Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Stargazing 135mm and f2.8 lens

  1. Sep 19, 2005 #1


    User Avatar

    Staff: Mentor

    I bought a new toy to play with. It's a 135mm/f2.8 lens I found used at B&H Photo for $39. I cut a hole in the back lens cap, glued a t-thread spacer to it, and built a little piggy-back rig for it and my DSI. Its pretty heavy, so I was a little worried that my ETX wouldn't be able to handle it (I have a counterweight on the bottom-aft), but it did a pretty good job (my usual periodic jerkiness tracking problem notwithstanding). I get about 2 degrees of field of view, which is about perfect for most open clusters and larger galaxies/nebulae.

    The piggyback rig isn't quite aligned with the telescope, so I don't think I actually got M-39 in the one pic, but it's a decent pic anyway: lots of stars in the area - so many, in fact, that I can't really identify the area on my planetarium program. Clouds were moving in so fast, I made no attempt to be sure I was on target after slewing away from M-13. From centering M-13, it looks like the piggyback rig is canted up about a degree, and I made some adjustments for next time.

    The weather should be clearer in the next few days(and the moon will be moving out of the way), so I'll hopefully get some better pics. I'll set up in a better area, too - these pics were taken outside my apartment, with a big floodlight 20 feet away, pointed directly at the scope.

    If I can get my tracking issues worked out and get some luck with long exposures, I may buy a DSI Pro. I'm a little disappointed that the DSI uses a grid of filters to get color, making the effective resolution about a quarter what the advertised resolution is. Otherwise, I may just go back to a webcam for planetary work.

    Attached Files:

    Last edited: Sep 19, 2005
  2. jcsd
  3. Sep 20, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    Try 'tracking' a fixed object first [streetlights are good] and see what happens.
  4. Sep 20, 2005 #3


    User Avatar
    Gold Member

    Hey, Russ. To get some experience with piggybacking and tracking, you might want to haunt the pawn shops a bit and get an OM-1. These are the perfect cameras for astrophotography. Light, with removable focusing screens, AND mirror lock-up to minimize vibration. Best of all, you don't need the meter for astrophotograpy, and can get one with a broken meter really cheap. Yes, film is not as user-friendly as digital imagery, but the resolution is wonderful, and it's easier to determine the source of periodic tracking errors with a long film exposure. You could also take some adult-ed courses in film photography and get some free (or cheap) lab time to process your own film and make prints. I've done this at home and it's a blast.
  5. Sep 20, 2005 #4


    User Avatar

    Staff: Mentor

    I'm not really sure what you mean, but I use streetlights to align my finderscope all the time. It really is a tracking problem, not an unsteady mount problem, if that's what you mean, and I describe it starting with post #59.

    An update, though: I spoke with Meade tech support a week ago and the guy I talked to said its periodic error and I should use Autostar's periodic error correction (newly available for the ETX) to correct it. I think he's wrong, as it is my understanding that periodic error occurs slowly over several minutes. My problem happens in less than two seconds: The image jumps ahead (I think its ahead, but need to verify that) in half a second or so, holds for a second, then jumps back. I'll try it, though, just to cover my bases.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: 135mm and f2.8 lens
  1. Lens cleaning (Replies: 0)

  2. Luneburg lens (Replies: 5)

  3. Gravitational lens (Replies: 1)

  4. Gravity Lens loop (Replies: 3)