Thursday, January 24, 2008

Light bucket

I have a a 12 inch Newtonian telescope on a Dobsonian mount. It's a cheap light bucket. Over the weekend I tried hooking up a DSLR camera to it.

Two initial results:
  1. The adaptor physically connects the DSLR to the scope, but it doesn't guarantee that it will work. In my case, it doesn't. The problem probably applies to most telescopes designed to be used with eyepieces:
    • The eyepiece, e.g. a 30mm eyepiece, is a 30mm focal length lens designed to take an image 30mm in front of the lens and make it appear to be at infinity. Your eye looks through the lens to see the image at infinity.
    • The image that the eyepiece is focussed on is 30mm in front of the first nodal point of that lens. It appears that the standard for telescope eyepieces is to have that image in front of the shoulder of the eyepiece. When you remove the eyepiece and put a plain piece of paper on the image, you find that it is 5-10mm inside the focussing tube.
    • The focussing tube has some range, such that your can rack it forward and get the image to be 10mm behind the focussing tube, but:
    • DSLRs all want about 42mm between their front flange and the sensor plane. You focus them by placing the image on the sensor plane.
    • There is no way to focus a camera attached to this thing at infinity, without altering the telescope to move the image focus out.
    • If the image focus is moved out, some sort of extension tube, about 2 inches long, will be necessary with all eyepieces to make it possible to focus them.
    • I'll simulate it, but I suspect that extension tube will then limit the field of view of some of the larger FOV eyepieces.
  2. Surprise, the DSLR will focus on things at finite distances! If you pull the imager on a 1500mm scope out 2 inches from focussed-at-infinity, you are focussed 43.5m away. So, I took some shots of some tree branches at about that distance while pointed close to the sun.
    • This was a little dangerous, because if I'd accidentally pointed it at the sun while looking into it I could have hurt myself. I got lucky this time, and I'll not be impatient again.
    • Depth of field is awful. Spot size is about 10 microns (pixel size x 1.8 for the Bayer sensor), so an f/5 scope focussed at 43.5m away has a depth of focus of +/- 5*10 microns which corresponds to +/- 45 mm out by the tree branches.
    • The focus was only okay, not great.
      • Global contrast issues below
      • The scope has, at minimum, nasty coma which will smear images. I had a Paracorr lens between the camera and the telescope, but I don't think I had it adjusted properly, and I have not verified that the scope actually has a parabolic and not spherical mirror.
    • Contrast was ridiculously bad. If you saw a photographic lens this bad you might chuckle, but you would never, ever consider buying it. Saturday night I tried looking at the moon, and found that anywhere within 20 degrees of the moon the sky had a uniform grey background that hid most of the stars.
    • I need to clean my optics, there is dust on them.
    • I need to flock the interior of the telescope.
Maybe amateur telescopes all have terrible contrast because amateur astronomers are used to looking at stuff that's mostly black, so that a little scattered light is no problem, compared to daytimes scenes where it does matter.

So, photography through the telescope is not a trivially implemented idea. It does have me thinking about how to design a Newtonian telescope that can do photography, daytime or night, as well as stargazing. I know a thing or two about flare suppression and camera design as a result of my work on Street View, and I can see a bunch of obvious problems that might be fixable.
  • The secondary mirror is not balanced on the spider, so that it twists as the telescope is changed in altitude. I can actually see this with the autocollimator in the scope, which measures maybe 4mm of drift between horizontal and vertical. That's an angle of 2.67 milliradians. The autocollimator doubles the actual angle, so it's about 1.33 milliradians. Across the 43mm field of a DSLR, that's 57 microns of tilt, which is a smidge more than half the focus budget of +/- 50 microns. It would be good to balance the secondary on the spider with a counterweight.
  • The eyepiece should not view the opposite side of the tube around the secondary mirror. Instead, it should view a recessed surface which is itself shaded from both the aperture and the mirror.
  • The eyepiece tube should have a baffle, which is recessed from the tube so that it is not lit by the aperture, and which prevents the eyepiece from seeing anything but the secondary mirror and the recessed light trap behind it.
Hmm.... this is looking like a lot of work.