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Nice targets to measure scope limiting magnitude?

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#1 Ocelot

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Posted 18 January 2020 - 09:58 PM

I want to measure, for fun, limiting magnitude of my 8" scope, used visually with different magnifications. I've red of a trick to roughly measure the limiting magnitude for an unaided eye, by counting the number of stars visible in the pegasus square. Has anyone heard of a similar trick to use with a telescope?



#2 viewer

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Posted 18 January 2020 - 10:26 PM

This is the theory, wonder what it is in practice, good question!

 

http://www.cruxis.co...ngmagnitude.htm

 

Anyhow starting from your NELM can give the ballpark.


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#3 Knasal

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Posted 18 January 2020 - 10:26 PM

My suggestion:

 

Get the sky safari app, aim telescope with any of your eyepieces in the focuser, and compare what you see to what the app displays...

 

Kevin



#4 Keith Rivich

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Posted 18 January 2020 - 10:48 PM

Here is probably bible on limiting magnitudes:

 

http://adsabs.harvar...PASP..102..212S

 

If you can wait for m57 to climb back up in the sky:

 

https://www.cloudyni...und-the-nebula/


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#5 555aaa

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Posted 19 January 2020 - 01:41 AM

M67 is a standard photometric open cluster. You can plot a finder chart with V magnitudes here

 

https://www.aavso.or...00.0&type=chart

 

Another maybe more interesting challenge is to work through the Astronomical League quasar list. It is in a program which is asking for a bigger telescope but the quasar list starts around 13th mag. 3C273 in Virgo is about 13th magnitude and is about two billion light years away so that is pretty much the oldest light you can see with your eyeballs.

 

The quasar list is appendix A here. You'll need some nice finder charts or a charting program to find these objects since they will look like very faint but ordinary stars and not particularly red.

https://www.astrolea...serving-program


Edited by 555aaa, 19 January 2020 - 06:32 PM.

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#6 Redbetter

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Posted 19 January 2020 - 03:07 AM

I want to measure, for fun, limiting magnitude of my 8" scope, used visually with different magnifications. I've red of a trick to roughly measure the limiting magnitude for an unaided eye, by counting the number of stars visible in the pegasus square. Has anyone heard of a similar trick to use with a telescope?

 

It is useful to know how deep you can go with a given aperture in dark sky.  Unfortunately, NELM magnitude doesn't help much in estimating that.  NELM is measured with your widest pupil opening, with undimmed background, using both eyes.  That is low contrast and subject to the greatest aberration of your vision (both for inability to focus at infinity, and for astigmatism.)  The telescopic limiting magnitude is best achieved with small exit pupils which dim the background sky, greatly reduce the impact of astigmatism, and allow focus at infinity.

 

To measure TLM you need a calibrated chart.

 

 

Here is probably bible on limiting magnitudes:

 

http://adsabs.harvar...PASP..102..212S

 

While useful, Schaefer's paper is superseded by Crumey's paper:  "Human contrast threshold and astronomical visibility."  (link)   My own results have coincided with Crumey's correlations once I stripped away a few of the limitations:  

  • Moving the "magnitude cut off" for dark sky background several magnitudes (rather than 25 MPSAS) to match what I have visually measured as detectable black cut off, and updating the limits accordingly.
  • Eliminating the observer factor derate.  A 1.0 factor matches my TLM observations in various small apertures.

Schaefer's paper and the Sky & Telescope request for data are helpful resources for a TLM chart of M67.  The issue can be found in a search online, but I won't link to it because I don't believe it is an authorized copy of the issue.  I used an image from the article and supplemented with AAVSO data for individual stars to create my own chart of the open cluster with many more steps.  I had to identify and label these by hand to make a chart. 

 

M67 is a great cluster for this in many ways since it has a wide range of star magnitudes, separated well enough from one another to identify in patterns.  Unfortunately, as hinted above the steps are somewhat wide and lack duplicates.  However, a serious negative is the time of year when M67 is best placed:  winter, when seeing is at its worst.  The seeing effect in winter becomes increasingly problematic in larger apertures and greatly diminishes limiting magnitude from what I have experienced.  (There also tend to be fewer dark sky observers and fewer clear nights in winter.)  I believe the seeing effect explains the surprisingly mediocre results of even many well known observers who submitted data.

 

I ended up culling Schaefer's data for only the better results seen in dark sky (NELM 6.5 or greater in the data...both to screen for folks with weaker night time vision and brighter skies.)  I probably could have included some others in the 6 to 6.4 range, but that would have left in more that might have either been in bright sky or had poorer eyesight.  I was after what was possible with good eyes in good conditions, not anything else.  This greatly reduced the data set.  It was reduced even more when I then screened for observations made at appropriate high levels of magnification (small exit pupil) to obtain optimal limiting magnitude.  Skiff's observations most closely matched my own results (adjusted for aperture.) 


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#7 j.gardavsky

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Posted 19 January 2020 - 10:16 AM

Helo all,

 

and here are some more standard fields for the photometry from AAVSO,

https://www.aavso.or...s/vsd/stdfields

 

Best,

JG


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#8 S.Boerner

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Posted 19 January 2020 - 12:40 PM

You can create finder charts using the AAVSO's Variable Star Plotter to create customized fields to suit your needs: 

 

https://www.aavso.org/apps/vsp/

 

You can input the RA/Dec, the FOV, and magnitude limit. 


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#9 vsteblina

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Posted 19 January 2020 - 07:18 PM

years ago, sky and telescope published the visual magnitudes around M57.

 

Look it up, you get to look at the Ring Nebula while trying to determine your limiting magnitude.

 

I even use it for my CCD imaging.


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#10 Ocelot

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Posted 19 January 2020 - 10:13 PM

Thanks for great suggestions, love the AAVSO plotter!



#11 mogur

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Posted 19 January 2020 - 10:53 PM

In any event, you'll actually be measuring the acuity of your vision, since the limiting magnitude of any scope is governed by hard and fast optical laws.



#12 B 26354

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Posted 20 January 2020 - 12:31 AM

In any event, you'll actually be measuring the acuity of your vision, since the limiting magnitude of any scope is governed by hard and fast optical laws.

Well... the "theoretical" limiting magnitude is. In the real-world, atmospheric conditions heavily override the math... along with visual acuity. I'm sure the OP is aware of this, given their desire to make these measurements "for fun".  grin.gif


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#13 clearwaterdave

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Posted 20 January 2020 - 02:11 AM

I often use Skysafari to check a stars magnitude.,not sure how accurate it is but it's also just for fun to me.,If I notice a star coming and going in the fov it's easy to check.,Usually I see/do this when I am studying the "nest".,what I call the stars in the fov surrounding a target.,cheers.,

 



#14 Redbetter

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Posted 20 January 2020 - 03:46 AM

In any event, you'll actually be measuring the acuity of your vision, since the limiting magnitude of any scope is governed by hard and fast optical laws.

 

It isn't acuity (which is the ability to resolve fine detail.)  It is the sensitivity of one's eye (and ability to interpret it) to a given photon flux at some contrast level with background sky.  This can be somewhat impacted by acuity in non-limiting magnitude situations, but at limiting magnitude the exit pupil should be small enough that acuity is not a major factor.

 

Limiting magnitude is not governed by hard and fast optical laws (unlike resolution or flux or distribution functions which are.)  Limiting magnitude is more a matter of the nature of the detector (e.g. eye), how the data is interpreted (e.g. brain/visual system), and how the detector interfaces with the scope.  For the eye such limits are more empirical and approximations and, as you note, they also vary among individuals. 

 

By comparison an old scope can be paired with ever improving hardware/software/filters, techniques, and longer integration times to reveal ever fainter objects.  With respect to integration time and processing:  think of the Hubble Deep Field. 


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