesting answers so far - I did not expect opinions to be so mixed. I suppose I should lay all my cards on the table at this point. I have had the opportunity to look at Jupiter through a 10" Dob, a 4" achromat, and an 8-10" SCT. I own the 4" achromat, the 10" Dob is owned by a family member, and I used the SCT at an outreach event and had no control over it. I honestly did not like the image from the SCT, it was suffering from the bad seeing conditions, and the image was definitely much "softer" (not very bright) compared to the Dob and achromat. The 10" Dob seemed every bit as bright and sharp as my achromat, but I can't say I noticed any new details.

As for my personal considerations... I am ruling out Dobs for convenience reasons. I am very happy with my 4" achromat, except for the lack of detail of course. That goes for DSO's too, where brightness also seems to be a big problem. I would like any second scope to have GEM or GoTo to track objects. I have seen several mentions of Maks, which seems interesting. Would an 180mm MCT like the Orion Skyview Pro be brighter than an SCT like the C8? It could be that my bad first impression of SCT's is making them hard to accept.

Christopher Bryant

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Re: "Refractors are best for planetary viewing"
« Reply #14 on: January 25, 2018, 10:14:38 PM »
For me personally my Tasco 10 TE (76mm) gives fantastic views of the bright planets the contrast is amazing but its no match for a 8 inch scope but quite close which says something for F15 + achromats I just love the old refractors