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Is there a magnifications vs. altitude rule of thumb?

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#1 the Elf

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Posted 16 October 2021 - 07:12 AM

Hello all,

 

from my location at 52°N the planets are low this year. Jupiter at 22°, Saturn at 18° above the horizon. I was wondering how much magnification makes sense at the low position and if there is a rule of thumb for maximum magnification for a given altitude. Last night even 200x seemed to be too much.

I could also put the question like this: what is the minimum altitude to see some details on Jupiter? All I could see last night was a disk with a single dark line near the equator plus the 4 Galilean moons.

 

CS

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#2 bigdob24

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Posted 16 October 2021 - 07:55 AM

When I had the big dobs the rule to go by was 30 degrees above the horizon. 
‘Even with that standard seeing and transparency come into play. 
You just have to adjust to conditions for where your at and what your trying to look at, less mag in the muck below 30 degrees.
 



#3 PKDfan

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Posted 16 October 2021 - 05:52 PM

Hi the Elf!

I am at 53.5°N.

I have unsteady skies and cannot use anything more than 150× and usually 135× works better more often.

Usually only get good crisp views 20 minutes before and after culminating. I was surprised one night when I got the best view an hour after it crossed the meridian.

Really looking forward to 2023 when Jupe will be much higher in the sky.

Using 4" F/9 apo.



Clear skies & Good seeing

Edited to add, been observing Jupiter since March and probably had 30+ nights of very enjoyable views but only a few times did I have SPECTACULAR moments. Brief times of Hubbelesque views lasting whole seconds at a time!

Otherwise its spending time waiting, waiting and some more tìme waiting for those flashes of unaberrated views, lasting for very brief parts of a second.

Well worth the wait!

Edited by PKDfan, 16 October 2021 - 06:01 PM.

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#4 Jon Isaacs

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Posted 16 October 2021 - 07:09 PM

Hello all,

 

from my location at 52°N the planets are low this year. Jupiter at 22°, Saturn at 18° above the horizon. I was wondering how much magnification makes sense at the low position and if there is a rule of thumb for maximum magnification for a given altitude. Last night even 200x seemed to be too much.

I could also put the question like this: what is the minimum altitude to see some details on Jupiter? All I could see last night was a disk with a single dark line near the equator plus the 4 Galilean moons.

 

CS

the Elf

 

CS:

 

I am about 20 degrees south of you so Jupiter and Saturn are about 40 degrees at culmination.  But I think the rules are the same wherever one observes, trial and error.  Have a set of eyepieces that cover the range of possible magnifications and experiment continuously.  Each night is different, each hour, each moment.  

 

I have had nights with Jupiter and/or Saturn were just rising and were just amazing and I have had nights like last night when they were both near 40 degrees and very disappointing. 

 

Jon


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#5 the Elf

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Posted 17 October 2021 - 04:34 AM

My eyepiece lineup is in the signature. Imho it fits my f/7 100mm refractor quite well. For the planets several people recommended using the RC instead for the higher resolution and despite the low contrast. Not sure this really is a good idea. If I use the RC there is a gap between the 8mm and 18mm eyepiece. If the situation is changing all the time it is quite difficult to compare two telescopes. It takes about two minutes to swap scopes and re-arrange the counterweights. If this is enough time to change everything how do I know which socpe is better suited? I think I will continue observing with the refractor. The ES 6.7mm seems to show more detail than the much cheaper 4mm TS planetary. Maybe I should add a high quality eyepiece like the Tak TOE 4mm.

Thoughts? RC or refractor? Tak or ES eyepiece? Televue? Does it make sense to use high end eyepieces in this cheap refractor?

https://www.teleskop...ularauszug.html

 

When I ordered I did not know how deep into visual I would want to go so I did not invest thousands of Euros. Is this thing suitable for planetary at all?



#6 Jon Isaacs

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Posted 17 October 2021 - 05:20 AM

My eyepiece lineup is in the signature. Imho it fits my f/7 100mm refractor quite well. For the planets several people recommended using the RC instead for the higher resolution and despite the low contrast.

 

 

Planetary contrast is a function of aperture.  The large secondary obstruction definitely reduces the contrast but my intuition says the R-C would probably still have better fine scale planetary contrast than a 4 inch ED.  

 

That's just a guess but my main point is that the aperture is normally more important that the CO in determining the planetary contrast.. 

 

Jon 


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#7 REC

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Posted 17 October 2021 - 11:54 AM

This situation is best served using a zoom EP. That's what I use on planets, it lets you zoom in and out to match the seeing conditions. When using my manual Dob, after I find the sweet spot with the zoom, I'll sometimes switch out to a 82* EP to keep it in view longer in the EP.


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#8 the Elf

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Posted 17 October 2021 - 12:50 PM

Funny. I was told not to use zoom eye pieces for planetary because a prime is always superior within a given price level. I also learned that the apparent FOV changes while you zoom. After using zooms in photography for years and years I recently changed to using primes only for the better quality. Thus it was quite natural to go for prime eye pieces as well. Maybe I have to get rid of my photography concepts first to understand observing.


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#9 Jon Isaacs

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Posted 17 October 2021 - 07:09 PM

Funny. I was told not to use zoom eye pieces for planetary because a prime is always superior within a given price level. I also learned that the apparent FOV changes while you zoom. After using zooms in photography for years and years I recently changed to using primes only for the better quality. Thus it was quite natural to go for prime eye pieces as well. Maybe I have to get rid of my photography concepts first to understand observing.

 

I would say this:

 

In astronomy, no one uses objectives that zoom.  The objective is the analog to the lens in photography.  All objectives are primes in astronomy.

 

Zoom eyepieces are used but the role of the eyepiece is less important than the role of the objective. And zoom eyepieces can be quite good.  They're main limitation is generally that the field of view is narrow at lower magnifications.  Their advantage for viewing the planets is that you can dial in the magnification in real time.  

 

The Baader Hyperion zoom is quite good.

 

Jon


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#10 Nicole Sharp

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Posted 24 October 2021 - 02:38 PM

You can decrease your magnification by approximating seeing with the Fried coherence length using the following formula:

 

(sin(altitude2)/sin(altitude1))^(3/5)

 

As the object moves from 45 degrees to 23 degrees, you can decrease magnification by (sin(22.5*deg)/sin(45*deg))^(3/5) = 0.69X, or from 100X to 69X.

 

https://www.wolframa...(3/5),{x,0,90}]

 

https://en.wikipedia...Fried_parameter


Edited by Nicole Sharp, 24 October 2021 - 02:41 PM.

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#11 the Elf

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Posted 24 October 2021 - 02:47 PM

Thank you very much! I read the article a few years ago an totally forgot about it. At the time I was wondering which physical effect makes my 15 min Ha subs be better than my 3 min OSC subs using a mirror optic that cannot have CA. I found the lambda^(6/5) relation and was happy. Of course the main topic is seeing as a function of altitude. Thank you for posting and thanks for the plot!


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#12 ButterFly

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Posted 24 October 2021 - 06:25 PM

Larger apertures are more resolved, and thus bad seeing gets in the way more.  An 80mm is almost immune to seeing because it's not very resolved.  It takes some rather atrocious seeing to mess up its views.  It's also looking through a smaller column of air, which is very much related to Fried's coherence length.  Local conditions can drastically affect what that is.  Hundreds of miles of open ocean with a slight breeze will do better then looking toward the Alps.  On the Alps will do better than next to them.



#13 the Elf

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Posted 25 October 2021 - 02:57 PM

Theory proved correct last night. Wrong forum because I took a photo but anyway: my usual FWHM (star diameter full width at half maximum) in photos is 2.3 arcseconds in the best sub exposures in Ha, 10 to 15 minutes long near the zenith. The Helix Nebula 15 to 17° above the horizon spoiled me with an FWHM of 5.7 to 7.2 arcsec. That is pretty much in accordance to the graph Nicole posted, 3x worse. Reducing the image size to about 1/3 makes it look sharp. I think I will try that again with my 65mm quad and I think I will continue my visual work with the 100mm refractor rather than with the RC8. Have to wait two years until Jupiter is high in the sky. That is what I like in astronomy, if you missed something it will be back next year and the year after and the year after (unless it was an eclipse).


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#14 Bob Bunge

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Posted 26 October 2021 - 02:36 PM

Just an old fashion visual planetary observer here... but... unless your telescope has been outside for at least 2 hours and you've spent at least 30 minutes with your Mark I eyeball smacked up against the eyepiece (assuming visual observing), ideally using at least two different magnifications, you aren't seeing all there is to see, regardless of conditions.  3-5 hours is better for cool down, especially for more larger systems like that RC8, and observing after midnight is even better.  

 

Also, seeing is usually poor the night after a weather system has moved through and better a day or two later and best after a week or so after a weather system having gotten stalled.  

 

After you have observed through the telescope, grab a chair, sit, look up and spend 30 seconds or more looking at the brighter stars.  After doing this a few times, you will start to develop a sense of what the seeing is like just by looking at how much the brighter stars twinkle.

 

Just some thoughts.  :-)

 

Steady Skies,

 

Bob


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#15 Chucky

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Posted 26 October 2021 - 07:38 PM

(( Also, seeing is usually poor the night after a weather system has moved through ))

 

Right Bob.  Just like here in Columbus, Ohio at the moment.  Our seeing is currently just horrible!


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#16 Jon Isaacs

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Posted 26 October 2021 - 08:16 PM

(( Also, seeing is usually poor the night after a weather system has moved through ))

 

Right Bob.  Just like here in Columbus, Ohio at the moment.  Our seeing is currently just horrible!

Chucky:

 

The sun is just setting here in San Diego.. Reading your post, I thought I would see what GoodToStargaze said about the seeing.. We had some rain yesterday, cleared up to today. 

 

2.0".. oh well..  

Then I noticed for some reason, the location was set to somewhere in Germany.. For my location, It's predicting 0.7" for the early evening, deteriorating to 1.1" at midnight. 

 

Not too bad but this afternoon I did something to my wrist so I have to baby it.. tonight, 80mm is the max. 

 

Jon



#17 the Elf

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Posted 27 October 2021 - 12:58 AM

Just an old fashion visual planetary observer here... but... unless your telescope has been outside for at least 2 hours and you've spent at least 30 minutes with your Mark I eyeball smacked up against the eyepiece (assuming visual observing), ideally using at least two different magnifications, you aren't seeing all there is to see, regardless of conditions.  3-5 hours is better for cool down, especially for more larger systems like that RC8, and observing after midnight is even better.  

 

Also, seeing is usually poor the night after a weather system has moved through and better a day or two later and best after a week or so after a weather system having gotten stalled.  

 

After you have observed through the telescope, grab a chair, sit, look up and spend 30 seconds or more looking at the brighter stars.  After doing this a few times, you will start to develop a sense of what the seeing is like just by looking at how much the brighter stars twinkle.

 

Just some thoughts.  :-)

 

Steady Skies,

 

Bob

Thank you very much. I am well aware of all these points but this is just not my reality. If there is 1 h of clear sky and the last 20 hours before that it is raining I cannot put the scope outside to cool down in the rain. If temperature drops from 18°C before sunset to 6°C 2h after sunset I cannot let the scope cool down at all. If I have to get up for work at 4:30 am I cannot observe after midnight. We have about 2 nights with little cloud coverage or high clouds only per month. There are not more then about 5 nights per year where there are absolutely no clouds. I have a feeling for the twinkle of the stars and they almost always do. If I stay inside because the seeing is poor I will never observe at all. As it seems a lot of CNers are retired and live under steady skies in the US. I envy you. My reality is different. 2.0'' seeing is about the best I can get here. There is no Bortle 1 or 2 in Europe. We are so densely packed that we just don't have this even in the mountains. I have seen a clear view of the Milky Way with the dark clouds visible only once in my life near Yosemite NP in the US. Over here all I see is a bit of a brighter patch in the zenith. Most people in Europe never see the MW in their whole lives.

When and if I can retire early in 12 years I will probably spend 3 months in the US or in the Alps or in NZ during the summer and enjoy the best views ever. For the time being I have to deal with a lot of limitations. That is why I find astrophotography more satisfying. It is just more tolerant to all the bad conditions. When the object is large seeing does not matter. The rig does it's job while I'm in bed sleeping and it catches the few hours of clear sky for me. I can look at it the next day after work. But a DSO on the screen is not the same as a view in the eyepiece. That is why I try observing now and then in the conditions that I have. With my initial question I just wanted to know what I can expect and how I can make the best of the conditions I have.



#18 Chucky

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Posted 27 October 2021 - 06:31 AM

<<  I did something to my wrist so I have to baby it  >>

 

OK Jon, what happened?  Let me guess.....some type of yard work or home chores.



#19 Jon Isaacs

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Posted 27 October 2021 - 07:01 AM

<<  I did something to my wrist so I have to baby it  >>

 

OK Jon, what happened?  Let me guess.....some type of yard work or home chores.

You see, there was this car on fire and I had break the window out with my fist, reach inside, open door and rescue a family of five..

 

Actually, I was opening a can of tuna and felt a sharp pain in my wrist.. 

 

As far as the seeing, it wasn't that good.. 

 

Jon


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#20 Bob Bunge

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Posted 27 October 2021 - 01:42 PM

Thank you very much. I am well aware of all these points but this is just not my reality. If there is 1 h of clear sky and the last 20 hours before that it is raining I cannot put the scope outside to cool down in the rain. If temperature drops from 18°C before sunset to 6°C 2h after sunset I cannot let the scope cool down at all. If I have to get up for work at 4:30 am I cannot observe after midnight. We have about 2 nights with little cloud coverage or high clouds only per month. There are not more then about 5 nights per year where there are absolutely no clouds. I have a feeling for the twinkle of the stars and they almost always do. If I stay inside because the seeing is poor I will never observe at all. As it seems a lot of CNers are retired and live under steady skies in the US. I envy you. My reality is different. 2.0'' seeing is about the best I can get here. There is no Bortle 1 or 2 in Europe. 

...

That is why I try observing now and then in the conditions that I have. With my initial question I just wanted to know what I can expect and how I can make the best of the conditions I have.

Yes, one has to make the most of what they have access too!  Of course light pollution has little or nothing to do with good seeing conditions and a steady view at high magnifications.  Other factors, in my experience, that lend to steady skies - most of which won't help you in Germany but can help when you pick where to go on your observing holiday - include being near large (as in oceans and large bays) bodies of water and a lack of temperature change between day and night.  Islands like Las Palmas, Barbados and the Florida Keys aren't famous for great seeing because of smooth airflow, but because the water has a moderating affect on the temperature delta.  That delta not only affects your telescope but everything around it as well.  

 

You may consider if there are actions you can take to pre-adapt your telescope to predicted temperatures for when you can observe.  The big monster professional scopes (especially the newer ones) are climate controlled during the day to match predicted conditions after sunset.  A example of an amateur doing this - unlikely in your case - would be to use an A/C unit to cool a trailer where a scope is stored.  

 

When it comes to the twinkle, they always twinkle, but a question of how much and how fast.  Consider this, as you describe your conditions, getting a feel for how much they are twinkling could have a big impact as to if you decide to set up or not just for a view or a series of images.  There have been many nights where I'll walk out and in a minute know that it isn't worth my time to pull out a scope or uncover one if it's already out.

 

My favorite planetary target is Mars.  During this past opposition event, I had fun comparing my observations - where I have sketches and records going back to 1994 - including records of seeing conditions with the seeing conditions predicted by some of the models in various apps.  Some were better, but all missed the boat at least once or twice during the 35 observations I was able to make.  Hopefully the modelers will get better at this!

 

For many years, I arose at 5:00am in order to do battle with Washington DC Beltway traffic and work.  I was blessed to be in a position where I could set up the telescope the evening before with proper protection for the optics and get up at 4am for an observation before heading to work. Historically, those have been my most productive observing sessions!

 

Clear and Steady Skies,

 

Bob



#21 Bob Bunge

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Posted 27 October 2021 - 01:55 PM

(( Also, seeing is usually poor the night after a weather system has moved through ))

 

Right Bob.  Just like here in Columbus, Ohio at the moment.  Our seeing is currently just horrible!

Ha!  My experience is the seeing is almost always bad in Columbus, Ohio, perhaps much like Mr. Elf's experience in Germany!  Lots of temperature delta there, even during the hot and humid months.  Spent many hours doing planetary observing off a south facing balcony in Short North.

 

Bob



#22 Chucky

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Posted 27 October 2021 - 02:10 PM

<<  My experience is the seeing is almost always bad in Columbus, Ohio  >>

 

Well Bob, at least you moved away!  I'm stuck here.

 

In many respects, I'm in this hobby thanks to you.  My first astronomy talk EVER (circa 1986) was yours on observing from a Navy ship in the North Atlantic.  And I knew my 10 Coulter optics were bad when I observed through your custom 10.  Your observing sessions at Hidden Hollow were legendary.  I still sometimes read your feature articles/reports in the old Deep Sky Magazine!

 

Hope all is well Bob.

 

Chuck Gulker- Dublin, Ohio


Edited by Chucky, 27 October 2021 - 02:22 PM.


#23 the Elf

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Posted 27 October 2021 - 03:35 PM

Bob,

 

thank you so much. Your story is an inspiration. I hope that I'll have the opportunity to make a trip to a place as you describe and spend some time imaging and observing. For the time being I'm stuck here as many countries still have restrictions due to Covid. But this pandemic will end as all pandemics before did and we will be back to normal. Until then I will make the best of what I have. I had a few nights with great lunar observing. I tried some DSOs as well and the must stunning was the double cluster at only 19x magnification.

If I travel the US any time soon, which place would you recommend in summer or fall? Florida Keys? Great Lakes? A dark NP?

 

CS

the Elf



#24 Bob Bunge

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Posted 27 October 2021 - 04:14 PM

<<  My experience is the seeing is almost always bad in Columbus, Ohio  >>

 

Well Bob, at least you moved away!  I'm stuck here.

 

In many respects, I'm in this hobby thanks to you.  My first astronomy talk EVER (circa 1986) was yours on observing from a Navy ship in the North Atlantic.  And I knew my 10 Coulter optics were bad when I observed through your custom 10.  Your observing sessions at Hidden Hollow were legendary.  I still sometimes read your feature articles/reports in the old Deep Sky Magazine!

 

Hope all is well Bob.

 

Chuck Gulker- Dublin, Ohio

I thought that might be you!  Great to hear from you!  I almost made it to Hidden Hollow this year, but it wasn't too be.  Perhaps 2022!

 

All is well here and now with the kids more grown up and mostly out of the house, hoping to get out some more.

 

Bob



#25 Bob Bunge

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Posted 27 October 2021 - 04:22 PM

Bob,

 

thank you so much. Your story is an inspiration. I hope that I'll have the opportunity to make a trip to a place as you describe and spend some time imaging and observing. For the time being I'm stuck here as many countries still have restrictions due to Covid. But this pandemic will end as all pandemics before did and we will be back to normal. Until then I will make the best of what I have. I had a few nights with great lunar observing. I tried some DSOs as well and the must stunning was the double cluster at only 19x magnification.

If I travel the US any time soon, which place would you recommend in summer or fall? Florida Keys? Great Lakes? A dark NP?

 

CS

the Elf

 

For dark skies, out west.  Two years ago, I was driving through Utah and managed to stop for a few minutes to witness some truly dark skies, so many places in the Western US would knock your socks off.  If you are in the Eastern US, there are some spots in West Virginia (Spruce Knob, a mountain ridge) that are about as dark as you can find east of the Mississippi river.   While I believe there are closer places to you then the Florida Keys, they have provided for me the best seeing conditions for planetary work I've experienced in the US, or at least the most reliable and likely to happen within a period of a few days.  I've had just as good nights in the Mid-Atlantic region, but they are quite rare and a lot of conditions have to align just right.  The Keys have the advantage of being in the south, which makes a huge difference.

 

Bob




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