This is the first update following the
initial period of use with the Toscano 8 inch RCT. I have managed
to acquire three images to date using the scope, two at the native
f9 and one using the AP 0.67 focal reducer.
When the scope first arrived I was impressed with
the overall quality and finished. For the price it represented
good value for money. The scope was supplied with a William Optics
SCT focuser and a set of good quality tube rings. However, I replaced
the focuser with a TCFS focuser.
Initial visual use.
After mounting the scope I set about checking the
optical quality. A quick check showed that the scope needed some
post travel collimation. This was achieved visually using the
secondary adjustments only. Although I have owned SCT type scopes
in the past, I was inexperienced at the time and had never really
carried out a star test so had no real idea what to look for except
through my experience with refractors. my first thoughts were
that the out of focus image was not ideal. However, after some
consulting with Ian King and a bit of research it was decided
that the out of focus image was perfectly normal for an RC.
Getting the focal plane
After some initial use with my ST2000XM and filter
wheel, I determined the correct spacing required and had some
extension tubes made to get correct focus position.
The next job was to pin down the collimation to
as good as I could get. For this purpose I purchased a Takahashi
collimating microscope. However, this was dogged by a few problems
that were not the fault of the scope, rather of the TCFS focuser
that I had fitted. It quickly became apparent that the focuser
was not on the optical axis as checked with a collimated laser.
Unfortunately the model of scope that I had did not have any adjustment
to the rear focuser attachment tilt / tip as with previous models.
In an effort to get the optical axis correct I needed to adjust
the spider centering and use some carefully placed sticky tape
on the TCFS SCT mounting adaptor. Unfortunately this condition
was not ideal and while it gave reasonably good collimation, it
produced double spider diffraction spikes on bright stars due
to the bend in the spider vanes. I decided that the only way to
get it right was to climb a very steep learning curve involving
the various steps and procedures needed to collimate these types
of scopes using the Tak collimating scope, and what the various
collimation images mean in terms of the information they give
you and the adjustments required. Along the way I have a few recommendations
to make regarding the scope and the methods that I use for collimating.
The first thing I decided was that the existing
secondary adjustment screws on the scope needed replacing. They
were of a Philips head type and this meant that the cross head
screw driver was prone to slipping when the screws got close to
being tight enough to hold collimation. I replaced the screws
with some socket head 4 mm x 20mm screws - much better. After
a bit of conversation with Ian King, the latest scopes use a push
pull adjustment that is much better that the spring loaded system
on my version.
The next job was to get the focuser on the optical
axis with the spider mechanically centered so that I got rid of
those double diffraction spikes. This was achieved my carefully
measuring the position of the secondary using some engineers calipers
and by placing some spacer washers behind the rear cell focuser
mounting plate. I would hope that newer models of the scopes re-introduce
the adjustable rear cell - a very useful feature that I feel is
essential to getting the best collimation possible. However, it
is simple to make a modification to the existing plate with the
addition of some slightly longer screws to replace the existing
ones, and the addition some threaded holes and screws for the
plate. I intend to make this modification at a later date.
After all the learning and adjusting I have achieved
what I feel is very good collimation. Along the way I have written
a collimation article that sums up my experience and the methods
I used during the collimation process.
images taken through the Tak collimating scope can be seen here
The collimation and
field according to CCD
I had also been trying out CCD
Inspector by CCDWare.
This is an excellent program and I feel that it provides a very
good analytical tool, not only for image selection and processing,
but also for helping to determine various optical errors and
corrections that can be made. Initially I have been cautious
about publishing data regarding my optical system using such
a tool, but after a period of use that shows consistent, repeatable
results, I believe that the data is valid.
The images below are links to larger versions. They show a
series of data taken from my luminance sub exposures for the
image of M56 with the scope operating at f9. The curvature and
3D images show good collimation, minimal tilt and tip for the
image sensor, and a nice flat field for the ST2000XM image size.
On the whole I think that they are a testament to the good quality
optics and value for money of these scopes. While the curvature
and 3D map were determined using an average of all the sub images,
the individual data does not show a marked deviation from the
average, therefore I would say that it is valid. As a point
of interest reduction of various data shows that the scope is
operating at a true f9.1 and focal length of 1820mm.
The next set of CCD
Inspector data images show similar information
except that this time an AP 0.67 focal reducer was added to
the optical system. Again the curvature and 3D maps are averages
of the data set. Using the focal reducer also gives a good
flat field with a focal length of 1288mm at f6.44
Summary so far.
Overall my impressions of the scope are good and I am pleased
with the results so far. It has been a bit of a difficult road
getting the collimation sorted but mostly due to the TCFS focuser
issues, although some modifications to the scope adjustment
system would / will help. Of course the collimation is always
going to be critical to these types of scopes but that is just
inherent in the design and you have to accept that you will
need to be meticulous and patient. Other areas of interest are
that the scope has held collimation well, needing no further
adjustment, the temperature variation in focus seems minimal,
there does not seem to be any detectable flex at different pointing
directions, and the finish is holding up OK. By the way I have
not had any dew issues so far, even when using the scope and
flexi-dew shield on muggy nights.