Many astroimagers will acquire an 8 inch catadioptric telescope (using both mirrors and lenses) with a focal length of ~2,000 mm to observe and image the Moon, Planets and smaller deep sky objects. The three most commonly bought optical tube assemblies are:
The Meade LX65 8-inch ACF (Advanced Coma Free) OTA costing ~£1,500 (but sometimes on offer for £1,127 from Meade UK). This new design eliminates coma towards the edge of the field by using an aspheric corrector plate and hyperbolic secondary mirror.
The Celestron Celestron C8 XLT Optical Tube Assembly costing ~£1,299 is the original SCT design and will suffer from coma towards the edge of a wide field.
Celestron EdgeHD 800 Optical Tube Assembly costing ~£1,900. This new design incorporates optical elements within the baffle tube to give a wide, aberration free, flat field – so can also be called an astrograph which is ideal for astroimaging.
This article is to showcase an interesting alternative design, which, like the Celestron Edge HD, also uses optical elements in the baffle tube to give a wide, aberration free, field of view – the Vixen VC200L OTA which I have used for many years with great success. It costs ~£1,538 so significantly less expensive than the EdgeHD. (Vixen is a highly regarded Japanese company.)
The Optical design
The optical design is unique. It features a high precision sixth order aspherical primary mirror, a convex secondary mirror along with a triplet lens mounted within the baffle tube. In combination, these optical elements form Vixen’s VISAC (Vixen Sixth-Order Aspherical Cassegrain) design. There are two advantages from using this internal corrector lens system; firstly, there is no full aperture corrector plate to dew up as in standard Schmidt-Cassegrains and secondly, it gives an open tube assembly so cool down times are considerably less. It does, however, mean that the primary mirror can collect dust though I have not found this to be a problem. The primary mirror’s aspherical shape is produced by controlling the depth of the mirror’ reflective coating so, should it ever need ‘re-silvering’ it would need to go back to Japan.
[“Vixen has created a unique aluminum vacuum evaporation system to produce extremely precise mirror surfaces. More accurate than the error-prone process of grinding material to form a mirror surface. Vixen’s revolutionary system works by controlled layering of aluminum film. The primary mirror of the “popular” 200mm newtonian R200SS and the unique 6th-order aspheric Cassegrain primary mirror of the VC200L are produced with this aluminum vacuum evaporation system”.]
Rack and Pinion Focuser
The Vixen primary mirror is fixed and focussing is done using a duel speed rack and pinion focuser at the rear of the tube assembly. This has two advantages over a SCT as there will be no image shift when focusing nor any ‘mirror flop’ when, for example, the mount performs a meridian flip. [ I have equipped my 9.25 inch Celestron with an additional Starlight instruments focuser which eliminates image shift but not mirror flop – but, pleasingly, my mount does not do meridian flips.]
The secondary mirror assembly.
This is mounted on a very rigid, cast metal, four vane spider – the arms being quite thick. This has the advantage that collimation is held well and rarely required and I have never had to collimate mine. The disadvantage is that brighter stars can become a ‘bit blocky’ but this can be fixed in post processing.
The central obstruction is 40 % by diameter compared to 33.8% for a Celestron EdgeHD 8. This will reduce the micro contrast to a somewhat greater extent than the EdgeHD as more light is diffracted away from the central peak of the Airy Pattern into the first ring. This is most significant when observing planetary features so the Edge HD might be slightly better for such observations under superb seeing conditions. Under typical conditions, the central peak (the Airy Disc) of the Airy Diffraction patterns is spread out to reach the first ring of the Airy Pattern
The advantage of a larger secondary is that vignetting will be less and the VC200L is able to virtually cover a full frame sensor. (As seen in the ‘flat field’ image below, there is essentially no vignetting across a Micro 4/3 sensor.)
[“The VISAC system provides high-definition star images to the edge of a wide viewing field and offers exceptionally outstanding performance in astrophotography. Even at the edge of the 35mm film format (larger CCD or CMOS chips) stars are sharp (smaller than 15 micrometers). With its elaborate aspherical optical design, it achieves excellent image correction throughout the large illuminated field. (42mm diameter fully illuminated.)”]
Imaging with the VC200L
I have used it extensively for lunar imaging with a small monochrome webcam (Point Grey Chamelion USB2 monochrome camera with a Sony ICX445 1/3” sensor employing 1296 x 964, 3.75 micron, pixels). To reduce the effects of atmospheric turbulence which are less at longer wavelengths I sometimes use an Astronomik Planet IR Pro 742 filter to image in the near infrared as used in the images below. The final one was composited from 54 panes and achieved a resolution of ~ 0.7 arc seconds. The composite of 4 areas shows the detail achieved.
M13 Globular Cluster
Used with a camera using a cooled Micro 4/3 sensor, the telescope nicely encompassed the M13 globular cluster in Hercules.
Very pleasingly, the image showed the 15th magnitude Galaxy IC 4617 as seen in a 200% crop whose location is given by the plate solved Astrometry.net image.