Sky & Telescope - November 2019 - 70

S&T Test Report

p Focusing the RASA 8 by hand is a challenge.
Adding Celestron's optional Focus Motor for
SCT and EdgeHD telescopes, along with the
author's favorite focusing program, made the
task quick and repeatable.

This leaves DSLR users out of the
RASA 8 game, as no current DSLR will
work with this little back focus, though
Celestron makes this very clear. However, mirrorless cameras shouldn't have
problems being set at the right distance
to reach focus. Celestron recommends
mirrorless cameras as well as smaller
astronomical CCD and CMOS cameras,
with the detector mounted close to the
front ﬂange of the camera housing.

pu A removable clear optical window
located just in front of the corrective
optics can be replaced with an optional
Light Pollution Imaging Filter. Both
use a proprietary thread rather than a
standard 2-inch filter thread.

70

N OV E M B E R 2 019 * SK Y & TELESCOPE

One-shot color cameras are the best
match for this instrument, though a
monochrome camera with a very thin
third-party ﬁlter tray could work; you'll
need to look carefully at the total back
focus for your particular combination
of equipment.
For my own tests, I used an engineering-grade Starlight Xpress Trius-SX814C
one-shot-color camera. Its round body
introduced no additional obstruction
in the optical path, particularly when
I removed its side fan. The camera's
3.39-micron-square pixels offered a
resolution of 1.9 arcseconds per pixel
when combined with the RASA 8.
The spacing of the camera from the
front corrective elements is absolutely
critical to achieve Celestron's stated
usable image circle of 32 mm. In fact,
my ﬁrst few attempts were quite disappointing because of spacing issues. The
SX814C has a back focus of 17 mm,
which meant I needed exactly 8 mm
of spacers on top of the M42 adapter.
Initially I was off by quite a bit and this
caused a couple of effects. Although
I could still achieve good focus in the
middle of the ﬁeld (or any part of the
ﬁeld where I wanted to focus), stars
appeared progressively worse as you
looked farther from this point. Even
being 1 mm off in the camera spacing
produced distorted stars around the
outer edge of my already small chip. To

further complicate things, the chip in
my camera was not quite square to the
camera housing, so I needed to adjust
the tip/tilt of the camera mounting.
Fortunately, very few users will have to
deal with this kind of scenario.
In general, you should never need
to collimate the RASA 8, as everything
inside is pretty locked down. However,
the tip/tilt of the camera might still
need to be tweaked if your chip is not
100% perpendicular to the RASA 8's
optical axis. There are three push/pull
screws that require 2-mm and 3-mm
hex key (or Allen) wrenches to adjust.
I found some cameras had just enough
room to make these adjustments,
but with my Starlight Xpress, there
wasn't enough room to ﬁt standard
hex key wrenches. Fine-tuning the tip/
tilt by removing the camera, making
an adjustment, replacing the camera,
taking an image, then repeating the
operation was frustrating and took
quite some time. I eventually found a
set of L-shaped hex key wrenches with
a shorter arm that ﬁt. It would be great
if Celestron provided these with the
scope, as I suspect many users may
encounter the same issue.
q In most cases you won't need to adjust the
tilt of your camera to get round stars across the
RASA 8's entire 32-mm image circle. But in the
event that you do, look for short L-shaped hex
key wrenches.
Sky & Telescope - November 2019

Table of Contents for the Digital Edition of Sky & Telescope - November 2019
