Moravian Instruments filter wheels are always computer controlled like cameras themselves, so they can be integrated into complete imaging setups.

There are three classes of Moravian Instruments filter wheels:

• Internal filter wheels (IFW)

• External filter wheels (EFW)

• Standalone filter wheels (SFW)

There are fundamental differences among these three filter wheel types:

• Internal and external filter wheels are always controlled and powered by the camera, there is no power or USB plug. Also, these filter wheels do not need own drivers; commands, controlling their operation, are passed through the camera driver.

• Standalone filter wheels on the other side contain own electronics and thus require standalone power line and communication line to the host PC. They also need own drivers, beginning with USB system driver, user space application driver etc.

Cameras with Internal Filter Wheels

The C2 and C3 camera lines, combining asymmetrical camera head design and relatively small sensors, can incorporate filter wheels directly into the front camera shell, which is then manufactured slightly thicker.

Internal filter wheels bring several advantages:

• Camera head is only slightly thicker and heavier, compared to camera without filter wheel, head width and height remain unchanged.

• Camera with IFW make up a single device without obvious mechanical or electrical connections.

• There is more space for filters inside the camera shell compared to External filter wheel shell — up to 10 mm space is available, compared to just 6 mm in the EFW.

But integrating the filter wheel into camera also brings disadvantages:

• IFW dimension and thus also number of filter positions are limited by the camera head front cross-section.

• IFW cannot be integrated into cameras incorporating large sensors (C4 and C5 lines), despite the asymmetrical design of the camera head.

• Changing filters require opening the camera head, exposing mechanical shutter and camera electronics, which require great care not to damage these components. Also, closing the camera is more demanding as it is necessary to properly arrange internal cabling etc.

So, in general, IFW is suitable for applications using relatively small number of filters, which are exchanged rather exceptionally. For instance, photometric camera operating with Johnson-Cousin UBVRI filters only is a good example.

Cameras with External filter wheels

All Moravian Instruments cameras are equipped with electronics, capable to drive the filter wheel. Also, the camera firmware and command set include instructions to control the wheel.

Cameras without IFW are manufactured with thinner front shell to save weight and to shrink the camera back focal distance (BFD), which means the internal wheel cannot be added later. Filter wheel controlling electronics remains unused in such case.

So, cameras without IFW are equipped with an external connector containing all signals necessary to control a filter wheel. If an External filter wheel is mechanically attached to the camera head, it is possible to connect it to the camera head using single 8-pin wire to provide power and control signals to the wheel.

From the software point of view, there is no difference between IFW and EFW. Software only sees the filter wheel controlled by the camera, regardless if the wheel is inside camera shell or in a separate shell, bolted to the camera head.

External filter wheels bring many advantages:

• EFWs offer wide range of filter sizes and filter slot positions.

• As EFWs can incorporate very large filters (up to 65 × 65 mm), they are offered also for cameras with large and very large sensors (C4 and C5 series).

• EFWs are compatible with both asymmetrical cameras (C2, C3, C4, and C5A) and corresponding symmetrical models (C1+, C1×, C5A).

• Opening the EFW shell and exchanging filters is easier, as there are no mechanical of electronics devices inside the shell but the wheel motor drive.

Using of an External filter wheel also brings some negatives:

• Camera with EFW is bulkier and heavier compared to the IFW variant.

• While the IFW is accessible from the back side when the camera front shell is removed and filter wheel itself need not to be removed from the shell, EFWs are opened from the front side and filter wheel must be removed from the shell to be able to install or replace filters.

• Only 6 mm space is available for filters inside the EFW shell, compared to 10 mm space in the IFW.

Cameras with Standalone filter wheels

The external filter wheel diameter has construction limits, it is not possible to simply increase both EFW shell and filter wheel itself. The diameter and weight would be so big, that usage of such wheels would not be practical.

Natural solution of this problem is placing of two smaller filter wheels beside each other. The total number of filter positions is lower than the sum of positions in both wheels from obvious reason: one position in each wheel must remain empty (clear) and it is set as an actual position when a filter from the other wheel is to be used not to interfere with it. So, if for instance two filter wheels with five positions each are used, we denote such wheel as “8+Clear” positions. This means four positions in both wheels may be used for any filter, which makes 8 total filter positions. When both filter wheels are set to index 0 (the clear position), one additional clear position is added to the total number of filter positions.

The filter wheel controlling electronics, incorporated into Moravian Instruments cameras, cannot control two independent filter wheels simultaneously. So, the new line of Moravian Instruments filter wheels is equipped with own control electronics and naturally require own control and power lines, fully independent on the camera. This is why the new line is called Standalone filter wheels (SFW).

Using of SFWs brings also changes to the driver structure. While IFWs and EFWs are controlled through a camera, SFWs need own driver stack. This means a Windows or Linux system USB driver must be installed, as well as a new user space libraries (e.g. ASCOM in Windows, INDI or driver libraries in Linux) etc.

Also, client software may handle the SFW differently. For instance, SIPS differentiates between filter wheel drivers handled by camera and fully independent filter wheel drivers:

Fixing filters into filter wheel

There are two standards of filter mounting mechanism used in astronomical instruments:

• Filters mounted in threaded cells are intended to be screwed into a threaded socket in the filter wheel.

• Unmounted filters (either glass-only, or with metal rings around the glass) have to be fixed into the wheel socket using screws.

Filter in threaded cells

Only two diameters of filter cells standardized over years, in fact copying the standard diameter of telescope focusers and eyepiece mounting barrels (many new filter diameters and formats, introduced with ever increasing size of astronomical camera sensors, remained unmounted):

• 1.25” diameter cells.

• 2” diameter cells.

Filters in 1.25-inch diameter cells

While the outer diameter of these cells is well defined (1.25” or 31.75 mm), attempts to standardize the thread, used on the 1.25” cells, failed:

• The thread diameter converged to 28.5 mm and all filter manufacturers are at or very close to this diameter.

• Thread pitch on the other side quite varies among various companies:

  • Some European companies standardized on 0.5 mm pitch.

  • Some American companies use 40 TPI, which leads to 0.635 mm pitch.

  • Some companies manufacture the cell thread with pitch somewhere between these two extremes.

Luckily, the thread length is typically very small (just 2 mm on Moravian filter wheels) and thus the thread pitch differences play only small role.

Moravian Instruments filter wheels are designed to be as compatible with various standards as possible, using M28.5 × 0.55 thread. Major filter brands, both European and American, can be used with them.

Filters in 2-inch diameter cells

Luckily, the M48 × 0.75 thread, used on 2” cells, is the same regardless of the filter manufacturer.

Unmounted filters

Unmounted filters must be fixed in filter wheel socket using screws. Filters are often just a piece of glass, which could be easily damaged if it is tightened only by metallic screw head. Moravian Instruments filter wheels are always accompanied with a set of plastic washers to be used between the filter itself and the fixing screw.

Moravian Instruments filter wheels use socket allowing to fix filters only 1 mm thick. But on the other side, much thicker filters must also remain compatible with the filter wheel. This is why different types of screws and washer orientation is recommended according to the thickness of the used filters:

• M3 × 4 screw with cylinder head and a plastic washer oriented “upside down” is recommended for 1 mm thick filters.

  

• M3 × 4 screw with cylinder head and a normally oriented plastic washer is recommended for 2 mm thick filters.

  

• M3 × 8 screw with countersunk head and a normally oriented plastic washer is recommended for 3 mm and thicker filters.

  

Internal Filter Wheels

Internal filter wheels are available for C2 and C3 cameras only, as the C4 and C5 camera require 50 × 50 mm or even 65 × 65 mm square filters, which are too big to fit into relatively small internal filter wheel.

C2 camera model with Internal filter wheel accepts two sizes of filters:

• 5 positions for unmounted D31 mm filters or filters in 1.25” threaded cells.

• 6 positions for unmounted D26 mm (or 1”) filters.

C3 camera model with Internal filter wheel accepts two sizes of filters:

• 5 positions for unmounted D50 mm (or 2”) filters or filters in 2” threaded cells.

• 7 positions for unmounted D36 mm filters .

Filter space

The space available for filters is the same for both C2 and C3 cameras, but it differs depending on the filter wheel type. The 3 mm thickness of the filter wheels designed for both unmounted filter and filter in threaded cells is divided to 2 mm of thread and 1 mm of a deepening for unmounted filter. The same 3 mm thickness of the filter wheels intended only for unmounted filters offer 2 mm deepening. This means the unmounted filters can be up to 1 mm thicker in this filter wheel compared to filter wheel also compatible with threaded cells.

• The barrel portion of the filter threaded cells, used in the Internal wheel, can be up to 8 mm thick. The thread can be up to 3 mm long. So, the total thickness of the cell can be up to 11 mm.

  

• Unmounted filters, but fixed in the wheel designed also for threaded filter cells, can occupy up to 8 mm space. Note such space includes not only the filter itself, but also the fixing screw head and washer. As the head and washer thickness typically slightly exceed 2 mm, it is safe to use up to 5 mm thick unmounted filters.

  

• Filter wheels designed for unmounted filters only (either D26 mm/1” or D36 mm filters) can accommodate up to 1 mm thicker filters.

  

Installing or exchanging filters

It is necessary to open the camera head to change filters or the whole filter wheel.

Opening the C2 camera head

To open the head, unscrew the six bolts holding camera head together.

The blade shutter rotates 180° between individual snapshots. Camera cover could be opened only when the shutter is fully closed (covers the sensor). If for instance the camera is unplugged from power adapter while exposing, the shutter remains open. Camera cannot be opened in such case.

After removing the screws carefully turn the camera head by the telescope adapter upward. Gently pull the front part of the case. Notice there are two cables, connecting the filter wheel motor and the filter position optical bar, plugged into the electronics board. It is not necessary to unplug these cables to change filters, but if you unplug them, take care to connect them again in the proper orientation!

Opening the C3 camera head

To open the head, unscrew the eight bolts holding camera head together.

After removing the screws carefully turn the camera head by the telescope adapter upward. Gently pull the front part of the case. Notice there are two cables, connecting the filter wheel motor and the filter position optical bar, plugged into the electronics board. It is not necessary to unplug these cables to change filters, but if you unplug them, take care to connect them again in the proper orientation!

Changing the Whole Filter Wheel

The whole filter wheel can be changed at once. It is necessary to remove the front part of the camera case the same way as in the case of changing filters. The filter wheel can be removed when you unscrew the bolt on the center of the front part of camera case. Take care not to damage the horseshoe-shaped optical bar when replacing the filter wheel.

External Filter Wheels

The External filter wheels are designed for the broad range of Moravian instruments cameras, from the C1+ up to the large sensor C5 series.

The External filter wheels are manufactured in several sizes, from the smallest “XS” (Extra Small) up to “XL” (Extra Large). But regardless of the filter wheel size, all filter wheels are designed to preserve the same back focal distance.

The most obvious difference among the EFW series is of course the diameter and thus number of filter positions. But individual series also differ in telescope adapter tiltable base dimension.

EFW with “S” (small) adapter base

The “S”-sized adapter base is intended for adapters limited to 2” (50.7 mm) diameter, which include M42 × 0.75 (T-thread), and M48 × 0.75 threads, but also Canon and Nikon lens bayonets.

The External filter wheels with the “S” adapter base are attached to the camera head using four metric M3 screws in the corners of the 44 mm square pattern. Such mounting interface is used on C2 and C3 camera heads, as well as on the C1+ and C1× cameras.

The “XS” filter wheels

The “XS” filter wheels are compatible with C1+, C1×, and C2 cameras:

• 8 positions for unmounted filters D31 mm or filters in 1.25” threaded cells.

• 7 positions for unmounted D36 mm filters.

The “S” filter wheels

The “S” filter wheels are compatible with C1+, C1×, C2, and C3 cameras:

• 12 positions for unmounted filters D31 mm or filters in 1.25” threaded cells.

• 10 positions for unmounted D36 mm filters.

• 7 positions for unmounted D50 mm or filters in 2” threaded cells..

• 5 positions for unmounted 50 × 50 mm filters.

EFW with “L” (large) adapter base

The “L”-sized adapter base is intended for greater adapters based on M68 × 1 thread, offering more robust connection between the camera and telescope. The Canon and Nikon lens bayonets are available also with the “L”-sized adapter base.

The “M” filter wheels

The “M” filter wheels correspond to the “S”-sized ones, only offer the “L” adapter base instead of the “S” adapter base. So, these filter wheels are compatible with the same camera series C1+, C1×, C2, and C3 and accommodate the same filters:

• 12 positions for unmounted filters D31 mm or filters in 1.25” threaded cells.

• 10 positions for unmounted D36 mm filters.

• 7 positions for unmounted D50 mm or filters in 2” threaded cells.

• 5 positions for unmounted 50 × 50 mm filters.

The “L” filter wheels

The “L” filter wheels are always equipped with the “L” adapter base. They accommodate these filters:

• 9 positions for unmounted D50 mm or filters in 2” threaded cells..

• 7 positions for unmounted 50 × 50 mm filters.

Both “M” and “L” sized EFW are made compatible with C2/C3 camera series as well as the C4 series, designed for greater sensors. However, while the C4 cameras also use four M3 threaded holes to attach EFW to camera head, these holes are 52 mm apart, compared to 44 mm of the C2/C3 series. Breaking the mechanical compatibility was necessary because of the size of sensors, used in C4 line.

So, both “M” and “L” wheels are offered in -3 version, compatible with C2 and C3 cameras, and -4 version, intended for C4 cameras.

EFW with “XL” (extra-large) adapter base

The “XL”-sized adapter base is designed for the M85 × 1 thread, used solely on C5 series cameras. Also, the Canon and Nikon lens bayonets are available with “XL”-sized adapter base, but photographic lenses always cause vignetting on very large C5 sensors.

The “XL” filter wheels

The “XL” filter wheels roughly equal to the “L”-sized ones, only offer the “XL” adapter base:

• 7 positions for unmounted 50 × 50 mm filters.

• 5 positions for unmounted 65 × 65 mm filters.

The “XL” filter wheels use a pattern of six M3 threaded holes on the camera head, spaced by 60° around the 70 mm diameter circle:

The pattern is the same on both asymmetrical C5A and symmetrical C5S camera versions.

Filter space

The space available for filters is the same, regardless of the EFW size. However, comparing to internal filter wheels, external filter wheels are thinner and thus also the space for filters is narrower.

The difference between filter wheels designed for both unmounted filters and filters in threaded cells, and filter wheels intended only for unmounted filters, also apply. This means the filters can be up to 1 mm thicker if the filter wheel is designed for unmounted filters only.

• The barrel portion of the filter threaded cells, used in the external wheel, can be up to 5.5 mm thick. The thread can be up to 3 mm long. So, the total thickness of the cell can be up to 8.5 mm.

  

• Unmounted filters, but fixed in the wheel designed also for threaded filter cells, can occupy up to 5.5 mm space. Note such space includes not only the filter itself, but also the fixing screw head and washer. As the head and washer thickness typically slightly exceed 2 mm, it is safe to use up to 3.5 mm thick unmounted filters .

  

  Remark:

  Standard photometric Johnson-Cousin filters typically can be used in the EFW, despite they are defined as 4 mm thick glass. The space between the screw head and EFW shell is stated very conservatively and also, the fixing screw head and washer are typically thinner than 2 mm.

• Filter wheels designed for unmounted filters only (either D26 mm/1” or D36 mm filters) can accommodate up to 1 mm thicker filters.

  

Installing or exchanging filters

Installing or exchanging filters requires opening of the EFW shell. As there are no mechanical of electronics devices inside the shell but the wheel motor drive, opening the EFW shell and exchanging filters is easier compared to IFW.

Moravian Instruments filter wheels are always designed so the aperture manufactured in the wheel, not the inserted filter, faces the incoming light cone. So, the complex shapes like unmounted filter edges, holding screws, etc., possibly causing reflections, are always hidden from the incoming light.

The geometry of the EFW requires to remove the filter wheel from the shell to insert or replace filters. Otherwise, when the filter wheel is removed, fixing of individual filter types in the wheel remains the same like in the case of IFW.

Standalone Filter Wheels

While the external filter wheels can accommodate more filters compared to the internal ones, even the external filter wheel diameter has construction limits. It is not possible to simply increase both EFW shell and filter wheel itself. The diameter and weight would be so big, that usage of such wheels would not be practical.

Natural solution of this problem is placing of two smaller filter wheels beside each other. The total number of filter positions is lower than the sum of positions in both wheels from obvious reasons: one position in each wheel must remain empty (clear) and it is set as an actual position when a filter from the other wheel is to be used not to interfere with it. So, if for instance two filter wheels with five positions each are used, we denote such wheel as 8+Clear positions. This means four positions in both wheels may be used for any filter, which makes 8 total filter positions. When both filter wheels are set to index 1 (the clear position), one additional clear position is added to the total number of filter positions.

The Moravian Instruments Standalone Filter Wheel (SFW) comes in the “XXL” flavor, which indicates it is even greater than the larges “XL” sized EFW available. Number of available positions depends on filter size:

• 8+Clear positions for “65 × 65 mm” square filters, suitable for C5 cameras (both asymmetrical C5A and symmetrical C5S variants)

• 12+Clear positions for 50 × 50 mm square filters, suitable for C5-100M cameras (with some vignetting over the sensor short edge), but also for C4 and C3 cameras, also using 50 × 50 mm filters

• 16+Clear positions for unmounted D50 mm or filters in 2” threaded cells for C3 cameras.

While the SFW filter wheels are always equipped with the “XL” sized telescope interface, offering M85 × 1 or M68 × 1 threaded adapter, as well as the Canon EOS and Nikon lens adapters, the SFW back plate is manufactured in three versions:

• The -5 version with six holes for M3 screws for C5 cameras.

• The -4 version with four holes for M3 screws 52 mm apart for C4 cameras.

• The -3 version with four holes for M3 screws 44 mm apart for C3 cameras.

Power and control interface

As opposed to IFW and EFW, the SFW filter wheels need separated power and control lines. The FSW control unit is equipped with 12 V DC 5.5/2.5 mm power plug as well as USB 1.1 data connection.

The SFW control can be also handled by the Moravian Camera Ethernet Adapter together with a main imaging, guiding, and context cameras. Only make sure the Moravian Camera Ethernet Adapter device is updated with the latest firmware available.

Back focal distance

While all EFWs from “XS” to “XL” size are designed to preserve the very same BFD on all Cx cameras, the SFW naturally increased the BFD because of the design using two overlapping wheels. The resulting BFD depends on the adapter used.

Filter space

The space for filters in the SFW corresponds to the space available in EFW wheels.

• The barrel portion of the filter threaded cells, used in the standalone wheel, can be up to 5 mm thick. This is 0.5 mm less compared to EFW; the reason is up to 1 mm space of the threaded part of filters in the second wheel. The thread can be up to 3 mm long. So, the total thickness of the cell can be up to 8 mm.

  

• Unmounted filters, but fixed in the wheel designed also for threaded filter cells, can occupy up to 5.5 mm space. Note such space includes not only the filter itself, but also the fixing screw head and washer. As the head and washer thickness typically slightly exceed 2 mm, it is safe to use up to 3.5 mm thick unmounted filters.

  

  Remark:

  Standard photometric Johnson-Cousin filters typically can be used in the EFW, despite they are defined as 4 mm thick glass. The space between the screw head and EFW shell is stated very conservatively and also, the fixing screw head and washer are typically thinner than 2 mm.

• Filter wheels designed for unmounted filters only (either square 50 × 50 or 65 × 65 mm filters) can accommodate up to 1 mm thicker filters.

  

Filter positions

Distinguishing to which position should be each wheel rotated is handled by the filter wheel driver and from the user's perspective such wheel acts as one wheel with 9 positions, filters are indexed from 1 to 9.

It is necessary to open the SFW front shell prior to installing filters. There are two filter wheels inside, one is on top (closer to the telescope), another is on the bottom of the shell.

8+Clear filter wheel

12+Clear filter wheel

16+Clear filter wheel