What is the clear aperture of the EFW2?

astrovienna

The mechanical drawing of the 7 x 2" version of the EFW shows that the filter openings are M48x0.75 threaded. What is the clear aperture of those openings? For my 12" f/4 system, the opening needs to be 48.08mm to avoid vignetting the ASI6200 sensor, per this calculator:

http://www.astrofriend.eu/astronomy/astronomy-calculations/minimize-vignetting/minimize-vignetting.html

Some web research suggests M48 openings can be as small as 43mm. But in any case, it doesn't seem like 48.08mm is possible.

w7ay

astrovienna The mechanical drawing of the 7 x 2" version of the EFW shows that the filter openings are M48x0.75 threaded. What is the clear aperture of those openings?

An M48 opening has a thread with about 48mm in diameter.

However, you then need to subtract the thickness of the spacer from the radius, which depends on the thickness of the wall of the spacer that you use, but typically 2.5mm to 3mm.

That means the hole will typically be between 42mm and 43mm in diameter.

The location that you place the filter depends on the exit pupil of the final lens (or reducer) of your OTA, and that will affect the size of the hole. With an exit pupil that is smaller than the diagonal of the sensor for example, you want the filter to be closer to the exit pupil than to the sensor.

I usually just draw out a diagram for geometric optics, and give it an extra half a millimeter to account for diffraction optics.

So yes, to avoid aperture vignetting with a full frame camera (42mm diagonal), an M48 hole is pretty much "unmöglich," unless the exit pupil of your OTA is much smaller than 42mm, and you place the filter wheel near the exit pupil of the OTA). "Conventional wisdom" is to place the filter as close as possible to the sensor, but that only works if you don't have a reducer (or a Petzval arrangement). This is where following old wives' tales blindly without thinking will lead to problems.

You want the optical path to fully clear the tubes. Otherwise you will not only get just an intensity fall-off (correctable with flat frames and/or cropping), but other ill effects of aperture vignetting from diffraction effects (that are not correctable with flat frames and/or cropping). Here is one of them:

https://www.cloudynights.com/topic/547028-anyone-know-what-causes-this-artifact-on-bright-stars/

You might be able to find really thin-walled spacers, but then, you could have flexure. Just shop for a different filter wheel. The ZWO wheels with M48 openings are just not suitable for full frame cameras. Find one with at least an M54 opening.

On top of that, the metal rings from mounted 2" filters (M48 thread) can be limiting the aperture too. This is why IDAS makes a set of 52mm filters in addition to 48mm (so called "2inch" filters).

Chen

astrovienna

Thanks for the reply, Chen. My question was about the openings in the filter carousel itself. The filters I'm using are 50mm unmounted. (I'm not sure what you mean by the "thickness of the spacer.") I was hoping ZWO could give the actual figure for the opening in the carousel.

w7ay

astrovienna I'm not sure what you mean by the "thickness of the spacer."

The 48mm spacer tube that connects the filter wheel to other parts of the telescope.

Chen

astrovienna

Oh, I get it. Fortunately I won't be using any spacers at all. The front opening of the wheel is M54, which should be fine. And the OAG will be M68. So I think the only potential "bottleneck" in the imaging train is the M48 opening in the filter carousel.

w7ay

astrovienna

By the way, the filter wheel should come with these filter masks:

https://astronomy-imaging-camera.com/product/zwo-filter-masks

The drawing for the 2" mask shows the clear diameter to be 48.0 mm.

You should be able to sketch the rays using geometric optics (based on the ray diagrams from your OTA manufacturer) to see if all light rays will clear. (A f/1 optics will be more critical than an f/15 optics.)

By the way, even if you have a filter position with no glass, you may need to install a good filter mask with a clean hole too, or the M48 filter thread itself will cause weird diffraction effects (random spikes emanating from bright stars)if the threaded hole is the one causing the vignetting.

I have no idea how precise the center of the filters in the ZWO carousel are. If they are not centered perfectly, and if the carosel aperture is creating aperture vignetting, the the flat frames for each filter will be slightly different.

So, it is best to make sure the filter and filter wheel do not block any of the rays; then you won't have to worry.

Chen