| review: Skywatcher 120/600 Achromatic Refractor (appr. € 450,-) |
This is a chinese rich-field Achromat intended for low power observations.
The basic dimensions are:
- 2 lens achromatic design
- aperture 120 mm
- focal length 600 mm
- f-stop f/5
- rack and pinion focuser with 2 inch + adapter to 1 1/4 inch and T-thread
- mounting rings included
Introduction
Another reviewer came to the conclusion that this refractor is of no use at all because the f/5 Achromat must have too much chromatic aberration - by design.
I cannot agree with that.
This refractor is intended for low power wide-field views.
With a good eyepiece this refractor is performing like a giant binocular.
When it is used like that I could not find to much or troublesome chromatic aberration.
It is true that the color becomes annoying in mid to high power views without filters.
But that is exactly what you have to expect from an Achromat at f/5.
It's simply trading a wide field against a color free high power view.
So this refractor is simply not entended for planet watching.
At f/5 I was mainly interested in the photographic capabilities of that fast refractor.
Photographic Usage
After the first tests some trouble showed up:
The straylight baffle is way too short for long time exposures with CCD.
The main problem is the streetlights shining into my observatory.
But the nearby moon is causing the same trouble.
They are shining to the front lens and causing the straylight to bring down image contrast.
This effect was almost not visible in my little Takahashi.
I guess it comes from a lack of correctly applied internal baffles.
Because of the restricted field of view of my CCD camera (12mm diagonal) the cure was very easy:
I made a much longer tube from cardboard (black inside, silver outside) - as long as possible not to introduce vignetting at the edge of the images.
Additionally I covered the original straylight shielding tube with black velevet at it's inside area.
The result is very good: no annoying straylight anymore.
The focuser is tilting the camera away from the optical axis when racked to outside.
I checked that first with my laser collimator.
When racked to inside the focuser's axis is very well aligned to the optical axis.
Because the movement of the focuser is otherwise quite OK my solution is just to focus to the inside.
With a star on the CCD I could judge the tilting to something like 20 seconds of arc.
As a comparison value take a good japanese rack and pinion focuser with a tilt of 4 seconds of arc.
Or the JMI DFX1 Crayford focuser with tilting less than 1 seconds of arc.
But keep in mind that the DFX1 is costing more than the whole Skywatcher optical tube including it's focuser!
The chromatic aberration of that refractor is obviously corrected for visual usage.
That means that red and blue is in the same focus while green has it's own focus and infra red as well as violett are way off.
For taking color photos that means that you have to refocus each color channel and it is quite hard to take a sharp luminance frame for LRGB at all.
As an example that's how mag 10 stars look like after only 10 seconds of exposure without a filter:
You can see the focused and reasonable sharp core formed by the green light and a halo formed by out of focus light from all the other colors.
So the first thing to do is to include an IR- and UV-blocking filter.
Additionally I used a SkyGlow Filter to reduce the green light to bring green closer to red and blue.
Now the red and green frames were really sharp.
This is a proof for low values of other problems like spherical aberration.
No astigmatism was visible.
But the result was still not very pleasing in the blue channel.
So I stopped the aperture down from 120 mm to 100 mm.
The refractor is now working at f/6 which is still reasonable fast and just the same like my Takahashi APO.
The slower system now is more tolerable to defocusing in general or in other words it has a bigger depth of focus.
And that is why the effective chromatic aberration is reduced as well.
Now it is possible to take reasonable sharp luminance frames.
As an example here is a mean of 25 luminance frames, each 120 seconds long.
This gives a total exposure time similar to 50 minutes - enough for the most objects of interest.
Red and white stars are OK.
Only the blue ones still creating a halo.
This is mostly from the violett light which is still way out of focus and unfortunately my CCD camera is very sensitive here.
With bigger pixels and a non-blue sensitive CCD camera the overall sharpness should be even better.
Example Images
Images are LRGB composites and were taken with Starlight XPress HX916 at full resolution with IR-blocking filter and with a Skyglow filter.
The moon was taken in red color only.
Conclusion
The Skywatcher 120/600 is a very nice rich-field refractor for low-power views.
It is not intended for high power visual work.
This is by design and nobody should complain about that.
On the other hand the Skywatcher is a fast and reasonable sharp deep sky objective for an unbeatable price.
If someone is devoted to H alpha photography the performance should be almost undistinguishable from an APO of a similar or slightly smaller size.
Please use the link below to find more astro photos in higher resolution taken with that scope:
Search more Photos taken with this Skywatcher
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