Fieldcorrector/Reducer for small-pixel CCD-Cameras ED Design
I wanted to have a small sized
newtonian for transportable
work and about 750mm focal lenght to get
a field with my ST10 of roughly
1 degree. I made some calculations and found a very
good design. It will work on nearly all focal ratios and
sizes. The reduction factor is about
0.73x and removes coma.
It fits in a 2" barrel, has a back-focus of 64mm (enough for the
ST7/8/10 + Filterwheel) and
gives a perfect corrected field of 20mm diameter. The length of the
corrector is 68mm. It has a male T-Thread (42mm x
0.75).
This reducer, combined with a VERY GOOD parabolic mirror will beat every single
Apochromat out there and
you will have a much faster focal
ratio.
This is the second design we are producing of this corrector. The first design used CaF2 and was very expensive. The performance was slightly better than with the current design but we think that the current design is still good enough not to show any errors even on good seeing conditions and so we decided to use an ED glass instead of CaF2. For those of you interested in the old CaF2 design we will make another set of the CaF2 lenses if there is enough interest, so drop me a mail in case you may want to spend the double price for even better spot sizes.
You can see the design above. For the Sbig-Cameras we can supply a suitable extension barrel to put it exactly in the optimum focus place.
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Above you can see the
performance of the Reducer (left
picture) compared to a typical Schmidt-Cassegrain with optimized (even better than the
common achromats used very often
!) 2-lens Reducer. You can see why
its so hard to get crisp
stars with SC and reducers. You
can also compare to the spot
diagrams of a newton 300mm f/4.5
without any corrector here.
Here you can get detailed
information about how the reducer
will work with your system. Please
note, that the box size is
25 microns.
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Spotdiagrams |
Other Diagrams |
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Optimum Back-Focus Distances from T-Mount to Focal plane:
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200mm
f/3 = 65.9mm |
250mm
f/3 = 65.4mm |
300mm
f/3 = 66.3mm |
400mm
f/3 = 66.1mm |
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200mm
f/4 = 65.2mm |
250mm
f/4 = 65.5mm |
300mm
f/4 = 65.6mm |
400mm
f/4 = 66.0mm |
500mm
f/4 = 65.9mm |
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200mm
f/5 = 65.1mm |
250mm
f/5 = 65.4mm |
300mm
f/5 = 65.5mm |
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the Tolerance is around +-1mm if you want optimum performance. Please take into account your filter thickness. Be aware, that a 2mm thick filter increases the distance by 0.7mm. So for example for the 250mm f/4 mirror the back focus distance is 65.5mm. If you have a 2mm filter in place, this distance is increased to 66.2mm.
How to order:
Please order here:
ASA
Astrosysteme GmbH
Galgenau 19
A-4212
Neumarkt
Austria
german http://www.astrosysteme.at/de/korrektoren.html
englisch
http://www.astrosysteme.at/eng/correctors.html
They also sell great ready to go telescopes that use these correctors !
Please be sure to read the below FAQ bevor you start to ask questions:
1.What is
the In-Travel that I need for my
system ?
The focus with the reducer will move about 16mm towards the primary
mirror for the Newtons. So your focal plane without the reducer should
be at least 85mm above the fully
racked in focuser to be able
to get into
focus.
2.Will it
work with larger Newtonians ?
Yes, you will have about the
same spot-diagrams
3. How about the Infrared region ? What happens
above 700nm
Not very much. With 1000nm the spot size is
still below 20 microns and you will not see any difference
between using a block filter or not. At 1000nm the spot
size is still smaller than in an Schmidt-Cassegrain.
4. Is there some vignetting inside the Camera, Filter wheel
Yes, you will get a vignetting below f/4 that comes from the
pick up mirror for the autoguider-chip
in the STX Cameras. Also the filter wheel starts to vignett
below f/3.5. However it is in a way
that you can get rid
of it with
flats. If you want to
image without vignetting at f/3 you should consider
to use a 2" filter wheel.
5. Are there some
ghost images
We checked for double bounce ghost images, including
the filter position of the CFW8. There
are no ghost
images that are imaged into
the focal plane. Since the corrector
is multi-coated, there are no
strong ghost images anyway.
6. Why is it so expensive compared to an eyepiece
If you try to find a manufacturer
for 10 pieces you will see that
it is a bargain.
Every lens gets its own coating
etc. It is still cheap if you
consider that with a 250mm parabolic mirror you get
a system that will blast away every APO availible used with a ST10 or other small pixel
cameras.
7. Can it be use also with other
cameras
Of course. If you have
the back-focus (65mm) you can use
every camera. Also the FLI cameras and Apogee Cameras
will work with this reducer.
8. How good should my parabolic
mirror be ?
You can be
sure that 90% of all parabolic mirrors out there are not suitable for imaging with
small-pixel ccd cameras. Since years, parabolic mirrors have to
be cheap to be successfull
and that automatically leads to bad quality.
Be sure you
have a good one before you
start to use this reducer.
Otherwise it is like putting
new tires on a wreek.
9. What size of diagonal mirror do I need ?
Since the focal plane is 85mm above the focuser
racked in, you need a larger diagonal mirror than that for
visual use. Here is an example:
Assume we have a 300mm f/4.5 Parabolic mirror with a 350mm tube OD and a NGF-DX1 (40mm fully racked in). The minimum diagonal size to get the
full main mirror into focus
is (350/2+40+84.3) / 4.5 = 66.5mm. You would have
to add the
full size of the CCD Chip to this diameter
x 1.33 to get zero vignetting. For the ST10 with
a diagonal of appr. 17mm, this would mean
a diagonal size of 88mm
(3.5"). However if you accept a small
amount of vignetting you can go with
a 3.1" (12mm full illuminated).
Here
is the vignetting
including the 78mm diagonal
mirror for the 300mm f/4.5 Newtonian reduced to f/3.3. Please note, that
the vignetting somes very slow
and smooth. This comes from the
fact that it is far
enough away from the focal
plane.
10. What else can cause vignetting ?
If you have a very long
drawtube you will get some vignetting
that you always get if
you use fast systems with long
drawtubes.