Re: [APML] OT: M109, NGC 2403 and other collaboration images

[Ray Butler <ray.butler@nuigalway.ie>]

Hi Jason,

>Hi Greg,
>
>Yes I agree, except that the common method to expand an image these days is to scan, crop, and zoom in something like Photoshop.  Photoshop does interpolation when changing image size, which doesn't affect the brightness.  But you don't get the details of the larger-aperture instrument, 
>
- and not just the details: you can't add anything to the signal to 
noise either...

>so "aperture rules" is still valid, but for different reasons.
>  
>
...indeed. Interpolation is just distributing the same original data 
around to different places. Only by the acquisition of additional data 
and/or achieving higher true angular resolution can real benefits be 
accrued.

Ray "who had to stay indoors during a _perfectly_ clear, dark night last 
night, sniff" Butler

>-Jason
>
>
>-----Original Message-----
>From: astro-photo-bounces@seds.org
>[mailto:astro-photo-bounces@seds.org]On Behalf Of Greg Hartke
>Sent: Thursday, March 03, 2005 8:50 AM
>To: Discussion of Film Astrophotography
>Subject: Re: [APML] OT: M109, NGC 2403 and other collaboration images
>
>
>Hi, Jason,
>
>Well... Let's think about it. Suppose you imaged the same extended object 
>with the 2 scopes you proposed as examples. For simplicity consider the 
>object to be of uniform brightness. Assuming no losses, the brightness 
>density at the focus plane would be the same in the 2 cases because 
>(obviously) the focal ratios are the same. However, the image scales are 
>drastically different because the focal lengths are different by a factor of 
>4. If the image from the smaller scope were expanded to have the same scale 
>as that of the larger, the brightness density would decrease by a factor of 
>4.
>
>Now suppose the imaged object has interesting details we would like to 
>resolve. When we matched the images scales from the 2 instruments, the 
>imaged size of the Airy disk of a point target (which would be important to, 
>say, the ability to capture detail in a spiral arm, etc.) from the smaller 
>scope would also increase by a factor of 4. Aperture still wins. Bummer!
>
>The old dictum from observational astronomy that "aperture rules" is still 
>true for imaging but how vital it is it depends on what you're trying to do 
>and how you present your results.
>
>Regards,
>
>Greg
>
>----- Original Message ----- 
>From: "Lane, Jason R" <jason.lane@navy.mil>
>To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
>Sent: Thursday, March 03, 2005 11:27 AM
>Subject: RE: [APML] OT: M109, NGC 2403 and other collaboration images
>
>
>Hi Greg,
>
>You're right, the Airy disc diameter increases as a function of f/#.  But 
>that still doesn't explain the larger aperture.  A 32" f/2 scope and a 8" 
>f/2 scope would have the same-sized star images assuming seeing is limiting 
>the resolution of the images.  But the larger aperture scope has a longer 
>focal length, so you get a narrower field of view.
>
>I think this is why there are only a few locations on earth where large 
>telescopes are used professionally...there really is no advantage to them 
>(for imaging in particular) if seeing is the limiting factor.  I guess my 
>thought is that you've gotta consider seeing as well as FOV, optical 
>quality, and f/# when you're getting ready to buy an imaging setup.  Then 
>you've got mechanical issues, mounts, gears, stability, etc.  That's a lot 
>of factors....you're probably just as well off buying the scope that looks 
>the coolest.  :)
>
>-Jason 
>
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