Distance from 2600MM sensor to Filter in EFW when Bolted

jc-astrophotos

I am interested in this since I cannot find any documentation about how the distance from the sensor to the filter changes once the camera is bolted directly to the 36x7 EFW. I know the sensor sits at 12.5mm inside the camera body.

Can someone chime in with a measurement?

w7ay

jc-astrophotos Can someone chime in with a measurement?

Here goes.

Assuming that you orient the 7x36 wheel the "normal way" (the stepper motor bulge sticks out towards the camera), this is what I measured from one of mine. All numbers in mm. Picture not to scale, for illustration of the numbers only.

Since we know the thickness of the EFW, you can deduce the dimensions of the glass from the other surface of the EFW (assuming that you know the thickness of the glass), in case you wish to reverse the orientation of the EFW to better minimize vignetting, etc.

In the "normal" mount configuration, from above, the surface of the glass that is closest to the EFW flat flange that is closest to the camera is therefore approvimately 10.5 mm.

The 7.19 mm value was done through a couple of indirect measuments on the position of the carousel bearings (that thing with a central hole, with three flat head screws that hold the carousel to it.

Notice the above takes into account the recess of the glass into the carousel.

The corousel plate thickness is probably nominally 4 mm thick, even though I'd measured 4.07. mm So you may want to factor in the inaccuracies.

Good luck.
Chen

enaiman

You don't need the actual distance to the filter itself.
The tilt plate on the camera is 5 mm thick and you have to take down the plate in order to be able to bolt the EFW on. For backfocus calculation you only need to factor in the 12.5 mm.
Backfocus will be 12.5 mm (camera) + 20 mm (EFW) + any other adapters you might have. Also you need to factor in about 1/3 of the filter thickness.

w7ay

enaiman You don't need the actual distance to the filter itself.

Actually, you do, if you are close to aperture vignetting the light cone to the sensor.

If the exit pupil of the telescope is smaller than the sensor diagonal, you would want to place the filter closer to the metal back of the telescope to minimize aperture vignetting, but usually, with the exit pupil larger than the sensor, you would want the filter to be as close as possible to the sensor.

Chen

enaiman

w7ay splitting hairs much Chen? 😛