Hi,

I attached a ZWO ASI 120MM-C to a William Optics Spacecat (F4.9) with a 40 mm extension tube in between. The Spacecat has 77.7 mm back focus, excluding 24 mm of rings behind the point indicated in a diagram, and the sensor sits 12.5 mm inside the camera, so I thought I could compensate for the remaining 1,2 mm by adjusting the focus on the Spacecat.



However, when I turn on Ekos during daylight, instead of the buildings I'm pointing the lens at, all I see is a white image, which turns dark when I cover the lens with my hand. At night it is completely black, no matter how I adjust the focus.

Did I understand this wrong, or do I need to change some settings in Ekos?

Perhaps a silly question: what is your exposure time?

I'm really new to this kind of camera, so, no, not silly! Please ask every question.

The exposure time is set to 1. When I change it to shorter times, the feed becomes darker (first yellow, then blue) and spotted, but I don't see any details.

We use a RedCat with an ASI2600 and that focuses perfectly well with 55 mm from the end of the telescope to the sensor of the camera. With a DSLR and the WO adapter for that camera we just don’t reach focus so we added 2 rings of 1 mm and now it’s fine.

I can't quite follow your math. If you connected the extender at 55.2 (59.7 minus 4.5), then subtracted 40 for the spacer and 12.5 more for the sensor recess in the camera housing, doesn't that still put you 2.7mm shy of intended backfocus position? Did I calculate this wrong?

Who are you asking?

I was asking Kleinebeer. I was typing my post and hadn't paid enough attention that you responded prior..... In any case, I think we're likely in agreement that he needs more spacers.... Cheers, Doug

So I need spacers to 55mm (55-40=15mm left)? So what is wrong with my calculations then?

And " the end of the telescope", is that the numbered ring, as indicated by the arrow in the diagram, or further away?

If you want to be precise, look at the diagram again and my post. You are likely 2.7mm away. Wouter suggests you can focus with 2mm more, and that might be possible. If you're going for precision, you'll want to get as close to that 2.7mm needed as possible. Slightly different may get you to focus, but with risk of slight optical aberrations. Best to try and get as close as reasonably possible IMO.

Leave the back focus as is, the exposure time is too long, 1.0 seconds for daylight exposures, set the exposure to the minimum the camera will allow and adjust the focus until you reach focus, alternatively put the camera in video mode and adjust focus until you get an image. That should sort it.

And end of the telescope means BEFORE the thread starts. The thread goes inside the spacers so it doesn’t count.

According to the diagram, the image focuses at 77.7 mm from the edge of the ring with the numbers on it. Attached to that ring, I have 24 mm worth of other rings, the last one of which I added a 40 mm extender to, to which I added the camera, (12.5mm), so 77.7 - (24 + 40 + 12.5) = 1.2. I figured I could compensate for the 1.2 using the manual focus of the Spacecat.

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