Diameter question

[craigcullum]

Hi,

My understanding is, to see deep space, diameter is key as the greater the width the better it's light gathering capabilities but how can the high end refractors such as the Skywatcher Black Diamond have such excellent deep space capabilities with such a small diameter.

On the Skywatcher website, the Black Diamond has a Faintest star magnitude of 13.1, where a 10" Dob has 13.9.

[Robh (Rob)]

Originally Posted by craigcullum

Hi,

My understanding is, to see deep space, diameter is key as the greater the width the better it's light gathering capabilities but how can the high end refractors such as the Skywatcher Black Diamond have such excellent deep space capabilities with such a small diameter.

On the Skywatcher website, the Black Diamond has a Faintest star magnitude of 13.1, where a 10" Dob has 13.9.

The limiting magnitude of 13.1 is for a 120mm (5 inch) Black Diamond ED refractor. A 10 inch Skywatcher dob is quoted as having a limiting magnitude of 14.7.
This is in line with the theoretical figures normally quoted and roughly using the formula m=9.6+5*log(d) where m is the limiting magnitude and d the aperture size in inches (log is in base 10). This assumes an ideal dark site.

In practise, the skyglow from light pollution in suburbia will reduce your limiting magnitude and the formula may be more like m=8+5*log(d). So the 13.1 will reduce to 11.5. In city areas the formula will change again for the worse.

So basically, aperture size and skyglow are the main determinants of limiting magnitude.

Regards, Rob

[renormalised (Carl)]

And to add to what Rob has posted there, the difference between the 5" refractor and the 10" dob is a factor of 4 in the light gathering capability, so that the 10" will collect 4 times the light of the 5". Not surprisingly, the difference between a mag 13.1 and a mag 14.7 star is approximately 4 times the amount of light (4.3 to be more exact....2.512^1.6).

However, there are other factors that come into play in the comparison, such as focal length (f), any aberrations that maybe present in the optics (chromatic/spherical aberration, coma etc), quality of the optics, obstructions in the light path etc etc etc.

[Robh (Rob)]

Originally Posted by renormalised

And to add to what Rob has posted there, the difference between the 5" refractor and the 10" dob is a factor of 4 in the light gathering capability, so that the 10" will collect 4 times the light of the 5". Not surprisingly, the difference between a mag 13.1 and a mag 14.7 star is approximately 4 times the amount of light (4.3 to be more exact....2.512^1.6).

Hi Carl,

I like that approach.

For those unfamiliar with the number 2.512 (Pogson's ratio).
A magnitude 1 star is 100 times as bright as a magnitude 6 star.
This is a magnitude difference of 6-1=5.
The 5th root of 100 is approximately 2.512 or 2.512^5=100.
Thus, a magnitude 1 star is 2.512 times as bright as a magnitude 2 star, a magnitude 2 star is 2.512 times as bright as a magnitude 3 star and so on.

Regards, Rob

[OneOfOne (Trevor)]

A refractor will also gather slightly more light than the same sized reflector because it does not have a secondary mirror sitting in the middle of it, the difference, however, would only be a tenth or so of a magnitude...not particularly important.

The quality of the scope may also make small differences, more expensive ones having better coatings or mirror finish, which may add a fraction of a magnitude.