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Filter drawers for ZWO ASI and RASA 8

Differences between M87 and M42 filter drawers

A seemingly trivial thing such as the size of a mounting thread has a number of implications for both design and use. In this article I will discuss the differences between the M42 and M87 versions. However, before I go on to discuss the distinguishing features, I will briefly mention what both versions have in common.

First of all, both versions can be used with the same camera models and mounted on the RASA 8 telescope. Also, both drawers have the same adjustability. There is no difference. Both drawers accept 2″ filters and are made with the same technology. Here the similarities basically end, so I will move on to discuss the differences.

The drawers with the built-in M87 nut were designed by me as dedicated drawers for RASA 8 telescopes. They are designed in such a way that they should eliminate most of the backfocus and tilt problems. Their distinguishing feature is that between the telescope and the camera ONLY ONE component can bring uncertainty to the alignment: the force of tightening one huge nut. Everything else is rigid: there are no extra connections and the camera sits perfectly in the optical axis of the telescope. In our opinion, all drawers for RASA should have this feature: with their dimensions they should be almost welded to the telescope and have as few moving connections as possible.

Such a construction, in spite of its great advantages, has one possible disadvantage: if you want to use the camera with another telescope, you have to unscrew this drawer from the camera, screw in the original tilt-plate, maybe also adjust it and only then screw in another drawer that will enable you to work with another telescope. Some people may find it inconvenient. However, this is the cost of dedicated design – less flexibility for more targeted functionality.

Demonstration of double filter system

The M87 drawers also have the unique feature of being able to mount TWO independent filters at once.

One, interchangeable, you can use in the slider. The other one you can mount on the front, for when the camera remains mounted on the telescope.

First, using broadband filters (like R, G, B), you can attach a UV, IR or other filter to the front of the camera to extend the backfocus. By attaching a second filter, you can change the effective optical path length without any mechanical adjustment. This works just like when you leave the original glass window screwed into the telescope, but it gives you much more possibilities. Unlike the only glass window available, you can use different filters, with different properties and different thicknesses. This gives a lot of room for experimentation and dramatically changes the flexibility of the whole system.

Secondly, when photographing with R, G, B filters (or similar broadband) you can try to place a light pollution filter in front, which will selectively cut out the harmful mercury, sodium and argon lines. Of course, success will depend on your choice of filters and to what mutual effect optical length they add up, but the bottom line is that with this drawer it is possible. It is probably the only solution in the world for RASA 8 that allows you to mount two filters at once, including one in a drawer and in addition in a system that works with cameras with a backfocus of 17.5 mm!

Despite all these possibilities you may find that you can use any filter “just like that”, without any combination. The resulting aesthetic effect may be completely satisfactory to you, whether you use narrow band filters or expose your photos without any filter at all. To learn more about what kind of results you can expect, click here and read the article on this very topic.

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