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Observing Jupiter: Magnification and Contrast

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#1 QuinnD

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Posted 01 December 2013 - 03:54 PM

This is my first forum post after months of learning from this very helpful community.

So, Jupiter is just starting to be observable over the stand of trees behind my house at times I'm usually out. I have NexStar 8SE and the Celestron eyepiece kit, and I've been experimenting with various eyepiece combos.

Last night I started out with my 17mm ep, and pushed the magnification up one ep at a time all the way up my 6mm ep. My scope had been outside all day, so it was well-cooled, and the seeing seemed fine. It was neat to see the planet so apparently large in the eyepiece.

The question I have is why the contrast was so much worse as I pushed up the mag. When I prepared to go the next object I put in my 32mm, and even though the planet was tinier, I felt like I saw just as much detail as with the higher mag eyepiece.

I enjoy learning about all the optical tradeoffs in this hobby. Is there a tradeoff between power and contrast? How do experienced planetary observers address this in terms of optimal ep selection?

I did a quick check if collimation, and it is a bit off center. I'm not quite brave enough to take a screwdriver to my scope yet, but would improving collimation improve contrast? I know that SCTs aren't optimal planetary scopes, but any other tips would be appreciated.

#2 T1R2

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Posted 01 December 2013 - 04:10 PM

333x is a lot of mag, it seems you've overpowered your target for the seeing conditions and collimation, I'd suggest dropping down in the area of 165-180x, if stars are twinkling more than 5 times a sec. seeing is not that good, more X's dim the image, so you want to balance between image scale and brightness, collimation is critical in a SCT, when nights are calm, I'd suggest practicing collimation and may invest in Bob's Knobs collimation screws, it makes it easier than fumbling in the dark with a screw driver close to the corrector plate, SCT's are great planetary scopes if you got a good one, and if properly collimated and cooled will give exquisite planetary views, my C11 is awesome. just look at all the great planetary pics taken through C11-14's

#3 QuinnD

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Posted 01 December 2013 - 04:29 PM

Thanks! This is extremely helpful. Can I ask two follow ups?

First, on the seeing conditions. I had thought that if seeing was bad I would see it as motion in the image. I've experienced this sometimes. Can seeing affect the view even if I can't see the rippling air distortions directly?

Second, for collimation. What counts as good enough? When I put in an 8mm eyepiece and de-focus, I get a difraction pattern, and I'd say the center is shifted maybe 10% of the distance to the last diffraction ring (I can see about 5 or 6 rings). Is that worth messing with?

Didn't mean to knock SCTs for planetary viewing, BTW. (I bought one after all, and I love it.) I just know there are some who prefer other designs.

#4 GlennLeDrew

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Posted 01 December 2013 - 04:30 PM

If you can tell that collimation is at all off, then tweaking further is very important to getting the most out of the instrument. Especially at high magnifications and when scrutinizing such small objects as planets.

The apparent decrease in contrast is mostly the combination of atmospheric seeing limitations and the resolution of diffraction effects at the small 0.6mm exit pupil. And if collimation is on the poor side, it softens the image as well.

#5 brianb11213

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Posted 01 December 2013 - 04:36 PM

I did a quick check if collimation, and it is a bit off center. I'm not quite brave enough to take a screwdriver to my scope yet, but would improving collimation improve contrast? I know that SCTs aren't optimal planetary scopes, but any other tips would be appreciated.

Accurate collimation is CRITICAL when observing at anything but the lowest powers. It doesn't matter how you do it, but do it you must.

As for contrast & magnification: overdoing the magnification reduces apparent contrast especially when the details are fine and the contrast relatively low as is the case with Jupiter, for everything apar from the major belts and (sometimes) the Great Red Spot. Jupiter does not stand magnification as well as Mars because the contrast of the disc features is lower. 20x to 25x per inch of aperture is about right with medium sized scopes like your 8" SCT i.e. a focal length somewhere in the 12mm - 10mm range. 6mm is definitely too much, even when the seeing is perfect & the collimation spot on.

The contrast of the details on Jupiter is usually improved by the use of a blue, blue-green or green filter. In a dark sky, a neutral density filter might be needed (as well as any coloured filter) to reduce glare, which tends to mask detail.

#6 T1R2

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Posted 01 December 2013 - 04:44 PM

it depends on how bad it is, usually its hard to get a night that's totally still and we have to make compromises, a little turbulence we all have to deal with on most nights, but drop down on the mag until its acceptable, and sometimes its heat currents in the tube that causes wavey images

yes, your scope is out of collimation, significantly I'd say, and needs to be adjusted, if you scope came with instruction on collimation follow them, place your finger in front of the corrector without touching it, pointing at the secondary screws and run you finger around the edge to point at the slim part of the rings(the way the image is skewed( this will be the one to adjust, then repeat with a smaller diffraction pattern until satisfied or it eliminated the best you can get it.

#7 acochran

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Posted 01 December 2013 - 05:59 PM

The higher in the sky Jupiter, or any object is, the better. Close to the horizon, the air is too squiggly (tech. term). Also, "seeing" conditions are generally better after midnight.
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#8 brianb11213

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Posted 01 December 2013 - 06:47 PM

Also, "seeing" conditions are generally better after midnight.

The rest of your advice is good & accurate but "better seeing after midnight" is not true in many, perhaps most, locations. Lots of reports of this phenomenon are likely caused by telescopes which have been out for some hours finally approaching thermal equilibrium.

I have found, at several sites, there is a fairly reliable "steady" interval about 20 - 40 minutes after sunset - of course the sky is still very bright then, but it's ideal for observing moon & planets when well placed. I think this is because the air currents caused by solar heating are losing their power but the currents caused by overcooling (by radiation) of the ground haven't yet started.

However the smeared seeing caused by upper atmospheric turbulence (at the height commercial jet transports fly) is not at all affected by the time of day, and moving to a "better" nearby site won't help you much as the jet streams are hundreds of miles across and thousands of miles long. You can find predictions of jet stream position & strength on several sites. These predictions are fairly but not completely accurate so it's always worth having a look, but if there is a strong jet stream overhead you will never get sharp images of planets etc. at high magnification. In this case, stick to low power objects & make the best of the conditions.

#9 Tony Flanders

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Posted 01 December 2013 - 06:59 PM

I had thought that if seeing was bad I would see it as motion in the image. I've experienced this sometimes. Can seeing affect the view even if I can't see the rippling air distortions directly?


Yes, there are broadly two different kinds of bad seeing -- which can of course occur in combination as well. In "slow" bad seeing, the air currents tweak the image at a speed the eye can follow, and you see the image dancing around. In "fast" bad seeing, the image is either moving too fast to follow or it's just smeared out by multiple different air currents all at the same time.

In my part of the world (U.S. Northeast), an 8-inch scope is almost always limited by the atmosphere. That's especially true when an object is low in the sky (e.g. Jupiter in early evening), when there's more air between you and your target.


#10 Joe Aguiar

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Posted 02 December 2013 - 08:54 AM

think about a ballon with a picture on it when the ballon gets bigger and bigger the picture also gets bigger and bigger, bit its spread over a bigger area so it gets a darker as it gets bigger. The image is spread wider and or bigger making it dimmer.

So get the mag to where its clear but still as big as you can get it, if the image breaks down back down the power.

#11 GeneT

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Posted 02 December 2013 - 01:03 PM

SCTs perform fine as planetary telescopes. I viewed for 10 years using only a C8. It performed very well on the planets, to include a close approach of Mars. What happened to you is that you used too much magnification based on your optics and seeing conditions. When the image blurs out, start dropping the magnification. Collimation is important, but that might not be your major problem. I shipped my C8 to Germany and back, and both times it needed only minor tweaking. Check out the collimation, but get out and view several more times. Also, let Jupiter rise higher in the sky. You may have been viewing when it was too close to the horizon.

#12 penguinx64

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Posted 02 December 2013 - 05:53 PM

Yes, collimation did improve contrast for me. I used 75x tonight and it Jupiter looked great. First time I had a clear view of the 2 belts too.

#13 NorthernSoul man

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Posted 03 December 2013 - 06:16 AM

Hi QuinnD, just try to collimate your cope a little, this may help a lot. Just adjust a screw at a time using very small adjustments, remembering which you moved and what way, if its worse, then just move it back, using very small adjustments will soon show you which way to adjust, remember, its only 3 screws - not at all a big deal!!!
Not sure how you observe - a quick look, then move on - if so,change this - spend a long time just looking at Jupiter, nothing else - just Jupiter - as the minutes tick by more and more detail will show - irregularities in the belts begin to "appear", white ovals become more noticeable and if the Great Red Spot is around, the region preceding (in front of)will stand out a lot more than the actual spot itself - a very turbulent area with a number of white ovals here (well, they were the last time I looked).
You can usually judge the seeing by actually focusing on the limb of Jupiter, seeing how "precise" the line is, but the main thing that has helped me is the most simplest - just spending a lot of time just observing Jupiter - not slewing away to other objects, just concentrating, seeing detail, then lowering mag and picking out the same detail (all be it smaller)but still there - as time goes by you can build a mental picture of the Planet and watch the detail move off the disc, and even more detail move onto the disc. Paul.






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