REPORT ON THE SPECTRA SYSTEMS SURE SHARP AND DUO-VUE


As any astrophotographers knows, there are many factors that
contribute toward the success of prime focus astrophotography. 
Tracking must be within a few arc seconds of perfect, polar
alignment must be accurate, collimation set, composition nice,
and of course, the focus has to be exact.

Perhaps more amateur shots are ruined by poor focus than any
other reason.  I've seen too many times folks show up at our club
meetings all excited to show me their latest prime focus shots
with say their C8's, and I grimace at the images on the prints
when all of them are various sized doughnuts!  The problem is two
fold.  First you've got to be able to find focus exactly, and
second, be able to repeat that trick over again, with out any
doubt.  

I'd say 99 percent of astrophotographers attempt to locate the
focus by just looking into the cameras viewfinder at a bright
star and attempting to minimize the image on the ground glass. 
The eye will accommodate small differences in such types of
images, and even a shot that is way off base may look really
great.  

To attempt to quantify this situation, I put my Pentax K1000 on
my 14" f/5 newtonian, and using my spiral focuser as a guide,
tried to measure the variation I got in focus at the point where
it looked the smallest on the ground glass.  Knowing the threads
per inch, and how much of a turn it varied, I could get an exact
number as to this variation.

The Results.  

I kept throwing the star out of focus, and refocusing again and
again, noting the position of the focusing ring.  The bottom line
is a focus variation of .010 inches all looked great to my eye. 
This is roughly three diameters of a human hair in length. 
Photographically, the variation was more obvious.  Careful
examination of the negatives using the ultra fine grained 2415
tech pan film showed assorted sized star images, when examining
the smallest images on the negatives.  

I found that I could derive an interesting equation to show the
enlargement on the film, knowing just how far off focus you are. 
It is D=distance off focus/focal ratio of system.  For my
example, if I was .01 inch off in focus, and I'm using an f/5
system, D=.01/5 or .002 inches in diameter of the smallest star
images.  This is quite large for 2415!  We must keep it under
.001 inches for the best results.  Wallis and Provin indicate
that .0008 inches or 20 microns (1 micron is 1/1000 millimeter)
is ideal for most systems.  Here I was getting 50 micron images
and it all looked beautiful in the camera viewfinder!

The SureSharp

When I saw the adds in S&T for a focusing device that claimed to
absolutely guarantee perfect focus EVERY TIME without fail, I was
quite interested.  I contacted Dan Gordon at Spectra Systems, and
placed my order for a generic Pentax thread mount Sure Sharp.  A
week or two later, the unit arrived.  (Just in time for the full
moon too)  I soon discovered that he had accidentally sent me a
unit for an Olympus camera, and the unit was exchanged for the
correct unit.  When the unit was ordered, I specified the back
focus of MY Pentax such that it would be perfect for my camera. 
To make this measurement, I borrowed a machinists depth gauge
from a friend, and found that the camera was very close to the
specified 1.790 depth.  

The new unit arrived promptly, and I began testing of the unit
that evening.  First I focused the camera by eye on a bright star
using the usual camera viewfinder.  Next, I put in the Sure
Sharp, and compared.  Using the "knife edge" focusing method
described in the instructions, I found that my attempt at focus
was a bit off.  By repeating the focus tests with the Sure Sharp
that I had done earlier with the Pentax, I discovered almost NO
variation in focus in the rotation of the focuser.  In other
words, I was repeatedly getting the focus within .001 inches in
depth, or my star diameters (theoretically) should be 5 microns. 

Seeing, and my guiding ability, combined with the rather average
quality of my stopped down DS16 mirror ( I can just resolve 3
arcsecond doubles!)  will limit the minimum star size seen on the
film.  I next composed the Lagoon nebula carefully with an
accessory called the "Duo Vue", which allows an inch and a
quarter eyepiece to be installed where the camera will go,
without affecting focus.  Although I found that my unit would not
accept a 2 inch ocular,  Dan Gordon said I could send it back to
be updated.  For consecutive shots, this is the only way to go,
since the focus change between shots will be extremely small, and
easy to tweak in.

I quickly got out the camera, and shot four exposures, ranging
from 15 minutes to an hour in duration.  When the negs were
printed up, I was truly amazed!  The stars were the smallest I
had ever achieved with my telescope in the five years that I have
had it, and ALL FOUR were the same!  Measurement on the prints of
the smallest star size revealed that the images on the negatives
were 25 microns.  I calculated that this corresponds to about 3.5
arcseconds in the sky.  In other words, I had achieved nearly the
best resolution possible with my instrument, only limited by the
seeing, my guiding, and collimation.

The 8x10's look very good.  The star images are microdots all
across the frame.  The Sure Sharp has changed forever the way I
do my prime focus astrophotography, and I would heartily
recommend this product to anyone who wishes to produce the best
possible results from their prime focus systems.   --- Chris
Schur