Starlight Xpress Mini Filter Wheel with OAG
A slim device that changes filters quickly and accurately. Combining a filter wheel and OAG in a single unit is such an obvious idea, and Starlight Xpress has turned it into an elegant product
Mono CCD cameras have many advantages over one-shot colour devices, such as DSLRs and colour CCD variants, mainly in terms of sensitivity and versatility. The ability to use a wide range of different filters – both broadband (normal colours) and narrowband (concentrating on very specific wavelengths of light) – has a great deal of appeal. However, swapping filters in and out of the light path by screwing and unscrewing each one in turn quickly becomes a chore. Manually rotated filter wheels holding five or more filters are one solution, but these are neither dust nor light proof, as they normally have an opening to allow you to move the wheel.
Automated filter wheels like this one from Starlight Xpress not only allow you to change the filters quickly, but also enclose them in a compartment to provide full protection against dust and light. Starlight Xpress has increased the usefulness of its mini filter wheel even more by incorporating a built-in off-axis guider (OAG).
An OAG is one of the two methods of autoguiding with a guide camera; the other is using a second telescope (called a guidescope). Guidescopes are easier to set up and operate, and give a wide selection of potential guide stars, but they often suffer from something called differential flexure. Differential flexure occurs when there’s a small amount of movement between the two scopes, so the imaging scope doesn’t quite follow the guiding corrections applied to the guidescope. An OAG resolves this issue as the guide camera is coupled so closely to the imaging camera that they both collect light from the same telescope, resulting in better guiding performance.
The Starlight Xpress mini filter wheel is built to a high standard and its shiny black finish matches the other CCD-imaging cameras in the Starlight Xpress range. The design ensures that the company’s imaging and guide cameras are parfocal with one another without requiring additional spacers, which makes fitting the filter wheel quick and easy.
Getting at the filters is a simple task, you just undo four thumbscrews to remove the front plate before unscrewing the single bolt that keeps the carousel on its bearing. Surprisingly, the filters go into the rear of the carousel even though the filter numbers are stamped on its face. It’s a minor issue but it doesn’t make installing them any harder.
We would normally adjust the individual focus of the imaging and guide cameras on a new OAG during the daytime by aiming the telescope at a distant object and then focusing both cameras on it. However, we were confident that the Trius-H814 CCD camera and Lodestar guide camera used for this test would be close to focus as both are made by Starlight Xpress, and we were right. We used a star field in Cygnus to check the focus; the only adjustment we had to make was a small tweak to the focus collar on the guide camera turret.
We had no problems installing the drivers on the supplied CD on our Windows 7, 32-bit laptop. A virtual controller that can select the required filter during an imaging run is included (the Starlight Xpress software can automate this process) but we opted to use MaxIm DL for our test. There’s an option to make filter changes faster by allowing the carousel to rotate in whichever direction brings the required filter into position the quickest, but filter placement was much more accurate with this feature switched off.
Using three-second exposures through our William Optics FLT98 refractor gave us a wide choice of guide stars, which made guide-star calibration easy and resulted in excellent guiding.
Combining a filter wheel and OAG in a single unit is such an obvious idea, and Starlight Xpress has turned it into an elegant and cost-effective product. It’s one that should appeal to astrophotographers making the leap from DSLR to long-exposure mono CCD imaging.
Fitting into tight spaces
The industry standard spacing for focal reducers, field flatteners and coma correctors is 55mm (±1mm) from the camera sensor to the mounting face of the corrector. This figure is derived from the nominal sensor to lens-mounting face distance of a typical DSLR camera of 45mm plus the 10mm depth of a typical T adaptor. This is fine for connecting a DSLR camera directly to a corrector but leaves no room for an OAG. This critical spacing problem becomes even more of an issue for CCD camera users wishing to use both a filter wheel and an OAG.
Guide camera port - The top of the adjustable prism turret has a height-adjustable collar and a standard C thread around its perimeter to accept any Starlight Xpress guide camera. The vertical movement of the collar allows the guide camera to be focused and a small grub screw allows its orientation to be adjusted.
Adjustable camera orientation - To ensure that the pick-off prism doesn’t intrude into the light cone falling on the imaging camera’s sensor, it’s important to align the prism with the long edge of the camera’s sensor. This simple slotted adjustment plate with its camera T thread makes setting the correct orientation easy.
Pick-off prism - This tiny, 45° prism takes a sample of the light that falls outside the light cone reaching the imaging camera and diverts it, via an adjustable turret, to the guide camera. Guide star shapes are slightly distorted but perfectly usable for guiding where just the star’s geometric centre is analysed.
Carousel - The carousel holds up to five 1.25-inch filters and is kept in alignment by a plain-nylon bushed bearing running on a brass pillar to produce a secure installation. Drive is applied by a small wheel from underneath, which is held in tension by the sprung drive motor to keep the carousel’s movement smooth.
USB-powered motor drive - Power comes via the same USB port through which the device receives its control instructions. This convenient method of connection eliminates yet another cable from the usual tangle. The motor inside the device has a beautifully made reduction gearbox.
This review first appeared in the November 2013 issue of BBC Sky at Night Magazine. Prices correct at time of going to press.