Category: Astrophotography

Review first published in 2010

Kit Reviewed

Scope under Review:	Vixen VC200L
Accessories Supplied (as standard)	Optical Tube Assembly Dovetail plate for GP, Sphinx, many EQ mounts 10×50 illuminated reticle viewfinder Flip Mirror
Extras Supplied	73868 Focal Reducer for VC200L f/6.4 73523 Camera Adapter 43mm Deluxe 72954 DC Ring 60mm to T2-thread 73876 Canon EOS Focal Adaptor
Country of Manufacturer	Japan
Aperture / focal length / f	Aperture 200mm / focal length 1800mm / focal ratio f/9
Limiting magnitude / resolution	Visual limiting magnitude 13.3 mag / theoretical resolution 0.63″
Tube Weight	6.3 kg. (13.2 lbs)
Tube Length/Diameter	486mm / 232mm (9.1 x 24.4 in)

First Impressions

Vixen describe this scope as a Vixen 6th-order Aspherical Catadiaoptric.  So what does that mean?  Its all to do with the configuration of the mirrors and lenses.  Catadiaoptric means that the scope is a combination of mirrors and lenses.  It allows the light to be folded in the tube, which means you can have a short tube with a long focal length.  The mirror is aspherical, which means it is not perfectly rounded and needs some correction.  Sounds complicated.  Which normally means its expensive.  Actually, this particular model is known in some circles as the poor mans Ritchey–Chrétien, and once I played for a bit, it was easy to see why.

The design is quite simple.  A normal Schmidt-Cassegrain telescope (SCT) is setup with the primary mirror moving to focus and the corrector lens at the front of the telescope.  The VISAC design attaches the primary mirror to the back of the telescope. Unlike a normal SCT, to achieve focus there is a single speed rack and pinion arrangement at the back.  The primary mirror can 39;t move, so there is no mirror flop.  The secondary mirror is held in place with a spider.  The corrector is a series of lenses inside secondary tubes inside.  This scope is its own dew shield!

Focusing is via a single speed rack and pinion focuser at the back.  I was a little worried that there wouldn’t be enough in-focus at first, but this proved not to be the case.  The scope focuses perfectly with my Canon 350D, Starlight Xpress SXV-H9 and SXVR-H18, and with a normal 2″ diagonal and eyepiece, so no problems there.

The scope is well built – I got just the OTA to review. There is a carry-handle at the top with a standard camera attachment screw, so mounting cameras for widefield use it a piece of cake. The unit also came with a pre-attached Vixen dovetail bar, so mounting it on Skywatcher or Vixen mounts is easy.

Collimation

When I recieved the scope, Vixen Opticron had already collimated the scope.  My initial tests showed that the collimation is still perfectly set, even after delivery.  Its also been in the back of my car to and from the Stargazers Lounge 5 Star Party and Salisbury Star Party, and hasn’t required any adjustments.  Low Maintenance, perfect for Star Parties!  I don’t think my eye is quite up to the Nick Howes or Damian Peach standard of collimation though!

Collimation is achieved by adjusting the primary mirror, instead of the usual SCT arrangement of adjusting the secondary.  Some people have suffered problems with this setup, however, there are several good guides on the internet to help you out if you need it.

Initial Tests and a Planetary Encounter

First light was a good look at Mars and Saturn.  Both resolved well in a 7.5mm eyepiece, and I was itching to get the camera onto the scope.  Before that though, I had a good look at the moon with a 40mm Meade 5000.  The view of the terminator was very nice and crisp.  Turning the power up on the moon showed great detail.  I must confess at this point, I couldn’t resist any longer and put the Skynyx onto the scope!  My planetary imaging skills are not up to much though, so it wasn’t until later in the year when Jupiter put in an apperance that I managed to capture anything worth showing you.  Here is Jupiter from 2010-08-09 – I still didn’t have a suitable IR/UV filter though at that point.  It was taken using a Lumenera Skynyx 2-1C and a 2x Barlow.

Jupiter

Imaging Deep Sky Objects

Vixen Opticron also supplied an F/7.1 Focal Reducer.  This reducer screws onto the back of the focus tube.  This brings the scope to the usual F/6.4 that we all know and love. Natively, this is an F/9 scope, not F/10 – 2000mm aperture and 1800mm focal length.  It would be nice to get an F/5 reducer to bring the Field of View wider and give the scope a ‘faster’ Focal Ratio.  The lower the number, the ‘faster’ the scope is, and the less time needed on targets.

I also have acquired a Starlight Xpress SXVR-H18 to replace my SXV-H9 Astronomical CCD camera recently, which worked extremely with the Vixen and the focal reducer.  Reducing the effective focal length, focal ratio and field of view of the telescope when used with a camera.

Prime Focus

As usual for my imaging runs, the camera was placed at prime focus.  I did have a google around for a suitable explanation, but it appears one isn’t readily available.  So here goes for my attempt!

Any camera which can have its lens removed can be used with the Prime Focus method.  You simply connect the camera to the telescope viewing end without any lenses in place – no eyepieces and no camera lens.  Focusing is achieved in the normal way.  Here is an example of a Prime Focus setup with my SXVR-H18, Truetech Filter Wheel and various connectors.  The Focal Reducer is also used in this case.

SXVR-H18 attached to the Vixen VC200L

First Deep Sky Image

I decided to return to an old friend.  I have never been satisfied with the images I achieved of the Crescent Nebula (NGC6888.)  To be honest, I still am not, but that is the fault of my processing and not the SXVR-H18 or VC200L.  I have recently gone through my raw data again, and I think I can put a much better version together.  However, that is having to wait at the moment.

Crescent Nebula in Sii/Ha/Oiii

That being said, look at the field of view!  The view is approximately the same as with the SXVR-H18 on my old 102mm F/7 Astro-Professional scope.  The Vixen is also giving extremely sharp and crisp detail for the H18 to work on.  I love the way the diffraction spikes work with the Narrowband as well.  These are caused by the 4-vane spider in the front of the scope.

4 Vane Spider

To be honest, that is the only critiscism I have of this scope.  The spider vanes are very wide, and therefore cause quite ‘blocky’ diffraction spikes.  However, I do happen to like diffraction spikes in my images, and the Vixen certainly delivers on that front.

Conclusion

If you want to ask whether I would recommend this scope, the answer is yes.  In fact, I brought it after the review.  The very few dis-advantages are outweighed by the superb benefits in my view.

Description of Parts 

CCD 1 is a Starlight Xpress SX814 camera. It is a peltier cooled CCD chip with a black and white Sony HDR sensor. To reduce electrical noise, it is cooled to -30 degrees below zero using an electrical cooler. Professional observatories use liquid nitrogen to do the same job, but that is not really practical for amateur astrophotography.

Calibration

Before I can start taking exposures, some initial calibration of the equipment is required. The computer needs to know where it is on Planet Earth as well as what time it is. GPS can be used is used to get that information, or it can be entered manually, using Google Maps, or the Internet to find the correct Latitude and Longitude.  Once entered, the computer stores this information for future reference.

Initial Setup

Starting with the telescope in the home position, pointing North and with the weights down.  Using the computer sky chart to control the mount to move the telescope to a bright star, and line it up on the camera chip.  Repeat that three times and the computer knows how reality relates to its internal model of the sky.  Luckily, if I “park” the mount after use, and don’t knock it, it keeps pretty good alignment night after night, so I only need to repeat the full Initial Setup procedure once or twice a year.

Focusing the Camera

Before taking any exposures for use in an image, the telescope and camera need to be focused.  Focus changes with temperature, this also means that each night the focus position will be slightly different.  To add complexity, as the equipment cools, and contracts, the focus point changes.  The automatic focuser I use, a Starlight Instruments (no relation to the camera manufacturer) Focuser Boss II Electronic Focusing Control and stepper motor.  This unit learns the change in focus over a 5 degree celsius temperature drop, and using that calibration compensates for changes as the temperature drops automatically.

Before any of that, I need to find a star, and carry out an initial focus run.  This is complicated further as each filter has a slightly different focus point.  Each time I change the filters in the filter-wheel, I need to recalculate the relative differences in position between the Red, Green, Blue, Clear, Hydrogen Alpha, Oxygen 3, Sulphur 2 and Hydrogen Beta filters.  Each one has a slightly different focus point.  More calibration, but the relative calculations stored. I usually redo that once a year, or if I change the filters around.

Slewing to target

Once the stars are focused, I pick a target, say a galaxy or nebula, and tell the scope to slew (move) to point at that location. The guide camera (CCD2) now needs to be told how it is aligned.  This is a simple software routine which involves the computer making small movements to figure out which way the motors move in relation to the stars.  Once done, it locks on and removes tracking errors automatically. CCD1 is then ready to take a long exposure.  At this point, CCD2 is taking pictures every second, sending commands to the mount motors to correct for errors.  This keeps the stars on the same pixels on CCD1 during a long exposure!  Otherwise you get lines, not points of light, and those do happen but go in the bin. I take many exposures because the objects are so very very dim that long exposures to collect as much light as possible for a long time.  That also introduces random noise in the raw data.  To remove that, astrophotographers take multiple exposures for each filter. Some images require all the filters.

Example Results – Messier 51 – The Whirlpool Galaxy