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Bad Seeing beats Light Pollution by far

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#1 WRAK

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Posted 28 July 2013 - 05:18 AM

Yesternight at a rural site a quite dark sky compared to my usual site (I could even see hints of the Milkyway) but a rather quick temperature drop after an extremely hot day made seeing really bad - not even a hint of a crisp spurious disk but only fuzzy spots instead of a diffraction pattern.
Tried some doubles in Oph with a 120mm refractor but results were rather frustrating - best I could do were splits of wide pairs > 3" sep and TML was reduced to +11.3mag. Some examples:
- BAL2895 – 14.2" +9.15/11.30mag. x25 hint at 11 o'clock, companion disappears with higher magnification. Should be easy with 90mm with better seeing
- STF2187 – 3.2" +9.1/10.1mag. x100 hint at 5 o'clock, x180 does not make things better. Should be easy with 90mm with better seeing
- BAL2445 – 5.1" +10.17/10.67mag. x180 at 4 o'clock, very fuzzy. Should be easy with 90mm with better seeing
- STF2202 (HIP 86831, 61 Oph) – 20.6" +6.1/6.5mag. Easy one. x25 at 3 o'clock. Limit below 40mm but this is my smallest mask
- STF2186 (HIP 86106) – 3" +8.2/8.4mag. x100 at 1:30, x180 only fuzzy spot. Should be easy with 60mm with better seeing.
Seeing is really bad now and the half moon is rising in the east, so good bye dark sky and I quit.

Was a tad frustrating but very instructive - bad seeing reduces the performance of your scope up to 50% or even more.
What I did not try (I wonder why) was reducing the aperture of my scope for example to 90mm from the very beginning to counter the bad seeing - the results would not have been better but images would have less frustrating. Will keep this in mind for next time.
Wilfried

#2 Asbytec

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Posted 28 July 2013 - 06:09 AM

Wilfred, you had me worried. Bad seeing "beats" light pollution...by far? Okay, no I understand bad seeing is the worst of the two...by far. Thankfully, I just moved to a yellow zone and don't need the scare. :)

Hate giving up my grey zone, though, with excellent seeing. I suspect the seeing here will be good, too. November will tell...

#3 Ed Wiley

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Posted 28 July 2013 - 11:12 AM

Wilfried, I feel your pain. Last two imaging sessions were all snowballs. :bawling:

Ed

#4 azure1961p

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Posted 28 July 2013 - 06:36 PM

I agree bad seeing can truly hobble a scopes performance. Ill take some light pollution and haze for better seeing instead of pristine clarity and a boiling pattern.

Pete

#5 Ed Wiley

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Posted 29 July 2013 - 07:46 PM

2++ Pete!

Ed

#6 WRAK

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Posted 02 August 2013 - 03:58 AM

A few days ago same session with severe light pollution (NEML in the field of view ~2.75 ) but far better seeing Pickering ~6. 140mm refractor with iris diaphragm.
- BAL2895 – 14.2" +9.15/11.30mag. Nothing to see of a companion, must be significant fainter than +11.3mag
- STF2187 – 3.2" +9.1/10.1mag. Resolution x75 auf 5:30, limit aperture aperture 70mm
- A2248 (HIP 86065) – 2.2" +8.41/12.15mag. No resolution - required aperture probably >200mm
- BAL2445 – 5.1" +10.17/10.67mag. Resolution x200 very faint at 4:30. Companion seems much fainter than +10.67mag. Limit aperture 110mm
- BU961 (HIP 86424) – 8.3" +6.81/11.40mag. Should be easy with 140mm but no reslution. Apodizing mask does not improve the image. Companion fainter than 11.4mag?
- BAL2450 – 6.2" +11.2/11.5mag. Should be easy with 140mm but no resolution. No Tycho star in this position - bogus double? WDS lists 3 observations
- STF2201 (HIP 86820) - 7.2" +8.7/10.57mag. x70 at 10 o'clock, companion very faint, fainter than advertised? Limit aperture 130mm
- STF2202 (HIP 86831, 61 Oph) – 20.6" +6.1/6.5mag. x15 at 2:30. Limit aperture 15mm
- BAL1942 – 5.1" +11.1/11.4mag. Extremly faint, x140 hint at 5 o'clock – position would be correct, but this is no "resolution". Again fainter than advertised?
- A2249 – 1.5" +9.1/12.8mag. No resolution, requires certainly much larger aperture >200mm
- STF2186 (HIP 86106) – 3" +8.2/8.4mag. Resolution x75 at 7:30, limit aperture 70mm
- STT331 (HIP 85812) – 1" +7.7/8.8mag. x280 rod direction 11 o'clock, limit aperture 140mm
- BU1538 – 2.6" +11.5/11.5mag. Extremly faint, not even to resolve as single star. Check WDS: Note X for "bogus double".

To some degree I wonder if Baillaud is just another Jonckheere regarding magnitude errors for companions.
Wilfried

#7 Ed Wiley

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Posted 02 August 2013 - 01:55 PM

Hi Wilfried:

Part of the problem may be that some of the magnitudes are Tycho-V rather than Johnson V. At least for the one case I looked up in VizieR, BAL2445, the reported mags are Tycho-V. The scatter increases between the Johnson and Tycho Vs as star get fainter and/or the color difference are greater. I found this article to be very interesting:

http://britastro.org... Photometry.htm

So, I suspect you are 'on target' with your visual impressions and conclusions.

Ed

#8 WRAK

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Posted 03 August 2013 - 10:09 AM

Ed, thanks - most interesting paper. Slowly I begin to accept that the unreliability of advertised magnitude data especially for faint companions has to be part of a probability approach when working on enhancements of the RoT model.
Wilfried

#9 Ed Wiley

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Posted 03 August 2013 - 11:14 AM

Right, Wilfried: That is why your observations are valuable. Those who do visual double star observations need to know what to expect visually and if a pair or one of the stars is relatively fainter than the typical catalog value then this is needed information.

Another thing to think about is difference in magnitude. Perhaps this would be more reliable, I know that the pros make much of if (just read the intro to the DM3 catalog). Frankly I am terrible at estimating dM, but perhaps with some training? If you look at the histories of observation sometimes you see estimates of dM rather than estimates of magnitude. I wonder, for example, if any of the old observations of your pairs with a "D" listed in the notes column listed a dM greater than the differences implied by the Tycho-V magnitudes? It might be worth investigating. There is a dM catalog, but its a photometric catalog mostly based on JHK. For the visual dMs one would have to access the original history of observation and I have no idea what you might find.

Ed

#10 WRAK

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Posted 03 August 2013 - 02:29 PM

Estimating dM - feel rather uneasy with such a task as I use different scopes from different locations so there is no standard setup. I tend to rely on my experience what is possible for me with the scope I use near the Telescope Magnitude Limit. While there is a huge TML variation between different observers I hope there is less variation regarding myself.
Wilfried






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