RC's and Newts and Theory

[Peter Ward]

Originally Posted by dpastern

But (excuse my non astro logic here) doesn't make sense. It is completely the opposite to terrestial photography. Physics are the same (at least that's how I was taught) - photography and photons are the same at night or day. Sorry, I'm just having a very hard time wrapping my head around it all Peter.

Dave

With camera lenses the aperture can be varied with a constant FL.
So a 'wide open' a lens is operating at its maximim aperture. Not surprisingly it gives its brightest image at this point....hence the amount of flux coming down the pipe, so to speak, is soley (give or take) determined by the aperture. Does that help?

[coldspace]

Originally Posted by sjastro

That's correct. The f/2.5 won't go any deeper.

As you know the rule doesn't apply to photographic emulsions. The f/5 will require 4X the exposure.

A good example of Quantum Mechanics at work.

Regards

Steven

Hi Steven,

Just so I can get my head around all this, the f2.5 for a given aperature won't go any deeper than say an f7 or f10 for the same aperature and same exposure time with the same camera. Only change is the field of view with less noise?

Just when you see companies like Starzona marketing their hyperstar lenses to make your f10 to F2 and over 30 times faster I wonder if they are not telling you the whole story on this. Their speed comparisons and their images and other advantages really make you want to buy one, not that I will be.
I might email a couple of guys I know in the states who got these hyperstar lenses in the last year and see what they really think.
http://starizona.com/acb/hyperstar/whatis.aspx

If you look at the comparison shots of M8 are not really good comparisons as one shot is using a hyperstar C14 with a top CCD camera and the other is using a 6 inch scope and film.
Its a pity they don't post comparisons of objects on their web site showing same scope,camera,exposure times with only the f ratio changes.
Interesting topic and thanks everyone for the input.
Maybe someone if they can get some time put up some images of a well known object using the same scope, camera and exposure time with only the f ratio decrease to see what happens.

Matt.

[gregbradley]

Hi Matt,

Firstly they compare a CCD image to a film image in the M8 example. Film is different because it loses sensitivity over longer exposure times.

Also 2ndly you would have a very widefield image at F1.8 compared to a small slice of sky at F10.

Greg.

[sjastro]

Originally Posted by coldspace

Hi Steven,

Just so I can get my head around all this, the f2.5 for a given aperature won't go any deeper than say an f7 or f10 for the same aperature and same exposure time with the same camera. Only change is the field of view with less noise?

Just when you see companies like Starzona marketing their hyperstar lenses to make your f10 to F2 and over 30 times faster I wonder if they are not telling you the whole story on this. Their speed comparisons and their images and other advantages really make you want to buy one, not that I will be.
I might email a couple of guys I know in the states who got these hyperstar lenses in the last year and see what they really think.
http://starizona.com/acb/hyperstar/whatis.aspx

If you look at the comparison shots of M8 are not really good comparisons as one shot is using a hyperstar C14 with a top CCD camera and the other is using a 6 inch scope and film.
Its a pity they don't post comparisons of objects on their web site showing same scope,camera,exposure times with only the f ratio changes.
Interesting topic and thanks everyone for the input.
Maybe someone if they can get some time put up some images of a well known object using the same scope, camera and exposure time with only the f ratio decrease to see what happens.

Matt.

Hello Matt.

Greg has summarized it well.

Their f/ratio vs exposure data is simply misleading. It is only applicable to film.

On the comparison images, the Lagoon Nebula being a red emission nebula falls right in the sweet spot of the ST-X10ME's QE of 85%. Even if the film used was a hypered Tech Pan, the QE over a 70 minute exposure would probably be no more than 1% (and that is also considering the non linear behavior of film over long exposures).

Ultimately all it proves is how superior CCD's are to film.

An uncropped full resolution version of the Lagoon will not only reveal how noisy the image is, but also the off axis performance of the Hyperstar.

Regards

Steven

[dpastern (Dave Pastern)]

Originally Posted by Peter Ward

With camera lenses the aperture can be varied with a constant FL.
So a 'wide open' a lens is operating at its maximim aperture. Not surprisingly it gives its brightest image at this point....hence the amount of flux coming down the pipe, so to speak, is soley (give or take) determined by the aperture. Does that help?

yes, that sort of makes sense to this lad. So, if I understand it all correctly, since the f stop on an astro setup is fixed and non changeable, then the amount of light coming through the aperture is fixed, only the exposure length and aperture width will affect the final exposure. Correct?

Dave

[Peter Ward]

Originally Posted by dpastern

yes, that sort of makes sense to this lad. So, if I understand it all correctly, since the f stop on an astro setup is fixed and non changeable, then the amount of light coming through the aperture is fixed, only the exposure length and aperture width will affect the final exposure. Correct?

Dave

Pretty much. Provided you are not pushing the noise floor of your CCD, way oversampling the data or shooting from bright urban skies etc. etc. ..as covered earlier..

[dpastern (Dave Pastern)]

Well, I've learnt something today. Thanks Peter.

Dave

[coldspace]

Thanks Steven and Greg,

It all makes sense now.

Regards Matt.

[Satchmo]

Originally Posted by Peter Ward

.... but
a 10" is a 10" you will only get a marginal performance increase (using a CCD) with a faster system.

I can see now why short focal length /large aperture instruments with a well corrected field are so highly prized. Having field of view to play with is as good as money in the bank.

[gregbradley]

That's how I see it too Mark.

If F ratio makes no difference, well why not image DSO at F40 then with 8 inches of aperture. The extreme example helps show the point.

But the troubles with some ASA F3.6/3.8 scopes shows how critical everything becomes in quality required to achieve a practical scope at the faster f ratio. But see Wolfgang Prompers Namibia images using one and a FLI 6303 camera and look at how deep the images are and then look at how short his exposure times were. But they also aren't 12 inch RCOS closeup galaxy shots either and that scope would be no good for that sort of work. So it is hard to have scope that is all things to all people. Often 2 or 3 scopes are required to fit the sweet spot for different types of images. The Cervalo Astrograph is an attempt to capture both grounds fast F ratio for the wide field and slow f ratio
for the galaxies or closeups. You can see a difference in resolution between the 2 f ratios and that is from the same scope and same camera. A good example of this theory that it is a workable guideline but not a truth.

F5 -7 is not a bad range for imaging. The Tak 180ED is F2.8 and they routinely showed lots of faint nebula in the usual imaging targets not normally seen.

Greg.