Optics 45 Degree Erecting Prism
Daytime Use Only...
recently purchased a TeleVue-76 from a friend when he upgraded to
another scope. This little telescope is a pleasure to use and the
images are nothing short of beautiful when panning through a dark
Arizonan sky. The telescope was acquired to be used for daytime as
well as nighttime use as my wife and I also enjoy birding. To allow
for this I needed to equip the scope with a correct image diagonal.
Some research led me to decide on the Williams Optics 2" 45
degree diagonal, or as termed on the William's website a "45
Degree Erecting Prism". I was looking for a unit that matched
the TV-76 in quality and hoped it would perform well.
no particular preference in which internet vendor I used I chose to
order from Astronomics as they are prominently listed as a Cloudy
Nights sponsor. Their website was easy to navigate and the order was
processed quickly. A few days later the diagonal arrived as promised
on my doorstep. Shortly after my arriving home from work the
diagonal was quickly unpacked and installed in the TV-76. Focus was
reached near the middle of focuser travel with all eyepieces and all
looked good with a nice image of the iron sitting on the ironing
board at the far end of the hallway.
construction of the diagonal is strictly first class. The machining
beautifully done with deep black anodizing. Sliding into the TV-76
focuser with a tight, close tolerance fit that gave a nice
impression. The eyepieces likewise fit into both the open tube or
the supplied 1.25" adaptor with a nice tight feel. It was clear
that this diagonal could hold any eyepiece, including the big
'terminaglers', it certainly has no issue with the 35mm Panoptic.
Looking down the tube of the diagonal you can see the prism with
about 1.25" of clear aperture providing a fairly wide light
path. Good sized locking knobs retain the eyepieces, with both the
main tube as well as the supplied 1.25" adaptor using brass
night the diagonal arrived the weather was quite cooperative. While
high cirrus draped most of the sunset sky with beautiful orange
streamers the seeing was quite steady, a solid seven or better on
Pickering's scale. My wife knew exactly what was up as I disappeared
to the back yard with a tripod mounted scope over my shoulder.
tested the diagonal with several eyepieces, taking advantage of the
2" capability I mounted a 35mm Panoptic. For higher powers I
used my 8mm and 4mm Radians. The view at wide angle was quite
pleasing with a fully illuminated field. Out of focus stars seemed a
little odd but I couldn't quite determine what was wrong, in focus
the stars were sharp until near the edge, but that I expected with
the short focal length telescope. The difference really became
apparent at higher powers...
Saturn very high in the evening sky it was an obvious first target to
try out the new toy. Using a low power eyepiece to locate the view
was just fine, it is when I switched to an 8mm Radian that I started
to have a little trouble. Saturn would not quite focus, the view
seeming a little soft as I rocked back and forth through ideal focus.
There was a fair amount of glare in the image as well as a slight
purple haze of uncorrected color I would not expect in a full
apochromatic telescope like the TV-76. The glare was most noticeable
as a bright pair of wide horizontal bars each side of Saturn.
Running back inside I stole a decent quality Meade 2" 90 degree
mirror diagonal out of my other refractor. While not a premium piece
of gear, the diagonal does perform reasonably well. With the 90
degree diagonal installed the views were once again what I would
expect from the TV-76. A crisp, clear view of Saturn rewarded me
with both the 8mm as well as the 4mm radian eyepieces.
the scope at a high elevation I found myself lying in the grass to
get a good view with the 45 degree diagonal, simply adjusting the
legs in on the tripod to allow a reasonably comfortable view.
(Fortunately I had mown the lawn recently.) This is the main reason
I would recommend a 90 degree diagonal for astronomy use, the 45
degree diagonal is not a great deal better than straight through for
viewing angle, while the 90 degree diagonal provided a comfortable
sitting viewing angle at all but the highest elevations.
over to Procyon, likewise quite high in the sky, I examined the
focused and out-of-focus star images with both the Williams 45 degree
diagonal and the standard 90 degree diagonal at high power. The
result was quite surprising at first until I though about the
structure of the erecting prism. With the 45 degree diagonal I saw
an odd pattern of vertical bars instead of the expected bulls-eye
pattern (see figure below), and the star would never form a decent
airy disk at focus. When far out of focus there was a thin vertical
bar in the out-of-focus image resulting from the prism structure.
With the 90 degree star diagonal the airy disks were textbook perfect
with the 4mm Radian eyepiece. This result completely agreed with the
soft view I had experienced with Saturn.
complete the testing I took the scope to a local birding hot-spot. A
large wetlands created using the outflow from a sewage treatment
plant, creating a lush oasis in otherwise arid Tucson that abounds
with all manner of avian life. I observed treats like a Great Horned
owl with it's large, fuzzy, awkward chicks and a beautiful American
Kestrel posed perfectly about 100ft away on a phone line. I was
using both 14mm and 8mm radian eyepieces for 35 and 60x respectively.
a daytime spotting scope, at lower magnification, the 45 degree
erecting prism performs quite well. It provides a sharp and bright
image when used at powers from 35x to 60x. For daytime use I had no
objective method to use in testing the scope. When evaluating optics
at night there are optical references scattered across the sky, these
perfect optical points called stars. Daytime provides no such
reference targets, all I can do is relay is my general impression of
the resulting images. I have nothing negative to note despite trying
to give the image a critical appraisal.
an erect image the TV-76 becomes a high performance spotting scope
with images to match any I have seen in purpose built spotting
scopes. The only issue somewhat it that it is somewhat heavy and not
as compact as a spotting scope with the same capabilities. The prism
equipped scope rewarded me with a wonderful day of memorable sights I
would have been unable to enjoy without it.
would consider the 45 degree diagonal for daytime use only. Keep in
mind this is the reason I had purchased it in the first place and I
am not disappointed with the device. I will need to remember to pack
both diagonals when traveling with the scope if I intend to use it
for both night and day observing.
do not believe the trouble is with the quality of the optics used in
the diagonal. I have had a chance to use top end Swarovski spotting
scopes at night, and was similarly disappointed, noticing some of the
same issues. Instead I believe the problem lies in the inherent
limitations of the prism design, too many reflections and too many
optical surfaces to get just right. For astronomy stick with a good
quality 90 degree mirror diagonal.