True Technology Custom-Wheel MK III
Selecting a filter wheel is not as simple as it may seem at first glance, and there are many parameters to consider. We hope that the following will help you make an independent and intelligent assessment to select a filter wheel for your system. It is easy to fall into the trap of over-valuing an item such as 'the effect of dust-spots on filters' (which may prove to be an imaginary 'problem' see later) or on the other hand, not fully appreciating the huge benefit of being able to instantly access or change filters...........
Inserting and Removing filters
Inserting and Removing Filters
The importance of this cannot be overstated, and one of the key feature to the design of the Custom-Wheel is that:
1. WITHOUT touching the FOCUSER or CCD camera, you can COMPLETELY REMOVE an internal wheel in a matter of seconds. The focus position and the CCD camera orientation are untouched, so that as soon as a filter is changed or cleaned, the internal wheel can be re-installed and you can continue imaging with a minimum of disturbance
2. With a range of internal wheels, one can load up separate internal wheels for different CCD cameras and/or applications, from colour work, to photometry to specialised narrow band.
Dimensions of the Custom-Wheel
1. Thickness from front to back. Ideally this should be as small as possible, while retaining rigidity, and of course the thickness of adapters front and back should be factored in. The Narrow Custom Wheel has a thickness of 27mm only, and the Wide Custom Wheel has a thickness of 50.8mm (2")
2. Clear aperture of system, includes the main threads on the wheel, clear aperture of adapters, and of course clear aperture of filters. The Custom-Wheels have a female SCT thread on front and back, which is a 2" diameter. We have designed our 2" adapters to have a clear internal aperture of 45mm, and that co-incides with the clear aperture of our 48mm filters. Our 1.25" filters have a clear aperture of 25mm
At first glance it may not appear too important that there are several options on internal wheels, but the following points should help you to assess what is most suitable for your application(s):
4d/v wheel. This is a 5 position wheel to take four 48mm filters, with the fifth position being a blank or dark frame position (for narrow wheels) for cameras which do not have a shutter (eg most Starlight Xpress cameras) or a diagonal mirror (for wide wheels).
5-way wheel. This is a 5 position wheel to take five 48mm filters
5d/v wheel. This is a 6 position wheel to take five 1.25" filters, with the sixth position being a blank or dark frame position (for narrow wheels) for cameras which do not have a shutter (eg most Starlight Xpress cameras) or a diagonal mirror (for wide wheels).
6-way wheel. This is a 6 position wheel to take six 1.25" filters
6d/v wheel. This is a 7 position wheel to take six 1.25" filters, with the seventh position being a blank or dark frame position (for narrow wheels) for cameras which do not have a shutter (eg most Starlight Xpress cameras) or a diagonal mirror (for wide wheels).
7-way wheel. This is a 7 position wheel to take seven 1.25" filters
8-way wheel. This is a 8 position wheel to take eight 1.25" filters
In some applications it can be extremely important, even critical, that the time to change between filters is as rapid as possible. It should be noted that in colour work, eg LRGB, each filter change, usually adjacent filters for three of the operations, and then a change of three positions to return to the first filter, the total change time is 6 x the figure given below, and this is lost time.
The Custom-Wheel was designed with a 'shortest route' philosophy. Thus if one wants to go from eg filter number 2 to filter number 8 of an 8-way wheel, the route will be down to 1 and down to 8, ie a distance of two filter positions only. The actual times to change one position for the different wheel types are:
5-way wheel 1.1 second
6-way wheel 0.9 second
7-way wheel 0.8 second
8-way wheel 0.7 second
The rotation is constant speed to to calculate the time to change more positions is pro-rata.
There is a lot of misunderstanding on this subject, but the good news is that the Custom-Wheel is an exceptionally accurate wheel.
A 'V' indent system is used on the circumference of the large diameter internal wheels. The wheels run on stainless steel shafts running in phosphor-bronze bushes. On the wheel housing is a ball spring plunger, which has no backlash, compared to nylon plunger types. In operation, photo-electric sensors determine the wheel position and when a desired filter position is reached the sensors not only stop the motor, but remove power altogether, such that the spring plunger will register the wheel precisely at the centre of the V indent.
The dust-spot argument. The most noticeable dust-spot donuts will come from dust on the CCD window of the actual camera, where the distance from the dust-spot to the sensor is least. This distance can be under 5mm for a video camera, and generally 15-35mm (sometimes more) for cooled astronomical CCD cameras.
By contrast, the distance from a filter (in a Custom-Wheel) to a CCD or video camera sensor would typically be in the range of 30-60mm.
If you think that you have an application where (a) the effect of dust spots on any filters will be detectable on your CCD set-up, then there are TWO bits of good news concerning the Custom-Wheel:
1. It takes seconds only to remove and clean filters (our first main point above), with no aggro or disturbance to your set-up, and/or
2. To measure the Custom-Wheel positional accuracy we did an optical test, using an off the shelf Wide wheel, where we put a video camera on the 'eyepiece' position, taking the image reflected from the diagonal mirror. We found that the BACKLASH was only 30 microns at the filter centre. But even this is a red herring when related to real-life imaging, because when doing repetitive or sequencing work, you will ALWAYS approach EVERY filter from one and the same direction only, eg RED-UP-GREEN-UP-BLUE-UP-CLEAR, then DOWN to RED, etc. We could not see any noticeable difference in image position, pixel for pixel, when we moved our wide wheel to the diagonal from the SAME direction each time. This means that for flat-fielding, the flat field for any particular filter will be identical each time that filter is accessed from the same direction.
This is another area where there appears to be confusion. We did a test to establish points of flexure in a system and found that the Custom-Wheel, whether narrow or wide version, showed virtually no detectable sign of flexure for a camera weighing 1.5Kg, while being able to support a camera of at least 2.5Kg. Even at the higher load, the deflection (static test done on a lathe with the top part of a WIDE housing, rotating the assembly 180° and looking at the 'sag' on the end of the CCD camera, using a centre-drill to make a dot on a label stuck on the end of the CCD camera) was well under 1mm. We found that even at the lower weight, the deflection of the tube of a quality 'crayford' design electric focuser was around 1.5mm, despite the focuser being in perfect adjustment and able to handle an 8lb load, and we found that it was easily possible to get this sort of flexure with 2" push-fit attachments, if fastened using a single screw, or if a two screw system was not tightened properly.
footnote: early on a few Custom-Wheels were supplied where the side panels had not been fully tightened, and these showed signs of flexure because of that !!! Our QA was immediately put right to ensure that all wheels go out of the door with side panels fully tightened.
The Custom-Wheel has its own stand-alone pc control program (Windows based), or can be controlled from MaxIm CCD or AstroArt software. In addition there is an SBIG mode for pulse control via ST7/8/9/10.
The Hand-Controller permits single UP-DOWN filter change, with indication of filter position
We can connect virtually anything to anything, and please refer to the thumbnail gallery for details and descriptions of adapters. If you can't see what you are looking for, or require something else, please click on the e-mail link at left and ask for further info