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how to check seeing?

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31 replies to this topic

#26 Pcbessa

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Posted 17 February 2019 - 03:11 AM

That unusual and seeing in clear sky weather, especially in winter, happens when the jet stream is right above us. Whilst calm at surface it can be very windy at altitude.

Seeing a weather chart can help determine good seeing...

#27 Jon Isaacs

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Posted 17 February 2019 - 06:09 AM

My approach:

 

- If it is clear, I will be observing something.  This means the most important thing is whether it will be clear or not.  

 

- When it comes to seeing, I use my own judgment based on my knowledge of the local weather conditions and add in Cleardarksky.  But since my scopes that are the best suited for excellent seeing conditions require some time to cool, waiting for it to be dark enough to actually judge the seeing means it's too late to setup. 

 

So, unless the seeing will obviously be poor, I setup with the assumption that the seeing will be reasonable. Poor seeing is quite rare and is almost always associated with a high pressure over the desert and a east-west wind flow over the coastal mountains.  This condition is known as a Santa Ana and Santa Ana's are easy to identify.. Warmer than normal with clear skies and windy during the day.  

 

Once out and under the night sky, I use doubles to gauge the seeing. 

 

Jon



#28 garret

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Posted 17 February 2019 - 06:10 AM

 

I ve been for a bit in this hobby now, but in some regards I still feel like a beginner. My plan is, to get some scope for planetary observing. My interest was previously DSOs, so different task now. How to check seeing, if your actual scope is smaller than the planned one? My biggest scope is an ED 120 binocular telescope, which gives 130x max with the N7. I live central in Germany and I can drive max 30 minutes to places and prefer my two DSO friendly places. I plan to observe the moon with it over the next 3 months to check, if it is worth at all to go beyond 120mm aperture. But how to know the limit? An APM 152 ED? Or a Mewlon 210? I think 8" might be the maximum...

 

Thanks for advice,

The smallest details you can see of an object like the moon is limited by long list of parameters;

1) Aperture

2) Optical quality of the telescope, eyepieces and diagonal

3) Thermal equilibrium of the telescope

4) Transparency of the atmosphere (thin icy clouds, moisture)

5) Stability of the atmosphere (amount of turbulence, seeing)

6) Characteristic of the turbulence (fast or slow)

7) Exit pupil size (2-3mm would be ideal)

8) Thickness and length of the atmosphere (observation site and object altitude)

9) Collimation and centering of all optical elements

10) Amount of obstruction

11) ...

 

Seeing prediction, seeing monitors they all inform just a small part of the whole story.

 

A 40cm Truss Newton is a very good choice, you can use a mask to get a 15cm obstruction-free planet killer, especial with Zambuto Optics (or equivalent).

 

Garrett vd Veen


Edited by garret, 17 February 2019 - 07:07 AM.


#29 salico

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Posted 17 February 2019 - 04:03 PM

Ok, imagine, you have a 12" Dob for mainly DSO and want to have at least 150x power. Some weather charts say 1" seeing, you assemble hopefully and realise even at 80 x the views are horrible...



#30 gnowellsct

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Posted 17 February 2019 - 05:49 PM

I myself consider myself a beginner and I can't stress how useful Stellarium is to determine what equipment is  needed to provide the view you want or will get with the equipment you have.  Stellarium has an ocular view mode in which you can configure the various equipment, telescopes, eyepieces, barlows/ reducers, CCD sensor size (for astrophotography).  If you haven't used Stellarium, I suggest downloading and playing around with it.

Well yeah, but any of the star software programs do this.  GN



#31 gnowellsct

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Posted 17 February 2019 - 06:02 PM

The smallest details you can see of an object like the moon is limited by long list of parameters;

1) Aperture

2) Optical quality of the telescope, eyepieces and diagonal

3) Thermal equilibrium of the telescope

4) Transparency of the atmosphere (thin icy clouds, moisture)

5) Stability of the atmosphere (amount of turbulence, seeing)

6) Characteristic of the turbulence (fast or slow)

7) Exit pupil size (2-3mm would be ideal)

8) Thickness and length of the atmosphere (observation site and object altitude)

9) Collimation and centering of all optical elements

10) Amount of obstruction

11) ...

 

Seeing prediction, seeing monitors they all inform just a small part of the whole story.

 

A 40cm Truss Newton is a very good choice, you can use a mask to get a 15cm obstruction-free planet killer, especial with Zambuto Optics (or equivalent).

 

Garrett vd Veen

 

All this is true.

 

What's also true is that the moon is so overwhelmingly rich in high contrast details that you have to work really hard to find the details in the big scope that aren't captured in the smaller scope.  The superior resolution of the big scope may capture a branch of a branch off a rille, for example, but both scopes will show the rille and the branch off the rille, and there may not *be* that many branch off-a branch -off a branches for you to find and evaluate.  And on top of it, if you *do* see these increases in detail, there's still going to be an insane amount of things going on in the field of view, so that your overwhelming impression will be that the smaller scope is "keeping up" with the larger scope.  

 

By contrast aperture increases will provide more readily detectable differences on planets and deep sky.  This is particularly true of color saturation.

 

I don't mind spending an hour or two here looking at favorably situated planets but I quickly want to move on to deep sky if seeing is poor.  I find the Clear Sky Clock predictions of seeing to be very unreliable in my area.  The best planet views tend to be dawn or dusk, so it's a good argument for getting set up before sunset so the scope is cooled and ready to go. 

 

Greg N   


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#32 Feidb

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Posted 18 February 2019 - 09:34 PM

Since I really have little interest in the planets or moon, or double stars, which are the most affected by seeing, it isn't my priority.

 

However, I still care about it to a point.

 

I'm more interested in transparency, because I'm into deep sky, and darkness and transparency are more critical to finding those faint fuzzies.

 

However, many times, seeing and transparency can go hand-in-hand.

 

Therefore, besides the initial darkness of the sky, if there's a planet handy when I first set up, I check it to see what it looks like. If it's a bloated gasbag, or dances like crazy and won't focus, I'm pretty sure the seeing is not going to be real great.

 

On the other hand, if said planet is crystal clear and shows lots of sharp detail, I don't have to worry about details and can push the magnification on deep sky objects.

 

Now, the more critical factor for me, transparency, is different. While the planet test may show incredible detail, if I switch to stars and though they may show nice sharp dots, but the background is gray, or the stars have nebulae around them that aren't supposed to be there, well...transparency is going to be a problem and those faint fuzzies are not going to show up.

 

THAT to me is more important than seeing.

 

In your case, if I get your drift, you are wondering how big of a scope you should go for, given your usual seeing conditions where you are in Germany?

 

The thing you have to remember is that those conditions are shifting. What they used to be and what they may be in the future may not match history. Therefore, the site you may consider for observing, while historically it may have mediocre seeing, to justifying going up to such and such of an aperture, that could shift to better seeing or even worse.

 

What you have to keep in mind is that you want to get the largest aperture you can afford, but also the largest you can handle. If you believe smaller is better, go for that. If your interests may change and you want to expand your horizons to deep sky objects, you want larger aperture. Seeing is not going to kill you one way or the other. Maybe you have to travel to a different spot, maybe not. It might be a gamble and maybe you'd still be better off smaller.

 

I'm not sure if I'm on the right track here, but hope this helps.


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