Limiting stellar magnitude, what's the story?

[emadmoussa]

I noticed that the smaller aperture Dobsonians have equal or  higher limiting magnitude than the slightly bigger aperture Celestron C11 XLT:

Celestron C11 (f/10) = Limiting Stellar Magnitude 14.7

Sky-watcher Skyliner 10" Dobsonian (f/4.7) =  Limiting Stellar Magnitude 14.7

GSO 10" Dobsonian (f/5) = Limiting Stellar Magnitude 15.1

What am I missing here?

[emadmoussa]

Also, the Sky-watcher flexitube 12" Dobsonian has a lower limiting magnitude than the 10" GSO Dobsonian

GSO 10" = 15.1

Sky-watcher Flexitube 12" = 14.9

[Macavity]

Error bars?

And specifically those of measurement, calculation, parameterisation - Or whatever gave those figures? Visual limiting magnitude can only ever be relative to local seeing and subjective to the observer. Certainly the effect of lenses versus mirrors, transmission, reflectivity, central obstruction etc. all play their part, but probably well within the uncertainty on the generally quoted figures?

http://en.wikipedia.org/wiki/Limiting_magnitude

[Prador]

Loads of different variables for manufacturers to manipulate. I think to the complete novice limiting magnitude means nothing. More unscrupulous scope makers go for the ridiculous power of magnification figures

[emadmoussa]

I think these figures are based on the average seeing conditions expected. And yes, there's probably an element in marketing in there. By all means, many of these scopes are not meant for complete beginners and the emphasis on higher and higher limiting magnitude is a selling point for the more experienced amateur astronomers.

According to Teleskop Service, the 10" GSO Dobsonian has a 94% multi-coating, increasing the brightness of viewed objects. This is perhaps part of their deciding factors.

[Macavity]

As a practical example, here are two different (equally justifiable) parameterisations of visual limiting magnitude:

http://twcac.org./Tutorials/limiting_magnitude_table.htm

Note, at the apertures you are considering, there is a difference of more than one magnitude in the values.

[crashtestdummy]

You also have to consider that 15 is 100 times fainter than 14.so it's a big leap

[Superdavo]

Different quoted numbers, simply from different measurements?

David

[BinocularSky]

Limiting visual magnitude tables are at best an estimate, give that most telescopes, most observing sites/conditions, and all observers are not "normal".

Tab on Tapatalk

[Macavity]

These scales have a difference, that remains constant with aperture. I just look at the naked eye starting point.

Sir Patrick's eyeball limit is (was) +6.3! My own (rather naff) eyesight, location and skills yields about mag +5.

So I just subtract ONE from SPM's values. But I can add about three for my VIDEO astronomy setup.

[Prador]

You also have to consider that 15 is 100 times fainter than 14.so it's a big leap

I think its a difference of 100 in brightness is equal to five steps in magnitude 

[Moonshane]

yep. 1 step in magnitude = 2.512x brighter or dimmer.

[crashtestdummy]

Think I may have been getting mixed up with sqm readings

[Moonshane]

you were right in a way in that the magnitude scale was originally conveived with mags 1-6 and there was 100x difference between them. to get from 6-1 it's 1x2.512 = 2.512 then 2.512x2.512 = 6.31 than 6.31x2.512 = 15.85 then 15.85 x 2.512 = 39.82 then 39.82 x 2.512 = 99.54

[BinocularSky]

 1x2.512 = 2.512 then 2.512x2.512 = 6.31 than 6.31x2.512 = 15.85 then 15.85 x 2.512 = 39.82 then 39.82 x 2.512 = 99.54

Ah, the dangers of premature approximation! (As I try to impress into my maths tutees)

2.512⁵ = 100.02 (to 2dp)

[Moonshane]

yeah, OK you big smart Alec. I have had an early approximation problem for a number of years now. there are a number of cures such as attention to detail but this is hard to come by

[AlexB67]

I hope this is one area my calculator can answer in a more transparent manner and can be used for when finished and released  . There are a lot of variables in there that sellers/manufacturers do not clarify. In any case they are often theoretical magnitudes and not an estimate of actual magnitudes that can be reached under most observing conditions in practice. To throw in a few things that are never stated by vendors,  dilated eye pupil size of observer,  assumed zenith angle, magnification and eyepiece used, since the sky background comes into it, and exit pupil comes into it also. Naked eye limiting magnitudes are often assume to be 6, but may be 6.5. The dilated eye pupil could be 6mm, but  it may be 7mm, or they  neglect things such as star colour and so on and so on,  usually the reference star is a white star but it may be yellow ( B-V Indices etc) .

In a nutshell, the more simplistic methods quoted above usually fix such figures  to whatever they may be.  Much of this stuff has  been discussed in a papers by Clark, Schaefer etc, for example  

http://adsabs.harvard.edu/abs/1990PASP..102..212S

A lot of the stuff coded in my calculator is based on that paper in fact.

[AlexB67]

Hope this is not seen as spam or show-off as it is kind of my pet hobby project  , but see the two graphs attached as an example, all the parameters  in the calculator can be varied  and it shows how they affect limiting magnitudes.  My calculator can also show this in the form of the interactive graphs, change any parameter on the right hand side of the graph using the controls, for example aperture size, or whatever it may be, and see how it changes the results in the graphs instantly. The two graphs attached are just a sample, many more you can select from and have a play with when it is all is done.