SCT vs DOB on planetary viewing??

robz (Robert) · #1 02-03-2014, 02:29 PM

Hi everyone. Would like to throw a question out there for you guys on planetary viewing and telescope suitability.

I own an 12 inch LX200 ACF SCT which is well collimated and has good optics from what I have seen. I have observed Jupiter and Saturn on many occasions. 380X seems to be the sweet spot here in Perth where seeing is good enough to crank up the power to get in close on your object.

Saturn has always produced a real wow factor without any trouble.
Jupiter, however, has been either just o.k. or disappointing....... even after sufficient scope cool down time and average to good seeing and transparency.
For some reason, I have yet to see the small white ovals and other fine details that people report on the forums.

My question is................should I be considering a 20 inch GSO dob(coming soon) or even a 16 inch as a better option to obtain higher resolution through larger aperature/smaller central obstruction?

How do the 16 inch Skywatcher and lightbridge perform at higher power on planets?.............or are the optics only good for DSO's.

Would love to hear your thoughts

AstroJunk (Jonathan) · #2 02-03-2014, 04:12 PM

I would dare to say that in my experience, seeing plays the greatest part in resolution. I have been fortunate in enjoying a view of Jupiter nearly as lovely as the images posted here, but that was once in 5 years from my site! Saturn always looks great because there are few details to blur...

There is plenty of talk of why a refractor is better than a newt, and why a newt is better than an SCT, all of which has good evidence in theory to back it up. But nothing less than perfect sky will allow you to resolve the details you want!

(But ultimately, I recon that a large Newtonian makes the best planetary scope visually)

Allan · #3 02-03-2014, 07:28 PM

As Jonathon says, medium/large newtonians hit the sweet spot for resolving planetary detail. But, only if they have high quality optics on board. Merely upgrading to a larger aperture will achieve nothing or worse, if the optics are poor. There is a lot of information out there that indicates the quality of the mass produced mirrors deteriorates as the aperture increases.

If deep sky is your passion then the 20"GSO would be worth a look when they come out. But for lunar and planetary, the best setup is a premium mirror in a Dob platform that tracks for easy high power observing.

robz (Robert) · #4 02-03-2014, 10:09 PM

Yes, gents I realize that seeing is the key to a blissful night of high resolution viewing. However those nights are rare in the case of Jupiter in particular for some reason.

I agree that a large dob will do nothing if the optics are below par. At present it seems to be a lottery at best and this is at the risk of the buyer.

I guess that that one night in 5 years is quite possibly the ratio to finally seeing what you hope to see on that massive gas giant.

I have owned a large newtonian during the Hayleys comet of 86 (?) mania and it did indeed show great DSO's, but for planetary it was a total abomination

!!!!

It seems a shame that going up in aperture withe the various brands available at present is o.k. for the faint fuzzies, but not a guarantee for good planetary viewing also

Camelopardalis (Dunk) · #5 02-03-2014, 10:30 PM

You'll see more in almost any scope with good seeing, but since we can't buy that we're left debating your scope is better than mine

Jupiter has low altitude this time around which - from Sydney at least - doesn't help the viewing

But if you want a quality scope be prepared to pay for it no matter what

There's anecdotal evidence suggesting some of the mainstream manufacturers, at least Synta brands (Skywatcher, Celestron), are upping their game a bit in recent years and putting out scopes with more consistency, but the jury is still out whether they're consistently better

Each design and it's merits and weaknesses, but you get what you pay for regardless of design, and probably only by winning the lottery will most of us get a chance to compare and contrast all the possibilities here...or see what selection shows up at IISAC

I've seen arguments for and against central obstructions, how aperture affects resolution, contrast, etc... in the end the important thing is to enjoy our time out under the stars and planets.

pdalek (Patrick) · #6 02-03-2014, 10:47 PM

The best Jupiter I have seen was with a 16" f/16 classical Cassegrain.
Also could get really bad images with the same scope.

glend (Glen) · #7 03-03-2014, 10:28 AM

In reading the article "Get the Right Telescope" by Rod Mollise in April issue of Aus Sky and Telescope (page 44), I ran across some comparative statements related to newts (ie Dobs) and SCTs:

SCTs: Fairly high cost per inch of aperture and longest cool down time of any design. In their favour most are sold on motorised mounts

Newt (dob): Lowest cost per inch of aperture,and obviously Dobs don't require mounts like the SCTs do.

The article is worth a read if your in the market.

Shiraz (Ray) · #8 03-03-2014, 02:44 PM

stick with the SCT if it works on other objects. Jupiter is so low currently that you will probably only see a disk and a couple of bands through the atmospheric turbulence. The people reporting good viewing on other forums are from the northern hemisphere?

firstlight (Tony) · #9 03-03-2014, 05:08 PM

Quote:

Originally Posted by robz

Hi everyone. Would like to throw a question out there for you guys on planetary viewing and telescope suitability.

I own an 12 inch LX200 ACF SCT which is well collimated and has good optics from what I have seen. I have observed Jupiter and Saturn on many occasions. 380X seems to be the sweet spot here in Perth where seeing is good enough to crank up the power to get in close on your object.

Saturn has always produced a real wow factor without any trouble.
Jupiter, however, has been either just o.k. or disappointing....... even after sufficient scope cool down time and average to good seeing and transparency.
For some reason, I have yet to see the small white ovals and other fine details that people report on the forums.

My question is................should I be considering a 20 inch GSO dob(coming soon) or even a 16 inch as a better option to obtain higher resolution through larger aperature/smaller central obstruction?

How do the 16 inch Skywatcher and lightbridge perform at higher power on planets?.............or are the optics only good for DSO's.

Would love to hear your thoughts

Hi Robert, it's funny to hear what you said about Saturn looking stunning and the other planets not so hot. Jeff Ryder, former curator of the Sir Thomas Brisbane Planetarium (AKA the Skydome) commented that Saturn seems to be just the right apparent size in a scope that seems to almost always have great results.

BTW, a refractor will give you the best planetary views.

John K · #10 03-03-2014, 05:29 PM

The point about Saturn looking better then Jove is a good one - I think even showing someone Saturn with an 80mm refractor who has not seen Saturn before and seeing the rings etc always creates an impression.

Jupiter will always be a tougher object - have you tried using some colour filters? Also remember, that for several years to come, Jove will be low for us and Saturn will be high.

My view is that you will nearly always get sharper views in a Newtonian vs an SCT. I don't however know about the optical quality of your SCT.

The issue you will also have with a Dob especially something as large as a 16" or 20" is tracking. At high magnifications, no matter how great the view, it will get annoying after a while trying to track at high magnifications - gather you have observed through large Dob's? Perhaps an equatorial platform is the way to also go for you.

Down the track are you also interested in doing some planetary astrophotography?

So to answer your question, optically speaking, a well made Newtonian mirror should give you way better views in my mind vs an SCT. But Jove will be a tougher object as it will be low down.

My money would go towards a slightly smaller Newtonian (say 12" to 14") with maybe a new EQ8 type mount, or if you want to go Dobsonian, then build in the cost of an equatorial platform.

But, if you simply want more light gathering power to also do deep sky stuff that doubles up as a better visual scope for planets, then a 16" to 20" Dob with a well made mirror and good tracking will satisfy you - not sure about the optical quality of some of the Chinese mirrors - also consider that cooling ect will come into play more and more with an increase in aperture - many years ago when I had my 10" Dob with a well made Parks mirror, I remember being at the ASV's Parer St observatory in Melbourne and comparing the view with the society's 20" fork mounted reflector which had been heating up all day in the observatory - the 10" won hands down! It was a sharp reminder on the other things that come into play with larger telescopes - more complexities to deal with - so larger is not necessarily better.

Good luck with your decision.

John K.

sn1987a (Barry) · #11 03-03-2014, 05:36 PM

I get great views of the planets in my dobs when the seeing is good even in my suburban backyard. The bigger the Dob the better the view. I'd be interested to compare with a top shelf refractor side by side they must be awesome to beat a good reflector optic on a good night.

icytailmark (Mark) · #12 03-03-2014, 05:46 PM

SCT's are the closest to a perfect scope. They are compact,high focal length(great for planets),and hold collimation very well. The only bad thing is they are expensive. You get what you pay for!!!

Allan · #13 03-03-2014, 07:38 PM

These discussions are always interesting. In just a few posts we have Newtonians, Refractors and SCT's all claimed as the best planetary scopes.

Any comparison is like comparing apples to oranges unless the telescopes have top quality optics. A 4" APO sure will show more detail than a large Dob if the Dob mirror is a poor one. Spend a couple of thousand dollars on a premium Zambuto mirror and the comparison ends. An 8" or 10" APO would level the playing field again, but at 10 times the cost though.

AstroJunk (Jonathan) · #14 03-03-2014, 07:46 PM

Quote:

Originally Posted by Allan

In just a few posts we have Newtonians, Refractors and SCT's all claimed as the best planetary scopes.

This is the internet, surely you wouldn't expect anyone to agree with one-another would you?

Allan · #15 03-03-2014, 08:39 PM

Quote:

Originally Posted by AstroJunk

This is the internet, surely you wouldn't expect anyone to agree with one-another would you?

Cloudy Nights separates the Reflector, Refractor and SCT's forums so everyone can play together nicely. A little debate always makes life more interesting though in my opinion.

Gem (Grant) · #16 04-03-2014, 07:00 AM

As an owner of a 9.25" SCT and 16" dob, I would say I prefer the dob. However, the difference varies. You see I can put the cooling fan on my dob several hours in advance. My SCT is a standard Celestron, not an EdgeHD that has quicker cooling. So, if I am patient and use my SCT around midnight - the results are quite good.
My SCT is more expensive than my dob, and in terms of bang for buck - the dob wins.
To those who prefer refractors - yes I agree they are wonderful. But comparing my 16" dob to a similarly priced 110mm refractor, and I still prefer my dob!
That's my opinion anyway.

Gem (Grant) · #17 04-03-2014, 07:08 AM

Quote:

Originally Posted by Allan

As Jonathon says, medium/large newtonians hit the sweet spot for resolving planetary detail. But, only if they have high quality optics on board. Merely upgrading to a larger aperture will achieve nothing or worse, if the optics are poor. There is a lot of information out there that indicates the quality of the mass produced mirrors deteriorates as the aperture increases.

It is also worth remembering that half of the optics in a dob, and nearly half in the others, is your eyepiece. A very good EP goes a long way to maximising the potential of any scope.

BlackWidow (Mardy) · #18 04-03-2014, 01:51 PM

I own an SCT and love it! I have also owned a Dob in the past an liked that also. Sky condition make a large diff, but I have also found it depends on whatt Eye I use. I seem to get better results out of my left eye than my right eye.

I am not sure if I have two different brands of eye, or if one is from my Father and the other from my Mother.. Dad has bad eyes so it might be his fault.

So if you are going to compare make sure you have good optics attached to your scope and always keep and eye out..... Well at least the bad eye.

So many variables

robz (Robert) · #19 04-03-2014, 02:34 PM

Thanks to all for your thoughts and input.
I agree, and realise that yes................Jove is very low compared to previous years so that may be a huge factor towards bad seeing(which is obviously the case here).
Secondly, I use an 8mm 72 degree Skywatcher E.P.(2 inch barrel) which has noticeable coma anywhere past the centre of the image. As the scope is an ACF, I know it's not the scope.

I haven't tried any other E.P.'s because I don't have any..... apart from some cheap super Plossl's that came with my other 6 inch achromat refractor.

Anyhow,for the moment, I will take a peep now and then at Jupiter, but I don't expect that magic night of great seeing this year around ?

Camelopardalis (Dunk) · #20 04-03-2014, 03:53 PM

Just a suggestion Robert...how about trying a longer magnification eyepiece? 380x might be a sweet spot on a good night, but on a not so good night I feel it's really high.

I've had many of my best planetary views at under 200x...to me, it's not the size that matters, it's the level of detail and crispness, vividness of colours, etc that are more important to me.

I'd also suggest something more in the direction of a premium eyepiece - some will disagree and say that slow scopes are more tolerant of poor eyepieces - but IMO _every_ scope benefits from better eyepieces.

Oh and Jupiter seems to have bit its low point and will climb by about 10 degrees in altitude for the next few oppositions