fivemack (fivemack) wrote,
fivemack
fivemack

How do you focus on a star?

Enlivened by the comet, I went up Castle Hill again, with my birding lens and nice new tripod.

After one or two bits of infrastructure-building - figuring out the format of the sensor-data part of Nikon's .NEF files (the last 9728000 bytes of the file contain 2000 lines, each of 3040 samples packed as 304 16-byte chunks, with each chunk containing ten 12-bit numbers packed two-in-three-bytes as 0xAA 0xAB 0xBB and a final 0x00) and implementing dark-field subtraction because the D100's sensor, especially at high ISO, is plagued with hot pixels, I got some reasonable shots.

The Hyades cluster in Taurus; Aldebaran bottom left



The problem is, the star images are big and ugly. I'd calculated, based on a pixel size of 7.8 microns, that a star would move three pixels in a 1.6-second exposure at 170mm, or one pixel in a 0.3-second exposure at 500mm. I'd noticed that it took a while for vibrations in the tripod to settle, so used self-timer to let it settle before taking the shot. But still a star at 500mm is fifteen pixels across on the 0.3s-exposed image. Seeing in Cambridge isn't great, but it's not likely to be as dreadful as 45 arc-seconds.



My guess is that I'm not focussing correctly. But I'm not sure how I focus correctly; there's no light, so auto-focus just hunts back and forth and gets nowhere, turning the focus dial until it hits the infinity end-stop doesn't seem to be sufficient; my eyes can't distinguish a perfectly-focussed star from a slightly out-of-focus star. Towards the end of the session, taking photos pointed pretty much straight up with the lens at 500mm, I was getting images trailed by movement of the tripod rather than the stars, but that's more a matter of waiting for a season where the interesting object isn't at the zenith.

[ For taking pictures of stars, I think the figure-of-merit is focal length / f_stop^2; the permitted exposure before the motion of the stars blurs them is proportional to 1/focal_length, and the amount of light that gets in is proportional to the lens area = (fl/ap)^2. For star-fields, it's 1/(fstop^2 * focal length), since the area of sky you see, and so the number of stars, is proportional to 1/fl^2. 50/1.4 lenses are what this measure tells one to lust after ]
Subscribe
  • Post a new comment

    Error

    default userpic

    Your IP address will be recorded 

    When you submit the form an invisible reCAPTCHA check will be performed.
    You must follow the Privacy Policy and Google Terms of use.
  • 3 comments