C9.25 image scale compared to MN190

[Tim]

Focal length of MN190 = 1000mm, the C9.25 is 2350mm iirc.

I was really surprised at how much larger the image of M1 appeared in the C9.25. I struggle with figures is the C9.25 version 235% bigger than the MN190 version?

And then, the C9.25 is working at F10. I have 1 x20 and 3 x 25 mins here with OIII filter.

The MN190 is f5.3, and 3 hr 45 min in 25 min sections, again OIII.

How much exposure time would I need in the C9.25 to gather the same amount of light as the MN190??? (equations are helpful, but please include the answer )

Pics as is, stacked and unprocessed except for levels. Full frame in each case, Sony 285 chip

Cheers

Tim

[ollypenrice]

About four times as long. Sorry, that may not be what you want to hear!! Exposure time goes as the square of the f ratio, with a few little bits of complication that I don't understand. Point sources don't follow that rule.

The logic is simple enough if you think of two scopes of the same focal length with scope A twice the aperture of scope B. Scope A has four times the area of mirror or lens (Pi R squared) so has four times the light grasp. Since the focal length is the same for both, the bigger one pours four times the light onto the same area of chip.

Edit; it would be interesting to test the resolution of both an a tight double...

Olly