FW-502 disk assembly for 50mm or 2" diameter unmounted filters
The FW-502 disk assembly will fit into any standard SupaSlim body from Serial Number 500 onwards. The identification of filter positions is by putting the two adjacent pin-holes near the periphery at the 4 'o clock position (above left-hand image). Filter No 1 is at the top, then you count anti-clockwise for filters 2-5, in this example, 2 being Red (cyan colour appears from the reflective coating), 3 being green (magenta), and 4 being blue (yellow). Filter No 5 in this example is actually H-Alpha. The right-hand image shows the gear side of the disk assembly.
Application for thin 50mm Diameter unmounted filters.
Configuration for 50mm unmounted filters up to 2mm thick
The image above shows the profile of one of the five filter holes in the 5mm thick disk, where the filters are inserted and secured in place by large (52mm dia.) 'O' Rings. Each filter hole has a 50.1mm diameter 'seat' to locate 50mm diameter glass. Filters up to 2mm thick can be retained in this fashion, but for filters over 2mm thick the solution below should be adopted.
The left image shows a thin 50mm filter secured by the large O-Ring. Up to 2mm thick glass can be accommodated using 'O' rings only to secure the glass. For thicker glass, up to 3mm thick, we can supply screws and fibre washers which overlap each adjacent 'O' Ring.
A set of 5 'O' Rings (right). The O-Rings are 52mm in diameter, so form a secure fit when pressed into the 50.8mm holes to secure the filter glass in place.
Application for thick 50mm and all 2" Diameter unmounted filters.
Configuration for 50mm filters over 2mm thick and all 2" diameter unmounted filters
The main filter hole is slightly over 2" in diameter. To retain 2" filters up to 3mm thick or 50mm filters which are 2-3mm thick it is necessary to use nylon retaining screws and washers, plus a smaller O-Ring (50mm diameter) placed under the filter. Four screws/washers are used per filter, and there is no intrusion into the light path as the overlap on the washers is kept to a minimum. See image below of disk with screws/washers inserted.
For 50mm diameter glass which is thicker than 2mm it is necessary to use slightly smaller 'O' Rings of 50mm diameter. These are placed in the holes first, with the filter glass on top, and then secured with four nylon screws and washers as shown. The FW-502 disk comes with M3 nylon washers and screws, plus 5 'O' Rings of 50mm diameter, and 5 'O' Rings of 52mm diamter. A carrying/storage case is also included as standard
SupaSlim wheels can be ordered with either SCT threaded (2"/24tpi) or M54 threaded (54mm x 0.75) sideplates. There is no difference in price, but plates can be ordered retrospectively if required and are easy to change. If no plates are specified then SCT plates will be supplied as standard for SS-8 and SS-5, but for SS-502 there will be an M54 plate on the front and a SCT plate on the rear unless otherwise specified. Plates are completely interchangeable, so you can have the best plates to suit your application, eg M54 on the front, to give widest aperture, and SCT on the rear. In addition to the range of adapters shown on the main page, new additional M54 adapters are detailed below. All the tests and information below are based on using a SupaSlim wheel with M54 plates unless stated otherwise
*** ADDITIONAL BENEFITS USING 50mm/2" UNMOUNTED GLASS ***
* A solution for use with full frame CCD cameras on fast focal ratio instruments
* 50mm/2" unmounted glass filters for maximum aperture
* Glass of 1-3mm thick to be secured by 'O'-Ring fixing (optional additional fixing screws/washers)
* Unlimited filter capability by use of extra internal disks, max sequence of 5 filters per disk
* Internal disks changed in seconds without disturbing telescope, camera or focus
* Very lightweight solution, no balance, sagging or mechanical interference that can happen with big heavy wheels
* For information on full range of disk assemblies, carrying cases and pricing, click here
*** THE VIGNETTING PROBLEM ***
A full frame CCD chip measures 36mm x 24mm, and from corner to corner measures 43.3mm. Now, most 48mm/2" filter have a clear aperture in the range of 43mm or 44mm (actually our True Tech ones have a 45mm clear aperture, but we have phased these out - maybe we will re-introduce them in time). This means that really only very long focal ratios, ie more or less parallel light, will be unvignetted using standard filters. The second important source of vignetting is the constriction or restriction of the apertures in the light path, usually towards the telescope, and the faster the focal ratio, the worse this problem becomes. The following diagram compares the Standard SupaSlim wheel, which has SCT thread on the front plate, requiring connection to either a SCT thread or a 2" push-fit system, and the SupaSlim with a M54 faceplate, with associated adapters. You will see that there is a huge difference in the 100% illuminated circle on the chip, simply due to the change in the faceplate and adapter(s). There is a further gain by using 50mm unmounted glass filters, which have a clear aperture of 47mm:
The three focal ratios covered are those used by Takahashi on the new FSQ106-ED, and the Epsilon 180. You can immediately see the terrible problem using a 2" push-fit connection, and in practice it is probably worse than the graphic implies, as the 2" focus tubes on most instruments are quite long.
*** HARDWARE ***
The outer images show the gear side and the boss side of the FW-502 disk assembly. The inner left image shows the FW-502 disk, which is 5.5mm thick, compared to 3.0mm of other disks. For comparison a standard disk for screw-in filters is shown at inner right.
50mm glass is inserted on the non-gear side of the disk, which is the side facing the CCD camera. Of course you can put the glass whichever way round you prefer, either with the main coating facing the incoming light, or the uv/ir block coating facing the incoming light. Maybe there is a difference in the halo effect when the glass is reversed ? Filters are held in place by rubber 'O'-Rings (Five 'O'-rings are supplied as standard with each disk). Currently we are offering Astronomik 50mm glass, and the price of these is the same as for the standard screw-in 48mm/2" filters. However, any glass of 50mm diameter, up to 3mm thick, can be accommodated.
*** INITIAL TEST RESULTS ***
Equipment used
SupaSlim wheel, with Astronomik 50mm 'L' filter installed ( http://www.astronomik.com/english/enghome.html ), M54 front plate and adapter, SCT thread standard plate on rear
Artemis Art11002 monochrome CCD camera, 4008 x 2672 pixels, 9µm square
Takahashi FSQ106-ED with focal reducer, f/3.6, focal length 385mm, field of view 6.5° on 44mm diameter image circle
MaxIm DL/CCD, v 4.53 for image acquisition and image processing ( http://www.cyanogen.com )
For information, the centre picture shows also a Sky 90 with SBIG STV attached, for autoguiding, though no autoguiding was required in this test and a Stellacam-EX video camera used as a video finder, all on a Takahashi NJP Temma mount. The polar alignment, though not spot on, was reasonable enough to do 5 minute unguided exposures for the test.
Note from the right-hand image, the very close coupling between the Art11002 and SupaSlim. The chip is only 15mm back inside the camera body, and this helps to minimise vignetting. The situation with Starlight Xpress ( http://www.starlight-xpress.co.uk/SXVF-H35-36.htm ) is that the chip is 17mm back from the front face, so vignetting results will be comparable.
The above demonstrates the superb profile feature available in MaxIm DL
The horizontal profile on the veil image shows a background level of 7000 in the centre, but at the very edges left and right is at 5900.
This is 84% of the centre value, and though not quite at the predicted level. is nevertheless very acceptable, considering the huge field covered at f/3.6, from a very short effective focal length of 385mm (only just over 15 inches). The original 11 Megapixel image is huge, note the scale of 4000 pixels across. The image is RAW, having had dark frame subtraction only carried out, with no other manipulation.
The diagonal profile, covering a full 6.5° field, remains at a level of 7000 in the centre, and drops to 5000 at the very corners of the field. This is over 71% of maximum, so stars should still appear half as bright as those of similar magnitudes, which are more central.
While one cannot do away without flat fielding, there is something that flat-fielding cannot do, and that is replace lost image content, so if there is a serious roll-off in intensity, much faint detail, especially nebulosity, can be lost altogether. By still getting 70% of the level to the very corners, coupled with good image processing (bias, dark and flat-fielding), one should be able to produce stunning images over the whole field of view
*** GETTING THE CORRECT METAL BACK DISTANCES ***
WITH FOCAL REDUCER AT F/3.6 ON FSQ106-ED
The most demanding test for accurate focus position is with the faster focal ratios, so this area has been tackled first, on the basis that longer focal ratios will pose less problems, especially in the areas of:
1. Obtaining a flat focus field, so that all objects in the field, right to the corners, are in sharp focus. This requires that the CCD sensor is at exactly the correct distance from the focal reducer. ie the correct metal back distance.
2. Minimising vignetting (covered in the section above)
Note: for information on the FSQ106-ED, including photo-visual system charts, click here
The Takahashi specification requires a metal back distance of 72.2mm, and this is the distance measured from the back of the focal reducer to the film plane (ie to the CCD chip). In the above image, the chip of the Artemis Art11002 is 15mm back from the front face (the ruler is actually 1mm out of position and reads 16mm). The filter wheel is 21.5mm thick, then the adapters make up the distance as follows: M56 spacer, adjacent to the focal reducer, of 17.5mm length, CA35 coupled to this is an adapter to convert from M56 to the Wide T-thread (M54), length of 16mm. Adding those all together gives a spacing of 15+21.5+17.5+16 = 70mm. Now, if you look just above the ruler scale at 37-39mm you will see the 2mm flange for the M54 male-male coupling adapter (between the CA35 and filter wheel), plus two light grey-blue shims. This is special shim material of approximately 0.38mm thickness (say 0.4mm), so adding these items also, will give a final spacing of 72.8mm, a little more than that specified, but looking at the ruler scale and deducting 1mm for the error reported above it is clear that the actual distance is very close to 72mm. The important item to report is that we tried 6 different spacings, using 0,1,2,3,4 and 5 shims, to vary the distance by 2mm, and found that the best position was with two shims. We are able to supply sets of shims for customers to do similar tests, as there is bound to be some variation. Shown below are the results we got with the best position
The above image is a one fifth size image of M31 and surrounding area. A full size image would measure approximately 1200mm x 800mm !
For a half-size version (500K) click here
For a full size version (1M) click here
You will note that even on the half scale image the star images look nice and small, and on the above just look like pinpricks. However, there is not absolute perfection in the corners, so 100% cut-outs of these corner regions, and the centre region, are shown below for reference. Remember that to look at these properly you should imagine a picture 4ft x 2ft 8 ins (1200mm x 800mm), so at normal viewing distance on that scale even the corner stars would appear like small dots.
Top left Bottom left Centre Top right Bottom right
AT PRIME FOCUS ON FSQ106-ED
The metal back distance at prime focus is quite hard to envisage, but the image above shows the distance from the front of the filter wheel to the main body of the FSQ. The focus tube is actually racked out 20mm, and this forms part of the specified metal back distance of 178mm. The reading on the ruler shows only 141.5mm, but to that has to be added the thickness of the filter wheel (21.5mm) and the distance from the CCD camera face to the CCD sensor (15mm). This is the actual position obtained at best focus, and not set for display purposes - the RGB image below was taken with the focuser at exactly this position.
The above is a fifth size RGB image of M31 at prime focus, using the Art11002 full frame CCD camera.
For a 50% full size image click here, and for a 100% size image click here. Details R: 5x210s, G: 5 x 240s, B: 5x300s
None of these images has been flat fielded, showing how good the illumination of the full frame chip was using the 50mm filters, even at f/5.
*** ADAPTERS ***
M54M-M M54-SCT 2PFM-M54M M54C
All the above are new adapters for use with the M54 plates:
M54M-M This is a male-male adapter, used to couple the wheel to (usually) Takahashi camera adapters (code CA35)
M54-SCT. This is a useful 'drop-down' adapter, which makes the wheel look like the standard SCT thread, so that any existing adapters with SCT thread can be used
2PFM-M54M. This is more or less identical to the 2PFM-SCTM adapter, but without having to use the above drop-down adapter, has a total flange thickness of 2mm compared to 4mm. This adapter is also threaded to take a standard M48/2" filter infront
M54C This adapter is used to couple Artemis, Starlight Xpress or other CCD cameras have a M54 thread connection. A very useful feature of the M54 adapter is that it is internally threaded to take a permanently mounted M48/2" filter, eg IDAS, CLS etc.
For full range of adapters see normal SupaSlim page for details
*** STL ADAPTERS/PLATES***
The left hand image shows a sideplate with the STL thread (2.156" @ 24 tpi), and this sideplate can be optionally fitted on either side of the SupaSlim wheel as required (or can be mixed and matched with the standard SCT threaded plate or the M54 plate). The Televue STL adapter, loaned courtesy of John Walton, has a male thread length of approx 1/4", so it is necessary to use a spacer (clear aluminium) to prevent the thread going inside the wheel and hitting the rotating disk or filters, when screwed in. The centre pic shows the adapter and spacer mounted on the wheel, and the right image shows the clear view through a SupaSlim with the TV STL adapter on the front, and a FW-50 disk installed
*** ORDERING INFORMATION ***
For prices on the SupaSlim filter wheel and adapters, plus prices on filters, CCD cameras, etc, go to our price-list page. (All price lists are in pdf format)
For further information or to order, e-mail : sales@trutek-uk.com
You can contact us by phone on [44]-(0)1189-700-777
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