Re: [APML] Re: Star Measuring WAS: Horsehead and Pleiades testshots with 6x9 camera/OAGand BRC

[Aplanatic@aol.com]

Matt,

> Is CTF the same as MTF?  

http://resolution.umn.edu/glossary/m.html

They define Modulation Transfer Function as:

"A mathematical function that expresses the ability of an optical or electronic device to transfer signals faithfully as a function of the spatial or temporal frequency of the signal. The MTF is the ratio of percentage modulation of a sinusoidal signal leaving to that entering the device over the range of frequencies of interest. The MTF is usually presented as a graph of MTF versus log (frequency). For a square wave signal, the function is known as the CTF."

Often you see MTF applied to the optics and CTF to the recording media. 

> That is very related to the grain structure, yes?

Yes.  Finer grain films will have a CTF that is higher at large spatial frequencies.

> If the film is scanned at 3000 DPI and 
> 3x3 pixel stars are seen, does that mean 
> the stars are about 25 microns in diameter?

Yes, that's about right.  One should really deconvolve the scanner pixelation to gain a better estimate of the the size of the star on film.

> The diameter can be determined by where the 
> intensity drops to half maximum?

Yes, theoretically.  But if the star images are small it's very hard to do in practice either from a scan or from direct measurement.

> For this analysis, one should select stars 
> that are not so bright as to be saturated?

Yes.

> By the way, one issue that hasn't been 
> mentioned in this thread is the star 
> size (and quality) as a function of off-axis 
> distance.  To characterize the imaging 
> quality of a system, no single number is
> sufficient.

Quite right.  A good f/5 astrograph should have an RMS spot diameter less than 10 microns at the stated edge of field, IMHO.

Jim,

> Dave you say you have measured 10 micron stars 
> on TP, what did you use to measure them?  How 
> visible are they to the naked eye?

We took the negatives into our company's test facility and used a high quality Zeiss microscope with a calibrated reticle.  Because of the grain, it's hard to make a valid measurement on one star image.  Instead, we visually scanned over the image to get an average feel of the image sizes.  The smallest stars, on average, were indeed on the order of 10 microns.  I actually didn't believe it was possible (in practice)until I saw it for myself.  Theoretical musing is one thing, having an actual example is quite another.  <g>  Schmidt cameras are amazing to me.
 
Without the aid of a good microscope I can't see the smallest star images.

Dave Rowe

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