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