Astronomy and Sky Website of Martin Lewis

Page written 20-1-2015 

General Introduction

This is a page giving more details about the different fish-eye lenses and ZWO cameras I have use with my all-sky camera. There is more on these related webpages;

  • AllSky Camera– a page giving more details of the construction of my all-sky camera with photos based on the initial incarnation with an ASI120MC camera and a Fujinon YV2.2×1.4A-2 lens
  • Time-Lapse Night Sky Videos– a page showing my best videos taken with my all-sky camera taken using the different cameras and lenses discussed below

Cameras and Lenses Introduction

When I first built my all-sky camera I used my ZWO ASI120MC colour camera and a Fujinon YV2.2X1.4A-2 zoom fish-eye lens which is a manual version of the auto-iris lens fitted to the now obsolete Orion AllSky Camera. This f1.4 lens has a focal length of 1.4-3.1mm and I used it set to about 1.55mm focal length where it gave a 185° view of the sky. I also tried this lens with a mono ASI120MM which has the same sized chip as the ASI120MC but gave increased sensitivity due to the absence of colour filters.

Fujinon YV2.2×1.4A-2 f1.4 1.4mm to 3.1mm fisheye lens

Although pleased with the night sky time-lapse videos I got with the above set-up I strove for better quality images, especially at low altitudes where there was a lot of distortion of the sky and coma seen in the stars.

A major step forward was the use of an Arecont 1.55mm f2 IR corrected fish-eye lens which is the same lens used in the Starlight Express Occulus camera. For lower altitudes this gave dramatically less sky compression and much less coma than the Fujinon lens but at the expense of smaller aperture.

Comparison of Fujinon zoom lens with Arecont lens on ASI120MC and ASI120MM

Click on images below to see each in more detail

Latest Fish-eye Lenses

In early 2015 I bought a Rainbow L163VCS lens which is an f1.4, 1.6-3.4mm zoom fisheye lens. Used at the shortest focal length this gives similar image quality to the Arecont but the images are brighter images due to its faster nature. Unfortunately though, the image circle was too large for chip on the ASI120 cameras and the image ran off the edge of the chip, losing the domed sky appearance that I found so appealing.

Later, married to the much larger chipped ZWO ASI174MM and ASI174MC cameras, which I bought later in 2015, the circular field of the L163VCS now easily fitted on the camera chip and even needed cropping down so that the sky circle was not lost in the large rectangular field. 

My latest fish-eye lens is a Fujinon FE185C086HA 2.7mm f1.8 lens bought from eBay. This now fills the large chip on the ASI174s and gives my best all-sky time-lapse videos yet of the night sky.

Trio of fish-eye lenses. Left to Right; Rainbow L163VCS f1.4 zoom 1.6mm to 3.4mm; the diminutive Arecont 1.55mm f2; Fujinon FE185C086HA 2.7mm f1.8

Lens and Camera Comparison Images- same pixel scale

Below you will see three different lenses on two ZWO different cameras all displayed at the same pixels per inch scale. These were all taken on the same winter night on 7/1/2016 all with a 5 sec. exposure and a gain of 10x (settings=200 for ASI174MM and 53 for ASI120MM) so you can realistically compare sky brightnesses with the different combinations. 

Please note that ASI120 images have a black surround added to keep the image scale in terms of pixel/cm constant on the page; the original chip frame is defined by a lighter border. Please click on each image to see it in more detail.

You see above the effect of the larger chip of the ASI174 (11.3mm x 7.1mm) compared to the ASI120MM (4.8mm x 3.6mm) allowing the full circle of some of the lenses to fall within the frame, whereas the frame for all lenses spills off the edge for the smaller chipped camera.

You also see the increased effective magnification for the ASI120MM due to the smaller pixel size for this camera (3.75um versus 5.86um).

Lens and Camera Comparison- Optimised Size for Each

Although the above images nicely compare the sizes of the different lenses in pixels for the two cameras what you probably want to see is the best view for each camera/lens combination. Your wish is granted below where some of the images are cropped so that the sky just fills the frame;

Above you can clearly see the effects of the different lenses on the sky brightness. The Rainbow is the brightest as it has an f1.4 lens but I would have thought that the Fujinon and the Arecont would have been more similar as both are supposed to be f2 – the Arecont fixed at f2 and the Fujinon was set to f2. The Fujinon is intermediate in brightness between the Arecont and the Rainbow with the settings used.

The two cameras have been set to nominally the same gain but the ASI120 is slightly brighter than the ASI174 for the same lens. In theory the ASI174 should be marginally brighter than the ASI120 for the same lens (see sub-section for same magnification under ‘comparative image brightness’ section on my CMOS digital video cameras page).

Do notice for the ASI174 that the stars in the Arecont at the top of the frame show noticeable coma whereas at the bottom of the frame they are much better. In the ASI120MM they are much better at the top. Experimentation has shown that for my sample there is an optimum rotational orientation of the top half of this lens compared to the bottom to minimise this asymmetry. The poor manufacturing control this demonstrates for this lens is a shame as otherwise this is a very nice lower cost fisheye lens.

Detail for each Lens/Camera Combination – on Orion

It is interesting to look more closely at the same section of sky with the different combinations to see what conclusions can be drawn regarding detail, star brightness and noise levels. Note bottom of ASI120MM with Rainbow lens lost off edge of frame. Click on images to enlarge

Camera/Lens Combination Rankings

Below is a table of results comparing the five Orion area images above, together with one or two other ratings based on earlier full all-sky images. I have given scores in different, somewhat arbitrary, ‘goodness’ categories, which allow me to give the combinations a relative ranking. Ratings in each category are 1 to 5, with 5 being the best and 1 the worst.

Although obviously subjective, this table may serve as a useful guide for those wishing to experiment with different lens and camera combinations.