Sky & Telescope - USA (2019-11)

S&T Test Report

70 NOVEMBER 2019 • SKY & TELESCOPE

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. How-

ever, 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 fl ange of the camera housing.

One-shot color cameras are the best

match for this instrument, though a

monochrome camera with a very thin

third-party fi lter 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 engineer-

ing-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 fi rst few attempts were quite disap-

pointing 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 fi eld (or any part of the

fi eld 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 fi t 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 fi t. It would be great

if Celestron provided these with the

scope, as I suspect many users may

encounter the same issue.

puA 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 fi lter thread.

pFocusing 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.

qIn 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.