10" f10 vs 12" f8

allan gould · #1 10-01-2014, 12:06 AM

I can't work out how much extra exposure I would have to do with a 10" f10 compared to a 12" f8 scope to give an equivalent ccd exposure.
Any geniuses out there that can assist?
Allan

clive milne · #2 10-01-2014, 01:09 AM

Approximately 40% all else being equal.

allan gould · #3 10-01-2014, 09:37 AM

Thanks for that Clive. Much appreciated.
Allan

cometcatcher (Kevin) · #4 10-01-2014, 10:56 AM

That doesn't sound right. f10 is 2/3 of an f stop dimmer than f8. A full f stop would require double the exposure, so 2/3 f stop make 66% more exposure needed?

RickS (Rick) · #5 10-01-2014, 11:13 AM

Approximately 1.57 times assuming a 50% obstruction by diameter.

allan gould · #6 10-01-2014, 11:29 AM

Quote:

Originally Posted by cometcatcher

That doesn't sound right. f10 is 2/3 of an f stop dimmer than f8. A full f stop would require double the exposure, so 2/3 f stop make 66% more exposure needed?

Kevin
I think you didn't see that I was comparing a 10" and 12" at different f ratios. But thanks any way.

Quote:

Originally Posted by RickS

Approximately 1.57 times assuming a 50% obstruction by diameter.

Thanks Rick, I thought that it would be in the ball park you and Clive have given.
Allan

cometcatcher (Kevin) · #7 10-01-2014, 11:42 AM

Quote:

Originally Posted by allan gould

Kevin
I think you didn't see that I was comparing a 10" and 12" at different f ratios. But thanks any way.

I did, but if you're shooting extended objects, the exposure is determined by f ratio not aperture. For instance a 3" f6 or a 20" f6 have the same exposure time since they're both f6.

But I've got a funny feeling I'm missing something.

Someone explain it to me.

RickS (Rick) · #8 10-01-2014, 12:03 PM

Quote:

Originally Posted by cometcatcher

I did, but if you're shooting extended objects, the exposure is determined by f ratio not aperture. For instance a 3" f6 or a 20" f6 have the same exposure time since they're both f6.

But I've got a funny feeling I'm missing something.

Someone explain it to me.

You're correct. The difference in exposure time (ignoring central obstruction) only depends on the f/ratios: 10*10/8*8 = 1.5625

However, if you're an astro imager you'll also want to think about image scale as well as exposure time. This will be very different when comparing a 3" f/6 and a 20" f/6.

cometcatcher (Kevin) · #9 10-01-2014, 12:21 PM

Quote:

Originally Posted by RickS

However, if you're an astro imager you'll also want to think about image scale as well as exposure time. This will be very different when comparing a 3" f/6 and a 20" f/6.

Yes. Image scale is very different but the "average" exposure will be the same. It will of course vary as the 20" could pick off bright and dimmer areas within the field of the 3". Allan's image scale is roughly the same, 100" for the 10" vrs 96" FL for the 12".

Aperture makes greatest impact on point sources like stars, although I've read f ratio affects this also.

Regulus (Trevor) · #10 10-01-2014, 12:30 PM

F ratio: f8 is f8 is f8 regardles of the lens size.

RickS (Rick) · #11 10-01-2014, 12:48 PM

I still don't buy the argument that f/ratio is terribly relevant for imaging. The two things that do matter are aperture (which determines how many photons you can capture) and image scale (which determines how those photons are distributed into pixels). Those two parameters determine exposure time and resolution - both fundamental concerns of the imager.

Cheers,
Rick.

cometcatcher (Kevin) · #12 10-01-2014, 01:03 PM

It's just part of the equation. I got a little confused comparing a decimal to percentage but other than that, we're all correct lol.

Geoff45 (Geoff) · #13 10-01-2014, 01:34 PM

Quote:

Originally Posted by RickS

I still don't buy the argument that f/ratio is terribly relevant for imaging. The two things that do matter are aperture (which determines how many photons you can capture) and image scale (which determines how those photons are distributed into pixels). Those two parameters determine exposure time and resolution - both fundamental concerns of the imager.

Cheers,
Rick.

I agree with Rick here. This topic keeps on recurring. People should read this article for some very useful info http://www.stanmooreastro.com/f_ratio_myth.htm.; There is a nice pic of the same field taken at 10 min with an 8.2" scope at f/12.4 and f/3.9. If anything, the longer f-ratio gives a better image. Here is another useful link to the same topic http://www.stark-labs.com/help/blog/...ioAperture.php.
Geoff

RickS (Rick) · #14 10-01-2014, 02:36 PM

Focal length and pixel size determine image scale. f/ratio has no role to play here

Geoff45 (Geoff) · #15 10-01-2014, 03:01 PM

Don't forget the camera here. A f8 scope and a camera with 8micron pixels will gather exactly the same amount of light per pixel as the same aperture f4 scope and a camera with 4micron pixels. Both systems will have the same "speed".

Don't forget binning. Binning 2 x 2 effectively halves your focal ratio.
Geoff

RickS (Rick) · #16 10-01-2014, 03:28 PM

Poor Allan's original question (sorry, mate!) was about imaging. We're talking about image scale in arcsec per pixel as it is conventionally used by imagers.

Quote:

Originally Posted by ghsmith45

Don't forget the camera here. A f8 scope and a camera with 8micron pixels will gather exactly the same amount of light per pixel as the same aperture f4 scope and a camera with 4micron pixels. Both systems will have the same "speed".

Don't forget binning. Binning 2 x 2 effectively halves your focal ratio.
Geoff

Exactly, Geoff! Pixel size is hugely important.

allan gould · #17 10-01-2014, 03:31 PM

Cr@p, Im glad I asked such a simple question!!!!
I assumed but did not state I would be using the same camera.
Essentially the image scale is the same for each scope, with the 12" F8 having a slightly larger obstruction than the 10" f10; which still makes the 12" gather more photons for the same length of exposure as its the bigger bucket. Very roughly a 10 min sub with the 12" would be equivalent to a 15 min sub with the 10" scope.
Think Ive got a handle on it now but very interesting all the same and thanks to all who replied.
Allan

RickS (Rick) · #18 10-01-2014, 03:41 PM

Not trying to be an ass here, but the original question was not about the abstract brightness relationship between two scopes. It was specifically about CCD exposure times:

Quote:

Originally Posted by allan gould

I can't work out how much extra exposure I would have to do with a 10" f10 compared to a 12" f8 scope to give an equivalent ccd exposure.
Allan

QED... I'll shut up now.

Peter.M · #19 10-01-2014, 04:12 PM

My terrible napkin math says for a given camera you would have to expose the 10 inch 1.5 times the length to get the same exposure, assuming that there is not obstruction.

clive milne · #20 10-01-2014, 08:19 PM

Quote:

Originally Posted by cometcatcher

That doesn't sound right. f10 is 2/3 of an f stop dimmer than f8. A full f stop would require double the exposure, so 2/3 f stop make 66% more exposure needed?

Teach me to write a 10 second reply... As I walked out the door I realised the error in my post.

So for clarity;
For (under-sampled) star images the exposure times will be approximately proportional to the aperture ratio squared;
ie) the 12" will collect 44% more light so the 10" will take 56% longer for the same depth of exposure. The unspoken assumption here is that a combination of background sky brightness and detector noise will determine the absolute magnitude limit. There is quite a difference between sensors (as much as 2 magnitudes) The Sony cmos chips being probably best in class within the amateur budget.

For extended objects the exposure times are proportional to the ratios of the f-numbers squared.
In this instance it will be the same as the numbers above (derived from relative aperture) but this isn't always the case so it is probably worth noting the distinction.

c