Once again, we have an other back focus question

[chris_g]

Hello all,

I just got my C8-A in with the Celestron F/6.3 reducer/corrector. The documentation for the reducer/corrector does not even mention back focus requirements, https://celestron-site-support-files.s3 ... 5%20IM.pdf. So off I go to do an Internet search and I'm finding 85mm or 105mm. I also found an interesting post on CN, finally I have my answer about the size of image circles, at least with focal reducers.

As for the vignetting, that's pretty common. A reducer can only produce an illuminated spot that is equal to the clear aperture of the reducer multiplied by it's reduction factor. So for the .63 reduction, and the 41mm aperture of the reducer, that results in a spot 25.83mm in size. That's a tad smaller than an APS-C sensor. So there will be vignetting for any sensor with a diagonal measurement that's close to 25.83mm.

My image train currently is a WO SCT to M48 adapter, 14mm, ZWO OAG, quality docs there, 16.5mm thick, Blue Fireball filter drawer, 16.4mm = 46.9mm

So here're my questions.

Do I need 55mm, 85mm or 105mm? With extension tubes I already have on hand, I can add up to another 52.5mm. That would take it to about 100mm total so I would need another 5mm

I also read that bringing the camera closer to the reducer decreases the reduction factor. Based on what I read above, that would be beneficial by creating a larger image circle. I would need to move inward until the reduction factor is .67. to cover the T2i 's sensor? Does decreasing the distance from the sensor to the camera actually reduce the reduction multiplier? My brain says yes as I visualize the camera moving inward in the light path, however, my brain has been known to be wrong. I'm more than willing to experiment with shooting different back focus lengths, experimenting is half the fun, if that is even a possibility.

As always, thanks for any input, good, bad or indifferent!

Clear Skies,
Chris

[chris_g]

I found the answer, back focus distance to the Celestron .63 reducer is 105mm, as far as I can tell, since Celestron tech support doesn't seem to know, according to one post I found. So subtract 44 for the camera body, 11 for the T-Ring (Canon), the adapter should be 50mm. I've got it to 50.25

Now I set up the tube and get all excited about shooting at 1280 and the 550D won't talk on the USB bus. Swapped cables, hubs and tried it on my machine, nope. It still turns on and works like a normal camera, just won't talk on the USB bus. Guess the astro gods aren't so nice after all. Had I known the camera wasn't working, it was last new moon cycle, I would have invested in a new camera instead of the C8, oh well. I still have the stock 600D and the Ha 6D. Maybe I can figure out what's wrong with it but I doubt it.

[SkyHiker]

What's wrong with the 6D, and did you modify the 550D yourself?

[chris_g]

What's wrong with the 6D, and did you modify the 550D yourself?

Nothing, the 6D was on the Z73 at the time getting data on the North America Nebula. No, I didn't, I don't have the manual dexterity any more to do that, actually, never did. It's a USB issue. Camera still functions, just won' talk to a computer any more on the USB bus, tried multiple computers and different cables and batteries, no joy, definitely a camera issue. It was just working three weeks ago. It goes busy as soon as you connect it to a USB port and Windows eventually reports a USB device failed. I still have my stock 600D, I'll shoot with it for a while, while I see if this can be fixed or not. Definitely time to start saving for a monochrome