How do you focus on a star?
Jan. 11th, 2007 12:28 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
fivemackEnlivened 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 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 ]
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 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 ]
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no subject
Date: 2007-01-11 05:17 am (UTC)With many ordinary lenses, just focusing to infinity *is* the winning option. But some lenses (mostly not ordinary ones though) have enough thermal expansion that they are made to focus beyond infinity at most temperatures, so that can cause trouble.
no subject
Date: 2007-01-11 05:18 am (UTC)I find it's a great available-light portrait lens, too.
no subject
Date: 2007-01-11 11:52 am (UTC)My own consumer grade pretend DSLR (looks like a DSLR so it can get a 10x zoom, but other than that, it's cheap plastic crap) has "manual" focussing that is two buttons that are *positively* useless. I mean, I can design better firmware! Seriously, just blow up what's in the centre until a few seconds after releasing one of the focus buttons. Idiots. Not that it reports how much focus has changed, so you can't do a binary search in low light levels.
Right now, at the Somewhat Bigger[TM] telescope, my autoguider consists of a CCD on a zoom lense, with filters sticky taped onto that. The exposures for the CCD are set by an RC network, with the R up here next to me 50m above the ground, and the C a couple of 10m floors below, on the other side of the coude 5 mirror. Precision high tech supremum. Doesn't matter when the seeing is 2". Ick.