What's the bigger limit for planetary observing?

[pipnina]

I was looking at saturn for the first time the other night and I thought "Is it that blury for everyone, or is my scope holding me back"

I used newt for the web to work out that my scope has an angular reolution of around 0.9 arcseconds. This would put jupiter at about 35 optical 'pixels' wide/high. Not too impressive. And then we must consider that my scope cost £135 including the EPs and the mount. I imagine the optics aren't quite living up to that 0.9 arcsec value.

So, do people with apertures of 8" (0.57 arcsecs) or 12" (0.38 arcsecs) get much better views of jupiter than two of the storm bands? Or does the atmosphere play more of a role than I think?

[jambouk]

I'm not sure what scope you are using as i can't see signatures in my iphone. Not sure where you are in the world either.

The more aperture you have the better, as greater light gather.

Newts tend to have shorter focal lengths than the so called planet-killers like the maks and sct scopes, and a longer focal length, and focal ratio of about f/20-f/30 helps (depending on numerous other factors).

Atmospheric conditions play a massive role. If you live under the path of the jet stream, and there is turbulent air in it between you and the object you are looking at, then the view will be more blurred than if that air is less turbulent. This website shows what the jet stream is doing and likely to do in the coming hours:

http://www.netweather.tv/index.cgi?action=jetstream;sess=

For imaging, focus is very important. I personally wouldn't get too caught up in the whole arcseconds per pixel thing for the time being.

James

[pipnina]

I'm not sure what scope you are using as i can't see signatures in my iphone. Not sure where you are in the world either.

The more aperture you have the better, as greater light gather.

Newts tend to have shorter focal lengths than the so called planet-killers like the maks and sct scopes, and a longer focal length, and focal ratio of about f/20-f/30 helps (depending on numerous other factors).

Atmospheric conditions play a massive role. If you live under the path of the jet stream, and there is turbulent air in it between you and the object you are looking at, then the view will be more blurred than if that air is less turbulent. This website shows what the jet stream is doing and likely to do in the coming hours:

http://www.netweather.tv/index.cgi?action=jetstream;sess=

For imaging, focus is very important. I personally wouldn't get too caught up in the whole arcseconds per pixel thing for the time being.

James

I'm using an f/6.9 130mm newt. (900mm fl)

I live in s/w england. So the jet stream is right above me    Is the wobblyness caused by the atmosphere noticable to the eye? Because I haven't ntoiced it at all I don't think. Jupiter/saturn/the moon don't seem to wobble or change focus randomly at all. Perhaps I need another observing session with the currently rising moon to make sure

[michael.h.f.wilkinson]

Saturn is low at the moment, so you are looking trough a very thick layer of atmosphere. That makes conditions much worse. One thing apart from a wobbly appearance is that there is atmospheric refraction. The red, green and blue images are shifted slightly. In principle you 130mm F=900 scope should be quite good at planets. Collimation might be a problem, and is always worth checking. Another point might be the EP. Most scopes come with a reasonable 25mm and so-so 10mm. You 10mm gives 90x which can resulve quite some detail on planets, although a 7mm might be better. Getting a better EP for planetary work might be necessary.

[dweller25]

Saturn is a very poor sight at the moment - it's too low for good visual detail.

Jupiter is better but it is well past opposition so it's getting tough to see details within the belts in my 8" scope.

[jambouk]

If the planet looks pretty small through the eye piece then the blurryness you describe could be due in part to the jet stream and atmospherics, the resolution of the inage on your retina might be such that you can't distinguish blurryness from wobble. I can certainly see the atmospheric effects on planets and the moon when i use my C11 scope, like heat waves washing over it. As michael says, it could also be in part due to poor collimation. One will never see it as clearly as the pictures people generate; well not with amateur telescopes like the ones we own, and not from the UK either

James

[paulastro]

With respect to jambouk, it's not always true that 'the more aperture you have the better' for planetary and Lunar visual observations. If the seeing is poor, and the altitude of the planet is very low, (as it is currently with Saturn) it will often be the case that a smaller good quality telescope will perform better as it is less affected by the poor seeing and low altitude of the planet.

The planetary seeing at my home site is, on average, such that my four inch refractor will often outperform my six inch and bigger scopes. It is wise to take into account the conditions most frequently experienced at your own site before opting for a telescope just for planetary viewing.

Of course, if your main interest is mainly deep sky, or you want a more general purpose scope, a bigger aperture may better suit your needs.

Also of course, the quality of the optics, the choice of eyepieces and the experience and skill of the observer all have a role to play in what your observing experience will be.

[jambouk]

Yes, i agree that convention says when seeing is poor, smaller aperture, fast scopes may offer superior views, which is why i added the caveat about numerous other factors having an influence (including cool down times, seeing, collimation etc). I've never tried this comparison for myself as if the seeing is really poor i pack up and go to bed

However, when the seeing is reasonable to good, then i find more aperture wins; the C11 gives much better (more detailed) views of Jupiter and the moon for me than say my 127 Mak or an old Vixen refractor i have.

Damian Peach would be using a small aperture scope if it allowed him to capture the best images possible, but he uses as much aperture as he can - though he also spends a fair amount of time near the equator... Lucky so and so

James

[John]

Saturn is a "special case" this year because it's never going to get too high in the sky. It may well be that refractors and other smaller aperture designs will punch through the atmospheric turbulance rather better this opposition because of that.

When the planet has been higher in the sky I've found it soaks up magnification better than Jupiter does so my 12" aperture dob does the business because it's "coasting" at, say, 250x-300x wheras my 4" and 4.7" refractors are at their limits there.

[paulastro]

Thanks for your response James. Yes, Damian does use larger aperture telescopes for his imaging - that's why in my post I was careful to make it clear I was talking about 'planetary and Lunar VISUAL observations'. (excuse the capitals!)

I must say, I don't go too much with 'convention' myself. My comments were based on information gleaned from numerous observing sessions over the years. It's often the case that many things are passed off as being factual information (by word of mouth, in books and other media) without really checking their validity in the real world. If possible I prefer to check things out myself - it's surprising the number of 'conventions' in astronomy that are rather ill-founded. It's also great fun checking things out for oneself, though it may not always be possible.

[hgjevans]

I used newt for the web to work out that my scope has an angular reolution of around 0.9 arcseconds. This would put jupiter at about 35 optical 'pixels' wide/high. Not too impressive. And then we must consider that my scope cost £135 including the EPs and the mount. I imagine the optics aren't quite living up to that 0.9 arcsec value.

The thing you need to keep in mind when considering a calculation of  resolution for any optical system is that you're only calculating the theoretical limit of the system - that is to say, the best resolution that the fundamental physics of light would allow that system to resolve, if it were a perfect system, and if the light reaching it were perfectly coherent. In practice, all sorts of things conspire to degrade performance from that limit - mirrors aren't perfectly figured, or 100% reflective, or perfectly collimated; eyepieces likewise aren't perfectly ground and polished, multicoatings still don't completely stop reflections, their overall design cannot completely eliminate both transverse and longitudinal chromatic aberrations, and so on, and on. And all of that is before you even start to consider atmospheric turbulence, or just the limits of the observer's eyesight.

As you say, an 8" scope (which is what I've got) supposedly has a theoretical resolution limit of a bit over half an arc-second. But from the few months experience I've had with it I'm quite pleased if I can resolve a double star pairing at 2 arc-seconds - and even then it's usually a fleeting moment of stillness, rather than a constant clearly resolved pair with dark space between. 

As it happens, I had a good look at epsilon Lyrae last night and was just about able to resolve both pairs, but briefly in each case. IIRC they are at about 2" and 3", which according to the theoretical limit ought to be easy for my scope. But I'm observing from a suburban back garden, looking out over probably about 3/4 of the town, with all the heat that's rising after a sunny day - all that bricks and concrete and asphalt acts as a storage radiator, and it really doesn't help.

So all in all, I wouldn't worry too much. Here on SGL I've lost count of the number of observing reports or first light reports I've read where fabulous sights were described, achieving amazing things, and it's tempting to think that eveyone else is getting much more from their equipment than I'm getting from mine. But on such a large and diverse forum, there will always be some people who have the 1/10-wave mirrors, and the eyepieces that cost more than my entire kit, and some are fortunate to live somewhere that looks out over miles of fields under beautiful dark skies, and so on. I'm sure most of us, most of the time, are having much more ordinary observing sessions, but we're probably less inclined to write and post an account of how we didn't manage to see this, or couldn't quite resolve that!

[Scott]

With respect to jambouk, it's not always true that 'the more aperture you have the better' for planetary and Lunar visual observations. If the seeing is poor, and the altitude of the planet is very low, (as it is currently with Saturn) it will often be the case that a smaller good quality telescope will perform better as it is less affected by the poor seeing and low altitude of the planet.

The planetary seeing at my home site is, on average, such that my four inch refractor will often outperform my six inch and bigger scopes. It is wise to take into account the conditions most frequently experienced at your own site before opting for a telescope just for planetary viewing.

Of course, if your main interest is mainly deep sky, or you want a more general purpose scope, a bigger aperture may better suit your needs.

Also of course, the quality of the optics, the choice of eyepieces and the experience and skill of the observer all have a role to play in what your observing experience will be.

There is certainly a school of thought that dictates smaller aperture can "punch through" bad seeing. I can't say with any certainty how true this is. If it is the case though, can you not put a focus mask on a large newt (for example)? Reducing a 450mm f4 down to 100mm will in effect be giving you an f18 scope. isn't a slow scope desireable for planetary? It then allows for the same scope to be used for deep space as well. This is a serious question as I truely have no idea. 

[jambouk]

Yes, i love the experimenting side of things too, just that with so few clear nights most of the time I just do what I know works. The guys and gals who have made advancements in astronomical science over the last 300-400 years must have enjoyed such great satisfaction when countless nights of experimenting eventually resulted in new technichogical discoveries or new astronomical ones. Something I suspect very very few people today experience.

James

[paulastro]

In reply to your post auspom, indeed you can put an aperture mask onto a large scope and it will then show a steadier image according to the size of the mask you use, along with lesser resolution than it's native aperture.

A slow scope was certainly considered better for planetary work in the past, generally because it was easier to make a longer focal ration refractor of good quality than a shorter one. Depending how far you go back of course there were less exotic glass types anyway even if you could figure a good shorter one. The longer focal ratios were of course also the only way of diminishing chromatic aberration to any great degree in a refractor.

In the present, one of the advantages of the longer focal ratio is in that it is more forgiving in that it will have a greater depth of focus (important in poor seeing) much as an f8 telephoto lens will have a greater depth of focus (depth of field) than an f4 lens. In practice this means you don't have to refocus a slow telescope as often as a fast telescope when the seeing changes. There are other factors but this is the gist of the difference between long and short f ratios in refractors.

It is true, as you say that if you have a larger reflector giving good views of faint objects stopping it down will give steadier views of the planets in bad seeing, but in practice I've met few people who have ever done this. I think the reason is probably because many long standing practical astronomer usually migrate to having two or more scopes for different purposes. For instance a short focal length smaller refractor can give you wider field views of extended deep sky objects than a large reflector - since when you stop a telescope down it's focal length remains the same even though it's effective focal ratio does not. (as you pointed out) This is only one example, but there are numerous reasons why astronomers have the need of a variety of intruments.

On the positive side, if you are a member of an active astronomical group, or can visit somewhere like The Astronomy Centre near Todmorden you can use a large variety of scopes without having to buy them all yourself. To get hands on experience with different scopes is by far the best to help people decide which is best for them.

[jambouk]

Paul, if you had say £250 to buy a scope (ignore the mount for now) for largely planetary and lunar observing under UK skies, what would you buy? For £500? For £1000?

James

[Scott]

Cheers for your response Paul. One of the reasons I enjoy star parties is the opportunity to look through many different 'scopes. .

I think I'll stick with  imaging though (dso)...it's far simpler...just go fast. and its cheaper to change 'scopes than ccd's to aquire different fov's

[ollypenrice]

Despite its low elevation we've had some utterly stunning views of Saturn recently. Certainly as good as I've ever seen.  We're south of the UK at Lat 44 (SE France) and Saturn doesn't become stable until a few hours after dark when it is almost as high as it's going to get. However, at this point the views have often been ridiculously good. I've been amazed by this given its unpromising altitude. So I'd advise you not to despair!

If the OP sees the image 'boiling' or moving then the seeing is indeed to blame. It the image looks disappointing but still and stationary then the telescope may need collimating or may just not be performing well. We've been using a 10 inch SCT but small apertures should still give a good view of Saturn when the seeing allows.

Olly

[Earl]

seeing has the biggest impact on astronomy as you cant fix that, collimation for example you can, if your well collimated regardless of apature etc, the seeing will be the potentiall game killer, be this unsteady skies or LP. (LP on planets not a major problme on the whole though)

Do you happen to be viewing over houses as that cab ruin seeing with the thermals.

[paulastro]

Hi James. Gosh, that's a difficult question. It's difficult because what suits me may not suit you. An element for many people in any telescope purchase is quite personal based on things which often aren't really much to do with what is the best telescope for their purpose in terms of performance.

Just some of the factors you need to consider are:

Where are you going to use the scope most of the time?

What are the seeing conditions like, on average, at this site?

Where are you going to keep the scope?

How big a scope can you carry from the storage place to where it is to be used?

Will you ever need to take it on public transport?

What will you ambitions be, if known, for your astronomical interest, will you ever want to image?

Does it need to go in the boot of the car to different sites?

Have you already got any preference for using any particular scope, and if so whey?

Is image quality or light grasp more important to you?

Do you want to use the scope for anything else other than astronomy?

Will you be having all night sessions or quick grab and grab sessions more frequently?

- and these are just for starters!

I could tell you what scopes I have, but the reasons I have them is connected to things that may not apply to you, such that I can go to The Astronomy Centre and use all sorts of scopes when I want to, but don't want to use them often enough to have at home.

I'm quite happy to answer your questions but it may be better if you personal message me and we can take it from there. I am also happy to speak with you on the phone if you wish which will save an awful lot of time.

I'll look forward to hearing from you.

Regards, Paul

[Dave Lloyd]

On the subject of aperture masks, with a refractor is there anything to be gained by getting it as close to the objective as possible? To eliminate scatter etc. I understand it would be impractical for a reflector.

[paulastro]

Thanks auspom (Scott?) Yes, imaging is a whole different kettle of fish, and I never was good at fishing! As you say, anywhere there are lots of telescope to use has to be the best way of finding out what suits. Star parties are the bees knees for this as you indicate. Good luck with your imaging, it's too much of a dark art for me, ha ha. (I sometimes go dabbling, but very seldom) Regards, Paul

[paulastro]

Hi Dave. I would agree that it's probably best to get as close to the objective for the reasons you state. Also to make it dark so there are no nasty reflections, and also having it close would prevent having air trapped between the mask and the objective. Particularly so if the mask is a much aperture than the objective size. It would be nice to experiment with different size masks and also to vary other things, you would glean a whole lot of useful information. It's just a shame we don't have more clear skies to try all these things out as often as we'd like.

The only masking I have done in recent years has been by accident! The most common one for me is seeing that the image looks a little fainter, and then noticing that the observatory wall is intruding on the light path so I am only using a fraction of the available aperture. The worrying thing is that when this happens I sometimes don't notice it until I'm down to a quarter or less of my aperture! Mmmm.

[Stu]

I have observed Jupiter with Shane's 16" dob at full aperture and with a 170mm aperture mask. The views in both were lovely. Ultimately there is more detail at 16" but the seeing plays a bigger role in disturbing the image. The periods of stable image are more frequent with the aperture mask so it does have its benefits.

The most detailed views I have had have been with my 16" under reasonable conditions, but I frequently observe with decent quality 4 and 6" ED refractors and very much enjoy the crisp clean views visible with them, and their ability to deal with poorer seeing. Recently I've had good colour in the GRS and nice detail in the belts with a 4".

[pipnina]

I managed to get outside not long after sunset and look at the very crescent moon, venus and Jupiter. I was looking specifically for atmospheric distortion this time (and enjoying the sights in the process, of course).

I used my 10mm EP (90x) and did this time notice wobbling on the moon and jupiter (with venus seemingly being immune, it is small after all). It was most noticable on the moon and only slightly with jupiter. Despite this I think I saw more detail in jupiter tonight than I've seen before, I normally only see two bands. Unfortunately the GRS wasnt out at the time (it never is when I'm looking!) so I can't judge by that.

[jambouk]

You should try and match your own observations of the seeing with the jet stream data and see if this is indeed influencing your seeing.

James