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Star pattern for TML test with light pollution

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#1 WRAK

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Posted 12 March 2013 - 06:08 AM

I have some ideas how light pollution influences the magnitude limit of a scope but not enough real data to put these ideas to test - so I am looking for a star pattern of use for this purpose.
This would ideally be a line (or another geometric pattern like an arc) of stars with confirmed magnitudes starting with a +10mag star (assumed TML for a 50mm scope with NEML of 3.2) going up in reasonable steps of ~0.2mag to about +12.5mag (assumed TML for a 300mm scope with NEML of 3.2) visible from the northern hemisphere.
The field around Alpha UMi has a rather random distribution of stars with magnitudes in this range so this is not so perfect.
Near Pollux (to be more spedific: about 15 arcminutes from HIP 38101) I have found an arc of stars from +10.85mag to +12.1mag covering only a part the the range I am looking for.
Any suggestions?
Wilfried

#2 Cotts

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Posted 12 March 2013 - 07:08 AM

Wilfried, there are charts in the excellent, and out of print, book 'Visual Astronomy of the Deep Sky" by Roger Clark which are exactly what you are looking for.

There are about a dozen or more open clusters around the sky which are shown with and without magnitudes. I have used them by taking a photocopy of the 'no-magnitude' diagram to the eyepiece and circling every star I could see. It is then a simple matter to compare what you saw with the chart with the magnitudes on it.

I could photograph a couple of pages and post them here but I suspect it would be a violation of the TOS. I will check with my fellow Mods and get back here. Since the book is out of print it shouldn't be a problem....

Dave

#3 WRAK

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Posted 12 March 2013 - 10:50 AM

Dave, thanks for your efforts, sounds good. No need to challenge copyright issues - just give me the NGC numbers for the mentioned open clusters and I will look myself for star maps.
Wilfried

#4 Cotts

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Posted 12 March 2013 - 03:01 PM

The maps are from a 1961 paper by Hoag et al. But you may find them lurking on the web somewhere...
NGC 225
1647
2129
2422
6494
6823
6910
7031
7235

That's it. Post here if you actually find any of these...

Dave

#5 Cotts

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Posted 12 March 2013 - 03:05 PM

Here's the book on Amazon. http://www.amazon.co...rk/e/B001H9TUB8

Nice price!

Dave

#6 WRAK

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Posted 12 March 2013 - 04:09 PM

Dave - NGC 2129 seems to be of special interest. Easy to locate near M35 and covering the range from some bright stars to very faint ones. Thanks.
Wilfried

#7 fred1871

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Posted 12 March 2013 - 10:55 PM

Wilfried, NGC 2129 has had photometry done on it since the paper of Hoag et al in 1961, and the more recent work should be an improvement given advances in instrumentation since then.

You'll find online at a few locations a paper by Carrarro, Chaboyer and Perencevich (2005) that uses CCD photometry for the cluster, and provides a list of stars (with Hoag's numbering) with new photometry. It's in pdf form for ready access.

The Hoag paper is also listed in the bibliography of the Carrarro paper if you need the source.

#8 WRAK

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Posted 13 March 2013 - 02:53 AM

Fred, found the Carraro et al article but so far no image reference for the used star numbers. But I think the DSS image on Sky-Map.org will do it: http://www.sky-map.o...6666667&zoom...
With the cursor over the individual star you get the data for it.
Wilfried

#9 fred1871

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Posted 13 March 2013 - 06:00 AM

It will depend on the source of their data.

#10 fred1871

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Posted 13 March 2013 - 07:27 AM

Wilfried, I've now located the Hoag et al 1961 paper - available online via the ADS - the paper is VERY long because they did photometry on many clusters, but a photo chart for NGC 2129 is there, with rulers each side, and a list of stars (location by grid, X and Y coordinates) with photometry in a table. This is done for each of the many clusters.

The original source is Publications of the US Naval Observatory, 2d series, volume 17, from page 347 onwards.
The NGC 2129 chart and data is on page 406.

#11 WRAK

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Posted 14 March 2013 - 03:51 AM

Fred, thanks for the hint. Found the article with the NGC 2129 chart - the grid system is a bit inconvenient to use but I will be able to countercheck the magnitude of at least some stars I have so far selected for a magnitude limit check.
Wilfried

#12 WRAK

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Posted 15 March 2013 - 07:44 AM

This seems to get tricky - just checked the magnitudes of some of the stars in NGC 2129 with the current AAVSO data (http://www.aavso.org...load-apass-data - works quite efficient, with a 0.001 radius degree you get usually only the data for the star at the given position) and got for several stars serious differences to the values of the other sources.
But I think the AAVSO values should be the most reliable ones.
Wilfried

#13 fred1871

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Posted 16 March 2013 - 01:42 AM

Wilfried, I expect some differences will be due to different filters used for photometry, quite apart from small errors. AAVSO are using a pretty standard set of filters for various wavebands, based on their description - you need to check against filters and methodology used in the science papers referred to - the older Hoag material, and the more recent paper I listed. The latter shows some of the differentials of their magnitudes compared to Hoag's for the cluster they re-studied. Newer material will often be better due to new instrumentation providing smaller errors if used well; but some differences will be due to filter differences (waveband centre and width).

#14 Ed Wiley

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Posted 16 March 2013 - 10:53 AM

M67 is a classic open cluster for photometry calibration and is well placed now.

http://binaries.boul...fields/m67.html

http://binaries.boul...elds/m67ids.txt

Would V10-13 be a suitable magnitude range?

Ed

#15 WRAK

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Posted 16 March 2013 - 12:45 PM

Ed, V10-13 is perfect. The documentation with numbers for the stars and the list with relating magnitudes is great. Checked a few stars with AAVSO and the data seems of very good quality.
M67 will soon come into my field of view and should then be certainly useful for my purpose. Thanks a lot.
Wilfried

#16 WRAK

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Posted 18 March 2013 - 01:53 PM

Good news first: Already the first observations with the topic of NEML influenced TML showed that I have so far overrated the impact of light pollution on TML by for example about 0.3 mag for a 140mm scope (TML 11.76 for NEML 3 instead of 11.46). The interest in this topic led to the inclusion of more doubles with faint secondaries near the TML of my scope into my session plans and I find the challenge to resolve doubles in this range as quite interesting.

The bad news: The TML part of my RoT model does therefore not work properly for NEML less than 3.5 leading under specific conditions to drastic too large required apertures for doubles with secondaries fainter than +11mag. But before I try to change this I will take some more time for sampling TML observations with regard to light pollution.

Any contributions especially for apertures larger than 140mm would be appreciated very much.
Wilfried

#17 Ed Wiley

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Posted 18 March 2013 - 07:45 PM

Glad to help! This is one of the standard star fields for checking photometry and its good to see it put to other good uses. I hope you continue to share your finding with us.

Clear skies, Ed

#18 WRAK

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Posted 11 April 2013 - 02:08 AM

Yesterday finally clear skies again. Transparency was high but seeing rather moderate (Pickering 5). NEML was +2.85mag as I could barely make out Beta CMi. Then I used the Sky Quality Meter and got values between +17.5 and +18mag for my field of view - indicating that NEML +2.85 was may be a bit on the pessimistic side.
Then I located M67 and checked TML for my 140mm refractor and got +11.83 as faintest star I could resolve indicating a loss of 1.6 magnitudes due to LP - not this bad.
Wilfried

#19 WRAK

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Posted 17 April 2013 - 07:10 AM

Had yesterday another opportunity to visit M67 and more time to make use of the iris diaphragm for my 140mm refractor. Quartermoon made the sky rather bright, SQM showed values about +16.5mag for my field of view. Despite this I had the impression that NEML was this time a bit better than +3mag, may be +3.1 or even better (not sure If I really could make out Eta Leo, this would even mean +3.45mag).
I got the following values for the different apertures:
140 - 11.83 again
130 - 11.64
120 - 11.64
110 - 11.4
100 - 11.3
90 - 11.3
80 - 11.3
70 - 11.3
60 - 11
50 - 10.6.

The value for 130mm should most probably be somewhere in the middle between 11.83 and 11.64 but I missed a star zu check this. The run of 11.3 for apertures between 100 and 70mm should be proof that the used reference star with +11.27mag at the position RA 132.860118 Dec 11.730742 is somehow questionable concerning advertised data or that I have simply failed in my observations - will have to check this again.
Wilfried

#20 fred1871

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Posted 17 April 2013 - 08:51 PM

A curious set of numbers, Wilfried. Obviously something has gone wrong, when a 100mm aperture goes no fainter than 70mm - despite twice the light gathering. Indeed, if 70mm gives mag 11.3, and 140mm only goes to mag 11.8, that's a mere difference of ~0.5 mag - however the difference in light-gathering area is a factor of 4:1, or about 1.5 magnitudes!

So either the magnitude sequences are faulty for visual magnitudes, or the gremlins are holding back the larger aperture, or the elves are helping the smaller aperture? :grin:

Or is there a problem with the iris diaphragm, perhaps placed where the real aperture effect doesn't match the intended effect?

Whatever the reason or reasons, the limiting magnitude sequence has problems.

I'm currently working on setting up my 80mm refractor side by side with the 140mm. That should allow me to make some interesting comparisons for magnitude limits and for resolution.

#21 WRAK

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Posted 18 April 2013 - 01:11 AM

Fred, I suspect the star in question is actually rather +11.1mag than +11.3mag and I will check this again.
Otherwise the idea with the gremlins has some charme.
Wilfried

#22 fred1871

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Posted 18 April 2013 - 10:19 PM

Even with one test star having a wrong magnitude value, there are still problems with the sequence.

What magnifications were used at various apertures? Equal magnifications don't give the same effect across apertures because the exit pupil is smaller with smaller apertures at the same power. And Bradley Schaefer's work twenty years ago demonstrated that increased power showed fainter stars for any particular aperture. For larger apertures, higher powers were needed to show the telescope limits than for smaller apertures.

That has to be factored in with limiting magnitude tests.

With my 140mm refractor much fainter stars are visible at 80x than at 33x; 114x improves on 80x; 160x goes fainter again.

One test you could run is to use, say, 80mm aperture, then see what the limiting magnitude is at various powers - Schaeffer's work showed a plateauing effect as one got into higher powers relative to aperture, but I'd expect that to happen with quite small exit pupil sizes, as his study indicates.

Looking at the graph in Schaefer's paper (PASP, 102:212-229, Feb 1990) - derived from modelling - there's a bigger improvement for 140mm than for 80mm due to magnification increase. With 80mm you go from about 12.1m at 30x to 12.7m at 100x (0.8mm exit pupil). For 140mm, you go from about 12.7m at 30x to about 13.8m at 200x (0.7mm exit pupil). So the magnification gain with the bigger telescope was greater. The gain for 140mm was small from 100x (1.4mm exit pupil) to 200x; most gain was from 30x to 100x.

The gain for 23.5cm (the SCT both of us have) was from about 13.8m at 60x to 14.9m at 400x (0.6mm exit pupil). 400x showed a small gain over 200x; 200x a bigger gain over 100x, which in turn was noticeably better than 60x.

Looking at those numbers, 80mm at 100x matches 140mm at 30x; 140mm at 200x matches 235mm at 60x. The smallest scope improves 0.6mag with greater magnification; the bigger ones about 1.1 mags.

Schaeffer includes a lot of data including observations used in the study, and gives a histogram of model errors, which shows a roughly Gaussian-shaped curve.

He also provides a histogram of "model errors after correction for experience." This was significant - "a very experienced observer may see over a magnitude fainter than a beginner".

A lot there to consider in working towards a model of TML.

#23 WRAK

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Posted 19 April 2013 - 03:22 AM

I have used x140 for all apertures with a field of view of about a half degree - big enough to show M67 complete so I have better orientation regarding the reference stars.
This may be a small advantage for the smaller apertures but I don't think x140 is not enough magnification for a 140mm scope regarding TML.
Wilfried

#24 WRAK

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Posted 22 April 2013 - 04:07 AM

The advertised magnitude data for faint stars seems even less reliable than the (despite the great efforts for the WDS catalogue) not this reliable double star informations.
Tycho, AAVSO, ... other catalogues or measurements - most of the time rather different values. There are about 40 stars in M67 in the range between +10mag and +12.5mag and about 4-5 are in the range of about +11.3mag. Two of them at the lower rim of M67 seem through the telescope a bit brighter than the other ones in the upper part of M67. This may be have something to do with the spectrum of the stars but then - we speak about visual magnitudes.
But it seems that this fact is the reason for the earlier mentioned "result" of +11.3mag TML over several classes of telescope aperture of the last TML observation. In future I will disregard this two lower "+11.3mag" stars for TML evaluation.

Yesterday with 3/4 moon and NEML of ~2.5 with Pickering ~4 I got with a 120mm refractor down to a TML of about +11.7mag. Seems resonable even if I have now to consider such results as very preliminary.
Wilfried
PS: No iris diaphragm available this time, so no results for other apertures

#25 brianb11213

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Posted 22 April 2013 - 04:34 AM

The advertised magnitude data for faint stars seems even less reliable than the (despite the great efforts for the WDS catalogue) not this reliable double star informations.
Tycho, AAVSO, ... other catalogues or measurements - most of the time rather different values.

Be sure to use "visual" magnitudes ... the catalogues are often using different photographic / photoelectric / CCD + filter combinations & vary condiserably.

IMVHO the AAVSO variable star charts - intended for visual estimation - are the most consistent over the whole sky (after allowing for extinction). Find a star which passes close to the zenith & use the charts for that. The AAVSO charts also have the benefit of being plottable for semi-inverted (with star diagonal) orientation ...






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