Both
spiders are ready, so we can paint them black and attach the secondary
mirrors. Double sided “sticky tape” from
Manco, Inc,
template
for center dot mirror is face
down trimming sticky tape ready to attach mirror
finished mirror/spider
Secondary
back side and spider were spray painted with
$.99 QuickColor flat black. I
marked a dot on the face of the secondary using same technique as used to mark
primary – giving a reference point to
“eyeball” align the center of the secondary with the center of the spider, and
later to align tertiary and secondary mirrors.
We figured the tertiary to be “down in the light cone” about 4 ½” and
distance from tertiary to tube wall interior is about 2 inches; tube wall interior
to mirror edge is 1”; mirror radius is 6”, so secondary is down in the light
cone about 13 ½”. The light cone at that
point is 13.5/60 of 12” or .225 of 12” or 2.7” in diameter. We would need a 2.7” diagonal to capture all
of the light. What do we lose or gain
using a 2.28” secondary? We lose light at the edge of the field, but not enough
to detect visually. We gain resolution
via a smaller central obstruction. Given
these two parameters, many divergent opinions are available. Starmaster Telescopes indicate they use
downsized secondaries to gain image quality.
I used an undersized secondary in a monocular scope I built to test the
mirrors for the binoc and found star images and extended object detail enhanced
when compared to a larger secondary catching “all the light.” Planetary observers want secondaries only
large enough to illuminate the central field of their eyepiece. Deep space people want all the light they can
capture. We are compromising between
these two extremes.