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STF1930

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#1 WRAK

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Posted 20 June 2014 - 06:46 AM

STF1930 near M5 in Ser is an 11.5" wide double (actually a quadruple but C and D are this far away that they are visually insignificant) with a quite large delta_m (+5.06/10.11mag). Despite this I would have expected to resolve this one with ~70mm aperture but needed actually 110mm. I had the impression that the companion is quite reddish and thus visually harder to resolve than the given magnitude suggests. But this was my first resolution of STF1930 so I will try it again.
Wilfried

#2 fred1871

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Posted 20 June 2014 - 09:31 AM

The Notes in the WDS give "Mt Wilson Spectral types" for this pair as F5 and M0 - the spectrum for the primary is now listed as F8V in the summary WDS, not much different; it's likely the companion is near M0. So "quite reddish" is the probable case.

I don't have any colour notes on the secondary star from my own observing; merely that it was an easy pair with pale yellow primary (150/180mm).

#3 WRAK

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Posted 25 June 2014 - 05:50 AM

Had last night again the opportunity to visit again STF1930, this time with my C925. Resolution was clearly no problem but the question of the magnitude of the companion was of interest. Observed therefore faint stars nearby: UCAC4-460-055152 +13.01mag seemend a tad fainter. UCAC4-460-055171 +12.08mag appeared similar faint like the companion. Same with UCAC4-460-055164 +12.47mag. Even if the companion is a tad reddish this seems to me to be no sufficient reason that the visual magnitude of the companion is with +10.11mag about 2mag brighter listed as it appears to the eye. I know the arguments that red stars are harder to resolve but this is no sufficient reason to violate the concept of visual brightness.
Wilfried

#4 fred1871

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Posted 25 June 2014 - 09:21 AM

The Purkinje effect is not likely to drop the apparent brightness by 2 mags; but you could see if any of the comparison stars might be orange or red, whether from finding spectral types, or seeing how bright (large) they look on a red plate compared to a blue plate - so that you might be able to compare similarly reddish stars.

#5 WRAK

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Posted 26 June 2014 - 02:04 AM

The primary is a variable star VSX34870 but the magnitude change is rather small from +4.99mag to +5.11mag. There is no AAVSO object for the secondary and despite the rather large separation of 11.5" there is also surprisingly no UCAC4 object for these coordinates. The 2MASS K-band image shows the secondary rather well with a diameter half of the primary thus confirming the +5mag delta_m in the infrared range. There is even a much fainter third component with about another +5mag delta_m to see at PA 180° (has nothing to do with the far away C and D components of STF1930).
Found also a sketch from Mariano Gibaia (Sketch STF1930) showing nicely that the visual magnitude of the secondary is compared with the stars nearby rather in the +12mag and not in the +10mag range.
The Star Splitters site also delivers a sketch (Serpens) and an historical quote from the " Venerable Admiral Wm. H. Smyth ... “A” 5½, pale yellow; “B” 10½, light grey. ". The 10½ value is obviously a crude estimation and interestingly here is the impression of a grey hue stated and not reddish as I find obvious.
Next the "33 Doubles Observing Project" - nice photo with an obviously huge and far larger than 5mag visual delta_m and reddish grey hue for the companion (STF1930).
Wilfried

#6 fred1871

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Posted 26 June 2014 - 02:11 AM

Admiral Smyth had a 15cm achromat refractor, so the light gathering would not be sufficient to allow seeing colour in the secondary star here if, as seems likely, it's around mag 12.

"All cats are grey in the dark". :grin:

#7 WRAK

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Posted 26 June 2014 - 02:46 AM

My statement that there is no UCAC4 object for STF1930B is wrong: UCAC4-459-054803 with model fit magnitude of +8.274 and interestingly no values for the other bands ( UCAC4 query Uni Heidelberg). While this value is completely obscure (as all UCAC4 magnitudes brighter than +10mag) this would to my experience be an indication for a magnitude not much fainter than +10mag. This is getting funny.
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#8 fred1871

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Posted 26 June 2014 - 05:12 AM

I've now called up data for this via Simbad, and it identifies a star as the companion of STF 1930 (among the identifiers is WDS J15193+0146B) - it gives a spectrum of K4 which fits for a fairly red star; and has a series of magnitudes in various bands. V is given as 10.11 (as in WDS); B as 11.45, expected for a K4 star.

So this might suggest the apparent faintness is an artefact of observing conditions combined with red colour making it look dimmer still.

I've looked at my observing notes again, and at the observing list (data) I used back when I observed this one - it gave mag 10.2 for the companion - so I suspect I didn't see it as a lot dimmer than that, or I would've noted the listed magnitude as too bright. That would only suggest I saw the companion as a fit for mag 10-point-something; if it had looked as faint as mag 12 that would be very obviously a lot dimmer than expected from the data list, so I would note that.

Therefore I'm inclined to think the magnitude might be closer to the quoted 10.11 than it appeared to you. If I get any clear weather here I'll have a look at it. Currently I'm having night after night of cloud, rain, and wind. Next week might be better.

#9 WRAK

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Posted 26 June 2014 - 07:18 AM

Fred, if you get this opportunity please compare the brightness of the companion with the Tycho II stars nearby (map attached).

And if you are already in this region you might as well visit TYC0362-00421-1 about 5° NE from STF1930 - this should be a 2.6" +11.7/12.58mag PA 115° double as UCAC4 has two stars here (UCAC4-479-056256 and UCAC4-479-056257 - tried to check proper motion, but this is only given for the "secondary", so this might be optical or physical). I tried this once with my C925 but due to not this good seeing conditions without conclusive result. Without better evidence I would suspect a "bogus" UCAC4 object - but when I checked the DSS and the 2MASS images I found in both cases a significant elongation in the right position so this seems solid. Yet a confirmation with a visual observation is necessary to get this confirmed.
Wilfried

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#10 ssmith

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Posted 27 June 2014 - 12:56 PM

Wilfried -

I managed to get out last night a get a few shots of STF 1930. Pretty spectacular view with M5 so close.

Wilfried - I sent you a wide-field shot in a separate email which may help you out in figuring out the magnitudes.

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#11 rcwolpert

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Posted 27 June 2014 - 11:26 PM

Wow...nice photo, Steve! That's very helpful!

- Bob

#12 WRAK

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Posted 28 June 2014 - 03:46 AM

Steve, your image shows for STF1930AB what would be a classic elongation in visual observing. Yet B shows enough of diameter to be compared with D indicating similar brightness. Interestingly the inset with clear separated A and B shows not this much reddish hue for B I would expect.

Last night I had again the opportunity to observe STF1930 with the C925 with not very fair seeing conditions (very thin clouds covered this area of the sky, NEML ~2.7 and Pickering ~4). Again I had the impression that STF1930B is far fainter than for example STF1930D +10.35mag or 13' north TYC0340-00647-1 +10.94mag or 25' south TYC0336-00039-1 +10.15mag.

Chris Thuemen provided me another interesting raw image of STF1930 (attached, had to make it a bit smaller to be able to upload it here) showing an intense orange hue of STF1930B but indicating also STF1930B being brighter than STF1930D.

Both images seem to confirm rather the WDS data. Compared with my visual impression this would simply mean that my eyes have a pronounced weakness regarding red light. On the other side the sensor of a camera might handle photons within the visual range less different than the human eye thus pronouncing photons with less energy besides the storing of photons over the exposure time in the chip thus enhancing visibility of fainter objects.

Steve sent me another image of STF1930 with obviously less exposure time giving a near visual impression (with the unavoidable bloating of the primary when imaging). Will study this in detail and report later.

Thanks to Steve and Chris for sharing their images - very helpful.
It would be nice the get also some other visual observation reports to compare visual impressions.

Wilfried

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#13 ssmith

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Posted 28 June 2014 - 10:51 AM

Chris' shot definitely shows a bit more color than mine. High thin cloud was moving in when I was shooting so it no doubt dulled the color a bit. Will make a point to revisit and shoot again the next chance I get.

Nice shot Chris.

#14 WRAK

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Posted 28 June 2014 - 04:14 PM

... TYC0362-00421-1 about 5° NE from STF1930 - this should be a 2.6" +11.7/12.58mag PA 115° double as UCAC4 has two stars here (UCAC4-479-056256 and UCAC4-479-056257 ...

Attached a snapshot from the SDSS-III Sloan Digital Sky Survey provided from ww.sky-map.org showing nicely that this is really a double. My second attempt last night to resolve this one visually failed again - seeing was again not good enough.
Wilfried

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#15 WRAK

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Posted 29 June 2014 - 08:19 AM

From the image Steve sent me by email I made a direct comparison of STF1930B with STF1930D and nearby TYC0336-00297-1 by inverting the colors and cutting out the disks of these stars and putting them side by side (see attachment).
Best color saturation has STF1930B in front of STF1930D in front of TYC0336-00297-1 - according to the advertised magnitudes. The diameters of the spurious disks (for me usually the relevant data regarding magnitudes - the larger the disk in an image the brighter the star) give a sequence exactly the other way around - but TYC0336-00297-1 seems despite the larger disk really faint in terms of saturation.
So imaging supports rather the WDS data by positioning STF1930B in the +10.5mag range.
Wilfried

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#16 fred1871

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Posted 01 July 2014 - 01:36 AM

Fred, if you get this opportunity please compare the brightness of the companion with the Tycho II stars nearby (map attached).

And if you are already in this region you might as well visit TYC0362-00421-1 about 5° NE from STF1930 - this should be a 2.6" +11.7/12.58mag PA 115° double as UCAC4 has two stars here (UCAC4-479-056256 and UCAC4-479-056257 - tried to check proper motion, but this is only given for the "secondary", so this might be optical or physical). I tried this once with my C925 but due to not this good seeing conditions without conclusive result. Without better evidence I would suspect a "bogus" UCAC4 object - but when I checked the DSS and the 2MASS images I found in both cases a significant elongation in the right position so this seems solid. Yet a confirmation with a visual observation is necessary to get this confirmed.
Wilfried


Wilfried, I'm not getting a response from the TYC number you've quoted, nor from the UCAC4 numbers via Simbad etc.

Can you give a position for this object?

#17 WRAK

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Posted 01 July 2014 - 02:48 AM

Fred, the catalog coordinates are:
TYC0362-00421-1 RA 15:32:40.929 Dec +05:43:37.251 +11.74mag
UCAC4-479-056256 RA 15:32:40.902 Dec +05:43:36.912 +11.70mag
UCAC4-479-056257 RA 15:32:41.077 Dec +05:43:35.689 +12.58mag.
Separation according to UCAC4 is 2.6" with PA 115°.
Wilfried

#18 Perseus_m45

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Posted 01 July 2014 - 09:17 AM

so good to see we are not chasing a ghost.

#19 David Gray

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Posted 04 July 2014 - 10:55 AM

Kaler has just added it to his site:

http://stars.astro.i...u/sow/5ser.html

DG

#20 fred1871

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Posted 04 July 2014 - 09:02 PM

Thanks, Dave - Kaler as usual gets a lot of information together in a compact article. Interesting that he assigns K7 as the likely spectral type to the companion - midway between the sources I found, M0 and K4....

Wilfried, I had a look at STF 1930 the last two nights. First night was with the 140mm refractor, companion a small speck clearly seen at 114x (I didn't try less magnification - was using that eyepiece as my "first look" eyepiece on various doubles). The M5 globular showed stars, partial resolution; better at 160x, which also made the STF 1930 companion more obvious. My estimate of the companion was that it was in the mag 10-11 range, definitely not as faint as 12.

Second night was with the C9.25. The STF 1930 companion was easy and obvious at 98x. Again, I'd estimate the magnitude as mid-10s. The M5 globular was very nice, with a lot of stars showing; 124x and 180x improved it further.

Unfortunately, thin cloud then came in, rapidly, and I didn't get to your 12th mag UCAC4 pair of stars. The previous night, with only 140mm aperture, I didn't look for those as I'd not expect 140mm to be adequate for such a faint fairly close combination (11.7, 12.6 at 2.6"). And on the first night there was moonlight (thick crescent) and some haze, so even less of a night for faint stars with 140mm. Seeing however was good - a 0.7" double was seen elongated and notched at 400x with 140mm (A 2076, mags 8.3 and 8.4).

Cloud and showers have returned here today, and by the time the weather clears the Moon will be too bright for the UCAC4 stars. After Full Moon, if the weather co-operates, I'll try for them.

#21 WRAK

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Posted 05 July 2014 - 08:58 AM

Thanks Fred. I consider now the topic of the brightness of STF1930B as settled - obviously my sensitivity in the reddish band range of photons is rather low with the consequence of needing a larger aperture for resolving as the advertised data suggests. To some degree this reminds me of my similar impression (companion fainter than advertised) of STF450 in the Pleiades. Although in this latter case I found meanwhile photographic evidence supporting my impression. This time it is the other way around.

Yet STF1930 will remain as Kaler describes an interesting target in my session plans not least due to its proximity to M5 - one of the few globular clusters offering the double star observer the chance to resolve single stars including some variable ones with a reasonable sized scope.

For the mentioned faint UCAC4 pair I will open a separate thread.
Wilfried






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