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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: fred1871]
      #5806021 - 04/18/13 02:11 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


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fred1871
professor emeritus


Reged: 03/22/09

Loc: Australia
Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5807911 - 04/18/13 11:19 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.


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: fred1871]
      #5808120 - 04/19/13 04:22 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


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5814655 - 04/22/13 05:07 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


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brianb11213
Postmaster
*****

Reged: 02/25/09

Loc: 55.215N 6.554W
Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5814676 - 04/22/13 05:34 AM

Quote:

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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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution [Re: brianb11213]
      #5815706 - 04/22/13 04:59 PM

Share your view that AAVSO is regarding magnitudes what WDS is for double stars - but this does in both cases not mean free from errors. For my purpose I used the Tycho star pattern for M67 (to keep the number of stars reasonable low) but the magnitudes from AAVSO and had the troubles mentioned above. But difficulties like this make it only more interesting to proceed further.
Wilfried


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5827845 - 04/28/13 07:51 AM

A few days ago another opportunity for a look at M67. More or less full moon from behind, NEML 2.5, SQM 17, Pickering 5-6, spurious disk crisp, bit dancing. Meanwhile I know the star pattern of M67 more or less by heart and some training effect is to notice - it seems easier to see the fainter stars now. Both dubious +11.3mag stars in the lower part of M67 are this time ignored. I got the following results:
140mm > 11.8 but less than 12.1 (11.9 and 12mag stars are mising in M67 and the 12.1 stars were not resolvable) - I assume 11.9
130mm 11.8
120mm 11.7
110mm 11.6
100mm 11.5
90mm 11,3
80mm 11.2
70mm 11.1
60mm 11
50mm 10.8
40mm 10.4
The smaller apertures below 60mm are somewhat difficult, as the star pattern changes due to the "loss" of the fainter stars so I will next time make a star chart with only the stars below +11mag.
This result seems very plausible and means that I have so far overestimated the magnitude loss due to light pollution by about 0.5mag for 140mm aperture. Interestingly also that the magnitude loss is clearly going down with aperture as was to be expected.
Wilfried


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5857963 - 05/13/13 05:09 PM

M67 is meanwhile rather low now for me and other observers in the northern hemisphere (last report was a +12.4mag TML for 152mm aperture and NEML of +4mag from Roberto) - may be I can have a last look at it with my 235mm SCT around dusk if the meanwhile again rotten weather allows.
Next good targets will be NGC6823 in Vul and NGC6910/NGC7031 in Cyg in September (Cotts: Thank you for suggesting these). I will prepare star maps with hopefully existing AAVSO magnitude values for these open clusters.
Wilfried


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WRAK
scholastic sledgehammer


Reged: 02/18/12

Re: Star pattern for TML test with light pollution new [Re: WRAK]
      #5861425 - 05/15/13 03:11 AM

Tried yesterday a bit late in the evening with the 235mm SCT. Despite a for my location rather good NEML of +3.45mag I got only a slightly better image than last time with the 140mm refractor. Bad seeing and altitude below 25 seems to have an erasing effect on fainter stars.
Wilfried


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