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.

Comp_SC.JPG (60396 Byte)

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.


Other Diagrams

200 f/3 reduced to f/2.25

Vignetting 200f3 reduced to f/2.25

200 f/4 reduced to f/3

Vignetting 250mm f/4 reduced to f/3

200 f/5 reduced to f/3.75

Encircled Energy for 250mm f/4 reduced to f/3

250 f/3 reduced to f/2.25


250 f/4 reduced to f/3


250 f/5 reduced to f/3.75


300 f/3 reduced to f/2.25


300 f/4 reduced to f/3


300 f/5 reduced to f/3.75


400 f/3 reduced to f/2.25


400 f/4 reduced to f/3


500 f/4 reduced to f/3


Optimum Back-Focus Distances from T-Mount to Focal plane:

200mm f/3 = 65.9mm

250mm f/3 = 65.4mm

300mm f/3 = 66.3mm

400mm f/3 = 66.1mm


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

200mm f/5 = 65.1mm

250mm f/5 = 65.4mm

300mm f/5 = 65.5mm



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







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.