[APML] New Horsehead - an interesting comparison?

[Philip Perkins <philip@astrocruise.com>]

I've been slow processing the images from southern France in December - too 
much time away on business trips unfortunately.  But anyway here is the 
latest of my images from southern France...  it is of the Horsehead (yet 
another!) taken with the RCOS 12.5" working at f/6.75 with the AP focal 
reducer.  Guiding was with the ST-4 using a custom OAG designed by Chuck 
Vaughn.  The camera was the Pentax 67 using hypered Kodak GPY 
(PPF).  During the trip I ran out of both freshly hypered film and N2 gas, 
so the last two exposures of the four stack were taken using film that was 
hypered in June 2001 with no gas purge.  One of the problems with this 
image is that I did not properly assess the effective f/ratio (T-stop) 
hence all of the negatives were somewhat underexposed (these were my first 
film images with the RC):

http://www.astrocruise.com/horserc.htm

I found it interesting to compare this image with a CCD image of the 
Horsehead which I took during the same trip (some of you who subscribe to 
the SBIG list may have seen this before).  The interesting thing is that 
the CCD image was taken using the exact same telescope and optical 
configuration just two nights later (conditions were very similar):

http://www.astrocruise.com/horse.htm

It seems to me that the two images could hardly be more different 
considering that they were of essentially the same object taken with 
exactly the same optics.  The huge additional field covered by the film 
image stands out strongly of course, as does the response to the very 
bright star Alnitak.  I don't know that a CCD camera would ever be able to 
record Alnitak in such an 'analogue' way.  The CCD image has its own 
strengths and would probably be considered as the more technically accurate 
rendition.  To me this is an interesting illustration of the vastly 
different results that can be obtained from film and CCD.  They both have 
their strengths but they serve a different purpose.

The important thing not to forget is the vastly different image scale of 
the two original images.  It is easy to lose sight of this when viewing 
images on a monitor.  Whereas the CCD image is displayed at its full 
original resolution, the film image has been downscaled massively to fit 
the monitor resolution.  The original film image will make a high quality 
20"x20" Lightjet print with no upscaling at all, whereas the CCD image will 
make a 6"x4" print with no upscaling.  Of course, the CCD image will 
tolerate upscaling rather better, but even so the CCD image will never come 
close to the print size obtainable with the film image.  The issue of 
detector size in relation to the making of large prints has no 
reasonable  answer with current CCD technology.  The number of CCD mosaics 
required to emulate the film image would be impractical.  Moreover the film 
image, with its rather saturated colours, will make a very fine Lightjet 
print.  With modest upscaling it will easily make a quality 30"x30" print.

Very different, I think you'll agree... comments welcome!

--Philip
Philip Perkins - philip@astrocruise.com
Wiltshire UK & Luberon France
Astrocruise - http://www.astrocruise.com


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