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Effects of bad seeing

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#1 John Fitzgerald

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Posted 25 December 2019 - 10:44 PM

1. Resolution of close pairs.  The stars will merge, as the size of the apparent disk becomes bloated by poor seeing.

2. Faint companions, that are otherwise wide enough to resolve, will disappear as they become bloated and defocused.  The light of a pinpoint is spread out to the size of a pinhead, becoming fainter, especially in light polluted or moonlit skies.

3. Your limiting magnitude for all faint stars will decrease due to bloating, as above.


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#2 Joe1950

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Posted 25 December 2019 - 11:53 PM

When I first got my C-8, John, I was trying it out on different targets one night including Epsilon Lyrae. Normally, it's a clean split with my C80ED. But that night, seeing was so bad, compounded by the fact that with the 8" I was looking through more of it, the view was not impressive at all.

 

Individual stars were so animated they were changing focus, size and position as I watched.

 

A weather front had just passed through, likely the cause of the unsettled air. And the jet stream is often not far from my location.

 

 

The scope shows a very nice Airy disk and a good first diffraction ring under normal circumstances, but nothing was normal that evening! It was a case where more aperture was a hinderance rather than a help.


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#3 Joe1950

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Posted 26 December 2019 - 12:20 AM

BTW: This visual sample of he Pickering seeing scale, by Damian Peach, is helpful assessing seeing, for me at least.


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#4 John Fitzgerald

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Posted 26 December 2019 - 08:52 AM

BTW: This visual sample of he Pickering seeing scale, by Damian Peach, is helpful assessing seeing, for me at least.

That's the one I use.  Very helpful.


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#5 John Fitzgerald

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Posted 26 December 2019 - 09:24 AM

Last night was a 3 with occasional  times of 4.  But, it was quite clear and temp in the upper to mid 50s while I was out.


Edited by John Fitzgerald, 26 December 2019 - 01:49 PM.

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#6 flt158

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Posted 26 December 2019 - 12:04 PM

I couldn't get my head around the fact that after sunset on Christmas Eve all double and triples stars had that "bloated" look for 1.5 hours which John Fitzgerald has raised. (Thank you, John.)

And yet, 2 hours a massive transformation took place. 

All the bloating was gone completely!

Therefore it pays to be patience and not to give up hope. applause.gif

 

Clear skies from Aubrey. 


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#7 Astro-Master

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Posted 01 January 2020 - 09:34 PM

I like using my 105mm APO on double stars over my larger scopes just because it gives nicer images even when the seeing is below average.  I still get nice round stars but the whole image kind of bounces almost like my clock drive is making it bump up and down a little.

 

An 8" scope will be affected 4 times as much as a 4" on the same night, so in this case aperture doesn't rule in below average seeing.


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#8 Jon Isaacs

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Posted 02 January 2020 - 07:03 AM

I like using my 105mm APO on double stars over my larger scopes just because it gives nicer images even when the seeing is below average.  I still get nice round stars but the whole image kind of bounces almost like my clock drive is making it bump up and down a little.

 

An 8" scope will be affected 4 times as much as a 4" on the same night, so in this case aperture doesn't rule in below average seeing.

 

If I am going specifically after doubles, I tend towards my 10 inch Dob rather than any of my refractors.  One reason is for it to perform it's rock solid best, I need to set out up before sunset and run the fans for considerably more than an hour.

 

If the seeing is on the good side and I started with a 4 or 5 inch, I'm stuck.  

 

For closer doubles, maybe 1.4" or closer, the 10 inch provides wider splits under most circumstances because it's Airy disk is so much smaller. A 1.14" double is a Dawes limit split in a 4 inch, in the 10 inch it's a wide split and I'm not fighting both the large Airy disk and the seeing.

 

I'm not sure about your factor of 4 in terms of the effect on the image. I am not looking so much for a pretty image, I'm looking for closer splits. Antares is usually a challenging split in a 4 or 5 inch but last year I made the split in my 22 inch with Antares quite low on the horizon, no way in a small scope.

 

It wasn't pretty but it was very wide, bright and apparent. It made me realize just how small the airy disks are in a scope that size.

 

With any scope but a larger scope in particular, the outer seeing aberrated region can as an extended object while it surrounds the region where the airy disk is brighter. Cranking up the magnification can increase the contrast by dimming that outer region.

 

Jon


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#9 Astro-Master

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Posted 02 January 2020 - 03:02 PM

If I am going specifically after doubles, I tend towards my 10 inch Dob rather than any of my refractors.  One reason is for it to perform it's rock solid best, I need to set out up before sunset and run the fans for considerably more than an hour.

 

If the seeing is on the good side and I started with a 4 or 5 inch, I'm stuck.  

 

For closer doubles, maybe 1.4" or closer, the 10 inch provides wider splits under most circumstances because it's Airy disk is so much smaller. A 1.14" double is a Dawes limit split in a 4 inch, in the 10 inch it's a wide split and I'm not fighting both the large Airy disk and the seeing.

 

I'm not sure about your factor of 4 in terms of the effect on the image. I am not looking so much for a pretty image, I'm looking for closer splits. Antares is usually a challenging split in a 4 or 5 inch but last year I made the split in my 22 inch with Antares quite low on the horizon, no way in a small scope.

 

It wasn't pretty but it was very wide, bright and apparent. It made me realize just how small the airy disks are in a scope that size.

 

With any scope but a larger scope in particular, the outer seeing aberrated region can as an extended object while it surrounds the region where the airy disk is brighter. Cranking up the magnification can increase the contrast by dimming that outer region.

 

Jon

 

I usually go for doubles 1.2" or wider in the 105mm APO, and a clean image with no 4 vane spider or central obstruction is what I like, I just like refractors best on doubles for that reason.  Most all of my double star observing is done in my backyard {Bortle 7 Red zone} and the small refractor has a much darker sky background to enhance the view.

 

I read somewhere that the seeing is like looking through 4" cells, so a 4" scope is like looking though one cell.  If the seeing is below average the one cell bounces around but the image inside the cell is still good.  I've observed this effect many times, the double star still looks good, but the whole image is moving up and down, almost like the clock drive is making it bounce a little.

If you are using an 8" scope on the same night, now you have 4 times the area of the 4", its like looking through 4 cells that are moving around and distorting the image up to 4 times as much.

 

I've experimented with this effect with my 18" Dob. on a night with poor seeing.  One night with a few friends, we were complaining about the poor seeing, so I put a 4" off axis aperture mask on the front of my 18" scope when my friends weren't looking and put it on the double double in Lyra.  I called them over for a look, the stars looked good, but the whole image was bouncing, they couldn't believe it, then one of them looked and saw the aperture mask, and took it off.  The image looked like crap without the mask, so IMHO the 4" cell theory seems to work for me.


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#10 Jon Isaacs

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Posted 08 January 2020 - 08:48 AM

Bruce:

 

A few thoughts:

 

- I think the cell size depends on the seeing or vise versa, it's not just 4 inches all the time.  

 

- I do most of my double star observing from my San Diego backyard.  Sky brightness is determined by exit pupil so I get dark background skies by increasing the magnification.  It's pretty dark at 800x in my 10 inch.. 

 

- I choose my scope based on the assumption that the seeing will be good to excellent. Probably my favorite double star scope is my 10 inch F/5 because it cools quickly and I have split doubles down to 0.5" and it splits 1.0" doubles wide, diffraction spikes are rarely visible and the stars are tight and round.  I enjoy all sorts of doubles but I particularly enjoy making splits under 1.0". 

 

My 13.1 inch F/5.5 splits closer doubles but it cools more slowly and is more effort to set up. I think scopes in the 10 inch-13 inch range are optimal for splitting close doubles.  Anything larger will almost always be seeing limited and scopes this size are capable of taking advantage of sub-arcsecond seeing. 

 

- A 1.2" double in a 4 inch is a Dawes limit split, it takes much better seeing to make a Dawes limit split in a 4 or 5 inch than it does to make that same split in my 10 inch.  That's because the 4 or 5 inch is fighting both the seeing and the overlapping Airy Disks whereas the 10 inch is only fighting the seeing, the airy disks are half the diameter.  

 

- I think of the double-double as a measure of seeing but it's an easy split in an 80mm and doable in a 60mm.  If I am not getting a decent split of the double-double in my 10 inch, it's not a night for doubles.

 

We all do this thing our own way..  My 120 ED does a good job on doubles and that's one of the reasons I bought it.  But the old 10 inch Dob does a better job on the close ones. 

 

Jon


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#11 SeaBee1

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Posted 09 January 2020 - 11:01 AM

A very interesting discussion gentlemen! I am now using a SW120ED for doubles work and I too have noticed the effect of seeing with it. At first, I thought something could be amiss with my optics, I was only able to see down to around 9.5 mag, and only able to split separations around 2.0 arc seconds, but I have had other observations with it that clearly indicated the optics are good. It has taken me a bit to come to grips with the seeing issues I have been encountering and not blame the scope. One clue for me is the image bouncing another commenter made upstream. I have noticed that with many of my observations and was concerned about the AVX mount I am using, but now realize it is the seeing that is bedeviling, more so than I have realized.

 

Good information guys, it has clarified some things for me!

 

Good hunting!

 

CB


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#12 John Fitzgerald

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Posted 09 January 2020 - 11:55 AM

If it's a moonlit night, and obvious bad seeing, I don't bother going out into the dome.  Bad seeing on moonlit nights means not seeing very much, at least to me.


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#13 The Ardent

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Posted 10 January 2020 - 09:33 PM

These are nights when I pull out the camera for double stars. My priority on good nights is visual telescopic observing of planetary nebulae. Or wide field observing with binoculars.

With either of these I tend to get distracted by doubles in the field! Cant get anything done. 

 

If it's a moonlit night, and obvious bad seeing, I don't bother going out into the dome.  Bad seeing on moonlit nights means not seeing very much, at least to me.


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