Planetary viewing

[Sagan]

While I'm impatiently waiting for a night when the clouds are at bay, I'm researching and trying to decide on the best kit for my goals. I own an Orion 6 inch (f/8) Dobs that came with a 25mm Plossl eyepiece and a Shorty 2x Barlow lens. I have a moon filter on the way and am looking to purchase one more eyepiece for the moment. I want to see pretty much everything I can in the skies, but my immediate goals are the objects in our own solar system. Top of the list is Jupiter. So, I'm looking for an additional eyepiece that will be the best for planetary viewing with the current setup I have now. After reading through some of the threads, am I right in thinking a 6mm Plossl is the best bet?

In addition, happy to hear any tips and advice for planetary viewing in general. Thanks.

[xtreemchaos]

i would go for a 10mm then with your barlow would make it a 5mm, then i wouldnt buy any more untill i got used to the scope.  clear skys charl.

[michael.h.f.wilkinson]

I used to have a 6" F/8 (home made with quality mirrors purchased), and they can be planet killers. The small central obstruction puts them close to apochromatic refractor performance. It is also an excellent scope for deep sky stuff as well. A 25 Plossl is very nice to begin with, and if you can stand the short eye relief (i.e.  do not wear glasses when observing) a 10mm Plossl is nice, but an 8mm might have the edge on nights of good seeing. Orthoscopic eyepieces are also great for planets, again, if you do not wear glasses. For those with (and without) glasses, the vixen LV, NLV and SLV series are all very good. I have the 5mm SLV and it gives near orthoscopic performance with great eye relief. I would try to buy second-hand, as that saves a lot of money. For my kids I got a few (second-hand) "Planetary" EPs and they work surprisingly well in their little F/4.3 Newton. In your slower F/8 they should be even better.

There are of course all sorts of exotic (=expensive) EPs around (Tele-Vue, Pentax, Zeiss, Nikon, .....), but I would not go for those. I spent many years using Plossls at the longer focal lengths, and Vixen LVs at the short, and had a great time. Only after gaining a lot of experience did I see any shortcomings, and only then did I start to upgrade.

[Stu]

Jupiter has quite low contrast features and doesn't benefit from over magnification. It is often best at around x180.

I would aim to have a couple of choices at perhaps x150 and x180 to take account of different seeing conditions. That would be an 8mm and probably a 7mm would best at x171.

Important things for planetary observing?

Cooling

Collimation

Seeing conditions

Planetary elevation

Patience

Get all these things right and you will get great views in your scope. At Opposition in March Jupiter will be at a reasonable elevation and around its closest so you should get best views then, but well worth taking opportunities in the months before and after.

Patience? Seeing conditions are often variable. Observe Jupiter for a few minutes and you will see the moons and a couple of bands. Observe for an hour or more and you will begin see festoons, barges, perhaps the GRS or shadow transits plus multiple bands. These become visible for short periods of time (seconds) when the seeing settles and after a while your brain begins to build up a much more detailed picture of what is visible.

Wrap up warm and put in the time at the eyepiece, well worthwhile.

[Charic]

If you keep within certain parameters you will avoid over magnification. Too much magnification just destroys the image.

You own a 6" f/8 telescope with a focal length of 1200mm!

Using these figures for the sums. Focal length 1200 divided by the aperture 150 equals  8? 8mm should be the perfect eyepiece to achieve a magnification of 150x power.  To obtain power, you divide the focal length of the scope 1200 by the focal length of the eyepiece 8mm equals 150, the aperture of your scope. So the label on the scope says it all. Your aperture is the designed maximum so 150mm Aperture = 150x power and f/8 Focal ratio  = 8mm eyepiece to achieve this maximum designed power.

Exceed these limits by a small amount and you wont go wrong, but a 6mm eyepiece giving 200x power will work, but reaching the extremes of the theoretical power of 300x your scope could achieve ( in a vacuum of nil weather and out in space?) 

An 8mm eyepiece alone will give you 150x power and if you Barlow that eyepiece, you could achieve the absolute limit of 300x power, but not much up there to see at 300x except the Moon, and that will be difficult to follow and the image will be washed out and blurred.

Astroboot do a very nice 9mm revelation Astro Plossl for £15. That will do you fine, and has a field of view of about 52° which ok .They do a 12mm too?

[Charic]

Patience? Seeing conditions are often variable. Observe Jupiter for a few minutes and you will see the moons and a couple of bands. Observe for an hour or more and you will begin see festoons, barges, perhaps the GRS or shadow transits plus multiple bands. These become visible for short periods of time (seconds) when the seeing settles and after a while your brain begins to build up a much more detailed picture of what is visible.

+1 couldn't agree more. But when it happens, its memorable. 

[Stu]

Just to put a little more context on this, the concept of exit pupil is worth getting your head around. This is the diameter of the beam of light exiting the eyepiece and entering your eye. Too large and it will be larger than your dilated pupil, wasting light, causing the sky background to be washed out, and in extreme cases making the shadow of the secondary support visible. Large exit pupils are associated with low magnifications and around 6 to 7mm is as big as you want to go, often 4 to 5 is better giving a darker sky background.

At the other extreme, too small an exit pupil can make the view too dim and cause floaters to be visible in your eye.

1mm is a good guideline minimum exit pupil in dobs, but in smaller refractors I find that going down to 0.5mm is possible and worthwhile for planetary observing.

You find the exit pupil by dividing the eyepiece focal length by the focal ratio of the scope.

In your case 8mm/f8 = 1mm which is ideal

7mm/f8 = 0.875, still quite acceptable

5mm/f8 = 0.625 which is getting a little small. The view would likely be a little dim and any floaters you have may be visible. This is still useful on the moon, double stars and also targets like Mars which needs a lot of power to see the detail well.

[michael.h.f.wilkinson]

Wen I had my 6" F/8, my 9mm symmetric EP was used most often on planets, the 5mm ortho rarely for reasons listed above. I built the scope as a teenager, and could not afford many EPs, but for planetary a range of EPs of 10 (120x), 8 (150x), 7 (171x), and 6mm (200x) would have been almost perfect, with 5mm (240x) as an extra option for those rare nights of excellent seeing, and high-contrast targets like the moon and Mars. If I could only pick one the 8mm would be ideal. If I could pick two, 8mm and 6mm.

As others have said: take your time observing. I have frequently had people at the scope first saying things like "oh, that's nice", and then suddenly exclaiming "WOW!!" as the image momentarily snapped into sharp focus. It is well worth waiting for those moments

An equally important skill is to learn to say "YES, YES, YES, YES!!!!" or "WOW!!!" in a hoarse whisper, whilst bouncing around the garden (or whatever observing patch you have), so as not to wake the neighbours .

[Owmuchonomy]

Great advice above.  If you have the inclination you might wish to supplement your Jupiter viewing sessions with some handy apps.  I use 'Jupiter's Moons', an app that does what it says on the tin, and more.  You can then predict such events as GRS transition, moon shadow transits and moon eclipses so that you optimise your precious time at the eyepiece you choose.  The much more comprehensive app Sky Safari does the same but you may have budgetary constraints.

[Sagan]

Thanks everyone for all the great advice. I didn't quite understand exit pupil before, but it all makes sense now. I don't wear glasses, so it sounds as if I should be fine with 1mm eye relief. Based on all the comments, it sounds like I should look at getting a 8mm eyepiece for now and then add to the collection as I get the hang of things.

An equally important skill is to learn to say "YES, YES, YES, YES!!!!" or "WOW!!!" in a hoarse whisper, whilst bouncing around the garden (or whatever observing patch you have), so as not to wake the neighbours .

 Haha, I really do wonder what the neighbors will think when I'm out in the garden in the early hours staring down a telescope that's almost as big as I am with a big grin on my face. 

[Stu]

Thanks everyone for all the great advice. I didn't quite understand exit pupil before, but it all makes sense now. I don't wear glasses, so it sounds as if I should be fine with 1mm eye relief. Based on all the comments, it sounds like I should look at getting a 8mm eyepiece for now and then add to the collection as I get the hang of things.

Haha, I really do wonder what the neighbors will think when I'm out in the garden in the early hours staring down a telescope that's almost as big as I am with a big grin on my face.

Don't confuse exit pupil with eye relief. Exit pupil is as I described, eye relief is the distance from the exit lens that the image is formed ie where you have to place your eye to see the image properly.

Some short focal length eyepieces such as Orthos and plossls have quite tight eye relief, meaning you have to place your eye very close to the eyepiece and impossible for glasses wearers.

I guess about 10mm or more is comfortable, (15mm better) without glasses but everyone is different in what they find comfortable.

[Charic]

Sagan....I predominantly use 60° BST Starguiders on my scope available  from here, http://stores.ebay.co.uk/Skys-the-Limit-Astro-and-Optical/BST-Starguider-ED-/_i.html?_fsub=2568750014 The 8mm and 12mm could be perfect for your needs.

If you bought the pair, and did not like them, return both for a  full refund, or just the one you don't like. But If like me you like both, then in your own time,  there are others to purchase. I would avoid the 3.2mm and the 5mm, as they won't get used so much with your scope, and you already have a 25mm. The BSTs are not Plossl type eyepieces. The BST's give good results and a decent field of view  compared to any of my Plossl's  and  good eye relief too.

[John]

Sagan....I predominantly use 60° BST Starguiders on my scope available  from here, http://stores.ebay.co.uk/Skys-the-Limit-Astro-and-Optical/BST-Starguider-ED-/_i.html?_fsub=2568750014 The 8mm and 12mm could be perfect for your needs.

If you bought the pair, and did not like them, return both for a  full refund, or just the one you don't like. But If like me you like both, then in your own time,  there are others to purchase. I would avoid the 3.2mm and the 5mm, as they won't get used so much with your scope, and you already have a 25mm. The BSTs are not Plossl type eyepieces. The BST's give good results and a decent field of view  compared to any of my Plossl's  and  good eye relief too.

BST Explorers / Starguiders are worth thinking about.

If you buy on line similar refund deals are available from any on line retailer by the way - they have to offer it under the UK Consumer Contracts Regulations.

[ronl]

The B S T Statguiders are a very good eyepiece, but this depends on your budget, they are £49,

the 8mm should work well in your scope, as Charic says, if it's not to your liking you can send it back,

but I am sure you will be over the Moon with it, pun intended, don't expect a huge Moon sized image

of Jupiter, but it's detail you need to see, not size, hope you find what you are looking for, and I forgot

to mention, the Starguiders are very good value for money.

Clear Sky's.

[Sagan]

Those 8mm BST Starguiders do look really good. Thanks for the recommendation! Definitely want the detail over the size.

[Mak the Night]

I was lucky in the summer and got to observe a twilight setting Venus and Jupiter. I used my Newtonian with a 3x Barlow and a 15mm EP to get 180x and was gobsmacked to see the Great Red Spot on Jupiter let alone the Galilean moons. I knew the spot would be visible as I'd checked on my software, but I was still impressed.  

It looked like this:

[Dave In Vermont]

If you'd like a free program to show you which moons are which around Jupiter:

http://www.astrosurf.com/rondi/jupiter/

Scroll to the bottom of the page for the download.

More proof the best things in life are free.....

Dave

[Peter Drew]

Sounds like a case for considering a zoom eyepiece, these work well at F8 and for the price of one Starguider you get several magnifications. The Seben 8-24mm zoom and its clones can be recommended, not the 7-21mm though. 

[Mak the Night]

If you'd like a free program to show you which moons are which around Jupiter:

http://www.astrosurf.com/rondi/jupiter/

Scroll to the bottom of the page for the download.

More proof the best things in life are free.....

Dave

That looks really interesting Dave. Not quite the same as this Jupiter 2 though lol.

[theghettostylus]

It's probably already been said but when I was using the old skywatcher 130 I sa

[theghettostylus]

blumming kids aha I saw Jupiter very well in that scope (haven't tried in the new one) it was fairly small and with my Barlow it killed the detail so hopefully your Barlow is of better quality as that may kill the detail if not. That's all I can offer.

[dweller25]

Here is my drawing done with a 7" scope at x188 - which is a good magnification for Jupiter which as others have said has subtle contrast so keeping the power low will help.

http://stargazerslounge.com/topic/239748-jupiter-21st-march-2015/

[Mak the Night]

Jupiter can be a bit hit and miss depending on conditions. When I saw the Great Red Spot earlier in the summer I used my 130mm Newtonian with a 3x TeleVue Barlow. I used several eyepieces including a 20mm Celestron erecting EP, a 15mm Kellner and a 17mm Celestron Plossl. The sky was still blue in the background and Jupiter was setting (with Venus in the constellation of Cancer) at around 29° about  21:45 BST. The detail, even with the erecting EP was quite remarkable considering its low angle in the sky.

About a month later I viewed the setting Jupiter and Venus again (now in Leo), but this time with my Mak combined with a 20mm TeleVue Plossl and TV 2.5x Powermate. I may have started observing earlier in the evening, I can't recall, but Jupiter looked great at first (even with no GRS visible lol) but it deteriorated very quickly as it got lower than about 14° in the still blue twilight sky. I'm guessing there was a lot of thermal rising with it being so low. 

I'm glad I caught Jupiter and Venus setting before it was too late in the year though.