More Canon 350D (Rebel
XT) wide field images with a Canon lens.
Based on the great results
with C-18 I decided to find some lenses to do wide field piggyback
type imaging with. I bought a modified body only from Hutech and
needed a decent lens. My Internet reading disclosed that the cheap
current 50mm series II Canon lens was a pretty good one in spite
of, or especially for the low cost. Mine is a Malaysian make.
I had already made a very difficult Olympus OM to Canon bayonet
adapter for manual operation and found that none of my old favorite
vintage 80s OM Zuikos were up to tiny CCD pixel sizes. Even my
all time favorite, the 100mm f/2.8 couldn't even cut it. Since
the CCD chip is smaller than 35mm film, this 50mm lens works more
like an 80mm fl. for image scale.
Below are two images done with
the Canon EF 50mm f/1.8 II lens stopped to f/4.5 and I adapted
a 48mm Lumicon Deep Sky filter for the Cygnus exposures. Exposure
times were 3 @ 10 minutes each, piggybacked on the C-18. These
are dark subtracted, but not flat fielded. I needed to combine
3 exposure sets to eliminate random pixel coloring since the star
sizes matched the pixel sizes and weird colors would result on
many smaller stars. Also, at large field angles, the Deep sky
filter exhibits color shift which makes the frame edge sky background
magenta hued. I cropped these to square to minimize the effect
and eliminate elongated stars. The Deep Sky filter really kicks
up the color saturation to where I almost have too much saturation
to find a black sky background. Other tricks were using raw data
capture with a PC using Maxim DL and also I found too much slop
between the plastic inner and outer focus barrel.
This was corrected by placing two overlapping thicknesses (.016")
of 3x5 card between them and Scotch taping it into a dew/glare
shield. The added stiffness helped achieve the necessary exact
manual focus sensitivity and better star collimation. A better
material would be .020" plastic shim stock available at automotive
supply stores. Exact manual focus is very critical. Unlike a 35
mm format lens that hit a hard stop at infinity, this DSLR auto
focus lens will go past infinity focus. It really takes a PC software
like Maxim DL where one can examine the stars intensity and size
to achieve the high level of exact manual focus.
I didn't experiment with wider
apertures since 10 minutes shows a lot of nebulosity. F/4.5 was
needed since M 31 was way out at the edge in order to fit M 33
in the same field. You can click on these smaller versions below
to get to a 100% magnification cropped version. North is up.
When you click to
zoom to the 100% version of the Cygnus area below, you will see
a result which reminds me exactly of what the 6X7 Pentax would
do with the big 165mm f/2.8 lens on gas hypered Fujichrome R-100
developed as a negative. That set-up astounded me in the 1980s.
A full color darkroom was needed then for large format processing
along with all the associated talent to make it happen. Actually
I just went back and looked in my 6x7 print binder and found some
400 f/4 6x7 shots with 45 minute exposures that are very comparable
to the 50mm DSLR results for M31 and NGC 7000!
To squeeze digital
detail, in color, out of a 50mm lens that is comparable to a 400mm
film lens displays how much the imaging technology has advanced
in 22 years and the potential to get thrilling results with basic
consumer level equipment. It also shows that lens makers have
met some very big challenges in order to actualize that huge potential.
When I started in with piggyback astrophotography in the 1960s,
I would never believe it could refine to this level. The below
images were done with less than a 1/2" aperture lens!
I add this page since
I really didn't see many definitive Canon DSLR wide field lens
images on the Internet and share these results in hopes of inspiring
many others to take advantage of the new possibilities. Christian
Buil has also posted some great wide field images taken with the
350D at http://www.astrosurf.org/buil/gallery/index1.htm
He also has some good lens information based on his own measurements
within his vast website.