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30 sec with 150mm vs 60sec with 100mm

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#1 caballerodiez91

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Posted 21 October 2019 - 08:48 PM

Hi,

Noob question here. 

What will it gather more light? A 150mm-aperture parabolic reflector with a exposure of 30 seconds or a 100mm-aperture apochromatic refractor with an exposure of 60 seconds? 

 

Asuming equivalent quality, camera, settings and sky.. same everything. Btw none of them with stacking.

 

I checked this calculator (http://www.nature1st...s/ccdlimit.html) and it seems that the 150mm-aperture with 30 seconds of exposure has a photometric aperture of 2025 counts, and the 100mm-aperture with 60 seconds of exposure has 980 counts.

 

Is it possible to say that the first one gathers twice the light of the second one?

 

Cheers!


Edited by caballerodiez91, 21 October 2019 - 08:48 PM.


#2 Brett Waller

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Posted 21 October 2019 - 09:06 PM

Same f-ratio for both scopes??



#3 17.5Dob

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Posted 21 October 2019 - 09:06 PM

Hi,

Noob question here. 

What will it gather more light? A 150mm-aperture parabolic reflector with a exposure of 30 seconds or a 100mm-aperture apochromatic refractor with an exposure of 60 seconds? 

 

You left out a 150mm SCT and a 150mm RC, the mirror doesn't have to be parabolic.....and the 100mm refractor could be an achro......

And by "settings" do you mean the same f-ratio, because it's awfully hard to manufacture a 100mm f3.9 APO vs a 6" f4 Newt ??

Just start HERE

 



#4 Brett Waller

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Posted 21 October 2019 - 09:11 PM

As to the original question you asked, the 150mm scope would gather 2.25 times the light that the 100mm scope would collect in the same amount of time.  However, doubling the exposure of the smaller scope would reduce that ratio to 1.25. Thus, it would not be possible to say the first cope would gather twice the light of the second.


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#5 17.5Dob

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Posted 21 October 2019 - 09:37 PM

As to the original question you asked, the 150mm scope would gather 2.25 times the light that the 100mm scope would collect in the same amount of time.  However, doubling the exposure of the smaller scope would reduce that ratio to 1.25. Thus, it would not be possible to say the first cope would gather twice the light of the second.

Assuming no secondary obstruction in the Newt and not factoring light loss from two mirrors.....

A typical 6" newt has a ~60-65mm secondary which gives an actual clear aperture of just 136mm. Throw in the light loss from two typical 91% coatings and it's even less.

The actual imaging difference between a 6" Newt and a 100mm APO is not that great



#6 Alen K

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Posted 21 October 2019 - 10:38 PM

Assuming no secondary obstruction in the Newt and not factoring light loss from two mirrors.....

A typical 6" newt has a ~60-65mm secondary which gives an actual clear aperture of just 136mm. Throw in the light loss from two typical 91% coatings and it's even less.

The actual imaging difference between a 6" Newt and a 100mm APO is not that great

Even including all of that and using your numbers, the 150mm Newt still has about 53% higher transmission, assuming use of four-element, two-group coma corrector for the Newt and a two-element field flattener for the refractor. Maybe that is "not that great" to you but it's hardly insignificant. Factor in that a decent 150mm Newt for AP (there are a few) costs significantly less than a 100mm APO refractor and it is easy to understand the appeal of the former. Back in the days of film before relatively fast APO refractors were available, amateur AP pioneers Wallace and Provin considered a 6-inch f/5 Newt to be a very effective astrograph (as they said in their seminal book).


Edited by Alen K, 21 October 2019 - 10:38 PM.

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#7 caballerodiez91

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Posted 22 October 2019 - 04:53 AM

As to the original question you asked, the 150mm scope would gather 2.25 times the light that the 100mm scope would collect in the same amount of time.  However, doubling the exposure of the smaller scope would reduce that ratio to 1.25. Thus, it would not be possible to say the first cope would gather twice the light of the second.

Thanks!

 

Yes, I meant with the same exposure time.



#8 caballerodiez91

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Posted 22 October 2019 - 04:53 AM

Even including all of that and using your numbers, the 150mm Newt still has about 53% higher transmission, assuming use of four-element, two-group coma corrector for the Newt and a two-element field flattener for the refractor. Maybe that is "not that great" to you but it's hardly insignificant. Factor in that a decent 150mm Newt for AP (there are a few) costs significantly less than a 100mm APO refractor and it is easy to understand the appeal of the former. Back in the days of film before relatively fast APO refractors were available, amateur AP pioneers Wallace and Provin considered a 6-inch f/5 Newt to be a very effective astrograph (as they said in their seminal book).

I agree, APO refractors are extremely expensive, even the second-hand ones. 



#9 caballerodiez91

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Posted 22 October 2019 - 06:28 AM

Just checked that even the 150 mm with a 60 sec exposure gathers 4 times more light than the 100 mm with the same exposure.. unbelievable!

 

To gather the same light as the 150 mm in 60 sec, the 100 mm would need an exposure of 4 minutes. The difference is huge.



#10 AlienRatDog

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Posted 22 October 2019 - 06:57 AM

It depends on f-ratio. If a smaller telescope is faster (lower f-ratio), then the exposure will be shorter (though it will have a smaller image scale). If they are the same focal ratio, the brightness will be the same except the larger telescope will have a larger image.

#11 caballerodiez91

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Posted 22 October 2019 - 07:15 AM

It depends on f-ratio. If a smaller telescope is faster (lower f-ratio), then the exposure will be shorter (though it will have a smaller image scale). If they are the same focal ratio, the brightness will be the same except the larger telescope will have a larger image.

Well, I was talking about same specs, also same f-ratio.

 

But still, even if the reflector has a f-ratio of 5, and the refractor an f-ratio of 4, the reflector still gathers way more light: http://www.stargazin...a/scopemath.htm



#12 AlienRatDog

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Posted 22 October 2019 - 07:54 AM

The image becomes larger due to increase in focal length. The f/5 (again I’m talking about using a sensor at prime focus) will “stretch” the light out by making it bigger (increased magnification). If the focal ratio is the same, the brightness is the same (photons per unit area), the increase in in focal length will make it bigger. An example, my f/2.8 camera lens will require a heck of a lot less exposure than my f/10 C8, even though the C8 is bigger. In visual use, you are using an eyepiece to make an image of the image at the focal point. If you have the same magnification, the bigger scope will be brighter due to more light being collected and distributed over that image (that is the same size). Here is an extreme case, get that 100mm refractor and 150mm reflector, point it at the Orion Nebula, on the 100mm refractor, put the lowest power eyepiece you can (say 15x) and on the reflector, put a very high power eyepiece (say 400x), you will notice that the smaller image will be brighter. Now place them at the same magnification (say 50x), in this case the 150mm reflector will be brighter as the image scale is the same and the 150mm collects more light. Point of the matter, the image scale (without an eyepiece) and your camera is a function of focal length. If you keep the focal length the same (hence image scale), the larger aperture will be brighter, the focal ratio is a handy way of quantifying this. In your case the 150mm would be brighter since it’s image scale is similar. Think about how much light you are collecting and how you are spatially distributing that light.

Edited by AlienRatDog, 22 October 2019 - 08:02 AM.


#13 caballerodiez91

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Posted 22 October 2019 - 01:19 PM

The image becomes larger due to increase in focal length. The f/5 (again I’m talking about using a sensor at prime focus) will “stretch” the light out by making it bigger (increased magnification). If the focal ratio is the same, the brightness is the same (photons per unit area), the increase in in focal length will make it bigger. An example, my f/2.8 camera lens will require a heck of a lot less exposure than my f/10 C8, even though the C8 is bigger. In visual use, you are using an eyepiece to make an image of the image at the focal point. If you have the same magnification, the bigger scope will be brighter due to more light being collected and distributed over that image (that is the same size). Here is an extreme case, get that 100mm refractor and 150mm reflector, point it at the Orion Nebula, on the 100mm refractor, put the lowest power eyepiece you can (say 15x) and on the reflector, put a very high power eyepiece (say 400x), you will notice that the smaller image will be brighter. Now place them at the same magnification (say 50x), in this case the 150mm reflector will be brighter as the image scale is the same and the 150mm collects more light. Point of the matter, the image scale (without an eyepiece) and your camera is a function of focal length. If you keep the focal length the same (hence image scale), the larger aperture will be brighter, the focal ratio is a handy way of quantifying this. In your case the 150mm would be brighter since it’s image scale is similar. Think about how much light you are collecting and how you are spatially distributing that light.

Thanks for the explanation!




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