exposure times

[alacant]

Hi everyone. Cloudy tonight. Dare I?

I have  152mm and 102mm diameter telescopes with 760mm and 600mm focus respectively. This gives me optical areas of 18148 and 8172 mm squared.

Could anyone give me a yes or no on any of the following?

1. To get the same number of photons, I need 18148/8172 or 2.2 times longer an exposure with the smaller telescope.

2. The smaller telescope has a wider field of view.

3. A 60 second exposure of M8 via the large telescope would have the same brightness as a 60x2.2 or 132s in the small telescope.

4. M8 would be smaller in the smaller telescope.

5. The focal ratios of f5 and f5.9 cannot help me determine the time needed for equivalent brightness when taking snaps of M8.

Cheers, clear skies and many TIA

 

[pipnina]

1 hour ago, alacant said:

Hi everyone. Cloudy tonight. Dare I?

I have  152mm and 102mm diameter telescopes with 760mm and 600mm focus respectively. This gives me optical areas of 18148 and 8172 mm squared.

Could anyone give me a yes or no on any of the following?

1. To get the same number of photons, I need 18148/8172 or 2.2 times longer an exposure with the smaller telescope.

2. The smaller telescope has a wider field of view.

3. A 60 second exposure of M8 via the large telescope would have the same brightness as a 60x2.2 or 132s in the small telescope.

4. M8 would be smaller in the smaller telescope.

5. The focal ratios of f5 and f5.9 cannot help me determine the time needed for equivalent brightness when taking snaps of M8.

Cheers, clear skies and many TIA

 

To get the same number of photons, you would indeed need twice as much exposure, however, it won't affect the image.

The smaller scope has a smaller field of view, but does not require 2.2 times the exposure for the same result as the larger scope. The f-ratio determines the brightness, not the aperture.

M8 would be smaller in the smaller scope, but not by a huge margin.

Hope this helps

    ~pip

[ChrisLX200]

You would also need to take into account the relative sizes of the image circles from each scope. Consider my NP127 refractor, that has a circle of around 60mm, so if use a CCD sensor of 15mm square well, you work out the percentage of light captured by the scope I'm recording! 

ChrisH

 

[alacant]

13 hours ago, pipnina said:

 

The smaller scope has a smaller field of viiew

Visually it looks bigger. Maybe on a camera sensor it's not. (?)

 

[pipnina]

9 hours ago, alacant said:

Visually it looks bigger. Maybe on a camera sensor it's not. (?)

 

The longer focal length means that a sensor of the same size will show less sky (but show objects larger). If the scope had a longer focal length but the sensor was larger, the image scale might stay the same. If the focal length were kept the same but the image sensor was smaller, the image scale would grow. (less FOV)

I don't know the maths but if you download "Stellarium" it will calculate the FOV of different scope / eyepiece / camera combinations and show you objects in the sky to scale. (You need to enable the feature, press "F2" when it loads, click "plugins" and tell "Field of View" to load on startup. Then restart the program (it should appear in the top right)

 

Hope this helps!

    ~pip

[alacant]

On 8/7/2016 at 00:48, pipnina said:

8 hours ago, pipnina said:

The longer focal length means that a sensor of the same size will show less sky

I don't know the maths but if you download "Stellarium" 

 

Hope this helps!

    ~pip

Thanks. I'd forgotten about planetarium fov thing. I have CDC and it shows that the smaller telescope has the bigger fov. The brightness f ratio and photons thing still gets to me. There doesn't seem to be any difference in the photos. A 2 minute exposure looks pretty much the same brightness using either. Usable, anyway...