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Filter drawers for ZWO ASI and Celestron RASA 8 - dedicated for 17.5 mm backfocus cameras

Filter drawers for ZWO cameras with tilt plate

If you have a ZWO ASI camera with a 17.5 mm backfocus, you probably have quite a problem to attach this camera to a Celestron RASA 8 telescope in such a way that you can determine the correct distance of the sensor to the front of the telescope. According to the manufacturer, this should be 28.73 mm. You can use various adapters for this, but the real trouble comes when you want to use the filter drawer. To solve this problem, I designed a brand new drawer, deduced just for such an astrophotography kit.

Our drawers work with the cameras:

  • ZWO ASI 2400 MC Pro,
  • ZWO ASI 2600 MM/MC Pro,
  • ZWO ASI 6200 MM/MC Pro,
  • ZWO ASI 071 MC Pro,

and, of course, with the Celestron RASA 8 telescope. Indeed, these drawers were specifically designed with RASA 8 in mind. You can also use my drawers with other telescopes, which is explained in detail later.

Filter drawer kit

Filter drawers for ZWO ASI cameras

Note: 

If you would like to connect other ZWO cameras to your telescope, you can select the appropriate adapter for the filter drawers.
Then you will also be able to connect :

  • ZWO ASI 1600 MM Pro
  • ZWO ASI 294 MC Pro
  • ZWO ASI 183 MM/MC Pro
  • ZWO ASI 533 MM/MC Pro

Drawer types and sizes

As already stated, the drawers are designed for use with ZWO ASI cameras having 17.5 mm backfocus. These are characteristic cameras, equipped with a black tilt plate on the front. ZWO manufactures these cameras in two sizes: 90 mm housing diameter and 86 mm housing diameter.

To be able to mount drawers on both types of cameras, it became necessary to make them in two diameterswider and narrower.

Each of the drawers (wider and narrower) is also available in two variants:

  • with M42 mounting thread allowing to universally mount it to existing adpaters,
  • with its own built-in nut M87, allowing to screw the drawer directly onto the telescope.

Both the M87 and M42 versions have their own special, distinctive features that are described in this article. You should familiarize themselves with this information so that you can make the most appropriate choice for yourself.

Here are my drawer designations:

  • FD 900 M42 – filter drawer having M42 external thread, for 90 mm cameras.
  • FD 900 M87 – filter drawer having M87 external thread, for 90 mm cameras.
  • FD 860 M42 – filter drawer having M42 external thread, for 86 mm cameras.
  • FD 860 M87 – filter drawer having M87 external thread, for 86 mm cameras.

90 mm cameras are: ASI 2400 MC Pro, ASI 2600 MM/MC Pro and ASI 6200 MM/MC Pro,

86 mm cameras are: ASI 071 MC Pro.

Drawers design

In the case of the RASA 8, the cameras 17.5mm backfocus gives very little room to mount any adapter, let alone a filter drawer. To deal with this limitation, I took advantage of the fact that the original tilt plate can be removed from the camera, gaining an additional 5mm of space. In exchange the drawer is equipped with its own tilt tuning system. In practice, it’s very simple: to mount the filter drawer, you will first need to unscrew the original black tilt plate from your camera. It is attached to the housing with three screws. It’s pretty straighforward, and exact instructions with step-by-step photos are included in the drawer manual. Then just screw on the drawer, which has its own tilt adjustment mechanism.

The drawer is also equipped with special factory-calibrated pairs of screws that ensure virtually perfect parallelism of the front and back drawer planes. Thanks to them, any possible dimensional inaccuracy is eliminated to the nearest hundredth of a millimetre. In addition to the factory calibration, the user has the opportunity to carry it out again using special screws included in the set.

Filter sliders are held in the drawer by special side magnets and removing the slider is facilitated by a special magnetic key included in the set. The whole set is complemented by blind filter that allows you to make dark frames without removing the camera and a special side blinder, which cuts off any false light coming from the side.

The drawer provides a backfocus of 28.73mm, which is exactly what Celestron recommends for the RASA 8.

Note, that this is the default recommended distance for H-alpha, S-II and O-III narrowband filters or possibly complex narrowband filters such as NBZ, Triad Ultra Quad-Band or similar. For broadband filters such as R, G, B or a situation where you are not using any filter at all, the effective telescope backfocus shortens. Read this article to learn more about this issue. For my drawers, you can adjust the backfocus using primarily the filter combination method, which you can read about here

The main suggested material of the drawer and its components is carbon fiber reinforced PET-G. The drawer is prepared to be made mostly by 3D FDM printing and should be hand assembled.

For a closer look at drawer construction, see photos below.

The information shown in the photos needs more discussion. Following are some explanations to help you better understand the idea of the project.

Mounting screws

The devices are equipped with five M2.5×8 screws attaching the drawer to the camera. On the sides of the mounting screw there are two calibration screws pre-set at the factory to ensure an ideal distance between the drawer and the camera. These calibration screws ensure that the front and rear drawer planes are virtually perfectly parallel and the distance is exactly 16.23 mm. In the photos, the mounting screws along with the calibration screws on the sides are marked YELLOW
 
The camera to which the drawer is mounted, after removing the original tilt plate has a backfocus of 12.5 mm. Consequently, when the drawer is mounted on the camera, the total backfocus of the optics is 28.73 mm, which is what Celestron recommends in the RASA 8 telescope documentation.
 
By default, the drawer does not require any calibration from the user and all adjustments are done at the factory. However, if you would like to change the drawer adjustment yourself, you can do it easily by using the special screws attached, marked on the first two figures in the top row with the letter F. Step by step instructions on how to do this can be found in the user manual of the device. To adjust these screws, use a 2 mm hex key (on FD 900 series drawers for cameras with 90 mm housing diameter) or a 1.5 mm hex key (on FD 860 series drawers for cameras with 86 mm housing diameter).
 
For legibility, the yellow color on the picture shows only one of the triplets of screws, out of five located around the perimeter of each drawer. Yellow arrows indicate holes for screw adjustments.

Tilt adjustment

The drawer is equipped with its own tilt adjustment screws. In the pictures the screws and access to them are marked in RED.

Because the original tilt plate must be removed from the camera to mount the drawer, the drawer has its own tilt adjustment screws. These can be adjusted from the front, as shown in figure 4.
 
Importantly, in my experience, the tilt adjustment is often not actually needed at all. The problems that users attribute to tilt often result from other factors, such as dimensional inaccuracy of the adapters. Just a tenth of a millimeter of dimensional deviation is enough to cause problems in the picture. The very precise factory adjustment with the screws marked in yellow (description above) ensures perfectly precise dimensionality, so the tilt adjustment is usually not needed. Nevertheless, the drawer is equipped with an appropriate adjustment – just in case. It’s worth your time to read the separate article where the tilt and backfocus issues are discussed, as well as the study on results and experiments

Threads

Drawers come in two versions, differing in how they are mounted to the telescope.
 
The first type is equipped with an integrated nut with M87  thread for a perfectly centered camera attachment on the RASA 8 telescope. With this version you do not use Celestron’s factory adapters, but screw the drawer directly onto the telescope. The drawer is equipped with a rotation mechanism, so you can position the camera at any angle. To use this version of the drawer, you must remove the glass window from the telescope. According to Celestron, if you want to use filters, you should do this anyway.
 

The second type is equipped with an internal M42 thread that allows it to be screwed onto the original Celestron adrapter.

On the one hand, this design is more universal, as it allows the drawer to be used not only for the RASA 8 telescope, but potentially for any optical system with an external M42 thread. On the other hand, however, the design dedicated exclusively to the RASA 8 with the M87 thread has some unique features such as perfect centering, the possibility of using two filters or a large clamping surface.

I have discussed the differences between these M42 and M87 versions in more detail. It is worth reading it in order to be able to choose the best solution for your individual needs. 

On the drawings, the mounting threads are marked in BROWN.

You can see how the camera drawers look mounted on the telescope in the pictures below.

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