60 replies to this topic

[JoeBlow]

I've been doing astronomy on and off for many years now, but its only been the past 2 years that I have taken a serous interest in observing planets, something I now regret. Though I had larger telescopes in the past, I currently only have an 8" newt. During nights of very good to excellent seeing I have been really impressed with what an 8" can show me; multiple cloud bands, complex detail in the NEB and SEB, some colour especially in the GRS, festoons, barges, white ovals, moons resolved as discs of different sizes and colours, and more details than you can sketch.

 

My question is what can a larger telescope show me on Jupiter during nights of typically good seeing that an 8" usually can't do? I'm interested as I plan to acquire a larger telescope in the next year or so (maybe a 14"), but plan to keep my 8" as a "grab and go".

 

Thanks,

 

Joe 

Edited by JoeBlow, 19 April 2017 - 08:31 AM.

[Special Ed]

Joe,

 

Well, you'll get better resolution (but only when the seeing allows) and better color--at least I did.  I used an 8" SCT for years on Jupiter before I got the C14.  I can perceive colors now that I couldn't before.

[Gipht]

Joe,

 

I am going through the  same thought process.  I do like the images I get from my 8 inch, especially of the moon.  I found an old blue box Coulter  with the one inch thick 13.1 inch mirror in good condition for $250.  Its a beast at  about 100 lbs, but it move fairly easily on a hand truck.  I sent the mirror out to have it checked and am currently having it refigured and a new coating put on, which will cost just over $500.  Some people like shiny and new though.

 

My main question would be if you want to deal with the additional weight, and if the extra weight would keep you from enjoying the new scope?

[Dave Mitsky]

My best planetary views have been through large apertures (truss-tube Dobs with premium mirrors and classical Cassegrains) during good seeing. Resolution is improved, of course, and color is definitely enhanced.

 

Dave Mitsky

[penguinx64]

I agree with Gipht.  Extra weight can be a problem.  My 6 inch reflector on an Alt/Az mount weighs 20 pounds, about 66% heavier than my Starblast 4.5.  I end up not taking out the larger scope as often because 20lbs is a lot to carry up and down stairs.  An 8 inch Dob would weigh twice as much, and larger scopes would weigh even more.  20lbs is plenty heavy enough already.  

[BillP]

My question is what can a larger telescope show me on Jupiter during nights of typically good seeing that an 8" usually can't do? I'm interested as I plan to acquire a larger telescope in the next year or so (maybe a 14"), but plan to keep my 8" as a "grab and go".

 

A scope smaller than 6" attains better than 1 arcsec resolution...and it is a rarety to find places on Earth where one can have a good stretch of seeing that good.  Altitude is also an important consideration as the more atmosphere you view through the more scatter and less contrast results.  However, even given this, my experience, like others, is that larger apertures can certainly be of advantage.  Even my 10" will show more than the 8" I had.  However, the important caveat is that on the majority of evenings, since I do not live at one of the rare best seeing sites on Earth, even a 4" will show substantially all that the 8" or 10" shows.  So I tend to use the small bore most of the time, and when those evenings roll around where I am being suitably impressed with teh detail I am seeing with the 4", then I know that the larger gun will be a productive switch and worth putting up with its less ergonomic form factor.  The trick of this though, to be successful, is to make sure the big gun is acclimated with the smaller one so it is "ready" should it be warranted.  There is a psychological consideration as well.  If you always use the big gun, then given that the seeing is rarely up to its capability, it becomes frustrating taking all the effort with the larger scope and not being able to reap the rewards too often.  It is opposite with the smaller scope as more times than not the seeing is great for its capabilities.  So the best solution, as always, is never the single do-it-all instrument but instead to have two or more so you can choose the one best for the current conditions.

Edited by BillP, 20 April 2017 - 11:28 AM.

[Special Ed]

Some people have success dealing with seeing issues and larger aperture by employing an apodizing mask.

[tsbikes]

Could you elaborate on what would be an anodizing mask?

 

Thanks,

Jim

[Special Ed]

Could you elaborate on what would be an anodizing mask?

 

Thanks,

Jim

It's apodizing​ mask.

 

See here:  http://csastro.org/a...apodizing-mask/

[Dave Mitsky]

I've had some excellent planetary views with apodizing masks.

 

http://www.graphiteg...i?a=diyapodmask

 

http://www.lcas-astr...ategory=general

 

Dave Mitsky

[Redbetter]

 

A scope smaller than 6" attains better than 1 arcsec resolution...and it is a rarety to find places on Earth where one can have a good stretch of seeing that good.  Altitude is also an important consideration as the more atmosphere you view through the more scatter and less contrast results.  However, even given this, my experience, like others, is that larger apertures can certainly be of advantage.  Even my 10" will show more than the 8" I had.  However, the important caveat is that on the majority of evenings, since I do not live at one of the rare best seeing sites on Earth, even a 4" will show substantially all that the 8" or 10" shows.  So I tend to use the small bore most of the time, and when those evenings roll around where I am being suitably impressed with teh detail I am seeing with the 4", then I know that the larger gun will be a productive switch and worth putting up with its less ergonomic form factor.  The trick of this though, to be successful, is to make sure the big gun is acclimated with the smaller one so it is "ready" should it be warranted.  There is a psychological consideration as well.  If you always use the big gun, then given that the seeing is rarely up to its capability, it becomes frustrating taking all the effort with the larger scope and not being able to reap the rewards too often.  It is opposite with the smaller scope as more times than not the seeing is great for its capabilities.  So the best solution, as always, is never the single do-it-all instrument but instead to have two or more so you can choose the one best for the current conditions.

 

 

Perhaps a premium 4" Apo will come close to a 10" Dob in lackluster seeing, but what I've seen of 80mm and 110mm ED's is that the ED's essentially top out for my eye at close to 50x/inch, while an 8 o 10" scope is still showing more detail even in poor seeing most evenings.  It takes very poor seeing or bad thermals (transient) to sufficiently cripple the 10".   A 6" Apo would be nice though...of course it's not going to be all that portable either.

 

I've used the 80 as a scout for the 110 and the 10" and brought them out non-equilibrated when the seeing looked promising or I noticed some event worth watching with more resolution.  Doing side-by-side checking of all three it takes some time for the 110 to cool and longer on the 10", but with the fan running it still isn't that long before the 10" pulls away from the other two and continues to improve until the seeing limit is hit.

 

The extra light at a given magnification improves the image, except when the seeing is bad enough to blur it into glare at the low magnification the scope will be limited to by the seeing.  Seeing might be limiting to around 150 - 200x for all, but the color is superior in the 10", the eye is operating at a more comfortable exit pupil, and there is more detail.  And on the nights where the seeing rises to mediocre or good the 110 is stuck under about 250 while the 10" isn't hitting a seeing limit until about 350x. 

 

I find the moving moons fascinating, and these are things that benefit greatly from aperture.  Things don't change much for Jupiter in this regard until you get to very large apertures and try to hunt down outer moons.  However, in town with the moon up I was finding it challenging to even find Saturn's Tethys and Dione in the 110 in bright conditions.  Last time I did this I could only find Dione by consulting software to pinpoint the position to search and even then it was just barely seen.  I've always found them easily in larger apertures in town.   They are farther from the limiting magnitude threshold for the light polluted conditions in larger apertures.  With the Z10 or 20" I've typically been finding Enceladus even in town, and if the seeing supports it Mimas.  Neptune's Triton benefits from 8 to 10".  Uranus just barely shows Titania and sometimes Oberon in the 8" SCT in darker skies.  A 10" will show both better, while the 20" will show Ariel and Umbriel as well if the seeing isn't too bad and the sky is reasonably dark.  During closer/better placed approaches of Mars, they big scope will show both moons at the same time.

 

From the psychological standpoint I accept that scopes much over 4" are typically seeing limited.  What I find though is that they still show about as much detail as a 4" in the same conditions.  And if I don't have a bigger scope ready when the seeing is good, I am kicking myself for being restricted to too little aperture.  My 20" is typically ready to travel to the dark site anymore, so it isn't ready to go quickly in the backyard.  On a night when the Z10 is humming along at 350 in the backyard I regret having the 20" ready to travel because I know it will do considerably more in the same conditions.  I don't find myself out at the dark site taking a look at Jupiter in poor seeing saying to myself, "I really wish I had the 110 here to look at Jupiter."  Instead, I go back to galaxy hunting and SN observing.  And there have been recent nights at the dark site where for a time things steadied.  I turned to Jupiter with the 20" revealing things the 110 doesn't even come close to showing and the 10" hasn't quite managed either. 

[TiSaph]

 

My question is what can a larger telescope show me on Jupiter during nights of typically good seeing that an 8" usually can't do? I'm interested as I plan to acquire a larger telescope in the next year or so (maybe a 14"), but plan to keep my 8" as a "grab and go".

 

A scope smaller than 6" attains better than 1 arcsec resolution...and it is a rarety to find places on Earth where one can have a good stretch of seeing that good.

 

It's important to know, but there are places and nights with good seeing. I'm in NC, and last night we had perfectly rock steady skies for viewing Jupiter.  I was limited by my primary mirror and aperture, and not the skies for sure.  I did a star test on Polaris at the end of the night and there was basically no movement whatsoever visible in either the in focus star or the "ripples" of air currents on the defocus image.  I was wishing for 16" Zambuto for sure last night.

[FarmerWhite]

Two nights ago we had great seeing here in Richland county, South Carolina and I was able to push my 12" dob to 447x (my highest current available magnification) with still clear viewing on Jupiter. So I was glad to have the aperture to do that.  Unfortunately this only lasted for all of 15 minutes until a total sky covering cloud bank rolled in and ended the night.

Edited by FarmerWhite, 27 April 2017 - 01:19 PM.

[Alan French]

I had the 15-inch out the other night intending to visit a number of galaxies. Although I did spend time on the galaxies I also spent a fair amount of time observing Jupiter. The seeing was good, but the 15-inch was certainly seeing limited. What impresses me about larger apertures and Jupiter is the prominence of the colors and the comfort of a larger exit pupil (at the same magnification that might be used on a smaller scope).

 

The best view of Jupiter I ever had was at Peach State with a 12.5" Portaball on an equatorial platform at 567x (10/10/2010). There was so much detail I tried to find a larger scope in use (no luck, alas). We could see albedo features on Ganymede and Callisto. Amazing what you can see on a night of exceptional seeing.

 

Even when the seeing is not at its best, I find there are often times on any night when a larger scope has an advantage.

 

Clear skies, Alan

Edited by Alan French, 27 April 2017 - 03:34 PM.

[Redbetter]

 

 

My question is what can a larger telescope show me on Jupiter during nights of typically good seeing that an 8" usually can't do? I'm interested as I plan to acquire a larger telescope in the next year or so (maybe a 14"), but plan to keep my 8" as a "grab and go".

 

A scope smaller than 6" attains better than 1 arcsec resolution...and it is a rarety to find places on Earth where one can have a good stretch of seeing that good.

 

It's important to know, but there are places and nights with good seeing. I'm in NC, and last night we had perfectly rock steady skies for viewing Jupiter.  I was limited by my primary mirror and aperture, and not the skies for sure.  I did a star test on Polaris at the end of the night and there was basically no movement whatsoever visible in either the in focus star or the "ripples" of air currents on the defocus image.  I was wishing for 16" Zambuto for sure last night.

 

Yep.  The areas with nights of steady seeing aren't as rare as some suggest.  The southern states in particular have some rock steady seeing at times, and much more steady seeing on average.  And the coastal Pacific areas have steadier seeing...inland from them, not so much. 

[Cpk133]

Wish I could say that.  We get some good seeing from time to time but most times it's short lived.  When it's good, I won't blink because I don't want to waste it.

Edited by Cpk133, 27 April 2017 - 10:00 PM.

[BillP]

It's important to know, but there are places and nights with good seeing. I'm in NC, and last night we had perfectly rock steady skies for viewing Jupiter.  I was limited by my primary mirror and aperture, and not the skies for sure. 

 

 

More aperture certainly affords more magnification and image scale at similar levels of brightness and contrast.  But I would bet that many folks feel the atmosphere is so steady that they are working at their scope's limit, when in fact they are not.  For a year or more I always brought my 10" out with my 4".  Yes there were nights when the 10" would outperform the 4" that is for sure...but it was not as many nights as I would have liked.  But I also know that a truely good sky for planetary is more than about seeing but also about water vapor and particulate levels and all the things that create scatter, which has a particularly great impact on all the fine contrast features on Jupiter.  As a point of reference, the absolute best view I've ever experienced of Jupiter was with my 4".  On that evening it showed more details than my 10" ever did.  The two links here represent almost what the image looked like, they show a little "less" details that I could see and they are also a little less "etched" than the view I had - so my view was better than these.  Only details on these pics that I could not see were any white ovals.  I've never achieved a white oval with my 4" yet.  They are easy with the 6" though.  Anyway, here's a view of Jupiter that is feasible through a 4" when the atmosphere is really operating at its best - http://pop.h-cdn.co/...ach-jupiter.gif and http://i.dailymail.c...3_634x468.jpg. 

Edited by BillP, 28 April 2017 - 12:36 AM.

[Redbetter]

 

 

 

More aperture certainly affords more magnification and image scale at similar levels of brightness and contrast.  But I would bet that many folks feel the atmosphere is so steady that they are working at their scope's limit, when in fact they are not.  For a year or more I always brought my 10" out with my 4".  Yes there were nights when the 10" would outperform the 4" that is for sure...but it was not as many nights as I would have liked.  But I also know that a truely good sky for planetary is more than about seeing but also about water vapor and particulate levels and all the things that create scatter, which has a particularly great impact on all the fine contrast features on Jupiter.  As a point of reference, the absolute best view I've ever experienced of Jupiter was with my 4".  On that evening it showed more details than my 10" ever did.  The two links here represent almost what the image looked like, they show a little "less" details that I could see and they are also a little less "etched" than the view I had - so my view was better than these.  Only details on these pics that I could not see were any white ovals.  I've never achieved a white oval with my 4" yet.  They are easy with the 6" though.  Anyway, here's a view of Jupiter that is feasible through a 4" when the atmosphere is really operating at its best - http://pop.h-cdn.co/...ach-jupiter.gif and http://i.dailymail.c...3_634x468.jpg. 

 

My experience in the land of poor seeing is that even on nights where the seeing is 4/10 at best rated in the 110mmED (4.33") the 10" Dob is showing more on Jupiter, although the white ovals are missing in the Z10 in such poor conditions.  And on nights when the seeing is halfway decent the difference is even greater with the Z10 easily showing the white ovals and myriad other details.  I have had hints of a white oval in the 110 at times but nothing certain, even when I knew where to look based on the four ovals showing at the time in the Z10.  However, I do expect a premium 4" refractor would do better than the 110ED in this regard.

 

If a 10" consistently isn't able to show white ovals in good seeing, then the optics are likely poor.  I can see them in the 21 year old 8" SCT when set up next to the Z10...although not as many at the same time.  It tends to show the ones closer to the meridian and is losing too much contrast as they approach the limbs.  My second sketch of Jupiter as a newbie with the SCT (in a land of better seeing) made note of the "white spots" in the STB seen briefly in highly variable seeing (rated from 4 to 9 that evening.) 

 

My best planetary nights were typically sticky still nights in East Texas.  The transparency was fair to good, but typically not great--nothing like a crisp winter night.  Those are the type of nights where the 20" was showing both of Mars moons at the same time, and details on the planet blew me away.  It is kind of obvious when the seeing is excellent in 8" aperture and more because the diffraction patterns around stars settle down.  When the diffraction rings don't move on bright stars you know the seeing is superb.  Planetary images take on that spaceship porthole quality, as if the telescope was no longer in between.  

 

FWIW the seeing here has been rather poor for the 20" but as usual it still has given the best views in the few minutes here or there that the seeing reached maybe 2/10 or 3/10 in that scope.  In a recent session at the dark site I was noticing how large one of the white ovals was near GRS Junior.  I wasn't actually seeing more than a vague impression of Junior in the weak seeing, but I was making note of the contrast against the zone, the dark trailing material in the zone, and white oval in the STB next to it.  The following link is to a video that shows what I was seeing:  link.   What isn't apparent in the video is the contrast of the dark edge on the backside of the white oval, it made it look more like a cut down into the zone, very interesting at the time.  Then a breeze kicked up and the seeing was awful, so I went back to galaxy hunting.

[Redbetter]

Bill, the second link isn't working.  It is picking up an extraneous period.  Once edited it looks closer to the current appearance of Jupiter. 

 

When was the image in the first link made?  It doesn't resemble Jupiter in recent years. 

[Dave Mitsky]

Speaking from my experiences with my 101mm Tele Vue refractor and other 4" refractors, I would choose a 6" or preferably an even larger Newtonian over the refractors any day of the week for planetary observing.

 

Dave Mitsky

[barbie]

Agreed!

[JoeBlow]

I noticed a big difference when I went from a 6" to an 8" on planets even under average seeing. That's why I was interested to ask how much better an even larger telescope could be.

[Illinois]

Speaking from my experiences with my 101mm Tele Vue refractor and other 4" refractors, I would choose a 6" or preferably an even larger Newtonian over the refractors any day of the week for planetary observing.

 

Dave Mitsky

Tele Vue 101mm  is short compare to Tele Vue 102 that is f8.6  I don't understand why Tele Vue don't do 102 f 8.6 anymore. Mine is Orion 100 ED is f9 is very nice so I jumped to ES 127 f7.5. Its nice but I still plan to buy 6 inch F8 ED refractor when I able to afford it in a few years.

 

My 180mm Mak-Cass is great  for Jupiter, Saturn and Mars during good clear night. Use ES 11mm eyepiece for 245 power is great! Cooling time is problem so  I left it outside in shade for one to two hours before sun set.

Edited by Illinois, 29 April 2017 - 12:36 PM.

[Dave Mitsky]

The 101mm Tele Vue apochromat is a four-lens-element Petzval design and works quite well at high magnifications but focal ratio is not the point.  I've used quite a few 4" refractors of varying focal lengths and the result, for me, is the same, insufficient aperture for worthwhile planetary observing.

 

I've also used some excellent 6, 7 and 8" apochromats and while the images are quite good I would rather being using a 20" Dob with a Zambuto mirror for making planetary observations, as long as the seeing is good.

 

Dave Mitsky

[CounterWeight]

Comparing to my 14" Dob/Newt to my smaller refractors

 

How to say this without getting into terminology hair splitting is for me difficult.  In my 14" Dob/Newt  I have a much better spread of larger exit pupil (using my 3.5mm ep I still have about 0.75mm exit pupil, in my 128 refractor it is at ~ 0.74 using my 6mm ep). It is a different aperture difference than 8" to larger but I hope the points are still valid. I could make a different spreadsheet for an 8" of whatever focal ratio but since we are talking more about aperture i hope it sits well. The three things of interest in comparing here to me are

 

-exit pupil size (here, Jupiter relative size for magnification used)

-mag/inch of scope used (how hard are my optics being pushed)

-magnification of scope to the sky seeing (what is reasonable to expect)

 

I'll stick to the 200 to 300 magnification range as that is most often where my seeing seems to sit.

 

here is the FS128 data

 

 

and here is the XX14g data

 

I hope the data helps make apparent the tradeoff of exit pupil size to the seeing limit, both scopes working to their respective possible resolution (Dawes or Rayleigh) limits at whatever ep is in the focuser as I understand that is based on aperture alone.  This I think gets lost in some discussions.

 

The larger scope has much more to work with, and I welcome the apparent dimming from decrease in ep focal length! I also enjoy trying out various filters of all types as losing apparent brightness not so much an issue as with my smaller refractors. This to me translates to a larger presentation of Jupiter under the potential seeing limits I usually experience.

 

What is interesting to me also is that as the 14" Dob/Newt is working at a higher resolution, seeing conditions are also more likely to impact, if that makes sense.  So I have a potential larger undimmed view of Jupiter that has a higher potential resolution than is possible with the smaller scope.

 

An explanation about the aesthetics of the view is that the smaller scope can work in it's upper magnification range (before dimming undesirable) as it is going to less sensitive based on it's resolution (object view becoming 'fuzzy').  The larger scope will always be working at it's resolution, but the presentation can be much larger - and it's no problem dropping down the magnification to where the fuzzyness can dissipate though Jupiter can appear uncomfortably bright. 

 

To me it helps explain why some people may be entirely happy with the view through a smaller scope, and I am in that group, I feel they never really disappoint.  The larger scope has far more potential, can be 'dialed in' to a much finer degree, and will have a larger presentation of the planet possible, which I also really like, so count me in that group too.

 

If anyone insists on comparing at same size exit pupil we go back to the large scope is still working at a higher potential resolution so I am not sure that really makes sense or is fair.