Friday, June 6, 2014

Killing time with the ST80, 85mm lenses, and GM8

Doing some practice work in the back yard as I prepare for GSSP, I've been fiddling with trying to get the most from what will be the film setup.

So far:

  • Tested polar alignment within PHD2 (2.2.2a) that it works well without an ASCOM mount.  Just point it at the proper parts of the sky, set the approximate values in the Drift Align tool within PHD2 and go.  The rest of the method seems to work.  What's nice is that the way PHD2 now has equipment settings that are savable and selectable, notes from the respective Drift Tool are persistent.
  • I have seen the return of a slow, consistent RA drift.  Considering that I've removed the three-point rings on the ST80 in favor of the clamping rings, I'm thinking that what's happening now is a refraction-based compression and expansion of the scene as the scope points higher into the sky.  Will need to run some math and see what the expected distortion should be and if it checks out.  Also, it would be worthwhile to check in other parts of the sky to see if the apparent movement tracks via RA or via refraction.  Testing near the pole, but low in altitude would be worthwhile.
  • Tightened up the DEC backlash that was causing issues with calibration.  Checked to ensure that the grub screws attaching the motor and worm gear were tight, then watched the spinning gear vs movement in the eyepiece.  Turned out that the backlash problem was irrespective of TVC or motor connection play.  What was the real problem was gear lash between the worm and the ring.  Tightened up this play via repositioning the worm blocks and offsetting the DEC balance.  Now the mount will move in DEC within 5 seconds of a change in direction.
  • Tested the performance of a couple of lenses.  Last year I picked up the smc PENTAX-A* 1:1.4 85mm lens for K mount.  I also had the S-M-C Takumar 6X7 LS 90mm f/2.8 lens.  The K mount lens performs well at F4 - sharp to the corners and minimal fringing once it's in focus.  I tried putting the B+W 486 filter on it to control the fringe, however this introduced star flare.  In reaction, I tried the medium format lens with an IDAS LPS P2 filter inside the adapter.  After a long time attempting to focus the 67 lens using the bahtinov mask, I was disappointed to see that the field flatness of the lens left a lot to be desired!  Also note the red doughnuts visible in the 67 lens image.  This was shot on a non-converted camera, so it seems the unfocused starlight is a problem even before the NIR range.
Upper left corner at 100%

Vega region, near center of FOV
Final testing to complete on the ST80:
  • Continue adjusting lens cell of the ST80 to improve star images for the SSAG.  I will loosen the holding screws on the lens cell and try tilting it to improve the out-of-round patterns of the stars.
  • Test different filters on the SSAG.  I have just a few available in the 1.25'' size.  Baader Moon and Skyglow, UHC, Baader Contrast Booster, Yellow, and Red 25.  The yellow showed some promise, but not as good as I'd like.  I think it was passing too much NIR.  The Moon and Skyglow filter has a built in IR block, so it should work well.