PLANETARY VIEWING ?? -aperature rules?or telescope type?

[Sylvain (Jon)]

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.

[casstony]

Here's one reviewers opinions on a range of scopes for planetary:

BEST PLANETARY SCOPES SO FAR..
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

[rmcconachy]

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.

[issdaol (Phil)]

Hi Rob,

The only prblem with all these reviews and opinions (including mine ) 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.

Cheers

Originally Posted by casstony

Here's one reviewers opinions on a range of scopes for planetary:

BEST PLANETARY SCOPES SO FAR..
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

Originally Posted by rmcconachy

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.

[robz (Robert)]

Thanks to all for your support on this journey to planetary bliss

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?

Cheers,
Rob.

[Alchemy (Clive)]

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.

[Sylvain (Jon)]

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!

[ngcles]

Hi Rob,

Originally Posted by robz

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 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.

http://en.wikipedia.org/wiki/Airy_disk

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!



Best,

Les D

[bobson (Bob)]

Robert,

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.

cheers

bob

[robz (Robert)]

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).

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

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).

Cheers,
Rob.

[robz (Robert)]

Originally Posted by bobson

Robert,

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.

cheers

bob

Thanks Bob, I may do that and also get in contact with you and the kind offer.

Regards,
Rob.

[casstony]

Originally Posted by robz

...............I just hope it's not an expensive dud or lemon (which has been known to be quite common these days).
Cheers,
Rob.

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.

[ngcles]

Hi Rob,

Originally Posted by robz

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

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).

Cheers,
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.



Best,

Les D

[robz (Robert)]

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

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.

Rob