View Full Version here: : PLANETARY VIEWING ?? -aperature rules?or telescope type?

09-10-2010, 04:04 PM
O.K guys, for as long as I can remember, I have allways wanted to be able to visually examine details in planets : the intricate structure of the belts and ovals on Jupiter and variations in the rings of Saturn.........bla, bla, bla,............you know what I mean :)

I am fully aware that seeing conditions play a big part in this particular field and know that some scopes need to be perfectly collimated/cooled down etc. to stand a chance at revealing any of the fine details I'm after.
I am also aware that what one sees through the eyepeice is not necessarily the same as what is captured and stacked with expensive astro cams and software:sadeyes:.

After my last humorous and light hearted post, I thought I might ask for some honest opinions from those who have owned the many various scopes available to the amateur astrononer and are aware of the different performance levels and the reality of what to expect :thanx:

The questions are as such :

does aperature still rule in revealing planetary details :question:

How does a 12 inch SCT compare against a 12 inch or even a 16 inch dob at the same magnification:question: Does the dob/newtonian need spectacular optics to begin to approach the SCT:question:

Will a 4-5 inch good/medium quality(and price) APO or ED refractor cream all the above at the same magnification:question:

Last of all, are high quality, small aperature(compared to the sizes mentioned above) refractors the ''ultimate'' for planetary viewing or is this just wishful thinking:question:

Love to hear your thoughts:thumbsup:


09-10-2010, 04:19 PM
From memory although not into planetary viewing myself, long focal length (preferably) larger aperture APO refractors have always been considered the pinnacle for this type of observing

09-10-2010, 04:34 PM
G'day Rob,

I suggest having a look at this thread <http://www.iceinspace.com.au/forum/showthread.php?t=66769> and this thread <http://www.iceinspace.com.au/forum/showthread.php?t=60382> for some more posts on this topic. I think that inch for inch a Newt is better than a SCT for planetary viewing (less optical surfaces to get right and easier to cool). However, a SCT will be easier to mount on a tracking mount than a Newt. Inch for inch a good refractor will be better still than the Newt but APO refractors get expensive quickly around the 5" aperture range and you can buy a much bigger and cheaper Newt that will perform better for less dollars.

In a nutshell I'd say that yes, good quality well-cooled aperture rules.

09-10-2010, 06:03 PM
The Newtonian will give you about the best bang for your buck, a 5 inch apo will cost you moderate amounts, I have a FLT 132 you're looking at 5 k plus unless the dollar goes gangbusters. For much less money you could get 16 inches of truss tube dob. Which will definitely give you better views at the eyepiece, as with all things there are scopes and scopes, you could buy a 16 inch meade for around 3k if you want a custom made SDM with superior optics and focuser etc it will cost you more.
You'd have to be able to pick up a 12 inch gso for around 1 k , if you get the bug then you can save for the scope younwill use for a lifetime.
Refractorsare great for imaging and inch for inch they are superior, but you won't find too many apo scopes above 6 inches around.
Then once you have the scope, there's eyepieces to think about, I'd be happy to have a box of televue eyepieces and no other brand, but thats just my thoughts.

09-10-2010, 06:24 PM
A 10" Dob will do the job. You'll also need a good quality apo barlow and a nice high power eyepiece and you'll be more than happy.
With a little practice, you can even do some basic planetary imaging.

09-10-2010, 08:55 PM
Thanks for your input guys and gals!:thumbsup:

I'll check out the thread links and will probably go and build or buy a large Newtonian for my EQ6PRO mount:)

09-10-2010, 09:52 PM
Don't forget to buy a stepladder then

10-10-2010, 06:46 AM
I have an 8" Newt and a 4.5" refractor. The refractor shows greater contrast and a 'crisper image', while the newt shows more detail but with less contrast and a slightly softer image. It can do your head in trying to work out the trade-offs!!
Personally I prefer the refractor, but as noted, a good one gets expensive very quickly.
If you are in the position to get one, one of the 'Intes-Micro'
x15 series of Maksutov Cassegrains are reputededly excellent for planetary viewing, without giving a lot away for deep sky.
They have a small (c.25%) central obstruction and an f15 focal length. The 6 & 7 inch seem reasonably priced for what they offer.
Have a boo at tetontelecopes.com for more info.
I have no connection with either company. Its just what I've garnered from reviews and gossip.

10-10-2010, 07:05 AM
I think one way to look at it is what type of scope are the very best planetary imagers using to capture their images?

Mostly they use Celestron SCTs - 9.25, 11 or 14 inch.

Bird uses his own made Newt.

Newts with that much aperture may be hard to handle physically.

The best views I have had was with a Celestron 11 inch SCT.

Refractors give you a sharper view but much smaller image unless barlowed to hell.

I have and have had many fine refractors. I remember a nice view of Jupiter under excellent seeing at my dark site with an FS152. But smallish evne is sharp and stable. The SCT has long focal length for its compact size.

My opinion would be to get a decent view of planets with an APO you'd need more than 4.5 inches of aperture. A 4 inch APO is a rather dim view unless you are looking at wide star fields. A 6 inch APO gives a similar brightness to around an 8 inch SCT as a guideline. It can at times appear brighter because APOs typically have such great contrast and have a very engaging sharp view.


10-10-2010, 07:09 AM
A Newtonian with a first class mirror is hard to beat on planets if the secondary is kept small, say under 20% obstruction by diameter. To ensure you do get a first class mirror I would recommend you have one made to your specifications than going with a mass produced mirror. It will cost a lot more and it will come down to how good an image you want and how much you want to pay for it. Note a first class mirror will perform on all objects not just planets. Stars will be tighter and fainter fuzzies will be seen.

A large Newtonian will take longer to cool but once stabilised it will out perform any smaller refractor for detail. However, for quick but quality views a small 4" APO is hard to beat. You will see all what the refractor is cabable of showing within minutes of setting up provided the seeing is good of course and this view often rivals larger 'scopes since those 'scopes are still stabilising or optics are mediocre.

Note too a large Newtonian on an equatorial mount really needs a set of rotating rings to ease access to the eyepieces and avoid difficult viewing angles.

10-10-2010, 10:19 AM
Well, now it's becoming clear thanks to some honest input :thumbsup:

Yes, I was thinking of a mass produced mirror for a Newtonian but knew at the back of my mind that it would most likely be of average optical quality.

I'm familiar with refractors(only achromats) and know that they can at times, even though they are not perfectly colour corrected, blow away a newt with much, much larger aperature.
I would not however spend thousands on a 5 inch overpriced APO..........this is just a rip off despite the makers claims on how costly the glass is.

I have a 60mm old tasco(yes,........60mm........with an obviously and unusually good objective) that shows the moons of jupiter as small dots at high power(dare I say disks?!) compared to my 15 year old 8 inch U.K made Orion ( excelent optics) ...........which no matter how well collimated, does not cut the mustard on planets and refuses to show more detail as the aperature should ????:screwy:????.......the moons look like spiky stars:(

I would love to own a SCT, but will not bother with anything under 10 inches in the aperature department.

Looks like a lot of money will be needed to get there by the sounds of it.

I guess you get what you pay for:rolleyes: :sadeyes:


10-10-2010, 02:34 PM
I've always found SCT's poor visually on the planets- partly due to the typically 40% obstruction when you take in to account the secondary baffle tube. For planetary photography however it is not so much a problem because image processing and stacking is so good at rendering contrasty images from more tenuous data. The eye of course can't do this in real time , so for visual observing of the planets I 'd recommend a good Newt with clean well collimated optics and less than 20% obstruction.

10-10-2010, 05:20 PM
Hmmmmm?.....that's interesting Mark, and I have heard this from time to time.

Does anyone know what the BINTEL mirrors for newtonians are like?
I know we're back to mass produced optics here again, but the specs are quite impressive (if true) : surface accuracy at least 1/16 wave RMS (typically better)BK7 substrate. :eyepop:

It all sounds better than your average plate glass slab of Chinese junk does it not?:question:

10-10-2010, 07:47 PM

For planetary viewing, aperture rules.
As long as we are comparing equipments of equivalent optical quality. There is no point comparing an apo to a dob since the apo has excellent optical quality and the dob average optical quality.

For a given night (= at a given seeing) there is a max useful magnification no matter the instrument type.
On a turbulent night, all instruments type perform the same (same maximum useful magnification). Smaller instruments might display an aesthetically more pleasant image due to the smaller aperture "seeing" less turbulences. A bigger instrument can be reduced in aperture to match the smaller one and will deliver the same images.

Under good seeing conditions, a small instrument in aperture might be limited by its resolution (below the maximum useful magnification on that night) whereas a larger instrument will not be.

A second parameter to consider is brightness and contrast. No matter the seeing conditions on a given day, a larger instrument will obviously deliver brighter images, which in turn helps seeing details of low contrast.
Consider the following images:


Obviously, the details are easier to figure out in the brighter images.
This is why, for planetary observations, aperture (and optical quality) always rules.

The Mekon
10-10-2010, 08:19 PM
The only times I have seen planetary images as good (and better) as in my 130mm Apo is through longer focus high quality Newtonians, (10" F6, 8" F7) At these times the Newts had had plenty of time to settle in temperature and the seeing was excellent.

In average conditions and straight out the box, the refractor blows all comers away.

Also I have never seen a 8" SCT that could better the 130 on any object, deep sky or otherwise.

10-10-2010, 08:33 PM
:doh:After reading forum member's rmcconachy's links (especially ''Damian Peach's VIEWS OF THE SOLAR SYSTEM'') there is no longer any question in my mind what I will settle on for planetary viewing and photography at a later stage.

It is clear that aperature is superior and that the B.S. spread on the central obstruction is not the'' image/contrast killer'' it's made out to be:mad:

As a Newtonian of ''adequate aperature'' (12 inch or more:rolleyes:)no matter how implemented, will be a long, cumbersome beast and PITA to keep in collimation, I'm will go for the latest 12 inch Meade LX200 - ACF SCT OTA on my EQ6PRO Mount.;)

I know it has excellent optics with minimized aberations and combines the benefits of many other optical systems. No,.......it's not the perfect scope and has it's drawbacks, but more SCT'S are sold world wide as the main telescope system in multiple scope set ups and are the choice of many small semi professional observatories.............and rightfully so....!:thanx:

10-10-2010, 08:50 PM
If you are just interested in planetary rather than wide field viewing then a normal SCT should be just as good optically as a Meade ACF or Celestron EdgeHD scope. Planets are small and the optical changes in both relative to a normal SCT are about improving the off axis views rather than the on axis views (although the EdgeHD is now better ventilated than the older Celestron SCTs I believe).

If you want to go into imaging, focus (bad pun) on obtaining a big steady mount with smooth periodic error. Talk to bird, gregbradley, strongmanmike and the other great imagers who frequent this place for recommendations.

Lastly, the links I posted in my previous message weren't mine, all of the good stuff came from other people. Kudos and thanks to them!

10-10-2010, 11:50 PM
Robert, the 12" SCT will do very nicely for lunar an planetary work. I have the 14" and it has very nice optics for the price.

If you really want to sell your soul, try a Takahashi Mewlon with which you can get the trifecta of aperture, quality and minimal central obstruction.

10-10-2010, 11:57 PM
rmcconachy is correct: the EdgeHD & equivalents are SCT with a built-in field flattener. Therefore they are not going to perform better than regular SCT when paired with tracking mounts.

There is no magic. All optical instruments obey the rules of physics. APOs included. They however are terrific instruments, in their aperture class.

10-10-2010, 11:58 PM
Yep, mewlons are said to be very nice, but apparently require fine tuning to deliver their best.

11-10-2010, 03:54 AM
They are (were) sourced from GSO and rebadged for Bintel. They do check them all before they go out.

I've been very please with my 8 inch Bintel reflecting telescope.

11-10-2010, 04:00 AM
Apples and Oranges using astrophoto as a proxy for visual evaluation...

Astrophotography benefits over visual in that you can stack, add/delete images to build a better one over time. Your eyeball can;t do that.

I'm not saying that an SCT is a bad type of scopes, they have some advantages in size relative to portability but from my own experiences I'd not list contrast as an advantage of the SCT design.

Anyway, enjoy your choice!

11-10-2010, 08:57 AM
For the sake of technical accuracy Bintel telescopes are collimated and mechanically checked -but not optically tested.

I don't know of any other shop in Australia besides Bintel that will perform this excellent service on a Chinese import.

11-10-2010, 09:03 AM
Yes- central obstruction is indeed a contrast killer. Damien Peach has access to exceptional seeing and does a load of image processing/stracking. You will never see views like that _through the eyepiece_ , but a high quality simple optical system with clean optics and a minimised central obstruction can give glimpses of it.

11-10-2010, 09:11 AM
Only if you don't understand about the tendency in the industry for `wave inflation' via terminology. '1/16 wave RMS on the surface' translates to about 0.5 P-V on the wavefront. I recently tested a couple of 16" Chinese mirrors for a well known Australian telescope builder. They came in at around 0.5 and 0.66 wavefront . The smaller mirrors may test out better than that I don't know.

Given the issues of a relatively thick non low expansion glass it is a moot point whether such a system would benefit from a higher accuracy mirror. If you are a casual observer and not out for high planetary performance they are great value there is no argument there. As always in life , you get what you pay for and usually no more.

11-10-2010, 10:07 AM
Rob, I like Schmidt Cassegrains but they do have a serious disadvantage with regard to high magnification in temperate climates - it's difficult to get the tube cooled. Typically the scope will lag falling temperatures until sometime around midnight and you won't get the best view until an hour or so after temperatures stop falling. Some owners cut holes in the tube to add cooling fans or other devices to get the main mirror cooled.

With that in mind the new Celestron Edge with it's cooling vents is worth considering.

11-10-2010, 01:39 PM
Good clarification! Thanks!

11-10-2010, 02:29 PM
Thanks to all again for your comments and assistance.

Yes, I have said that I have settled on the SCT at the present point in time until funds permit ..........daughter's wedding coming up in January.......DOH!!......having a hard time juggling finances!:juggle:
BUT...................until that purchase is made, there is a lot to think about........especially ''REFRACTORS'' :shrug: ?????????

I can't seem to get them out of my head knowing and seeing how good even the small aperature achromats can be :rolleyes::question:

What does concern me over this tremendous discussion is that optical quality of sufficient level in newtonians really needs to be in the hand made catagory over here in Australia which blows the price way past my budget in anything around the 16 inch mark.:(Plus I would have trouble keeping such a large tube steady on an EQ6PRO:rolleyes:

Then there's the cooling problem - SCT, MAK, R.C. NEWT. and so on. I have seen the differences in my 8 inch newtonian astrograph, F4.4 and know the amazing transformation that occurs :rolleyes:

This all brings back the PITA factor which I do not want........I'm getting too old, have two stuffed knees and little time to wait for optics to cool down all night with the fans blowing their guts out :mad2:!

Someone once said to me : '' if I were to go back before I bought and collected all my various scopes and spent the money on the ultimate scope, it would have been a TAK 150 and I would be done.......... and in heaven ever since''
Yeah well I can't afford a ''TAK'' and would be looking at ED's and APO's that are affordable and still perform well.

We can support or slam the Refractor as much as we want, but deep down inside, we all know how extraordinary they can be...............:sadeyes:

What''s the use of aperature when high power won't work due to temperature equalisation problems, sub standard optics and mis-collimation when ever a passing bat farts?

11-10-2010, 03:19 PM
The Tak150 is a superb refractor that will offer excellent views more consistently (generally speaking) than a Dob with large aperture on low contrast objects such as planets, or splitting doubles. But for the same price you can get a premium 15" dob which will blow the Tak out of the water on dso (every time) and on planets when seeing permits, there's no ifs or buts about it. Had this not been the case, there would be no point in people purchasing 20"+ dobs. It's not all about high power! Collimation is no where near as bad as some people make it out to be (trust me I've been there).

Unfortunately your going to have to compromise on something with each scope, that's the reality of this hobby. I've bought and sold so many scopes over the years that I've realised you can't have one that does it all (yes even an SCT is full of compromises). It takes time to work out what your observing habits are like and from there, you can settle on a scope which would best suit your observing needs.

I personally think the best combination is a 100-120mm refractor with a 10-12" dob. Those two scopes will cost less than an 11" SCT!

Best of luck.

11-10-2010, 04:20 PM
O.k................without the budget for a ''premium dob'', what brand that is readily available on the Australian market has good optics and reputation in a 16 inch????

We have the Meade Lightbridge, the Bintels, The SkyWatchers, the GSO's..............:confused2:

11-10-2010, 04:43 PM
In the 16" size all of the scopes you mention use the same optics supplied GSO.

11-10-2010, 05:01 PM
Must say I agree with mbaddah. A 12" dob and a 100-120mm ED refractor would be my ideal setup. The 8" sct I owned never came close to giving me the views from my old 10" skywatcher dob on planets, and just about everything else. Tube currents drove me nuts as it never seemed to reach equilibrium. On one occasion an ED80 gave a better view of mars in side by side comparison.

I've gone back to a 12" dob and now looking for a 100-110 mm ED refractor.

11-10-2010, 06:57 PM
Cooling down can be an issue it's true, but large refractors will suffer from the same problem though to a lesser extend. A 150mm refractor is a thick piece of glass. Takes time to cool down.
Easy solution to solve cooling down issues: take the scope out 2 hours or so before observing. Easy.

What's the use of aperture? Greater brightness at a same magnification will help you see tiny details of low contrast. (see the illustrative picture posted previously).
It is however true that instruments require proper collimation. This is also true for refractors although they typically do not require the user to perform collimation. That being said, my Mak does not require collimation either. Holds it extremely well. Regarding optical quality, maks are usually good performers due to the relative ease with which spherical optical surfaces can be produced.

For good optical quality (never tested it myself though), have a look at the orion optics website in the UK where they certify the optics up to lambda/10 for newtons. Some models are relatively affordable.

11-10-2010, 08:21 PM
Hi All,

Well lots of opinions here but for what it's worth, I agree with Satchmo -- spot on as usual. :thumbsup:

Yes, a 4-5" APO will be very good, but the smallest detail it will resolve on a planet is around the 1 arc-second diameter mark. Resolution is tied intrinsically to aperture. It doesn't matter how good the optics are or whether you've got 99.8% Strehl ratio optics etc, it won't (indeed can't) do better than this.

A clean, well-cooled and collimated, say 10" f/6 or f/7 Newtonian with quality optics around 1/25th wave RMS mark and a secondary obstruction <20% in near perfect conditions will do twice as well as the 5" APO in the same conditions -- ie in the smallest detail to be potentially visible. It will probably cost little more than half as much. If the seeing is mediocre or poor there will be little difference in detail visible except in those occasional moments when clarity prevails for a moment or two -- and the Newtonian wins again. The larger 'scope will produce a significantly brighter image that will take much more magnification before it becomes unacceptably dim and uncomfortable to view.

Don't get me wrong, refractors are beautiful telescopes inch for inch, but they are practically limited in aperture. (well they are aperture-limited by the depth of your pockets I guess). Aperture of the primary mirror/lens is the prime determiner of how much detail is potentially visible in a planetary image.

In poor, mediocre or average conditions a SCT of similar size to our Newtonian will perform about as well on planetary detail as the Newtonian. In very good conditions or excellent (rare) the Newtonian will produce a somewhat crisper image due to the much smaller secondary mirror used. It is a simple matter of physics due to the size of the secondary obstruction and the wave-nature of light. Increase the secondary mirror size and you push more light out of the Airy-disc and into the surrounding diffraction pattern. As Foghorn Leghorn said to the young chicken-hawk "Son yer can argue with me, but yer can't argue with figures" -- and that's a fact, not an opinion!

Also, it is a simple fact of life that in a typical commercial SCT used with a diagonal, you need to get 5 optical surfaces right for it to work well. In a Newtonain there are only two.

If you are looking for a quality, visual-use, portable "APO killer" for an Eq6 mount, get yourself an 8" f/7.5 Newtonian with a 25mm secondary. Longer focal length Newtonians are easier to collimate and much more forgiving of slight errors. Additionally, they are much easier to fabricate! Your eyes (and bank-balance) will thank you for a long time -- it will flog the pants off any 4" APO on any subject in the night-sky save perhaps ultra-wide field viewing. The image will be 4x brighter at a given magnification and will show twice as much detail in the right conditions.


Les D

12-10-2010, 12:00 PM
Getting back to the article by Damian Peach further back on this thread, it clearly seems to me that most have not paid much attention to it or am I wrong?

I realise that the images and comprisons are examples of perfect conditions(and are stacked and manipulated captures) with all necessary criterias met etc., but it is NOT the central obstruction that causes loss of fine details.

The main conclusion is what some have mentioned here : APERTURE .RULES...................end of story.

After some more Googling, let's now go back to the 12 inch Meade SCT I have been considering along the way :it has all that is required for planetary viewing : large aperture , large focal ratio so I don't need to barlow the crap out of it(talking about extra optical surfaces!)and high quality aberation reduced optics all in a compact tube and not an F7 or F8 long ,heavy, wind catching cannon.

Central obstruction????...............rated at 11.1%..........shocking!....ain't it???.............I'm sure that a newtonian with an average obstruction of usually more than 20% is muuuuuuuuuch better?

I appreciate the help of all and am grateful:thumbsup:..........but c'mon everyone...........let's keep the ''myths'' at bay next time as I have been confused to buggery:D!

12-10-2010, 12:32 PM
Rob, central obstruction is conventional stated as a percentage of the diameter of the objective. The Meade 12" SCT will have an obstruction of around 35%.

The 12" SCT will deliver greater resolution than a perfect 6" refractor, but only under perfect conditions (stable atmosphere, good collimation, temperature equalised). Your local conditions may or may not limit the magnification you can achieve; collimation is not a problem; cooling can be a big problem, which is why I would point you to the Celestron Edge(has vents) in preference to the Meade.

I like SCT's for their convenience (compact tube, comfortable observing position, can store the tube inside my house) but my local conditions rarely let me use high magnification so I'm not too bothered by tube currents - it's a trade-off I'm prepared to accept for convenience. An 11" or 12" aperture gives decent deep sky views at lower magnifications and I'm prepared to wait until midnight for occasional higher magnifications that come with those lazy high pressure systems.

Refractors, SCT's and Newts are all good scopes - you'll probably only know which one suits you best after you've owned each type.

12-10-2010, 01:27 PM
Hi casstony, thanks for your input.
According to the specifications on the Meade 12'' SCT, they state a 4 inch(physical?)/11.1% (optical ratio?) secondary obstruction figure................so what gives?

Anyhow, with all the compromises that exist with telescope types, I will most likely go the SCT route.

I have had a 16 inch Newtonian(pipe fitting mount:lol:), have at present,an 8 inch F4.4 Orion on a Gazer E.Q.(undriven) and an EQ6 PRO mount with standard and extra large dovetail heads ready for whatever new ''PLANET KILLER'' I decide upon and a TASCO 60mm refractor.

The newtonians have and are a proper nuisance, in having to check and collimate the optical path, rotate that bloody tube to get to the eyepeice and are heavy and long, making set up just that bit harder.

Have never owned a SCT, but have allways wanted a big one............wish me luck coz I'm gonna need it:question:

12-10-2010, 02:40 PM
Rob, 11.1% probably refers to the area of the secondary compared to the primary - equates to deceptive advertising. 4"/12" = 33% sceondary obstruction by diameter, which is about the same as my C11.

You'll still need to tweak the collimation regularly on the SCT to get the best high power views - and you'll certainly need to add some kind of active cooling if you get the 12" Meade and high power is the priority.

The Meade 12" also weighs about 38 lbs while the Celestron 11" weighs about 28 lbs.

12-10-2010, 05:02 PM
The C11 is already a pretty massive beast and it's not an easy task to move it around.
I suggest you really consider the weight issue. It's always something we think is going to be fine prior to purchase, but it ends up being one of the most (or the most?) annoying thing for daily use afterwards.

12-10-2010, 06:01 PM
Hi All,

Where do I start ?

I'm apologise Rob but I believe you are incorrect here. See this paper here:


and a slightly more lay-approach here:


The physics of this has not changed in the last 50 years (trust me!). Leaving aside the effects of seeing and imperfections in the optical surfaces, aperture determines the size of the smallest details that are resolved. However the size of the central obstruction expressed as a percentage of the diameter of the primary mirror has a substantial effect on contrast (produces contrast depletion) and the human eye's ability to discern a contrast difference between this "spot" and an adjacent "spot" on the disc of a planet.

This is not particularly noticeable on a target like the Moon which has high-contrast detail -- mainly in black & white, or splitting a close binary, but on say Mars, Jupiter and Saturn where most of the detail (particularly the fine detail) is low contrast it is significant. Below a 20% obstruction, the effects of this contrast depletion are not noticeable visually. For practical purposes it can be disregarded. Once you pass about 25% obstruction by diameter, they become increasingly significant. Most commercial Schmidt-Cassegrainians are between 35 and 40% obstructed by diameter.

You might legitimately ask at this point about why so many very high quality images are produced by significantly obstructed telescopes -- and it would be a fair question. The answer is simple. Pure, raw resolution is governed by aperture diameter. Commercially available software used in the processing of raw images manipulated and draws out the fine contrast that is there but hidden by the effects of a large secondary. Your eyes see in real-time. There is no image-processing software in your brain, or between the retina and brain apart from what God gave you. You can't un-muddy contrast depletion visually.

True, subject to what I've said above.

Most of this is perfectly fair comment. :thumbsup: Large aperture is desirable as it is the sole determiner as to theoretical resolution. However, even for a 12" telescope that produces an airy-disc around (I haven't done the maths -- this is from memory) the 0.35 arc-second size, the number of times you will get seeing that will permit the 'scope to resolve to that level is very small. Perhaps only a night or two a year (depending on where you live). If you are expecting to see fine detail on a windy night, dream on. But I agree completely that solid-tube Newtonians are more wind sensitive than a smaller package.

Long focal length is also desirable for a planetary visual-use 'scope because it means you won't have to use short focal length eyepieces to achieve moderately high and high magnification. Very short fl eyepieces usually have teensy-tiny eye lenses you have to screw your eye-up or squint to see through. This increases eye-strain and makes using the telescope uncomfortable. If the user is uncomfortable, you will find it hard to see ultra small low contrast detail.

High quality, aberration reduced optics? Just because you buy a commercial Schmidt-Cassegrainian, don't necessarily expect as a matter of course, higher grade optical surfaces than a home-made optic, or a Chinese made Newtonian. I've seen many optically good and occasionally very good S/C telescopes ... and I've seen several I'd describe as sub-standard and also a few out-and-out lemons.

A friend of mine spent two years fighting with a manufacturer (no names, no pack-drill) over his S/C that was obviously, clearly spherically aberrated (by about 1/2 a wave) -- a dead-set lemon. He sent it back overseas to the manufacturer once at his own expense to have it checked. They returned it (at his expense) and said it was fine. He had to send it back again several months later (at his own expense) with several testimonials as to the lack of optical quality. They re-checked it, apologised profusely, replaced the corrector and it is now a good telescope. It took 2 years of fighting to resolve (pun not intended :thumbsup:).

Hmmm ... that 11.1% figure is perfectly true -- as expressed as the area of the objective mirror aperture lost by the central obstruction. They express it that way on purpose, can you work out why??). However, it is misleading because the important factor here is the percentage of the diameter of the aperture. Expressed that way I'm certain the Meade 12 S/C it will be in the realms of 35-40%. This will cause contrast depletion, no ifs, no buts. That contrast depletion will be detectable in very good seeing and significant in excellent seeing.

Many commercial Newtonians (like the various Chinese brands and the Meade light-bridges) are typically 20-25% ocstructed. Good, could be better. If you want to optimise a Newtonian, unless the f/ratio is very, very short the obstruction is very commonly less than 20%. Examples: My old 25cm f/6 (254mm paerture, 54mm secondary) was 21.25% -- not optimal but close. My 31cm f/5.3 (307mm aperture, 54mm secondary) is 17.5% -- optimal. My 46cm f/4.9 (456mm aperture, 78mm secondary) is 17.1% -- optimal. It takes very little work or expense to optimise a commercial Newtonian. You can't optimise a commercial Schmidt-Cassegrainian.

No myths in what I've said, sorry.

Agree, correct.

Agree mostly here. If the seeing is the same as or worse than the Dawes limit for a 6" aperture, the resolved detail will be pretty much the same in both 'scopes. If the seeing permits smaller airy discs than the 6" can physically form, the 12" will achieve better resolution. Up to twice as good (depending on how much smaller) in fact because it is twice the diameter.

Good comments, agree!

Agree 100%. The best telescope for you is the one you will use the most often. If you feel its too heavy, too inconvenient, too fiddly etc etc you won't use it as often and thereby becomes an inferior choice for you. This is a perfectly good reason to go down that track -- no argument from me at all. In the end it is all about your priorities

As I said above and Tony has also said, that 11.1% figure is an "area" obstructed, not a percentage of the diameter and it is the latter figure (about 35-40%) that bears on the question of visual contrast depletion.

All telescopes are a compromise. You have to trade $ per inch of aperture -v- ease of use -v- portability -v- quality -v- image perfection. I've looked through a lot of S/C telescopes over the years and there are a large number of perfectly legitimate reasons to own one. They pack significant aperture into a small package (compared to a Newtonian or refractor of identical aperture), they are inch-for-inch much cheaper than a refractor all-up and because the tube is short and mass lowish for that aperture, don't need such a heavily engineered mount for their aperture and are generally low-maintenance telescopes. These are all perfectly desirable things and if they are near the top of you list of priorities when you shop will probably tip you to the Schmidt-Cassegrainian.

Sorry, no commercially manufactured Schmidt-Cassegrainian compared inch-for-inch against a similar quality Newtonian or Refractor could be described as a "PLANET KILLER". Due to the compromises made in that optical design, it will never compete because of contrast depletion induced by the relatively large central obstructions. There are a lot of perfectly valid reasons to choose a Schmidt-Cassegrainian. They may well be the telescope of choice for you having regard to all those things that must be considered as I've outlined. But, high contrast visual planetary observing is not one of them.

... Pt 2 follows


Les D

12-10-2010, 06:02 PM
Pt 2 ...

I can't (and won't) tell you what to feel and if these are your feelings, a Newtonian is probably not for you and there is nothing wrong with that at all.

Donít for a moment think Iím ridiculing your choice Ė Iím not. All I hope is that you choose for the right reasons. There are many reasons that make the Schmidt-Cassegrainian a legitimate choice for many people. No argument there at all. Going down that path because you hope for better visual planetary images is not one of them. Going down that path because you believe you are guaranteed higher quality optics Ö ditto. Some people also go down the S/C path because of peer-pressure or because they find them aesthetically pleasing (ie pretty). Personally I donít buy telescopes to look at, but if thatís important to you as a buyer, well thatís your choice and I wonít argue about your feelings. Personally I choose a telescope based on what itís like to look though, not look at!

Very best of luck with your choice and I sincerely hope that whatever you settle on, it will bring you many hours of observing pleasure.


Les D

12-10-2010, 06:13 PM
Interesting discussion. I understand all the theoretical reasons why a larger aperture gives better results but my personal experience hasn't really shown this in side by side tests. I have an 11" celestron and a 4" Tak refractor and even with careful attention to collimation and cooling in the SCT, it rarely provides more detail visually on Jupiter than the smaller refractor. Brighter, and larger image scale but much softer images 95% of the time. I'd also be wary of the EdgeHD Celestrons if visual planetary studies are your main interest. The vents are passive and not large so don't sound especially effective in assisting airflow, and the flattening optics in the baffle tube will stop you putting in a device like a Lymax cooler that greatly assists cooling.

12-10-2010, 06:29 PM
It only takes about 15sec to remove the secondary to assist air flow as they are Hyperstar enabled. This is actualy mentioned some where in a Celestron manual or on the web site. It won't be long before some one fits a fan to either the vents or at the secondary for more rapid cool down.Personaly give me a good quality Newtonian any day.-( I have an EDGE and there actually quiet good as far as SCT go for Planets but I find a well built newt hard to beat:thumbsup:.

12-10-2010, 07:35 PM
Hi Robz,

Les has done 2 great posts on the aspects thet need to be considered.

On the practical side it really comes down to how much budget and how many compromises you want to make.

I had a really nice US made 8" Meade SCT and a great CPC1100. I also had a very nice 4" refractor (though not as good as the Tak or AP refractors).

MY refractor gave me really nice sharp contrasty views and was simple and easy to setup and did not take much effort to maintain. Due to the lower aperture I found that it's performance on DSO's was lacking but it was a nice scope.

My 8" Meade was very easy to move around and setup. It also performed well across the board but images were "soft" and not as contrasty as the refractor.

My CPC1100 was an improvement on the Meade but again planetary views, while signficantly improved, were a little soft for my taste. Also the CPC1100 was heavier and took ages to cool down to ambient (over 2 hours). Once you get to this size SCT and above you really need to consider cool down times and mechanisms to help reduce this like filtered active fans/vents. As I did not have these, I used to setup my CPC1100 hours ahead of my visual sessions.

Both my Meade and CPC1100 held their collimation quite well so I did not personally find this a problem. Focus is an issue in both of these so I would consider a focuser upgrade mandatory.

When I first used a Mewlon 250 this scope blew away any of my prior scopes in terms of sharpness and contrast. The Mewlon 250 is also easy to setup and use at a reasonable price for the quality you get.

I now have a Mewlon 300 which completely blows away anything else I have used or owned. However, It can be a pain in the behind to setup and breakdown as the total setup weighs nearly 100kg but it is a sacrifice I am happy to make.

For Visual image quality/sharpness/contrast standard SCT's come nowhere near the Mewlons or similar Quality APO's and Similar Quality reflectors.

I have found focus on the Mewlon 300 to be very easy as it has a motorised secondary and the primary is permanently fixed. Also you really have to bounce these around to mess up your collimation.

Another critical thing I have learnt is quality of both the mount and tripod especially for portable systems. The last thing you want is a very high quality scope were the image jiggles around at the slightest touch or breeze. You can very quickly become frustrated.

- Get the best quality Mount AND Tripod you can afford
- Get the best quality Optics you can afford
- Get the best quality mechanical setup you can afford (ie structural rigidity/alignment, baffling etc)
- I would not try to choose a VISUAL scope based on Astrophotograhic images. Base your decision on real visual testing if you can.
- Go and watch other users setup and breakdown their systems at starparty events to gauge if this suits your circumstances.


12-10-2010, 09:42 PM
What would I obtain if I wanted the best planetary instrument for visual use ?
I like to keep my feet on the ground so a quality motor driven premium 16" or 18" F4 Dob Newt with low profile focusser,well ventilated thin mirror, below 20% obstruction , and a 2X Televue Powermate plus Clave Plossl eyepiece , would be somewhere near the top , but below the 16" Apo refractor which I could never afford :) The larger aperture will yield a whole level of detail, via differentiation of colour in features by being able to maintain a healthy exit pupil for colour perception- regardless of the qulity of the seeing. An SCT would be near the bottom of my list of choices for all the reasons set out in this thread. A classical or dall -kirkham cassegrain would come in below the Newt as it has a faster aspheric primary, larger obstruction , and an extra optical surface .

12-10-2010, 10:31 PM
Oh God................................ .............somebody kill me.................please?......... ......:bashcomp: :doh:

I guess we all know the truth don't we?................but most are afraid to admit it(read on below), and some have secretly regretted upping their aperture because of the highly contageous ''A.F'', and have ended up with an expensive and possibly dissapointing giant bucket of fiddly glass and metal with the dreaded ''central obstruction'' and the contrast loss as a consequence?

Say what you like about the beloved newtonian, but I hate them........especially the long F ratio ones.........the ''cannons''.
I have owned real big ones and have an 8 inch now - as mentioned (you should see the obstruction on this one..........you can hardly see the primary through the draw tube!)..........no wonder my 60mm refractor can match it in the planetary department:rolleyes:

As an Audiophile and having built my own amplifiers and speakers, the K.I.S.S. principle was no myth.It obviously also applies to the optical chain in a telescope doesn't it???:P

I now realise that large aperture may be of some benefit to see brighter fuzzies, but I have never seen a newtonian produce visual fine detail no matter how perfectly collimated, optically brilliant or cooled to within the required 0.5 degrees(!!!!)
I personally do not wish to at any time, set up the scope at breakfast or wait hours for the optics to cool down(fan assisted or not)....this is insanity.................surely?

K.I.S.S..........keep it simple stupid = REFRACTOR

12-10-2010, 11:11 PM
Hi Robz,

Your initial request for information is an interesting contrast compared to your latest post on this thread.

You initially asked members of the forum for advice based on their experience with different scopes of different size, quality optics, design etc for planetay use.

A lot of members have posted some great comments and recommendations based on their experiences.

If you already had all of the sage experience and opinions in the first place (like your last post implies) why ask for help then put others systems, recommendations and opinions down???

I am sure we are all just trying to help give you the advice you initially requested.


12-10-2010, 11:37 PM
:rofl: Don't ya just love open debates!

Just remember, there is no such thing as the right scope - only the least wrong one.

Go and buy something BIG!

13-10-2010, 07:51 AM
Hmm , what can I say but you clearly haven't owned the right instrument or been hanging around with the right people at star parties and taking note of how and why they do things.

It takes a little care and know how to tease the finest planetary detail from the sky- don't expect to get it all just by slapping down a wad of cash on an instrument and just expecting it to perform . If you are happy to be limited in resolution by a small aperture then a refractor is a good choice, it will give all it is capable of, or just go and stick an off axis mask on a reflector and you'll hardly pick the difference. Any large instrument no matter the type will require care , patience and TLC to wring performance out of. That is how you actually see stuff far beyond the `slap down a wad of cash' approach.

You seem a little confused - yesterday you were singing the praises of SCT's as your ultimate planetary instrument, and today it's refractors ? Methinks you are having fun with us.

A lot of 'good oil' has been patiently sent your way in this thread by a number of people ..go read it and my advice is think twice before make posts seeking advice on "best instrument for *****" if you really have pre -conceived immutable ideas , refuse to take in any detail, creating a pile of traffic from people getting stressed that the detail not be glossed over with sweeping statements. Like the `boy who cried wolf' be careful that people who honestly tried to give their considered opinion will ignore you next time.

13-10-2010, 04:42 PM
My sincere apologies to all and any that have been offended in some way by my last post. It was not meant as such in any way.

I was merely having a shot at myself in attempt at some dry humour as I indeed got so many faithful replies and assistance and good advice from all................so much so that it did my head in and the reality is that there is no perfect scope for all duties.

I guess that as mentioned by some during this discussion, the laws of physics can not be changed and they apply quite strongly in optics as they do in everything else.

All comments and replies on this thread were regarded as very useful information and a wake up call for me that I will need to make up my own mind on what I can afford and what I can handle physically.

Due to my failing health and my struggle with it over the years, it sometimes gets the better of me and dictates the extent of my ability to lift and move heavy items, causing frustration of body and mind.........such is life.

Best regards to all for your kind help.


14-10-2010, 12:12 PM
After careful consideration, I will be going for an 8 inch Maksutov or possibly the Russian 10 inch TAL 250K depending on total price including shipping (information pending at the moment).

14-10-2010, 08:40 PM
I'm only a beginner, but i'm surprised no one has mentioned a mak

Plenty of astro buffs all over the world say a well made 6in mak is as good as a 4in APO.

After trying to get good views from my used 8in saxon dob, i'm saving for a mak( i'll keep the dob for the kids and DSO)

14-10-2010, 08:49 PM
Actually Mak was mentioned in couple of posts, eg post #8.
TAL-200K or TAL -250K would be a nice choice, as Robert discovered.

14-10-2010, 09:06 PM
I would entirely agree, and there is a brilliant example for sale on this very forum at a great price ;)


14-10-2010, 09:47 PM
Yes, the MAKs do have many advantages over the standard Cass.,R.C. or SCT.

They're not perfect either, but the advantage of a small central obstruction, long focal length and in the case of the two brands I'm considering, no mirror shift.

They are well known for their sharp APO like performance.

15-10-2010, 11:04 AM
I must say, Maks have my preference when it comes to planetary viewing.

15-10-2010, 11:38 PM
it really comes down to the seeing conditions etc. No point having a huge telescope if u cant use the high mag on planets. However if you have very good conditions it best to get the largest aperture u can afford. Reflectors gather in the most light therefore u see the most detail on planets.

16-10-2010, 09:22 AM
Many people swear by their 4-5 inch APO refractors as the best for planetary viewing and claim ''high definition images''.

However this is no doubt inch for inch in aperture against other scope types.

I stated that I had settled on an 8 inch MAK as the scope of preference, but once again, I'm thinking twice before making the purchase as has been the same for SCT's after evaluating what has been said on this thread and Cloudy nights forum also.

So where do I go from here?

I wonder if a 16 inch dob would give me the occasional great view in good seeing and all other factors covered such as collimation, cooling etc.?

In a 60mm refractor at 175 x the moons of Jupiter are clearly defined ''disks'' and that's with a crappy SR eyepeice that came with it!

Can someone please confirm that a 16 inch dob can do this and more and not produce flared stars as moons, and can indeed show fine details on the planet's surface?

I am totally confused after so many opinions(the comments have been appreciated though) and am at the point where I am thinking of leaving amateur astronomy and selling off what I have before I even started back in to it again.

On top of that, I inadvertently got some members off side which is the furthest thing from my intentions when I started this thread, as I am not that kind of person.

All along, I only wanted to know what type/size optics are needed to visually see that detail when the seeing is right.


16-10-2010, 10:02 AM
Just go with your gut feeling Rob, most of the different telescope designs will give good views of planets: 5" refractor, 8" newt, 8" SCT(celestron edge with vents), 6" or 7" intes Maksutov Cassegrain(has very smooth, well figured optics and cooling fan).

The only thing I'd stress with a catadiotric scope is to get one with vents or vents and and fan, otherwise you might run into cooling problems. The Intes Maksutovs can be purchased with various focal ratios and secondary mirror sizes(eg. f15, 25% obstruction).

16-10-2010, 10:38 AM
Hi Rob,

The main limiting factors for choosing a scope (apart from technical capability) basically come down to Size/Manouverability and Budget.

It sounds like you have some good experience already but I would recommend that you consider your Scope Budget and Scope Manouverabiliy needs and base your decision on the following:

- Get the best quality Mount AND Tripod you can afford
- Get the best quality Optics you can afford
- Get the best quality mechanical setup you can afford (ie structural rigidity/alignment, baffling etc)
- I would not try to choose a VISUAL scope based on Astrophotograhic images. Base your decision on real visual testing if you can.
- Go and watch other users setup and breakdown their systems at starparty events to gauge if this suits your circumstances.

At the moment I use my system 100% for Visual Work and I have always been a Visual Observer. I have yet to look through another system that provides better Visual Planetary and DSO contrast and detail. (I am not saying there are not equal or better ones out there ;) but I would not like to think of the cost of those either :eyepop:)

There is another couple of members in Perth that each own both Tak APO's and Tak Mewlons. You may want to ask them and look though both.

If you can find a Visual Planetary scope of 4", 5" or 6" that is better than a Tak TSA120, TOA130 or TOA150 (or equivalent) then let us all know :).


16-10-2010, 11:06 AM
To make you mind up you should ask yourself: where will I be observing from? your backyard? some place else where you have to drive to?

Portability and manoeuvrability indeed is a very important factor: as they say: the best scope is the one you use the most. Very true statement. If you observe from home, it is ok to have a reasonably large/heavy setup, but if you plan on travelling, simply avoid anything too big if you don't feel you'll have the motivation to transport and setup everything every time.

The best would be for you to have a look through a couple of scopes...go to a star party or something and this will totally clear your mind as to what you like and what you don't.
Anyway, any type of decent quality scope will deliver good images of the planets. My advice is don't go for anything smaller than 6" if you wanna see nice details.
If you are unsure and cannot go to star parties and such, get yourself a 6" mak or SCT, preferably second hand so you don't spend too much money. These are terrific and flexible in what they let you see. I like the maks because they hold collimation very well and are very compact. The only thing you wanna pay attention to with these designs is to remember to leave your scope out in the backyard before you observe. It's no hassle: just take the scope out a couple of hours before you observe, go back to your movie or your dinner, and when you're done it's ready and in temperature. If you store your scope in your garage, you don't even need to do so.

Anyway, no matter which design you choose, you will get good images of the planets as long as you have a decent aperture.

16-10-2010, 11:38 AM
Here's one reviewers opinions on a range of scopes for planetary:

1. 14.5"
2. 16" & 18" Starmaster w. Zambuto and Pegasus optics 2. 12.5" Portaball from Mag1
3. 10" Teleport w. Zambuto optics
4. 8" Portaball with Zambuto optics
5. Takahashi FS-152mm & Currently Testing FCT150mm
5b. 6.1" D155 f/7 Astrophysics EDF
(Two above are close, TAK seemed better, Ed Ting is testing them against ea. other, and has come up
with the opposite conclusion)
(Takahashi TSC225 seems to fall approx. here.. as for the CN212, planets were not available)
6. C-14 SCT (not 100% positive, needed better seeing to confirm, brief test only, use this with skepticism)
7. C9.25" SCT Celestron SCT
8. 7" Starmaster with Zambuto optics, and 7" Teleport with Zambuto optics
9. Ultima 2000 8" SCT
10. 5" Takahashi refractor FS128 & 5.1" Astrophysics EDF (planets not avail. when FC125 was used)
11. Intes MN56 Mak-Newt (5" f/6)

Source: http://www.weatherman.com/scope.htm

16-10-2010, 12:06 PM
May I suggest an alternative Rob? Don't buy another scope at all just yet and join a local astronomical club instead. There are at least two in Perth - The Astronomical Group of Western Australia (AGWA) <http://www.astronomywa.net.au/> and The Astronomical Society of Western Australia <http://aswa.info/about.html>. Attend a few observing sessions with some of these guys and take the opportunity to look through a variety of other people's telescopes (most people are happy to let you look through their scope if you ask nicely). After you've done that you'll have a much better idea of what kind of telescope will work for you personally.

Happy observing whatever you wind up doing.

16-10-2010, 12:36 PM
Hi Rob,

The only prblem with all these reviews and opinions (including mine :innocent:) is that they are all done by different people, under different conditions, with different biases, needs and budgets. The reviews and opinions are only guides to consider.

You can end up going around in circles endlessly until you either look through something that you love straight away, test out several or just take the risk and buy something you have not seen before.

My SCT's were great scopes for the price and what they were, but I was never entirely happy with their visual perfomance no matter how well collimated and cooled down. For my latest scope I took the time to do extensive research and look through other peoples systems before buying.


16-10-2010, 12:52 PM
Thanks to all for your support on this journey to planetary bliss :D

Yes, It seems that I will need to look through some scopes to truly know what will give me the type of views I'm after.

As an example, I am able to get a William Optics M120 new for a very good price.

I know it's not a 6 inch aperture, but I'm hoping that it will provide some fine images as it seems like a very high quality refractor.
Or, is 6 inches in refractor aperture a ''minimum'' before I get to see small details in good seeing?


16-10-2010, 03:08 PM
Interesting you want to buy a smallish refractor for planetary viewing, I had a quick look at Jupiter through a bigger refractor, but the exit pupil diameter just had me seeing floaters....
On a light hearted note .... After 15 minutes of visual observing..... And I only did it because i needed to clean the CCD sensor....... I'm glad I haven't invested ANY money in a visual scope.:lol::lol::lol:

16-10-2010, 03:11 PM
6" refractors are not very accessible for most pockets to be honest. A 120 refractor is a pretty big refractor already. Rest assured: it will provide fine images. But at the end of the day it is "only" a 5" scope.

I cannot stress enough the previous recommendation: try various scopes for yourself first!
People have different tastes. Some like very clean, easy to read images even if it means less details can be seen. Others don't mind waiting for turbulence gaps to catch the finest details even if it means having to deal with a turbulent and sometimes unpleasant image.

What I am trying to say is that the refractor/reflector choice is a very personal one. Some people will love one type and others won't. And unless you have a look through both, there is no way of knowing which one you will prefer.
However, since you have not compared various designs, you will be happy no matter what you choose at this stage. Only later on, after you look through other scopes will you be able to say: I prefer my scope or I'd rather change for something else.

And one last thing: under good seeing, you'll always see good details :) The extend of the "good", this is what varies depending on the design. But the only way for you to feel how different or similar the images in various scopes are, is to find a way to compare them!

16-10-2010, 08:03 PM
Hi Rob,

I was going to leave it at that after my last novel-length post but there is one observation I’ll like to make here.

You cannot see the "disc" (which I take it you mean seeing it as a globe or other than a point-like star-lie appearance) of any of the Jovian satellites using a 60mm refractor. What you are almost certainly seeing at that magnification (x175) are "Airy Discs" formed by the telescope that are a natural consequence of the wave nature of light and the circular aperture.

In case you don’t understand what an "Airy-disc" this wiki article might help explain the basics.


The size of the airy disc formed by the optics of the telescope decreases as the aperture increases -- ie large 'scopes make the smallest theoretical Airy Discs. Small 'scopes the largest theoretical ones. They are so much easier to see in small 'scopes because they are so much bigger. This is basically why a good big telescope will always beat a good little telescope.

However, a well formed, stable airy disc formed by a star (a virtual point) is rarely seen in telescopes over about 10" because the seeing will very, very rarely be that good to show it -- you need quite high to very high magnification to see this diffraction pattern. They are much more often seen in small 'scopes because there is a high likelihood the seeing will be good enough. The seeing is rarely worse than a 60mm refractor can show. Airy discs are best observed at high or very high magnification (like x175 for a 60mm refractor).

A 60mm refractor forms airy-discs out of point-like sources (ie star-like) that are about 1.95 arc-seconds diameter. The largest of the Galilean Moons, Ganymede is around 1.6 arc-seconds diameter. The resolving power of 60mm is way, way insufficient to show it as a "disc" or globe". 25cm aperture is the realistic minimum to see the Galilean satellites as "globes", "discs" or "small-worlds".

Excellent 18" and above telescopes in very good conditions can show gross detail on the discs of Ganymede and Callisto.

Your 16" isn't showing globes (and hence they look star-like) probably due to a number of reasons (not in any particular order) Poor seeing, hot mirror, mis-collimation, insufficient magnification, inaccurate optics.

Just one (pick one, any one) or a combination of these foregoing factors can turn these little globes into spiky messes. From here, obviously I can't tell which is the problem for you. I saw the moons of Jupiter as tiny, crisp globes fairly frequently (say 40% of the time) in my 12" and maybe a little less frequently in my 18" (maybe 30% of the time). The times I failed to see thm as globes is probably due to seeing as I can eliminate the other factors I mentioned. The seeing is routinely crappy in suburban Sydney.

So in summary, it seems to me you are being fooled by the 60mm refractor into thinking it is showing these satellites as little globes (and believing it is therefore super-sharp) and then bemoaning the Newtonian. I think however (actually I don't think -- I'm certain in fact) you are seeing Airy-Discs in the refractor -- the diffraction pattern formed by a virtual point source passing through a circular aperture.

The Newtonian is actually far, far more capable of showing these moons as actual tiny little worlds (because the smallest Airy-Discs it can in theory form is in the order of 0.3 arc-seconds diameter). It is failing due to one or more of the factors I've outlined: Poor seeing, hot mirror, mis-collimation, insufficient magnification, inaccurate optics.

Large apertures, no matter what the design of the optical train, are far more "seeing sensitive" than small apertures, but when the seeing settles, a good big telescope will beat a good little telescope any day of the week.

Hope that helps!


Les D

17-10-2010, 10:51 AM

BTOW in Malaga ( http://www.btow.com.au/ ) used to have free planetary viewing every Thursday night. I believe Keith can give you any scope in the shop to test it and see if you are happy with what you see. Many AGWA members bring their scopes on Thursday nights and set up in front of BTOW for free public viewing. So, why not give Keith a call and ask him if they still run it on Thursday so you can take a look.

I own 12" Bintel Dob and you are welcome to come to my place and have a look through it at Moon and Jupiter. I am in Forrestfield.



17-10-2010, 10:51 AM
Thanks for all the information guys once again everyone.There really is a wealth of generous members here willing to help someone in need.

Yes, I checked the moon discs phenomenon, and Les is right, they are indeed airy discs (checked with inside and outside focus points).:rolleyes:

However they do look a lot better than my 8 inch newtonian (I no longer have the 16 inch........gone 15 years ago) which is an abortion of coma and possibly astigmatism............dissapointin g from older U.K. made optics:(:mad2:

I don't know where I will go from here, the last time I looked at a number of scopes was 12 months ago at my wife's school where they had a ''star night'' for kids and parents.

The seeing was average to good, but I did not see anything that came out as superior in the telescope type department.

I'll have to settle on something eventually...............I just hope it's not an expensive dud or lemon (which has been known to be quite common these days).


17-10-2010, 10:54 AM
Thanks Bob, I may do that and also get in contact with you and the kind offer.


17-10-2010, 01:32 PM
It seems rare to get a dud these days. The mid-tier manufacturers (Meade, Celestron/Skywatcher, GSO.....) seem to produce consistently decent optics versus the hit and miss of past years. They build to a price and polishing/figuring and mechanics are not as good as the top tier manufacturers, but I'm glad there are affordable but decent alternatives to Takahashi, etc.

17-10-2010, 02:30 PM
Hi Rob,

Yes they will be more aesthetically pleasing ie "better". But there is no "information" in them for your eyes to actually interpret as detail.

A good 8" will theoretically produce Airy Discs 0.57 arc-seconds in diameter. In order for them to be actually seen through the telescope, you will need magnification around x300 or higher and seeing that will allow a disc that small to actually form.

In theory and 8" has enough aperture that Ganymede would appear as a disc. It dosen't actually look like it because the "disc" of Ganymede is only about 3 "theoretical pixels" wide and not well enough "sampled" that is is actually seeable to the human eye as a proper disc (globe, whatever ...) in the real world. Once you get to a 10" it is around 4 "theoretical pixels" wide it becomes easier (and is therefore seen from time to time in that aperture). 12" it is 5 "pixels" etc.

Seeing of that order (around the 1/2 arc-second mark) is very uncommon where I live at least. You'd see it maybe a few times a year. For a pure planetary 'scope the returns you get from increasing aperture in increments above 10" become increasingly small because the seeing will not allow them to perform to their potential in resolution. Beyond 10", on 360 nights of the year, the only real advantage as you go to even larger apertures is higher/better colour saturation within the detail as Satchmo noted -- which makes contrast between this "spot" and that "spot" easier to percieve for the observer.

This is why my advice was to consider an 8" f/7.5 Newtonian. If well made, cooled and collimated, the view would be hardly different from a fine 7" APO refractor save for the diffraction spikes (which you can eliminate if you want to with a different design of spider -- though they make no difference to actual resolution). It would cost < 1/10th what a 7" APO does too.

The only real advantage in going over an 8" on planets (except on about a half-dozen nights a year) is in colour saturation. Assuming the optic is well made an 8" f/7.5 will be an inherently high contrast instrument because the secondary obstruction is in the order of 17%. It has longish focal length (about 1500mm), so to obtain a comfortable magnification on Jupiter of say x220 a 7mm ep is needed -- not so tiny that it is uncomfortable to use -- or a barlowed 12-14mm ep for even more comfort in terms of eye-relief and size of the eye-lens.

In fact if it was me and I wanted a pure planetary 'scope that was a permanent set-up, I'd go for a 10" f/7 with a 45-odd mm secondary. But, that's just me. You have to pick the right one for you and all your own pet likes and dislikes, your priorities and your pockets. As for what optical design and make that is purely your choice.There are a lot of good reasons why say a Maksutov, or a Schmidt-Cassegrainian, or a Newtonian, a Gregorian (or even a Schiefspiegler for that matter) might be the right choice for you. You've got to weigh them all up for yourself. I just want to make sure you pick what you pick for the right reasons.


Les D

17-10-2010, 03:32 PM
Thanks Les.
I now get the picture so to speak.

I have an EQ6 PRO ready and waiting, so I know that it's a resonably stable mount and not a pile of jelly like some that I've had over the years.

The scope choice will be a mixture of advice provided here on this thread, and my ability to handle a tube that is not too long, heavy, takes forever to adjust to temperature or just a PITA so that it gets used less frequently because of those exact reasons.

I find it interesting (and would not have known this previously) that large apertures can be your enemy in many ways when it comes to planetary viewing............that changes a lot in my way of thinking.

I also now know that optical quality is of prime importance(especially in high power planetary use...........without it, we may as well use our scopes as barbeque chimneys):)

It's been an interesting thread and I've learnt a lot from all that have been kind enough to contribute:thumbsup:

Many thanks to all the PM's sent to me and helping me with various specifics:)

I will let you all know what I decide upon when funds permit.:thanx::D