So I had started a thread under ATM asking about using a borescope camera on axis for dpac testing. Nobody seems to have tried it, and didn't seem to think it would work, since borescopes typically have wide angle lenses on them focused in the near field.
So I've come back here, and have some questions. I have tried several configurations with my firefly MV camera and get no better results than I showed above. Usually, they're a lot worse, if I can find the reflection at all, that is. Note that this test is of the 8" f//8 mirror that i've stripped the coatings from.
I find that, once I've gotten the optics collimated, I still can't see the ronchi bands over the whole mirror. As I said, the results I've been getting are worse than what I showed above, with the ronchigram being "football shaped" with the shadow of the secondary mirror on one side of the football. The whole mirror is visible in the camera, too (see attached pic taken with one shop light on so I could see the secondary hub at the same time view the ronchigram). Note that this was the best I could do with the adjustment screws on the primary cell. I never did get the bands on the "left" side of the mirror, unless I rotated the camera/light, but then got the same size or narrower football. So I don't know why I'm not seeing the ronchigram over the mirror.
I've tried just using the ronchi screen and LED and viewing by eye, but I can't find the reflection any easier by eye. I can't adjust the mirror support while viewing either. And I would rather not core the Perkin Elmer 12" flat if at all possible. It's just too nice (and about 2 1/2" thick). Being on the optic axis without having a **** secondary to adjust as well as the primary and flat, would certainly be my preference, so I've been thinking about building a support stand that would hold a 1 1/4" ID tube that I could put my laser in and then swap out for the camera/light/ronchi screen.
*How close does a camera lens need to be to the ronchi screen to image the ronchigram?
The firefly MV uses a webcam lens, so its' pretty small and the front element is close to the front of the lens barrel.
The Flea has a c-mount lens where the front element is a good 3/4" inside the light shield, so I can't get it any closer to the screen than that.
The iphone X's camera is only about 1/4" from the LED, and is even smaller than the webcam lens. It and the LED can be placed very close to the ronchi screen, so if I could mount the iphone in the focuser I've been using and not have it wiggle every time I go to image with it, that would still be my preferred option.
*When viewing by eye, there's obviously a lens involved for focusing. But this isn't afocal, since there's no eyepiece in the telescope. So, does the camera need a lens in order to reach focus? What are you focusing on when imaging the ronchigram? Infinity? Or the ronchi screen?
Here's what I saw with the firefly MV after I'd made a new "ronchi eyepiece" with it, the red LED, and a piece of ronchi screen taped to a 1 1/4" adapter for the 2" focuser I've been using.
Edited by tim53, 18 February 2019 - 06:53 PM.