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Resolving limits and Seeing

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#26 astrokeith

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Posted 30 July 2021 - 09:01 AM

Several hours?  I think somewhat longer.. how about a lifetime? 

 

Making difficult splits with larger aperture scopes are long term projects, waiting for that perfect night when the seeing is super, making sure the scope is fully thermally equilibrated, it's not something that just happens when you want it too.

 

A Rayleigh split in Kevin's 25 inch is 0.22" and that's only a 25% drop in brightness from the maximum. Twice the Rayleigh Criterion is a full contrast split, the first minima overlap, that's 0.44", such seeing is very rare..

 

My efforts in cooling my 13.1 inch are driven by my interests in splitting close doubles. I've managed a few under 0.5", most recently a 0.44" but they are projects..

 

Jon

Now we can see why the USAF test chart has some advantages! OK its got issues, but a horizontal and vertical measurement could be done in minutes.



#27 Jon Isaacs

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Posted 30 July 2021 - 09:43 AM

Now we can see why the USAF test chart has some advantages! OK its got issues, but a horizontal and vertical measurement could be done in minutes.

 

The problem is that it is seeing limited even for relatively small scopes.  

 

A few years back I was corresponding with a CN member who was observing in the mid-Atlantic area with an AP-175.. I was splitting double stars with my 10 inch GSO made in Taiwan Dob that he was unable to split.  Part of it was the fact that I can have super seeing and part of it was that resolution is directly related to aperture. 

 

Big scopes will always be more difficult to operate but they also have the potential to make the splits that smaller scopes can't.  Zeta Bootis is currently at 0.20".  I know that Redbetter was getting elongation and maybe partial separation with his 20 inch.

 

Jon



#28 Keith Rivich

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Posted 30 July 2021 - 12:56 PM

The double stars is probably the only way of achieving high altitude measurements, at the moment. I guess there are already reference lists of such doubles.

 

This would be a major exercise for each observer, probably taking some hours?

 

I would think a list of double stars would not be to difficult to prepare for next years TSP. Pick 4 pairs that culminate in the 11:00pm hour, 4 more for the 12:00am hour and 4 more for the 1:00am hour. Wouldn't take to much time to do the observations. 



#29 Migwan

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Posted 30 July 2021 - 07:48 PM

Interesting thread.   There's a list of acceptable doubles HERE.  

 

I managed a split to Reyleigh's Criteria (60% overlap) two of the three listed in Cass for 300mm.   Only had to wait two or three month's for a night of good seeing.   Did so in an actively cooled C11 at 461x.

 

I'm not really into splitting doubles, but this was an interesting exercise.


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#30 Jon Isaacs

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Posted 31 July 2021 - 05:17 AM

I would think a list of double stars would not be to difficult to prepare for next years TSP. Pick 4 pairs that culminate in the 11:00pm hour, 4 more for the 12:00am hour and 4 more for the 1:00am hour. Wouldn't take to much time to do the observations. 

 

For your 25 inch, a challenge double would be under 0.5" and a measure of the resolution of the optics would be the Rayleigh criterion, 0.22".  

 

Dialing up 0.2" seeing is probably not going to happen.  Star party locations are generally chosen for dark skies rather than super stable seeing. 

 

Currently, according to goodtostargaze.com, the seeing in Fort Davis is currently 1.4".  My San Diego backyard is predicted to be 0.6"

 

My point is that any test like this is very likely to be a measure of the seeing rather than the resolving capabilities of the scope. 

 

Jon



#31 astrokeith

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Posted 01 August 2021 - 04:35 AM

For your 25 inch, a challenge double would be under 0.5" and a measure of the resolution of the optics would be the Rayleigh criterion, 0.22".  

 

Dialing up 0.2" seeing is probably not going to happen.  Star party locations are generally chosen for dark skies rather than super stable seeing. 

 

Currently, according to goodtostargaze.com, the seeing in Fort Davis is currently 1.4".  My San Diego backyard is predicted to be 0.6"

 

My point is that any test like this is very likely to be a measure of the seeing rather than the resolving capabilities of the scope. 

 

Jon

So it could be the at TSP that the short path length to the target (~700m) meant that the atmospheric seeing problems introduced were less than looking straight up? For some apertures at least.

 

It was a while ago now but I did at the time look in to the Fried Parameter 'hypothesis' and thought it explained the big dob poor results, compared with the 150-300 mm class scopes.

 

The data I have, included multiple attempts by most observers, and appears to show 0.25" as an atmospheric seeing limit across to the target. There is one interesting outlier which is a premium refractor observing from the lower field. The result was quite poor. So I went down to have a look. The path length was a lot longer, but it was also straight across the top of the ranch buildings, including the swimming pool and snack bar. Problem solved!

 

One advantage of the target I hadnt mentioned yet - it worked on cloudy night!




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